path: root/contrib/awk/doc/gawk.texi

\input texinfo   @c -*-texinfo-*-
@c %**start of header (This is for running Texinfo on a region.)
@setfilename gawk.info
@settitle The GNU Awk User's Guide
@c %**end of header (This is for running Texinfo on a region.)

@dircategory GNU Packages
@direntry
* Gawk: (gawk).           A text scanning and processing language.
@end direntry
@dircategory Individual utilities
@direntry
* awk: (gawk)Invoking gawk.           Text scanning and processing.
@end direntry

@c @set xref-automatic-section-title

@c The following information should be updated here only!
@c This sets the edition of the document, the version of gawk it
@c applies to and all the info about who's publishing this edition

@c These apply across the board.
@set UPDATE-MONTH March, 2001
@set VERSION 3.1
@set PATCHLEVEL 0

@set FSF

@set TITLE GAWK: Effective AWK Programming
@set SUBTITLE A User's Guide for GNU Awk
@set EDITION 3

@iftex
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER @inmargin{@image{lflashlight,1cm}, @image{rflashlight,1cm}}
@end iftex
@ifinfo
@set DOCUMENT Info file
@set CHAPTER major node
@set APPENDIX major node
@set SECTION minor node
@set SUBSECTION node
@set DARKCORNER (d.c.)
@end ifinfo
@ifhtml
@set DOCUMENT Web page
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@end ifhtml

@c some special symbols
@iftex
@set LEQ @math{@leq}
@end iftex
@ifnottex
@set LEQ <=
@end ifnottex

@set FN file name
@set FFN File Name
@set DF data file
@set DDF Data File
@set PVERSION version

@ignore
Some comments on the layout for TeX.
1. Use at least texinfo.tex 2000-09-06.09
2. I have done A LOT of work to make this look good. There are  `@page' commands
   and use of `@group ... @end group' in a number of places. If you muck
   with anything, it's your responsibility not to break the layout.
@end ignore

@c merge the function and variable indexes into the concept index
@ifinfo
@synindex fn cp
@synindex vr cp
@end ifinfo
@iftex
@syncodeindex fn cp
@syncodeindex vr cp
@end iftex

@c If "finalout" is commented out, the printed output will show
@c black boxes that mark lines that are too long.  Thus, it is
@c unwise to comment it out when running a master in case there are
@c overfulls which are deemed okay.

@iftex
@finalout
@end iftex

@c Comment out the "smallbook" for technical review.  Saves
@c considerable paper.  Remember to turn it back on *before*
@c starting the page-breaking work.
@smallbook

@ifinfo
This file documents @command{awk}, a program that you can use to select
particular records in a file and perform operations upon them.

This is Edition @value{EDITION} of @cite{@value{TITLE}: @value{SUBTITLE}},
for the @value{VERSION}.@value{PATCHLEVEL} version of the GNU implementation of AWK.

Copyright (C) 1989, 1991, 1992, 1993, 1996-2001 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'', the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
``GNU Free Documentation License''.

@enumerate a
@item
``A GNU Manual''

@item
``You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.''
@end enumerate
@end ifinfo

@c Uncomment this for the release.  Leaving it off saves paper
@c during editing and review.
@setchapternewpage odd

@titlepage
@title @value{TITLE}
@subtitle @value{SUBTITLE}
@subtitle Edition @value{EDITION}
@subtitle @value{UPDATE-MONTH}
@author Arnold D. Robbins

@c Include the Distribution inside the titlepage environment so
@c that headings are turned off.  Headings on and off do not work.

@page
@vskip 0pt plus 1filll
@ignore
The programs and applications presented in this book have been
included for their instructional value.  They have been tested with care
but are not guaranteed for any particular purpose.  The publisher does not
offer any warranties or representations, nor does it accept any
liabilities with respect to the programs or applications.
So there.
@sp 2
UNIX is a registered trademark of The Open Group in the United States and other countries. @*
Microsoft, MS and MS-DOS are registered trademarks, and Windows is a
trademark of Microsoft Corporation in the United States and other
countries. @*
Atari, 520ST, 1040ST, TT, STE, Mega and Falcon are registered trademarks
or trademarks of Atari Corporation. @*
DEC, Digital, OpenVMS, ULTRIX and VMS are trademarks of Digital Equipment
Corporation. @*
@end ignore
``To boldly go where no man has gone before'' is a
Registered Trademark of Paramount Pictures Corporation. @*
@c sorry, i couldn't resist
@sp 3
Copyright @copyright{} 1989, 1991, 1992, 1993, 1996-2001 Free Software Foundation, Inc.
@sp 2

This is Edition @value{EDITION} of @cite{@value{TITLE}: @value{SUBTITLE}},
for the @value{VERSION}.@value{PATCHLEVEL} (or later) version of the GNU
implementation of AWK.

@sp 2
Published by:
@sp 1

Free Software Foundation @*
59 Temple Place --- Suite 330 @*
Boston, MA  02111-1307 USA @*
Phone: +1-617-542-5942 @*
Fax: +1-617-542-2652 @*
Email: @email{gnu@@gnu.org} @*
URL: @uref{http://www.gnu.org/} @*

@c This one is correct for gawk 3.1.0 from the FSF
ISBN 1-882114-28-0 @*

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'', the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
``GNU Free Documentation License''.

@enumerate a
@item
``A GNU Manual''

@item
``You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.''
@end enumerate
@sp 2
Cover art by Etienne Suvasa.
@end titlepage

@c Thanks to Bob Chassell for directions on doing dedications.
@iftex
@headings off
@page
@w{ }
@sp 9
@center @i{To Miriam, for making me complete.}
@sp 1
@center @i{To Chana, for the joy you bring us.}
@sp 1
@center @i{To Rivka, for the exponential increase.}
@sp 1
@center @i{To Nachum, for the added dimension.}
@sp 1
@center @i{To Malka, for the new beginning.}
@w{ }
@page
@w{ }
@page
@headings on
@end iftex

@iftex
@headings off
@evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @|
@oddheading  @| @| @strong{@thischapter}@ @ @ @thispage
@end iftex

@ifinfo
@node Top, Foreword, (dir), (dir)
@top General Introduction
@c Preface node should come right after the Top
@c node, in `unnumbered' sections, then the chapter, `What is gawk'.
@c Licensing nodes are appendices, they're not central to AWK.

This file documents @command{awk}, a program that you can use to select
particular records in a file and perform operations upon them.

This is Edition @value{EDITION} of @cite{@value{TITLE}: @value{SUBTITLE}},
for the @value{VERSION}.@value{PATCHLEVEL} version of the GNU implementation
of AWK.

@end ifinfo

@menu
* Foreword::                       Some nice words about this
                                   @value{DOCUMENT}.
* Preface::                        What this @value{DOCUMENT} is about; brief
                                   history and acknowledgments.
* Getting Started::                A basic introduction to using
                                   @command{awk}. How to run an @command{awk}
                                   program. Command-line syntax.
* Regexp::                         All about matching things using regular
                                   expressions.
* Reading Files::                  How to read files and manipulate fields.
* Printing::                       How to print using @command{awk}. Describes
                                   the @code{print} and @code{printf}
                                   statements. Also describes redirection of
                                   output.
* Expressions::                    Expressions are the basic building blocks
                                   of statements.
* Patterns and Actions::           Overviews of patterns and actions.
* Arrays::                         The description and use of arrays. Also
                                   includes array-oriented control statements.
* Functions::                      Built-in and user-defined functions.
* Internationalization::           Getting @command{gawk} to speak your
                                   language.
* Advanced Features::              Stuff for advanced users, specific to
                                   @command{gawk}.
* Invoking Gawk::                  How to run @command{gawk}.
* Library Functions::              A Library of @command{awk} Functions.
* Sample Programs::                Many @command{awk} programs with complete
                                   explanations.
* Language History::               The evolution of the @command{awk}
                                   language.
* Installation::                   Installing @command{gawk} under various
                                   operating systems.
* Notes::                          Notes about @command{gawk} extensions and
                                   possible future work.
* Basic Concepts::                 A very quick intoduction to programming
                                   concepts.
* Glossary::                       An explanation of some unfamiliar terms.
* Copying::                        Your right to copy and distribute
                                   @command{gawk}.
* GNU Free Documentation License:: The license for this @value{DOCUMENT}.
* Index::                          Concept and Variable Index.

@detailmenu
* History::                        The history of @command{gawk} and
                                   @command{awk}.
* Names::                          What name to use to find @command{awk}.
* This Manual::                    Using this @value{DOCUMENT}. Includes
                                   sample input files that you can use.
* Conventions::                    Typographical Conventions.
* Manual History::                 Brief history of the GNU project and this
                                   @value{DOCUMENT}.
* How To Contribute::              Helping to save the world.
* Acknowledgments::                Acknowledgments.
* Running gawk::                   How to run @command{gawk} programs;
                                   includes command-line syntax.
* One-shot::                       Running a short throw-away @command{awk}
                                   program.
* Read Terminal::                  Using no input files (input from terminal
                                   instead).
* Long::                           Putting permanent @command{awk} programs in
                                   files.
* Executable Scripts::             Making self-contained @command{awk}
                                   programs.
* Comments::                       Adding documentation to @command{gawk}
                                   programs.
* Quoting::                        More discussion of shell quoting issues.
* Sample Data Files::              Sample data files for use in the
                                   @command{awk} programs illustrated in this
                                   @value{DOCUMENT}.
* Very Simple::                    A very simple example.
* Two Rules::                      A less simple one-line example using two
                                   rules.
* More Complex::                   A more complex example.
* Statements/Lines::               Subdividing or combining statements into
                                   lines.
* Other Features::                 Other Features of @command{awk}.
* When::                           When to use @command{gawk} and when to use
                                   other things.
* Regexp Usage::                   How to Use Regular Expressions.
* Escape Sequences::               How to write non-printing characters.
* Regexp Operators::               Regular Expression Operators.
* Character Lists::                What can go between @samp{[...]}.
* GNU Regexp Operators::           Operators specific to GNU software.
* Case-sensitivity::               How to do case-insensitive matching.
* Leftmost Longest::               How much text matches.
* Computed Regexps::               Using Dynamic Regexps.
* Records::                        Controlling how data is split into records.
* Fields::                         An introduction to fields.
* Non-Constant Fields::            Non-constant Field Numbers.
* Changing Fields::                Changing the Contents of a Field.
* Field Separators::               The field separator and how to change it.
* Regexp Field Splitting::         Using regexps as the field separator.
* Single Character Fields::        Making each character a separate field.
* Command Line Field Separator::   Setting @code{FS} from the command-line.
* Field Splitting Summary::        Some final points and a summary table.
* Constant Size::                  Reading constant width data.
* Multiple Line::                  Reading multi-line records.
* Getline::                        Reading files under explicit program
                                   control using the @code{getline} function.
* Plain Getline::                  Using @code{getline} with no arguments.
* Getline/Variable::               Using @code{getline} into a variable.
* Getline/File::                   Using @code{getline} from a file.
* Getline/Variable/File::          Using @code{getline} into a variable from a
                                   file.
* Getline/Pipe::                   Using @code{getline} from a pipe.
* Getline/Variable/Pipe::          Using @code{getline} into a variable from a
                                   pipe.
* Getline/Coprocess::              Using @code{getline} from a coprocess.
* Getline/Variable/Coprocess::     Using @code{getline} into a variable from a
                                   coprocess.
* Getline Notes::                  Important things to know about
                                   @code{getline}.
* Getline Summary::                Summary of @code{getline} Variants.
* Print::                          The @code{print} statement.
* Print Examples::                 Simple examples of @code{print} statements.
* Output Separators::              The output separators and how to change
                                   them.
* OFMT::                           Controlling Numeric Output With
                                   @code{print}.
* Printf::                         The @code{printf} statement.
* Basic Printf::                   Syntax of the @code{printf} statement.
* Control Letters::                Format-control letters.
* Format Modifiers::               Format-specification modifiers.
* Printf Examples::                Several examples.
* Redirection::                    How to redirect output to multiple files
                                   and pipes.
* Special Files::                  File name interpretation in @command{gawk}.
                                   @command{gawk} allows access to inherited
                                   file descriptors.
* Special FD::                     Special files for I/O.
* Special Process::                Special files for process information.
* Special Network::                Special files for network communications.
* Special Caveats::                Things to watch out for.
* Close Files And Pipes::          Closing Input and Output Files and Pipes.
* Constants::                      String, numeric and regexp constants.
* Scalar Constants::               Numeric and string constants.
* Non-decimal-numbers::            What are octal and hex numbers.
* Regexp Constants::               Regular Expression constants.
* Using Constant Regexps::         When and how to use a regexp constant.
* Variables::                      Variables give names to values for later
                                   use.
* Using Variables::                Using variables in your programs.
* Assignment Options::             Setting variables on the command-line and a
                                   summary of command-line syntax. This is an
                                   advanced method of input.
* Conversion::                     The conversion of strings to numbers and
                                   vice versa.
* Arithmetic Ops::                 Arithmetic operations (@samp{+}, @samp{-},
                                   etc.)
* Concatenation::                  Concatenating strings.
* Assignment Ops::                 Changing the value of a variable or a
                                   field.
* Increment Ops::                  Incrementing the numeric value of a
                                   variable.
* Truth Values::                   What is ``true'' and what is ``false''.
* Typing and Comparison::          How variables acquire types and how this
                                   affects comparison of numbers and strings
                                   with @samp{<}, etc.
* Boolean Ops::                    Combining comparison expressions using
                                   boolean operators @samp{||} (``or''),
                                   @samp{&&} (``and'') and @samp{!} (``not'').
* Conditional Exp::                Conditional expressions select between two
                                   subexpressions under control of a third
                                   subexpression.
* Function Calls::                 A function call is an expression.
* Precedence::                     How various operators nest.
* Pattern Overview::               What goes into a pattern.
* Regexp Patterns::                Using regexps as patterns.
* Expression Patterns::            Any expression can be used as a pattern.
* Ranges::                         Pairs of patterns specify record ranges.
* BEGIN/END::                      Specifying initialization and cleanup
                                   rules.
* Using BEGIN/END::                How and why to use BEGIN/END rules.
* I/O And BEGIN/END::              I/O issues in BEGIN/END rules.
* Empty::                          The empty pattern, which matches every
                                   record.
* Using Shell Variables::          How to use shell variables with
                                   @command{awk}.
* Action Overview::                What goes into an action.
* Statements::                     Describes the various control statements in
                                   detail.
* If Statement::                   Conditionally execute some @command{awk}
                                   statements.
* While Statement::                Loop until some condition is satisfied.
* Do Statement::                   Do specified action while looping until
                                   some condition is satisfied.
* For Statement::                  Another looping statement, that provides
                                   initialization and increment clauses.
* Break Statement::                Immediately exit the innermost enclosing
                                   loop.
* Continue Statement::             Skip to the end of the innermost enclosing
                                   loop.
* Next Statement::                 Stop processing the current input record.
* Nextfile Statement::             Stop processing the current file.
* Exit Statement::                 Stop execution of @command{awk}.
* Built-in Variables::             Summarizes the built-in variables.
* User-modified::                  Built-in variables that you change to
                                   control @command{awk}.
* Auto-set::                       Built-in variables where @command{awk}
                                   gives you information.
* ARGC and ARGV::                  Ways to use @code{ARGC} and @code{ARGV}.
* Array Intro::                    Introduction to Arrays
* Reference to Elements::          How to examine one element of an array.
* Assigning Elements::             How to change an element of an array.
* Array Example::                  Basic Example of an Array
* Scanning an Array::              A variation of the @code{for} statement. It
                                   loops through the indices of an array's
                                   existing elements.
* Delete::                         The @code{delete} statement removes an
                                   element from an array.
* Numeric Array Subscripts::       How to use numbers as subscripts in
                                   @command{awk}.
* Uninitialized Subscripts::       Using Uninitialized variables as
                                   subscripts.
* Multi-dimensional::              Emulating multidimensional arrays in
                                   @command{awk}.
* Multi-scanning::                 Scanning multidimensional arrays.
* Array Sorting::                  Sorting array values and indices.
* Built-in::                       Summarizes the built-in functions.
* Calling Built-in::               How to call built-in functions.
* Numeric Functions::              Functions that work with numbers, including
                                   @code{int}, @code{sin} and @code{rand}.
* String Functions::               Functions for string manipulation, such as
                                   @code{split}, @code{match} and
                                   @code{sprintf}.
* Gory Details::                   More than you want to know about @samp{\}
                                   and @samp{&} with @code{sub}, @code{gsub},
                                   and @code{gensub}.
* I/O Functions::                  Functions for files and shell commands.
* Time Functions::                 Functions for dealing with timestamps.
* Bitwise Functions::              Functions for bitwise operations.
* I18N Functions::                 Functions for string translation.
* User-defined::                   Describes User-defined functions in detail.
* Definition Syntax::              How to write definitions and what they
                                   mean.
* Function Example::               An example function definition and what it
                                   does.
* Function Caveats::               Things to watch out for.
* Return Statement::               Specifying the value a function returns.
* Dynamic Typing::                 How variable types can change at runtime.
* I18N and L10N::                  Internationalization and Localization.
* Explaining gettext::             How GNU @code{gettext} works.
* Programmer i18n::                Features for the programmer.
* Translator i18n::                Features for the translator.
* String Extraction::              Extracting marked strings.
* Printf Ordering::                Rearranging @code{printf} arguments.
* I18N Portability::               @command{awk}-level portability issues.
* I18N Example::                   A simple i18n example.
* Gawk I18N::                      @command{gawk} is also internationalized.
* Non-decimal Data::               Allowing non-decimal input data.
* Two-way I/O::                    Two-way communications with another
                                   process.
* TCP/IP Networking::              Using @command{gawk} for network
                                   programming.
* Portal Files::                   Using @command{gawk} with BSD portals.
* Profiling::                      Profiling your @command{awk} programs.
* Command Line::                   How to run @command{awk}.
* Options::                        Command-line options and their meanings.
* Other Arguments::                Input file names and variable assignments.
* AWKPATH Variable::               Searching directories for @command{awk}
                                   programs.
* Obsolete::                       Obsolete Options and/or features.
* Undocumented::                   Undocumented Options and Features.
* Known Bugs::                     Known Bugs in @command{gawk}.
* Library Names::                  How to best name private global variables
                                   in library functions.
* General Functions::              Functions that are of general use.
* Nextfile Function::              Two implementations of a @code{nextfile}
                                   function.
* Assert Function::                A function for assertions in @command{awk}
                                   programs.
* Round Function::                 A function for rounding if @code{sprintf}
                                   does not do it correctly.
* Cliff Random Function::          The Cliff Random Number Generator.
* Ordinal Functions::              Functions for using characters as numbers
                                   and vice versa.
* Join Function::                  A function to join an array into a string.
* Gettimeofday Function::          A function to get formatted times.
* Data File Management::           Functions for managing command-line data
                                   files.
* Filetrans Function::             A function for handling data file
                                   transitions.
* Rewind Function::                A function for rereading the current file.
* File Checking::                  Checking that data files are readable.
* Ignoring Assigns::               Treating assignments as file names.
* Getopt Function::                A function for processing command-line
                                   arguments.
* Passwd Functions::               Functions for getting user information.
* Group Functions::                Functions for getting group information.
* Running Examples::               How to run these examples.
* Clones::                         Clones of common utilities.
* Cut Program::                    The @command{cut} utility.
* Egrep Program::                  The @command{egrep} utility.
* Id Program::                     The @command{id} utility.
* Split Program::                  The @command{split} utility.
* Tee Program::                    The @command{tee} utility.
* Uniq Program::                   The @command{uniq} utility.
* Wc Program::                     The @command{wc} utility.
* Miscellaneous Programs::         Some interesting @command{awk} programs.
* Dupword Program::                Finding duplicated words in a document.
* Alarm Program::                  An alarm clock.
* Translate Program::              A program similar to the @command{tr}
                                   utility.
* Labels Program::                 Printing mailing labels.
* Word Sorting::                   A program to produce a word usage count.
* History Sorting::                Eliminating duplicate entries from a
                                   history file.
* Extract Program::                Pulling out programs from Texinfo source
                                   files.
* Simple Sed::                     A Simple Stream Editor.
* Igawk Program::                  A wrapper for @command{awk} that includes
                                   files.
* V7/SVR3.1::                      The major changes between V7 and System V
                                   Release 3.1.
* SVR4::                           Minor changes between System V Releases 3.1
                                   and 4.
* POSIX::                          New features from the POSIX standard.
* BTL::                            New features from the Bell Laboratories
                                   version of @command{awk}.
* POSIX/GNU::                      The extensions in @command{gawk} not in
                                   POSIX @command{awk}.
* Contributors::                   The major contributors to @command{gawk}.
* Gawk Distribution::              What is in the @command{gawk} distribution.
* Getting::                        How to get the distribution.
* Extracting::                     How to extract the distribution.
* Distribution contents::          What is in the distribution.
* Unix Installation::              Installing @command{gawk} under various
                                   versions of Unix.
* Quick Installation::             Compiling @command{gawk} under Unix.
* Additional Configuration Options:: Other compile-time options.
* Configuration Philosophy::       How it's all supposed to work.
* Non-Unix Installation::          Installation on Other Operating Systems.
* Amiga Installation::             Installing @command{gawk} on an Amiga.
* BeOS Installation::              Installing @command{gawk} on BeOS.
* PC Installation::                Installing and Compiling @command{gawk} on
                                   MS-DOS and OS/2.
* PC Binary Installation::         Installing a prepared distribution.
* PC Compiling::                   Compiling @command{gawk} for MS-DOS, Win32,
                                   and OS/2.
* PC Using::                       Running @command{gawk} on MS-DOS, Win32 and
                                   OS/2.
* VMS Installation::               Installing @command{gawk} on VMS.
* VMS Compilation::                How to compile @command{gawk} under VMS.
* VMS Installation Details::       How to install @command{gawk} under VMS.
* VMS Running::                    How to run @command{gawk} under VMS.
* VMS POSIX::                      Alternate instructions for VMS POSIX.
* Unsupported::                    Systems whose ports are no longer
                                   supported.
* Atari Installation::             Installing @command{gawk} on the Atari ST.
* Atari Compiling::                Compiling @command{gawk} on Atari.
* Atari Using::                    Running @command{gawk} on Atari.
* Tandem Installation::            Installing @command{gawk} on a Tandem.
* Bugs::                           Reporting Problems and Bugs.
* Other Versions::                 Other freely available @command{awk}
                                   implementations.
* Compatibility Mode::             How to disable certain @command{gawk}
                                   extensions.
* Additions::                      Making Additions To @command{gawk}.
* Adding Code::                    Adding code to the main body of
                                   @command{gawk}.
* New Ports::                      Porting @command{gawk} to a new operating
                                   system.
* Dynamic Extensions::             Adding new built-in functions to
                                   @command{gawk}.
* Internals::                      A brief look at some @command{gawk}
                                   internals.
* Sample Library::                 A example of new functions.
* Internal File Description::      What the new functions will do.
* Internal File Ops::              The code for internal file operations.
* Using Internal File Ops::        How to use an external extension.
* Future Extensions::              New features that may be implemented one
                                   day.
* Basic High Level::               The high level view.
* Basic Data Typing::              A very quick intro to data types.
* Floating Point Issues::          Stuff to know about floating-point numbers.
@end detailmenu
@end menu

@c dedication for Info file
@ifinfo
@center To Miriam, for making me complete.
@sp 1
@center To Chana, for the joy you bring us.
@sp 1
@center To Rivka, for the exponential increase.
@sp 1
@center To Nachum, for the added dimension.
@sp 1
@center To Malka, for the new beginning.
@end ifinfo

@summarycontents
@contents

@node Foreword, Preface, Top, Top
@unnumbered Foreword

Arnold Robbins and I are good friends. We were introduced 11 years ago
by circumstances---and our favorite programming language, AWK.  
The circumstances started a couple of years
earlier. I was working at a new job and noticed an unplugged 
Unix computer sitting in the corner.  No one knew how to use it,
and neither did I.  However,
a couple of days later it was running, and
I was @code{root} and the one-and-only user.  
That day, I began the transition from statistician to Unix programmer.

On one of many trips to the library or bookstore in search of
books on Unix, I found the gray AWK book, a.k.a. Aho, Kernighan and
Weinberger, @cite{The AWK Programming Language}, Addison-Wesley,
1988.  AWK's simple programming paradigm---find a pattern in the
input and then perform an action---often reduced complex or tedious
data manipulations to few lines of code.  I was excited to try my
hand at programming in AWK.

Alas,  the @command{awk} on my computer was a limited version of the
language described in the AWK book.  I discovered that my computer
had ``old @command{awk}'' and the AWK book described ``new @command{awk}.''
I learned that this was typical; the old version refused to step
aside or relinquish its name.  If a system had a new @command{awk}, it was
invariably called @command{nawk}, and few systems had it.
The best way to get a new @command{awk} was to @command{ftp} the source code for
@command{gawk} from @code{prep.ai.mit.edu}.  @command{gawk} was a version of
new @command{awk} written by David Trueman and Arnold, and available under
the GNU General Public License.

(Incidentally,
it's no longer difficult to find a new @command{awk}. @command{gawk} ships with
Linux, and you can download binaries or source code for almost
any system; my wife uses @command{gawk} on her VMS box.)

My Unix system started out unplugged from the wall; it certainly was not
plugged into a network.  So, oblivious to the existence of @command{gawk}
and the Unix community in general, and desiring a new @command{awk}, I wrote
my own, called @command{mawk}. 
Before I was finished I knew about @command{gawk},
but it was too late to stop, so I eventually posted
to a @code{comp.sources} newsgroup.  

A few days after my posting, I got a friendly email 
from Arnold introducing
himself.   He suggested we share design and algorithms and
attached a draft of the POSIX standard so
that I could update @command{mawk} to support language extensions added
after publication of the AWK book.

Frankly, if our roles had
been reversed, I would not have been so open and we probably would
have never met.  I'm glad we did meet. 
He is an AWK expert's AWK expert and a genuinely nice person.
Arnold contributes significant amounts of his
expertise and time to the Free Software Foundation.

This book is the @command{gawk} reference manual, but at its core it
is a book about AWK programming that
will appeal to a wide audience.
It is a definitive reference to the AWK language as defined by the
1987 Bell Labs release and codified in the 1992 POSIX Utilities
standard.

On the other hand, the novice AWK programmer can study
a wealth of practical programs that emphasize
the power of AWK's basic idioms:
data driven control-flow, pattern matching with regular expressions,
and associative arrays.
Those looking for something new can try out @command{gawk}'s 
interface to network protocols via special @file{/inet} files.

The programs in this book make clear that an AWK program is
typically much smaller and faster to develop than 
a counterpart written in C.
Consequently, there is often a payoff to prototype an
algorithm or design in AWK to get it running quickly and expose
problems early. Often, the interpreted performance is adequate
and the AWK prototype becomes the product.

The new @command{pgawk} (profiling @command{gawk}), produces
program execution counts.
I recently experimented with an algorithm that for
@math{n} lines of input, exhibited
@tex
$\sim\! Cn^2$
@end tex
@ifnottex
~ C n^2
@end ifnottex
performance, while
theory predicted
@tex
$\sim\! Cn\log n$
@end tex
@ifnottex
~ C n log n
@end ifnottex
behavior. A few minutes poring
over the @file{awkprof.out} profile pinpointed the problem to
a single line of code.  @command{pgawk} is a welcome addition to
my programmer's toolbox.

Arnold has distilled over a decade of experience writing and
using AWK programs, and developing @command{gawk}, into this book.  If you use
AWK or want to learn how, then read this book.

@display
Michael Brennan
Author of @command{mawk}
@end display

@node Preface, Getting Started, Foreword, Top
@unnumbered Preface
@c I saw a comment somewhere that the preface should describe the book itself,
@c and the introduction should describe what the book covers.
@c
@c 12/2000: Chuck wants the preface & intro combined.

Several kinds of tasks occur repeatedly
when working with text files.
You might want to extract certain lines and discard the rest.
Or you may need to make changes wherever certain patterns appear,
but leave the rest of the file alone.
Writing single-use programs for these tasks in languages such as C, C++ or Pascal
is time-consuming and inconvenient.
Such jobs are often easier with @command{awk}.
The @command{awk} utility interprets a special-purpose programming language
that makes it easy to handle simple data-reformatting jobs.

The GNU implementation of @command{awk} is called @command{gawk}; it is fully
compatible with the System V Release 4 version of
@command{awk}.  @command{gawk} is also compatible with the POSIX
specification of the @command{awk} language.  This means that all
properly written @command{awk} programs should work with @command{gawk}.
Thus, we usually don't distinguish between @command{gawk} and other
@command{awk} implementations.

@cindex uses of @command{awk}
@cindex applications of @command{awk}
Using @command{awk} allows you to:

@itemize @bullet
@item
Manage small, personal databases

@item
Generate reports

@item
Validate data

@item
Produce indexes and perform other document preparation tasks

@item
Experiment with algorithms that you can adapt later to other computer
languages.
@end itemize

@cindex uses of @command{gawk}
In addition,
@command{gawk}
provides facilities that make it easy to:

@itemize @bullet
@item
Extract bits and pieces of data for processing

@item
Sort data

@item
Perform simple network communications.
@end itemize

This @value{DOCUMENT} teaches you about the @command{awk} language and
how you can use it effectively.  You should already be familiar with basic
system commands, such as @command{cat} and @command{ls},@footnote{These commands
are available on POSIX-compliant systems, as well as on traditional Unix
based systems. If you are using some other operating system, you still need to
be familiar with the ideas of I/O redirection and pipes.} as well as basic shell
facilities, such as Input/Output (I/O) redirection and pipes.

Implementations of the @command{awk} language are available for many
different computing environments.  This @value{DOCUMENT}, while describing
the @command{awk} language in general, also describes the particular
implementation of @command{awk} called @command{gawk} (which stands for
``GNU awk'').  @command{gawk} runs on a broad range of Unix systems,
ranging from 80386 PC-based computers, up through large-scale systems,
such as Crays. @command{gawk} has also been ported to Mac OS X,
MS-DOS, Microsoft Windows (all versions) and OS/2 PC's, Atari and Amiga
micro-computers, BeOS, Tandem D20, and VMS.

@menu
* History::                     The history of @command{gawk} and
                                @command{awk}.
* Names::                       What name to use to find @command{awk}.
* This Manual::                 Using this @value{DOCUMENT}. Includes sample
                                input files that you can use.
* Conventions::                 Typographical Conventions.
* Manual History::              Brief history of the GNU project and this
                                @value{DOCUMENT}.
* How To Contribute::           Helping to save the world.
* Acknowledgments::             Acknowledgments.
@end menu

@node History, Names, Preface, Preface
@unnumberedsec History of @command{awk} and @command{gawk}
@cindex recipe for a programming language
@cindex programming language, recipe for
@center Recipe For A Programming Language

@multitable {2 parts} {1 part  @code{egrep}} {1 part  @code{snobol}}
@item @tab 1 part  @code{egrep} @tab 1 part  @code{snobol}
@item @tab 2 parts @code{ed} @tab 3 parts C
@end multitable

@quotation
Blend all parts well using @code{lex} and @code{yacc}.
Document minimally and release.

After eight years, add another part @code{egrep} and two 
more parts C.  Document very well and release.
@end quotation

@cindex acronym
@cindex history of @command{awk}
@cindex Aho, Alfred
@cindex Weinberger, Peter
@cindex Kernighan, Brian
@cindex old @command{awk}
@cindex new @command{awk}
The name @command{awk} comes from the initials of its designers: Alfred V.@:
Aho, Peter J.@: Weinberger and Brian W.@: Kernighan.  The original version of
@command{awk} was written in 1977 at AT&T Bell Laboratories.
In 1985, a new version made the programming
language more powerful, introducing user-defined functions, multiple input
streams, and computed regular expressions.
This new version became widely available with Unix System V
Release 3.1 (SVR3.1).
The version in SVR4 added some new features and cleaned
up the behavior in some of the ``dark corners'' of the language.
The specification for @command{awk} in the POSIX Command Language
and Utilities standard further clarified the language.
Both the @command{gawk} designers and the original Bell Laboratories @command{awk}
designers provided feedback for the POSIX specification.

@cindex Rubin, Paul
@cindex Fenlason, Jay
@cindex Trueman, David
Paul Rubin wrote the GNU implementation, @command{gawk}, in 1986.
Jay Fenlason completed it, with advice from Richard Stallman.  John Woods
contributed parts of the code as well.  In 1988 and 1989, David Trueman, with
help from me, thoroughly reworked @command{gawk} for compatibility
with the newer @command{awk}.
Circa 1995, I became the primary maintainer.
Current development focuses on bug fixes,
performance improvements, standards compliance, and occasionally, new features.

In May of 1997, J@"urgen Kahrs felt the need for network access
from @command{awk}, and with a little help from me, set about adding
features to do this for @command{gawk}.  At that time, he also
wrote the bulk of
@cite{TCP/IP Internetworking with @command{gawk}}
(a separate document, available as part of the @command{gawk} distribution).
His code finally became part of the main @command{gawk} distribution
with @command{gawk} @value{PVERSION} 3.1.

@xref{Contributors, ,Major Contributors to @command{gawk}},
for a complete list of those who made important contributions to @command{gawk}.

@node Names, This Manual, History, Preface
@section A Rose by Any Other Name

@cindex old @command{awk} vs. new @command{awk}
@cindex new @command{awk} vs. old @command{awk}
The @command{awk} language has evolved over the years. Full details are
provided in @ref{Language History, ,The Evolution of the @command{awk} Language}.
The language described in this @value{DOCUMENT}
is often referred to as ``new @command{awk}'' (@command{nawk}).

Because of this, many systems have multiple
versions of @command{awk}.
Some systems have an @command{awk} utility that implements the
original version of the @command{awk} language and a @command{nawk} utility
for the new
version.
Others have an @command{oawk} for the ``old @command{awk}''
language and plain @command{awk} for the new one.  Still others only
have one version, which is usually the new one.@footnote{Often, these systems
use @command{gawk} for their @command{awk} implementation!}

All in all, this makes it difficult for you to know which version of
@command{awk} you should run when writing your programs.  The best advice
I can give here is to check your local documentation. Look for @command{awk},
@command{oawk}, and @command{nawk}, as well as for @command{gawk}.
It is likely that you already
have some version of new @command{awk} on your system, which is what
you should use when running your programs.  (Of course, if you're reading
this @value{DOCUMENT}, chances are good that you have @command{gawk}!)

Throughout this @value{DOCUMENT}, whenever we refer to a language feature
that should be available in any complete implementation of POSIX @command{awk},
we simply use the term @command{awk}.  When referring to a feature that is
specific to the GNU implementation, we use the term @command{gawk}.

@node This Manual, Conventions, Names, Preface
@section Using This Book
@cindex book, using this
@cindex using this book
@cindex language, @command{awk}
@cindex program, @command{awk}
@ignore
@cindex @command{awk} language
@cindex @command{awk} program
@end ignore
@cindex Brandon, Dick
@cindex sex, comparisons with
@quotation
@i{Documentation is like sex: when it is good, it is very, very good; and
when it is bad, it is better than nothing.}@*
Dick Brandon
@end quotation

The term @command{awk} refers to a particular program as well as to the language you
use to tell this program what to do.  When we need to be careful, we call
the program ``the @command{awk} utility'' and the language ``the @command{awk}
language.''
This @value{DOCUMENT} explains
both the @command{awk} language and how to run the @command{awk} utility.
The term @dfn{@command{awk} program} refers to a program written by you in
the @command{awk} programming language.

Primarily, this @value{DOCUMENT} explains the features of @command{awk},
as defined in the POSIX standard.  It does so in the context of the
@command{gawk} implementation.  While doing so, it also
attempts to describe important differences between @command{gawk}
and other @command{awk} implementations.@footnote{All such differences
appear in the index under the heading ``differences between @command{gawk} and
@command{awk}.''} Finally, any @command{gawk} features that are not in
the POSIX standard for @command{awk} are noted.

@ifnotinfo
This @value{DOCUMENT} has the difficult task of being both a tutorial and a reference.
If you are a novice, feel free to skip over details that seem too complex.
You should also ignore the many cross references; they are for the
expert user and for the online Info version of the document.
@end ifnotinfo

There are
subsections labelled
as @strong{Advanced Notes}
scattered throughout the @value{DOCUMENT}.
They add a more complete explanation of points that are relevant, but not likely
to be of interest on first reading.
All appear in the index, under the heading ``advanced notes.''

Most of the time, the examples use complete @command{awk} programs.
In some of the more advanced sections, only the part of the @command{awk}
program that illustrates the concept currently being described is shown.

While this @value{DOCUMENT} is aimed principally at people who have not been
exposed
to @command{awk}, there is a lot of information here that even the @command{awk}
expert should find useful.  In particular, the description of POSIX
@command{awk} and the example programs in
@ref{Library Functions, ,A Library of @command{awk} Functions}, and in
@ref{Sample Programs, ,Practical @command{awk} Programs},
should be of interest.

@ref{Getting Started, ,Getting Started with @command{awk}},
provides the essentials you need to know to begin using @command{awk}.

@ref{Regexp, ,Regular Expressions},
introduces regular expressions in general, and in particular the flavors
supported by POSIX @command{awk} and @command{gawk}.

@ref{Reading Files, , Reading Input Files},
describes how @command{awk} reads your data.
It introduces the concepts of records and fields, as well
as the @code{getline} command.
I/O redirection is first described here.

@ref{Printing, , Printing Output},
describes how @command{awk} programs can produce output with
@code{print} and @code{printf}.

@ref{Expressions},
describes expressions, which are the basic building blocks
for getting most things done in a program.

@ref{Patterns and Actions, ,Patterns Actions and Variables},
describes how to write patterns for matching records, actions for
doing something when a record is matched, and the built-in variables
@command{awk} and @command{gawk} use.

@ref{Arrays, ,Arrays in @command{awk}},
covers @command{awk}'s one-and-only data structure: associative arrays.
Deleting array elements and whole arrays is also described, as well as
sorting arrays in @command{gawk}.

@ref{Functions},
describes the built-in functions @command{awk} and
@command{gawk} provide for you, as well as how to define
your own functions.

@ref{Internationalization, ,Internationalization with @command{gawk}},
describes special features in @command{gawk} for translating program
messages into different languages at runtime.

@ref{Advanced Features, ,Advanced Features of @command{gawk}},
describes a number of @command{gawk}-specific advanced features.
Of particular note
are the abilities to have two-way communications with another process,
perform TCP/IP networking, and
profile your @command{awk} programs.

@ref{Invoking Gawk, ,Running @command{awk} and @command{gawk}},
describes how to run @command{gawk}, the meaning of its
command-line options, and how it finds @command{awk}
program source files.

@ref{Library Functions, ,A Library of @command{awk} Functions}, and
@ref{Sample Programs, ,Practical @command{awk} Programs},
provide many sample @command{awk} programs.
Reading them allows you to see @command{awk} being used
for solving real problems.

@ref{Language History, ,The Evolution of the @command{awk} Language},
describes how the @command{awk} language has evolved since it was
first released to present.  It also describes how @command{gawk}
has acquired features over time.

@ref{Installation, ,Installing @command{gawk}},
describes how to get @command{gawk}, how to compile it
under Unix, and how to compile and use it on different
non-Unix systems.  It also describes how to report bugs
in @command{gawk} and where to get three other freely
available implementations of @command{awk}.

@ref{Notes, ,Implementation Notes},
describes how to disable @command{gawk}'s extensions, as
well as how to contribute new code to @command{gawk},
how to write extension libraries, and some possible
future directions for @command{gawk} development.

@ref{Basic Concepts, ,Basic Programming Concepts},
provides some very cursory background material for those who
are completely unfamiliar with computer programming.
Also centralized there is a discussion of some of the issues
involved in using floating-point numbers.

The
@ref{Glossary},
defines most, if not all, the significant terms used
throughout the book.
If you find terms that you aren't familiar with, try looking them up.

@ref{Copying, ,GNU General Public License}, and
@ref{GNU Free Documentation License},
present the licenses that cover the @command{gawk} source code,
and this @value{DOCUMENT}, respectively.

@node Conventions, Manual History, This Manual, Preface
@section Typographical Conventions

@cindex Texinfo
This @value{DOCUMENT} is written using Texinfo, the GNU documentation
formatting language.
A single Texinfo source file is used to produce both the printed and online
versions of the documentation.
@iftex
Because of this, the typographical conventions
are slightly different than in other books you may have read.
@end iftex
@ifnottex
This @value{SECTION} briefly documents the typographical conventions used in Texinfo.
@end ifnottex

Examples you would type at the command-line are preceded by the common
shell primary and secondary prompts, @samp{$} and @samp{>}.
Output from the command is preceded by the glyph ``@print{}''.
This typically represents the command's standard output.
Error messages, and other output on the command's standard error, are preceded
by the glyph ``@error{}''.  For example:

@example
$ echo hi on stdout
@print{} hi on stdout
$ echo hello on stderr 1>&2
@error{} hello on stderr
@end example

@iftex
In the text, command names appear in @code{this font}, while code segments
appear in the same font and quoted, @samp{like this}.  Some things are
emphasized @emph{like this}, and if a point needs to be made
strongly, it is done @strong{like this}.  The first occurrence of
a new term is usually its @dfn{definition} and appears in the same
font as the previous occurrence of ``definition'' in this sentence.
@value{FN}s are indicated like this: @file{/path/to/ourfile}.
@end iftex

Characters that you type at the keyboard look @kbd{like this}.  In particular,
there are special characters called ``control characters.''  These are
characters that you type by holding down both the @kbd{CONTROL} key and
another key, at the same time.  For example, a @kbd{Ctrl-d} is typed
by first pressing and holding the @kbd{CONTROL} key, next
pressing the @kbd{d} key and finally releasing both keys.

@c fakenode --- for prepinfo
@subsubheading Dark Corners
@cindex Kernighan, Brian
@quotation
@i{Dark corners are basically fractal --- no matter how much
you illuminate, there's always a smaller but darker one.}@*
Brian Kernighan
@end quotation

@cindex d.c., see ``dark corner''
@cindex dark corner
Until the POSIX standard (and @cite{The Gawk Manual}),
many features of @command{awk} were either poorly documented or not
documented at all.  Descriptions of such features
(often called ``dark corners'') are noted in this @value{DOCUMENT} with
@iftex
the picture of a flashlight in the margin, as shown here.
@value{DARKCORNER}
@end iftex
@ifnottex
``(d.c.)''.
@end ifnottex
They also appear in the index under the heading ``dark corner.''

As noted by the opening quote, though, any
coverage of dark corners
is, by definition, something that is incomplete.

@node Manual History, How To Contribute, Conventions, Preface
@unnumberedsec The GNU Project and This Book
@cindex Torvalds, Linus
@cindex sex, comparisons with
@quotation
@i{Software is like sex: it's better when it's free.}@*
Linus Torvalds
@end quotation

@cindex FSF
@cindex Free Software Foundation
@cindex Stallman, Richard
The Free Software Foundation (FSF) is a non-profit organization dedicated
to the production and distribution of freely distributable software.
It was founded by Richard M.@: Stallman, the author of the original
Emacs editor.  GNU Emacs is the most widely used version of Emacs today.

@cindex GNU Project
@cindex GPL
@cindex General Public License
@cindex GNU General Public License
@cindex online documentation
@cindex documentation, online
The GNU@footnote{GNU stands for ``GNU's not Unix.''}
Project is an ongoing effort on the part of the Free Software
Foundation to create a complete, freely distributable, POSIX-compliant
computing environment.
The FSF uses the ``GNU General Public License'' (GPL) to ensure that
their software's
source code is always available to the end user. A
copy of the GPL is included
@ifnotinfo
in this @value{DOCUMENT}
@end ifnotinfo
for your reference
(@pxref{Copying, ,GNU General Public License}).
The GPL applies to the C language source code for @command{gawk}.
To find out more about the FSF and the GNU Project online,
see @uref{http://www.gnu.org, the GNU Project's home page}.
This @value{DOCUMENT} may also be read from
@uref{http://www.gnu.org/manual/gawk/, their web site}.

A shell, an editor (Emacs), highly portable optimizing C, C++, and
Objective-C compilers, a symbolic debugger and dozens of large and
small utilities (such as @command{gawk}), have all been completed and are
freely available.  The GNU operating
system kernel (the HURD), has been released but is still in an early
stage of development.

@cindex Linux
@cindex GNU/Linux
@cindex BSD-based operating systems
@cindex NetBSD
@cindex FreeBSD
@cindex OpenBSD
Until the GNU operating system is more fully developed, you should
consider using GNU/Linux, a freely distributable, Unix-like operating
system for Intel 80386, DEC Alpha, Sun SPARC, IBM S/390, and other
systems.@footnote{The terminology ``GNU/Linux'' is explained
in the @ref{Glossary}.}
There are
many books on GNU/Linux. One that is freely available is @cite{Linux
Installation and Getting Started}, by Matt Welsh.
Many GNU/Linux distributions are often available in computer stores or
bundled on CD-ROMs with books about Linux.
(There are three other freely available, Unix-like operating systems for
80386 and other systems: NetBSD, FreeBSD, and OpenBSD. All are based on the
4.4-Lite Berkeley Software Distribution, and they use recent versions
of @command{gawk} for their versions of @command{awk}.)

@ifnotinfo
The @value{DOCUMENT} you are reading now is actually free---at least, the
information in it is free to anyone.  The machine readable
source code for the @value{DOCUMENT} comes with @command{gawk}; anyone
may take this @value{DOCUMENT} to a copying machine and make as many
copies of it as they like.  (Take a moment to check the Free Documentation
License; see @ref{GNU Free Documentation License}.)

Although you could just print it out yourself, bound books are much
easier to read and use.  Furthermore,
the proceeds from sales of this book go back to the FSF
to help fund development of more free software.
@end ifnotinfo

@ignore
@cindex Close, Diane
The @value{DOCUMENT} itself has gone through several previous,
preliminary editions.
Paul Rubin wrote the very first draft of @cite{The GAWK Manual};
it was around 40 pages in size.
Diane Close and Richard Stallman improved it, yielding the
version which I started working with in the fall of 1988.
It was around 90 pages long and barely described the original, ``old''
version of @command{awk}. After substantial revision, the first version of
the @cite{The GAWK Manual} to be released was Edition 0.11 Beta in
October of 1989.  The manual then underwent more substantial revision
for Edition 0.13 of December 1991.
David Trueman, Pat Rankin and Michal Jaegermann contributed sections
of the manual for Edition 0.13.
That edition was published by the
FSF as a bound book early in 1992.  Since then there were several
minor revisions, notably Edition 0.14 of November 1992 that was published
by the FSF in January of 1993 and Edition 0.16 of August 1993.

Edition 1.0 of @cite{GAWK: The GNU Awk User's Guide} represented a significant re-working
of @cite{The GAWK Manual}, with much additional material.
The FSF and I agreed that I was now the primary author.
@c I also felt that the manual needed a more descriptive title.

In January 1996, SSC published Edition 1.0 under the title @cite{Effective AWK Programming}.
In February 1997, they published Edition 1.0.3 which had minor changes
as a ``second edition.''
In 1999, the FSF published this same version as Edition 2
of @cite{GAWK: The GNU Awk User's Guide}.

Edition @value{EDITION} maintains the basic structure of Edition 1.0,
but with significant additional material, reflecting the host of new features
in @command{gawk} @value{PVERSION} @value{VERSION}.
Of particular note is
@ref{Array Sorting, ,Sorting Array Values and Indices with @command{gawk}},
@ref{Bitwise Functions, ,Using @command{gawk}'s Bit Manipulation Functions},
@ref{Internationalization, ,Internationalization with @command{gawk}},
@ref{Advanced Features, ,Advanced Features of @command{gawk}},
and
@ref{Dynamic Extensions, ,Adding New Built-in Functions to @command{gawk}}.
@end ignore

@cindex Close, Diane
The @value{DOCUMENT} itself has gone through a number of previous editions.
Paul Rubin wrote the very first draft of @cite{The GAWK Manual};
it was around 40 pages in size.
Diane Close and Richard Stallman improved it, yielding a
version that was
around 90 pages long and barely described the original, ``old''
version of @command{awk}.

I started working with that version in the fall of 1988.
As work on it progressed,
the FSF published several preliminary versions (numbered 0.@var{x}).
In 1996, Edition 1.0 was released with @command{gawk} 3.0.0.
The FSF published the first two editions under
the title @cite{The GNU Awk User's Guide}.

This edition maintains the basic structure of Edition 1.0,
but with significant additional material, reflecting the host of new features
in @command{gawk} @value{PVERSION} @value{VERSION}.
Of particular note is
@ref{Array Sorting, ,Sorting Array Values and Indices with @command{gawk}},
as well as
@ref{Bitwise Functions, ,Using @command{gawk}'s Bit Manipulation Functions},
@ref{Internationalization, ,Internationalization with @command{gawk}},
and also
@ref{Advanced Features, ,Advanced Features of @command{gawk}},
and
@ref{Dynamic Extensions, ,Adding New Built-in Functions to @command{gawk}}.

@cite{@value{TITLE}} will undoubtedly continue to evolve.
An electronic version
comes with the @command{gawk} distribution from the FSF.
If you find an error in this @value{DOCUMENT}, please report it!
@xref{Bugs, ,Reporting Problems and Bugs}, for information on submitting
problem reports electronically, or write to me in care of the publisher.

@node How To Contribute, Acknowledgments, Manual History, Preface
@unnumberedsec How to Contribute

As the maintainer of GNU @command{awk},
I am starting a collection of publicly available @command{awk}
programs.
For more information,
see @uref{ftp://ftp.freefriends.org/arnold/Awkstuff}.
If you have written an interesting @command{awk} program, or have written a
@command{gawk} extension that you would like to
share with the rest of the world, please contact me (@email{arnold@@gnu.org}).
Making things available on the Internet helps keep the
@command{gawk} distribution down to manageable size.

@node Acknowledgments, , How To Contribute, Preface
@unnumberedsec Acknowledgments

The initial draft of @cite{The GAWK Manual} had the following acknowledgments:

@quotation
Many people need to be thanked for their assistance in producing this
manual.  Jay Fenlason contributed many ideas and sample programs.  Richard
Mlynarik and Robert Chassell gave helpful comments on drafts of this
manual.  The paper @cite{A Supplemental Document for @command{awk}} by John W.@:
Pierce of the Chemistry Department at UC San Diego, pinpointed several
issues relevant both to @command{awk} implementation and to this manual, that
would otherwise have escaped us.
@end quotation

@cindex Stallman, Richard
I would like to acknowledge Richard M.@: Stallman, for his vision of a
better world and for his courage in founding the FSF and starting the
GNU project.

The following people (in alphabetical order)
provided helpful comments on various
versions of this book, up to and including this edition.
Rick Adams,
Nelson H.F. Beebe,
Karl Berry,
Dr.@: Michael Brennan,
Rich Burridge,
Claire Coutier,
Diane Close,
Scott Deifik,
Christopher (``Topher'') Eliot,
Jeffrey Friedl,
Dr.@: Darrel Hankerson,
Michal Jaegermann,
Dr.@: Richard J.@: LeBlanc,
Michael Lijewski,
Pat Rankin,
Miriam Robbins,
Mary Sheehan,
and
Chuck Toporek.

@cindex Berry, Karl
@cindex Chassell, Robert J.@:
@cindex Texinfo
Robert J.@: Chassell provided much valuable advice on
the use of Texinfo.
He also deserves special thanks for
convincing me @emph{not} to title this @value{DOCUMENT}
@cite{How To Gawk Politely}.
Karl Berry helped significantly with the @TeX{} part of Texinfo.

@cindex Hartholz, Marshall
@cindex Hartholz, Elaine
@cindex Schreiber, Bert
@cindex Schreiber, Rita
I would like to thank Marshall and Elaine Hartholz of Seattle and
Dr.@: Bert and Rita Schreiber of Detroit for large amounts of quiet vacation
time in their homes, which allowed me to make significant progress on
this @value{DOCUMENT} and on @command{gawk} itself.

@cindex Hughes, Phil
Phil Hughes of SSC
contributed in a very important way by loaning me his laptop GNU/Linux
system, not once, but twice, which allowed me to do a lot of work while
away from home.

@cindex Trueman, David
David Trueman deserves special credit; he has done a yeoman job
of evolving @command{gawk} so that it performs well and without bugs.
Although he is no longer involved with @command{gawk},
working with him on this project was a significant pleasure.

@cindex Drepper, Ulrich
@cindex GNITS mailing list
The intrepid members of the GNITS mailing list, and most notably Ulrich
Drepper, provided invaluable help and feedback for the design of the
internationalization features.

@cindex Beebe, Nelson
@cindex Brown, Martin
@cindex Deifik, Scott
@cindex Hankerson, Darrel
@cindex Jaegermann, Michal
@cindex Kahrs, J@"urgen
@cindex Rankin, Pat
@cindex Rommel, Kai Uwe
@cindex Zaretskii, Eli
Nelson Beebe,
Martin Brown,
Scott Deifik,
Darrel Hankerson,
Michal Jaegermann,
J@"urgen Kahrs,
Pat Rankin,
Kai Uwe Rommel,
and Eli Zaretskii
(in alphabetical order)
are long-time members of the
@command{gawk} ``crack portability team.''  Without their hard work and
help, @command{gawk} would not be nearly the fine program it is today.  It
has been and continues to be a pleasure working with this team of fine
people.

@cindex Kernighan, Brian
David and I would like to thank Brian Kernighan of Bell Laboratories for
invaluable assistance during the testing and debugging of @command{gawk}, and for
help in clarifying numerous points about the language.  We could not have
done nearly as good a job on either @command{gawk} or its documentation without
his help.

Chuck Toporek, Mary Sheehan, and Claire Coutier of O'Reilly & Associates contributed
significant editorial help for this @value{DOCUMENT} for the
3.1 release of @command{gawk}.

@cindex Robbins, Miriam
@cindex Robbins, Jean
@cindex Robbins, Harry
@cindex G-d
I must thank my wonderful wife, Miriam, for her patience through
the many versions of this project, for her proof-reading,
and for sharing me with the computer.
I would like to thank my parents for their love, and for the grace with
which they raised and educated me.
Finally, I also must acknowledge my gratitude to G-d, for the many opportunities
He has sent my way, as well as for the gifts He has given me with which to
take advantage of those opportunities.
@sp 2
@noindent
Arnold Robbins @*
Nof Ayalon @*
ISRAEL @*
March, 2001

@ignore
@c Try this
@iftex
@page
@headings off
@majorheading I@ @ @ @ The @command{awk} Language and @command{gawk}
Part I describes the @command{awk} language and @command{gawk} program in detail.
It starts with the basics, and continues through all of the features of @command{awk}
and @command{gawk}.  It contains the following chapters:

@itemize @bullet
@item
@ref{Getting Started, ,Getting Started with @command{awk}}.

@item
@ref{Regexp, ,Regular Expressions}.

@item
@ref{Reading Files, , Reading Input Files}.

@item
@ref{Printing, , Printing Output}.

@item
@ref{Expressions}.

@item
@ref{Patterns and Actions, ,Patterns Actions and Variables}.

@item
@ref{Arrays, ,Arrays in @command{awk}}.

@item
@ref{Functions}.

@item
@ref{Internationalization, ,Internationalization with @command{gawk}}.

@item
@ref{Advanced Features, ,Advanced Features of @command{gawk}}.

@item
@ref{Invoking Gawk, ,Running @command{awk} and @command{gawk}}.
@end itemize

@page
@evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @|
@oddheading  @| @| @strong{@thischapter}@ @ @ @thispage
@end iftex
@end ignore

@node Getting Started, Regexp, Preface, Top
@chapter Getting Started with @command{awk}
@cindex script, definition of
@cindex rule, definition of
@cindex program, definition of
@cindex basic function of @command{awk}

The basic function of @command{awk} is to search files for lines (or other
units of text) that contain certain patterns.  When a line matches one
of the patterns, @command{awk} performs specified actions on that line.
@command{awk} keeps processing input lines in this way until it reaches
the end of the input files.

@cindex data-driven languages
@cindex procedural languages
@cindex language, data-driven
@cindex language, procedural
Programs in @command{awk} are different from programs in most other languages,
because @command{awk} programs are @dfn{data-driven}; that is, you describe
the data you want to work with and then what to do when you find it.
Most other languages are @dfn{procedural}; you have to describe, in great
detail, every step the program is to take.  When working with procedural
languages, it is usually much
harder to clearly describe the data your program will process.
For this reason, @command{awk} programs are often refreshingly easy to
write and read.

@cindex program, definition of
@cindex rule, definition of
When you run @command{awk}, you specify an @command{awk} @dfn{program} that
tells @command{awk} what to do.  The program consists of a series of
@dfn{rules}.  (It may also contain @dfn{function definitions},
an advanced feature that we will ignore for now.
@xref{User-defined, ,User-Defined Functions}.)  Each rule specifies one
pattern to search for and one action to perform
upon finding the pattern.

Syntactically, a rule consists of a pattern followed by an action.  The
action is enclosed in curly braces to separate it from the pattern.
Newlines usually separate rules.  Therefore, an @command{awk}
program looks like this:

@example
@var{pattern} @{ @var{action} @}
@var{pattern} @{ @var{action} @}
@dots{}
@end example

@menu
* Running gawk::                How to run @command{gawk} programs; includes
                                command-line syntax.
* Sample Data Files::           Sample data files for use in the @command{awk}
                                programs illustrated in this @value{DOCUMENT}.
* Very Simple::                 A very simple example.
* Two Rules::                   A less simple one-line example using two
                                rules.
* More Complex::                A more complex example.
* Statements/Lines::            Subdividing or combining statements into
                                lines.
* Other Features::              Other Features of @command{awk}.
* When::                        When to use @command{gawk} and when to use
                                other things.
@end menu

@node Running gawk, Sample Data Files, Getting Started, Getting Started
@section How to Run @command{awk} Programs

@cindex command-line formats
@cindex running @command{awk} programs
There are several ways to run an @command{awk} program.  If the program is
short, it is easiest to include it in the command that runs @command{awk},
like this:

@example
awk '@var{program}' @var{input-file1} @var{input-file2} @dots{}
@end example

When the program is long, it is usually more convenient to put it in a file
and run it with a command like this:

@example
awk -f @var{program-file} @var{input-file1} @var{input-file2} @dots{}
@end example

This @value{SECTION} discusses both mechanisms, along with several
variations of each.

@menu
* One-shot::                    Running a short throw-away @command{awk}
                                program.
* Read Terminal::               Using no input files (input from terminal
                                instead).
* Long::                        Putting permanent @command{awk} programs in
                                files.
* Executable Scripts::          Making self-contained @command{awk} programs.
* Comments::                    Adding documentation to @command{gawk}
                                programs.
* Quoting::                     More discussion of shell quoting issues.
@end menu

@node One-shot, Read Terminal, Running gawk, Running gawk
@subsection One-Shot Throw-Away @command{awk} Programs

Once you are familiar with @command{awk}, you will often type in simple
programs the moment you want to use them.  Then you can write the
program as the first argument of the @command{awk} command, like this:

@example
awk '@var{program}' @var{input-file1} @var{input-file2} @dots{}
@end example

@noindent
where @var{program} consists of a series of @var{patterns} and
@var{actions}, as described earlier.

@cindex single quotes, why needed
This command format instructs the @dfn{shell}, or command interpreter,
to start @command{awk} and use the @var{program} to process records in the
input file(s).  There are single quotes around @var{program} so
the shell won't interpret any @command{awk} characters as special shell
characters.  The quotes also cause the shell to treat all of @var{program} as
a single argument for @command{awk}, and allow @var{program} to be more
than one line long.

This format is also useful for running short or medium-sized @command{awk}
programs from shell scripts, because it avoids the need for a separate
file for the @command{awk} program.  A self-contained shell script is more
reliable because there are no other files to misplace.

@ref{Very Simple, ,Some Simple Examples},
@ifnotinfo
later in this @value{CHAPTER},
@end ifnotinfo
presents several short,
self-contained programs.

@c Removed for gawk 3.1, doesn't really add anything here.
@ignore
As an interesting side point, the command

@example
awk '/foo/' @var{files} @dots{}
@end example

@noindent
is essentially the same as

@cindex @command{egrep} utility
@example
egrep foo @var{files} @dots{}
@end example
@end ignore

@node Read Terminal, Long, One-shot, Running gawk
@subsection Running @command{awk} Without Input Files

@cindex standard input
@cindex input, standard
You can also run @command{awk} without any input files.  If you type the
following command line:

@example
awk '@var{program}'
@end example

@noindent
@command{awk} applies the @var{program} to the @dfn{standard input},
which usually means whatever you type on the terminal.  This continues
until you indicate end-of-file by typing @kbd{Ctrl-d}.
(On other operating systems, the end-of-file character may be different.
For example, on OS/2 and MS-DOS, it is @kbd{Ctrl-z}.)

As an example, the following program prints a friendly piece of advice
(from Douglas Adams's @cite{The Hitchhiker's Guide to the Galaxy}),
to keep you from worrying about the complexities of computer programming.
(@code{BEGIN} is a feature we haven't discussed yet.):

@example
$ awk "BEGIN @{ print \"Don't Panic!\" @}"
@print{} Don't Panic!
@end example

@cindex quoting, shell
@cindex shell quoting
This program does not read any input.  The @samp{\} before each of the
inner double quotes is necessary because of the shell's quoting
rules---in particular because it mixes both single quotes and
double quotes.@footnote{Although we generally recommend the use of single
quotes around the program text, double quotes are needed here in order to
put the single quote into the message.}

This next simple @command{awk} program
emulates the @command{cat} utility; it copies whatever you type at the
keyboard to its standard output. (Why this works is explained shortly.)

@example
$ awk '@{ print @}'
Now is the time for all good men
@print{} Now is the time for all good men
to come to the aid of their country.
@print{} to come to the aid of their country.
Four score and seven years ago, ...
@print{} Four score and seven years ago, ...
What, me worry?
@print{} What, me worry?
@kbd{Ctrl-d}
@end example

@node Long, Executable Scripts, Read Terminal, Running gawk
@subsection Running Long Programs

@cindex running long programs
@cindex @code{-f} option
@cindex command-line option, @code{-f}
@cindex program file
@cindex file, @command{awk} program
Sometimes your @command{awk} programs can be very long.  In this case, it is
more convenient to put the program into a separate file.  In order to tell
@command{awk} to use that file for its program, you type:

@example
awk -f @var{source-file} @var{input-file1} @var{input-file2} @dots{}
@end example

The @option{-f} instructs the @command{awk} utility to get the @command{awk} program
from the file @var{source-file}.  Any @value{FN} can be used for
@var{source-file}.  For example, you could put the program:

@example
BEGIN @{ print "Don't Panic!" @}
@end example

@noindent
into the file @file{advice}.  Then this command:

@example
awk -f advice
@end example

@noindent
does the same thing as this one:

@example
awk "BEGIN @{ print \"Don't Panic!\" @}"
@end example

@cindex quoting, shell
@cindex shell quoting
@noindent
This was explained earlier
(@pxref{Read Terminal, ,Running @command{awk} Without Input Files}).
Note that you don't usually need single quotes around the @value{FN} that you
specify with @option{-f}, because most @value{FN}s don't contain any of the shell's
special characters.  Notice that in @file{advice}, the @command{awk}
program did not have single quotes around it.  The quotes are only needed
for programs that are provided on the @command{awk} command line.

If you want to identify your @command{awk} program files clearly as such,
you can add the extension @file{.awk} to the @value{FN}.  This doesn't
affect the execution of the @command{awk} program but it does make
``housekeeping'' easier.

@node Executable Scripts, Comments, Long, Running gawk
@subsection Executable @command{awk} Programs
@cindex executable scripts
@cindex scripts, executable
@cindex self-contained programs
@cindex program, self-contained
@cindex @code{#!} (executable scripts)

Once you have learned @command{awk}, you may want to write self-contained
@command{awk} scripts, using the @samp{#!} script mechanism.  You can do
this on many Unix systems@footnote{The @samp{#!} mechanism works on
Linux systems,
systems derived from the 4.4-Lite Berkeley Software Distribution,
and most commercial Unix systems.} as well as on the GNU system.
For example, you could update the file @file{advice} to look like this:

@example
#! /bin/awk -f

BEGIN @{ print "Don't Panic!" @}
@end example

@noindent
After making this file executable (with the @command{chmod} utility),
simply type @samp{advice}
at the shell and the system arranges to run @command{awk}@footnote{The
line beginning with @samp{#!} lists the full @value{FN} of an interpreter
to run and an optional initial command-line argument to pass to that
interpreter.  The operating system then runs the interpreter with the given
argument and the full argument list of the executed program.  The first argument
in the list is the full @value{FN} of the @command{awk} program.  The rest of the
argument list is either options to @command{awk}, or @value{DF}s,
or both.} as if you had
typed @samp{awk -f advice}:

@example
$ chmod +x advice
$ advice
@print{} Don't Panic!
@end example

@noindent
Self-contained @command{awk} scripts are useful when you want to write a
program that users can invoke without their having to know that the program is
written in @command{awk}.

@c fakenode --- for prepinfo
@subheading Advanced Notes: Portability Issues with @samp{#!}
@cindex advanced notes

Some systems limit the length of the interpreter name to 32 characters.
Often, this can be dealt with by using a symbolic link.

You should not put more than one argument on the @samp{#!}
line after the path to @command{awk}. It does not work. The operating system
treats the rest of the line as a single argument and passes it to @command{awk}.
Doing this leads to confusing behavior---most likely a usage diagnostic
of some sort from @command{awk}.

@cindex portability issues
Finally,
the value of @code{ARGV[0]}
(@pxref{Built-in Variables})
varies depending upon your operating system.
Some systems put @samp{awk} there, some put the full pathname
of @command{awk} (such as @file{/bin/awk}), and some put the name
of your script (@samp{advice}).  Don't rely on the value of @code{ARGV[0]}
to provide your script name.

@node Comments, Quoting, Executable Scripts, Running gawk
@subsection Comments in @command{awk} Programs
@cindex @code{#} (comment)
@cindex comments
@cindex use of comments
@cindex documenting @command{awk} programs
@cindex programs, documenting

A @dfn{comment} is some text that is included in a program for the sake
of human readers; it is not really an executable part of the program.  Comments
can explain what the program does and how it works.  Nearly all
programming languages have provisions for comments, as programs are
typically hard to understand without them.

In the @command{awk} language, a comment starts with the sharp sign
character (@samp{#}) and continues to the end of the line.
The @samp{#} does not have to be the first character on the line. The
@command{awk} language ignores the rest of a line following a sharp sign.
For example, we could have put the following into @file{advice}:

@example
# This program prints a nice friendly message.  It helps
# keep novice users from being afraid of the computer.
BEGIN    @{ print "Don't Panic!" @}
@end example

You can put comment lines into keyboard-composed throw-away @command{awk}
programs, but this usually isn't very useful; the purpose of a
comment is to help you or another person understand the program
when reading it at a later time.

@cindex quoting, shell
@cindex shell quoting
@strong{Caution:} As mentioned in
@ref{One-shot, ,One-Shot Throw-Away @command{awk} Programs},
you can enclose small to medium programs in single quotes, in order to keep
your shell scripts self-contained.  When doing so, @emph{don't} put
an apostrophe (i.e., a single quote) into a comment (or anywhere else
in your program). The shell interprets the quote as the closing
quote for the entire program. As a result, usually the shell
prints a message about mismatched quotes, and if @command{awk} actually
runs, it will probably print strange messages about syntax errors.
For example, look at the following:

@example
$ awk '@{ print "hello" @} # let's be cute'
>
@end example

The shell sees that the first two quotes match, and that
a new quoted object begins at the end of the command-line.
It therefore prompts with the secondary prompt, waiting for more input.
With Unix @command{awk}, closing the quoted string produces this result:

@example
$ awk '@{ print "hello" @} # let's be cute'
> '
@error{} awk: can't open file be
@error{}  source line number 1
@end example

Putting a backslash before the single quote in @samp{let's} wouldn't help,
since backslashes are not special inside single quotes.
The next @value{SUBSECTION} describes the shell's quoting rules.

@node Quoting, , Comments, Running gawk
@subsection Shell Quoting Issues
@c the indexing here is purposely different, until we
@c get a way to mark the defining instance for an index entry
@cindex quoting rules, shell
@cindex shell quoting rules

For short to medium length @command{awk} programs, it is most convenient
to enter the program on the @command{awk} command line.
This is best done by enclosing the entire program in single quotes.
This is true whether you are entering the program interactively at
the shell prompt, or writing it as part of a larger shell script:

@example
awk '@var{program text}' @var{input-file1} @var{input-file2} @dots{}
@end example

@cindex @command{csh} utility
Once you are working with the shell, it is helpful to have a basic
knowledge of shell quoting rules.  The following rules apply only to
POSIX-compliant, Bourne-style shells (such as @command{bash}, the GNU Bourne-Again
Shell).  If you use @command{csh}, you're on your own.

@itemize @bullet
@item
Quoted items can be concatenated with nonquoted items as well as with other
quoted items.  The shell turns everything into one argument for
the command.

@item
Preceding any single character with a backslash (@samp{\}) quotes
that character.  The shell removes the backslash and passes the quoted
character on to the command.

@item
Single quotes protect everything between the opening and closing quotes.
The shell does no interpretation of the quoted text, passing it on verbatim
to the command.
It is @emph{impossible} to embed a single quote inside single-quoted text.
Refer back to
@ref{Comments, ,Comments in @command{awk} Programs},
for an example showing what happens if you try.

@item
Double quotes protect most things between the opening and closing quotes.
The shell does at least variable and command substitution on the quoted text.
Different shells may do additional kinds of processing on double-quoted text.

Since certain characters within double-quoted text are processed by the shell,
they must be @dfn{escaped} within the text.  Of note are the characters
@samp{$}, @samp{`}, @samp{\} and @samp{"}, all of which must be preceded by
a backslash within double-quoted text if they are to be passed on literally
to the program.  (The leading backslash is stripped first.)
Thus, the example seen
@ifnotinfo
previously
@end ifnotinfo
in @ref{Read Terminal, ,Running @command{awk} Without Input Files},
is applicable:

@example
$ awk "BEGIN @{ print \"Don't Panic!\" @}"
@print{} Don't Panic!
@end example

Note that the single quote is not special within double quotes.

@item
Null strings are removed when they occur as part of a non-null
command-line argument, while explicit non-null objects are kept.
For example, to specify that the field separator @code{FS} should
be set to the null string, use:

@example
awk -F "" '@var{program}' @var{files} # correct
@end example

@noindent
Don't use this:

@example
awk -F"" '@var{program}' @var{files}  # wrong!
@end example

@noindent
In the second case, @command{awk} will attempt to use the text of the program 
as the value of @code{FS}, and the first @value{FN} as the text of the program!
This results in syntax errors at best, and confusing behavior at worst.
@end itemize

@cindex shell quoting, tricks
Mixing single and double quotes is difficult.  You have to resort
to shell quoting tricks, like this:

@example
$ awk 'BEGIN @{ print "Here is a single quote <'"'"'>" @}'
@print{} Here is a single quote <'>
@end example

@noindent
This program consists of three concatenated quoted strings.  The first and the
third are single-quoted, the second is double-quoted.

This can be ``simplified'' to:

@example
$ awk 'BEGIN @{ print "Here is a single quote <'\''>" @}'
@print{} Here is a single quote <'>
@end example

@noindent
Judge for yourself which of these two is the more readable.

Another option is to use double quotes, escaping the embedded, @command{awk}-level
double quotes:

@example
$ awk "BEGIN @{ print \"Here is a single quote <'>\" @}"
@print{} Here is a single quote <'>
@end example

@noindent
This option is also painful, because double quotes, backslashes, and dollar signs
are very common in @command{awk} programs.

If you really need both single and double quotes in your @command{awk}
program, it is probably best to move it into a separate file, where
the shell won't be part of the picture, and you can say what you mean.

@node Sample Data Files, Very Simple, Running gawk, Getting Started
@section @value{DDF}s for the Examples
@c For gawk >= 3.2, update these data files. No-one has such slow modems!

@cindex input file, sample
@cindex sample input files
@cindex @file{BBS-list} file
Many of the examples in this @value{DOCUMENT} take their input from two sample
@value{DF}s.  The first, called @file{BBS-list}, represents a list of
computer bulletin board systems together with information about those systems.
The second @value{DF}, called @file{inventory-shipped}, contains
information about monthly shipments.  In both files,
each line is considered to be one @dfn{record}.

In the file @file{BBS-list}, each record contains the name of a computer
bulletin board, its phone number, the board's baud rate(s), and a code for
the number of hours it is operational.  An @samp{A} in the last column
means the board operates 24 hours a day.  A @samp{B} in the last
column means the board only operates on evening and weekend hours.
A @samp{C} means the board operates only on weekends:

@c 2e: Update the baud rates to reflect today's faster modems
@example
@c system if test ! -d eg      ; then mkdir eg      ; fi
@c system if test ! -d eg/lib  ; then mkdir eg/lib  ; fi
@c system if test ! -d eg/data ; then mkdir eg/data ; fi
@c system if test ! -d eg/prog ; then mkdir eg/prog ; fi
@c system if test ! -d eg/misc ; then mkdir eg/misc ; fi
@c file eg/data/BBS-list
aardvark     555-5553     1200/300          B
alpo-net     555-3412     2400/1200/300     A
barfly       555-7685     1200/300          A
bites        555-1675     2400/1200/300     A
camelot      555-0542     300               C
core         555-2912     1200/300          C
fooey        555-1234     2400/1200/300     B
foot         555-6699     1200/300          B
macfoo       555-6480     1200/300          A
sdace        555-3430     2400/1200/300     A
sabafoo      555-2127     1200/300          C
@c endfile
@end example

@cindex @file{inventory-shipped} file
The second @value{DF}, called @file{inventory-shipped}, represents
information about shipments during the year.
Each record contains the month, the number
of green crates shipped, the number of red boxes shipped, the number of
orange bags shipped, and the number of blue packages shipped,
respectively.  There are 16 entries, covering the 12 months of last year
and the first four months of the current year.

@example
@c file eg/data/inventory-shipped
Jan  13  25  15 115
Feb  15  32  24 226
Mar  15  24  34 228
Apr  31  52  63 420
May  16  34  29 208
Jun  31  42  75 492
Jul  24  34  67 436
Aug  15  34  47 316
Sep  13  55  37 277
Oct  29  54  68 525
Nov  20  87  82 577
Dec  17  35  61 401

Jan  21  36  64 620
Feb  26  58  80 652
Mar  24  75  70 495
Apr  21  70  74 514
@c endfile
@end example

@ifinfo
If you are reading this in GNU Emacs using Info, you can copy the regions
of text showing these sample files into your own test files.  This way you
can try out the examples shown in the remainder of this document.  You do
this by using the command @kbd{M-x write-region} to copy text from the Info
file into a file for use with @command{awk}
(@xref{Misc File Ops, , Miscellaneous File Operations, emacs, GNU Emacs Manual},
for more information).  Using this information, create your own
@file{BBS-list} and @file{inventory-shipped} files and practice what you
learn in this @value{DOCUMENT}.

@cindex Texinfo
If you are using the stand-alone version of Info,
see @ref{Extract Program, ,Extracting Programs from Texinfo Source Files},
for an @command{awk} program that extracts these @value{DF}s from
@file{gawk.texi}, the Texinfo source file for this Info file.
@end ifinfo

@node Very Simple, Two Rules, Sample Data Files, Getting Started
@section Some Simple Examples

The following command runs a simple @command{awk} program that searches the
input file @file{BBS-list} for the character string @samp{foo}.  (A
string of characters is usually called a @dfn{string}.
The term @dfn{string} is based on similar usage in English, such
as ``a string of pearls,'' or, ``a string of cars in a train.''):

@example
awk '/foo/ @{ print $0 @}' BBS-list
@end example

@noindent
When lines containing @samp{foo} are found, they are printed because
@w{@samp{print $0}} means print the current line.  (Just @samp{print} by
itself means the same thing, so we could have written that
instead.)

You will notice that slashes (@samp{/}) surround the string @samp{foo}
in the @command{awk} program.  The slashes indicate that @samp{foo}
is the pattern to search for.  This type of pattern is called a
@dfn{regular expression}, which is covered in more detail later
(@pxref{Regexp, ,Regular Expressions}).
The pattern is allowed to match parts of words.
There are
single quotes around the @command{awk} program so that the shell won't
interpret any of it as special shell characters.

Here is what this program prints:

@example
$ awk '/foo/ @{ print $0 @}' BBS-list
@print{} fooey        555-1234     2400/1200/300     B
@print{} foot         555-6699     1200/300          B
@print{} macfoo       555-6480     1200/300          A
@print{} sabafoo      555-2127     1200/300          C
@end example

@cindex action, default
@cindex pattern, default
@cindex default action
@cindex default pattern
In an @command{awk} rule, either the pattern or the action can be omitted,
but not both.  If the pattern is omitted, then the action is performed
for @emph{every} input line.  If the action is omitted, the default
action is to print all lines that match the pattern.

@cindex empty action
@cindex action, empty
Thus, we could leave out the action (the @code{print} statement and the curly
braces) in the above example and the result would be the same: all
lines matching the pattern @samp{foo} are printed.  By comparison,
omitting the @code{print} statement but retaining the curly braces makes an
empty action that does nothing (i.e., no lines are printed).

@cindex one-liners
Many practical @command{awk} programs are just a line or two.  Following is a
collection of useful, short programs to get you started.  Some of these
programs contain constructs that haven't been covered yet. (The description
of the program will give you a good idea of what is going on, but please
read the rest of the @value{DOCUMENT} to become an @command{awk} expert!)
Most of the examples use a @value{DF} named @file{data}.  This is just a
placeholder; if you use these programs yourself, substitute
your own @value{FN}s for @file{data}.
For future reference, note that there is often more than
one way to do things in @command{awk}.  At some point, you may want
to look back at these examples and see if
you can come up with different ways to do the same things shown here:

@itemize @bullet
@item
Print the length of the longest input line:

@example
awk '@{ if (length($0) > max) max = length($0) @}
     END @{ print max @}' data
@end example

@item
Print every line that is longer than 80 characters:

@example
awk 'length($0) > 80' data
@end example

The sole rule has a relational expression as its pattern and it has no
action---so the default action, printing the record, is used.

@cindex @command{expand} utility
@item
Print the length of the longest line in @file{data}:

@example
expand data | awk '@{ if (x < length()) x = length() @}
              END @{ print "maximum line length is " x @}'
@end example

The input is processed by the @command{expand} utility to change tabs
into spaces, so the widths compared are actually the right-margin columns.

@item
Print every line that has at least one field:

@example
awk 'NF > 0' data
@end example

This is an easy way to delete blank lines from a file (or rather, to
create a new file similar to the old file but from which the blank lines
have been removed).

@item
Print seven random numbers from 0 to 100, inclusive:

@example
awk 'BEGIN @{ for (i = 1; i <= 7; i++)
                 print int(101 * rand()) @}'
@end example

@item
Print the total number of bytes used by @var{files}:

@example
ls -l @var{files} | awk '@{ x += $5 @}
                  END @{ print "total bytes: " x @}'
@end example

@item
Print the total number of kilobytes used by @var{files}:

@c Don't use \ continuation, not discussed yet
@example
ls -l @var{files} | awk '@{ x += $5 @}
   END @{ print "total K-bytes: " (x + 1023)/1024 @}'
@end example

@item
Print a sorted list of the login names of all users:

@example
awk -F: '@{ print $1 @}' /etc/passwd | sort
@end example

@item
Count lines in a file:

@example
awk 'END @{ print NR @}' data
@end example

@item
Print the even-numbered lines in the @value{DF}:

@example
awk 'NR % 2 == 0' data
@end example

If you use the expression @samp{NR % 2 == 1} instead,
it would print the odd-numbered lines.
@end itemize

@node Two Rules, More Complex, Very Simple, Getting Started
@section An Example with Two Rules
@cindex how @command{awk} works

The @command{awk} utility reads the input files one line at a
time.  For each line, @command{awk} tries the patterns of each of the rules.
If several patterns match, then several actions are run in the order in
which they appear in the @command{awk} program.  If no patterns match, then
no actions are run.

After processing all the rules that match the line (and perhaps there are none),
@command{awk} reads the next line.  (However,
@pxref{Next Statement, ,The @code{next} Statement},
and also @pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).
This continues until the end of the file is reached.
For example, the following @command{awk} program contains two rules:

@example
/12/  @{ print $0 @}
/21/  @{ print $0 @}
@end example

@noindent
The first rule has the string @samp{12} as the
pattern and @samp{print $0} as the action.  The second rule has the
string @samp{21} as the pattern and also has @samp{print $0} as the
action.  Each rule's action is enclosed in its own pair of braces.

This program prints every line that contains the string
@samp{12} @emph{or} the string @samp{21}.  If a line contains both
strings, it is printed twice, once by each rule.

This is what happens if we run this program on our two sample @value{DF}s,
@file{BBS-list} and @file{inventory-shipped}, as shown here:

@example
$ awk '/12/ @{ print $0 @}
>      /21/ @{ print $0 @}' BBS-list inventory-shipped
@print{} aardvark     555-5553     1200/300          B
@print{} alpo-net     555-3412     2400/1200/300     A
@print{} barfly       555-7685     1200/300          A
@print{} bites        555-1675     2400/1200/300     A
@print{} core         555-2912     1200/300          C
@print{} fooey        555-1234     2400/1200/300     B
@print{} foot         555-6699     1200/300          B
@print{} macfoo       555-6480     1200/300          A
@print{} sdace        555-3430     2400/1200/300     A
@print{} sabafoo      555-2127     1200/300          C
@print{} sabafoo      555-2127     1200/300          C
@print{} Jan  21  36  64 620
@print{} Apr  21  70  74 514
@end example

@noindent
Note how the line beginning with @samp{sabafoo}
in @file{BBS-list} was printed twice, once for each rule.

@node More Complex, Statements/Lines, Two Rules, Getting Started
@section A More Complex Example

Now that we've mastered some simple tasks, let's look at
what typical @command{awk}
programs do.  This example shows how @command{awk} can be used to
summarize, select, and rearrange the output of another utility.  It uses
features that haven't been covered yet, so don't worry if you don't
understand all the details:

@example
ls -l | awk '$6 == "Nov" @{ sum += $5 @}
             END @{ print sum @}'
@end example

@cindex @command{csh} utility
@cindex @command{csh}, backslash continuation
@cindex backslash continuation, in @command{csh}
@cindex @command{ls} utility
This command prints the total number of bytes in all the files in the
current directory that were last modified in November (of any year).
@footnote{In the C shell (@command{csh}), you need to type
a semicolon and then a backslash at the end of the first line; see
@ref{Statements/Lines, ,@command{awk} Statements Versus Lines}, for an
explanation as to why.  In a POSIX-compliant shell, such as the Bourne
shell or @command{bash}, you can type the example as shown.  If the command
@samp{echo $path} produces an empty output line, you are most likely
using a POSIX-compliant shell.  Otherwise, you are probably using the
C shell or a shell derived from it.}
The @w{@samp{ls -l}} part of this example is a system command that gives
you a listing of the files in a directory, including each file's size and the date
the file was last modified. Its output looks like this:

@example
-rw-r--r--  1 arnold   user   1933 Nov  7 13:05 Makefile
-rw-r--r--  1 arnold   user  10809 Nov  7 13:03 awk.h
-rw-r--r--  1 arnold   user    983 Apr 13 12:14 awk.tab.h
-rw-r--r--  1 arnold   user  31869 Jun 15 12:20 awk.y
-rw-r--r--  1 arnold   user  22414 Nov  7 13:03 awk1.c
-rw-r--r--  1 arnold   user  37455 Nov  7 13:03 awk2.c
-rw-r--r--  1 arnold   user  27511 Dec  9 13:07 awk3.c
-rw-r--r--  1 arnold   user   7989 Nov  7 13:03 awk4.c
@end example

@noindent
The first field contains read-write permissions, the second field contains
the number of links to the file, and the third field identifies the owner of
the file. The fourth field identifies the group of the file.
The fifth field contains the size of the file in bytes.  The
sixth, seventh and eighth fields contain the month, day, and time,
respectively, that the file was last modified.  Finally, the ninth field
contains the name of the file.@footnote{On some
very old systems, you may need to use @samp{ls -lg} to get this output.}

@cindex automatic initialization
@cindex initialization, automatic
The @samp{$6 == "Nov"} in our @command{awk} program is an expression that
tests whether the sixth field of the output from @w{@samp{ls -l}}
matches the string @samp{Nov}.  Each time a line has the string
@samp{Nov} for its sixth field, the action @samp{sum += $5} is
performed.  This adds the fifth field (the file's size) to the variable
@code{sum}.  As a result, when @command{awk} has finished reading all the
input lines, @code{sum} is the total of the sizes of the files whose
lines matched the pattern.  (This works because @command{awk} variables
are automatically initialized to zero.)

After the last line of output from @command{ls} has been processed, the
@code{END} rule executes and prints the value of @code{sum}.
In this example, the value of @code{sum} is 140963.

These more advanced @command{awk} techniques are covered in later sections
(@pxref{Action Overview, ,Actions}).  Before you can move on to more
advanced @command{awk} programming, you have to know how @command{awk} interprets
your input and displays your output.  By manipulating fields and using
@code{print} statements, you can produce some very useful and impressive
looking reports.

@node Statements/Lines, Other Features, More Complex, Getting Started
@section @command{awk} Statements Versus Lines
@cindex line break
@cindex newline

Most often, each line in an @command{awk} program is a separate statement or
separate rule, like this:

@example
awk '/12/  @{ print $0 @}
     /21/  @{ print $0 @}' BBS-list inventory-shipped
@end example

However, @command{gawk} ignores newlines after any of the following
symbols and keywords:

@example
,    @{    ?    :    ||    &&    do    else
@end example

@noindent
A newline at any other point is considered the end of the
statement.@footnote{The @samp{?} and @samp{:} referred to here is the
three-operand conditional expression described in
@ref{Conditional Exp, ,Conditional Expressions}.
Splitting lines after @samp{?} and @samp{:} is a minor @command{gawk}
extension; if @option{--posix} is specified
(@pxref{Options, , Command-Line Options}), then this extension is disabled.}

@cindex backslash continuation
@cindex continuation of lines
@cindex line continuation
If you would like to split a single statement into two lines at a point
where a newline would terminate it, you can @dfn{continue} it by ending the
first line with a backslash character (@samp{\}).  The backslash must be
the final character on the line in order to be recognized as a continuation
character.  A backslash is allowed anywhere in the statement, even
in the middle of a string or regular expression.  For example:

@example
awk '/This regular expression is too long, so continue it\
 on the next line/ @{ print $1 @}'
@end example

@noindent
@cindex portability issues
We have generally not used backslash continuation in the sample programs
in this @value{DOCUMENT}.  In @command{gawk}, there is no limit on the
length of a line, so backslash continuation is never strictly necessary;
it just makes programs more readable.  For this same reason, as well as
for clarity, we have kept most statements short in the sample programs
presented throughout the @value{DOCUMENT}.  Backslash continuation is
most useful when your @command{awk} program is in a separate source file
instead of entered from the command line.  You should also note that
many @command{awk} implementations are more particular about where you
may use backslash continuation. For example, they may not allow you to
split a string constant using backslash continuation.  Thus, for maximum
portability of your @command{awk} programs, it is best not to split your
lines in the middle of a regular expression or a string.
@c 10/2000: gawk, mawk, and current bell labs awk allow it,
@c solaris 2.7 nawk does not. Solaris /usr/xpg4/bin/awk does though!  sigh.

@cindex @command{csh} utility
@cindex @command{csh}, backslash continuation
@cindex backslash continuation, in @command{csh}
@strong{Caution:} @emph{Backslash continuation does not work as described
above with the C shell.}  It works for @command{awk} programs in files and
for one-shot programs, @emph{provided} you are using a POSIX-compliant
shell, such as the Unix Bourne shell or @command{bash}.  But the C shell behaves
differently!  There, you must use two backslashes in a row, followed by
a newline.  Note also that when using the C shell, @emph{every} newline
in your awk program must be escaped with a backslash. To illustrate:

@example
% awk 'BEGIN @{ \
?   print \\
?       "hello, world" \
? @}'
@print{} hello, world
@end example

@noindent
Here, the @samp{%} and @samp{?} are the C shell's primary and secondary
prompts, analogous to the standard shell's @samp{$} and @samp{>}.

Compare the previous example to how it is done with a POSIX-compliant shell:

@example
$ awk 'BEGIN @{
>   print \
>       "hello, world"
> @}'
@print{} hello, world
@end example

@command{awk} is a line-oriented language.  Each rule's action has to
begin on the same line as the pattern.  To have the pattern and action
on separate lines, you @emph{must} use backslash continuation; there
is no other way.

@cindex backslash continuation, and comments
@cindex comments and backslash continuation
Another thing to keep in mind is that backslash continuation and
comments do not mix. As soon as @command{awk} sees the @samp{#} that
starts a comment, it ignores @emph{everything} on the rest of the
line. For example:

@example
$ gawk 'BEGIN @{ print "dont panic" # a friendly \
>                                    BEGIN rule
> @}'
@error{} gawk: cmd. line:2:                BEGIN rule
@error{} gawk: cmd. line:2:                ^ parse error
@end example

@noindent
In this case, it looks like the backslash would continue the comment onto the
next line. However, the backslash-newline combination is never even
noticed because it is ``hidden'' inside the comment. Thus, the
@code{BEGIN} is noted as a syntax error.

@cindex multiple statements on one line
When @command{awk} statements within one rule are short, you might want to put
more than one of them on a line.  This is accomplished by separating the statements
with a semicolon (@samp{;}).
This also applies to the rules themselves.
Thus, the program shown at the start of this @value{SECTION}
could also be written this way:

@example
/12/ @{ print $0 @} ; /21/ @{ print $0 @}
@end example

@noindent
@strong{Note:} The requirement that states that rules on the same line must be
separated with a semicolon was not in the original @command{awk}
language; it was added for consistency with the treatment of statements
within an action.

@node Other Features, When, Statements/Lines, Getting Started
@section Other Features of @command{awk}

The @command{awk} language provides a number of predefined, or
@dfn{built-in}, variables that your programs can use to get information
from @command{awk}.  There are other variables your program can set
as well to control how @command{awk} processes your data.

In addition, @command{awk} provides a number of built-in functions for doing
common computational and string related operations.
@command{gawk} provides built-in functions for working with timestamps,
performing bit manipulation, and for runtime string translation.

As we develop our presentation of the @command{awk} language, we introduce
most of the variables and many of the functions. They are defined
systematically in @ref{Built-in Variables}, and
@ref{Built-in, ,Built-in Functions}.

@node When, , Other Features, Getting Started
@section When to Use @command{awk}

@cindex uses of @command{awk}
@cindex applications of @command{awk}
Now that you've seen some of what @command{awk} can do,
you might wonder how @command{awk} could be useful for you.  By using
utility programs, advanced patterns, field separators, arithmetic
statements, and other selection criteria, you can produce much more
complex output.  The @command{awk} language is very useful for producing
reports from large amounts of raw data, such as summarizing information
from the output of other utility programs like @command{ls}.
(@xref{More Complex, ,A More Complex Example}.)

Programs written with @command{awk} are usually much smaller than they would
be in other languages.  This makes @command{awk} programs easy to compose and
use.  Often, @command{awk} programs can be quickly composed at your terminal,
used once, and thrown away.  Because @command{awk} programs are interpreted, you
can avoid the (usually lengthy) compilation part of the typical
edit-compile-test-debug cycle of software development.

Complex programs have been written in @command{awk}, including a complete
retargetable assembler for eight-bit microprocessors (@pxref{Glossary}, for
more information), and a microcode assembler for a special purpose Prolog
computer.  However, @command{awk}'s capabilities are strained by tasks of
such complexity.

If you find yourself writing @command{awk} scripts of more than, say, a few
hundred lines, you might consider using a different programming
language.  Emacs Lisp is a good choice if you need sophisticated string
or pattern matching capabilities.  The shell is also good at string and
pattern matching; in addition, it allows powerful use of the system
utilities.  More conventional languages, such as C, C++, and Java, offer
better facilities for system programming and for managing the complexity
of large programs.  Programs in these languages may require more lines
of source code than the equivalent @command{awk} programs, but they are
easier to maintain and usually run more efficiently.

@node Regexp, Reading Files, Getting Started, Top
@chapter Regular Expressions
@cindex pattern, regular expressions
@cindex regexp
@cindex regular expression
@cindex regular expressions as patterns

A @dfn{regular expression}, or @dfn{regexp}, is a way of describing a
set of strings.
Because regular expressions are such a fundamental part of @command{awk}
programming, their format and use deserve a separate @value{CHAPTER}.

A regular expression enclosed in slashes (@samp{/})
is an @command{awk} pattern that matches every input record whose text
belongs to that set.
The simplest regular expression is a sequence of letters, numbers, or
both.  Such a regexp matches any string that contains that sequence.
Thus, the regexp @samp{foo} matches any string containing @samp{foo}.
Therefore, the pattern @code{/foo/} matches any input record containing
the three characters @samp{foo} @emph{anywhere} in the record.  Other
kinds of regexps let you specify more complicated classes of strings.

@ifnotinfo
Initially, the examples in this @value{CHAPTER} are simple.
As we explain more about how
regular expressions work, we will present more complicated instances.
@end ifnotinfo

@menu
* Regexp Usage::                How to Use Regular Expressions.
* Escape Sequences::            How to write non-printing characters.
* Regexp Operators::            Regular Expression Operators.
* Character Lists::             What can go between @samp{[...]}.
* GNU Regexp Operators::        Operators specific to GNU software.
* Case-sensitivity::            How to do case-insensitive matching.
* Leftmost Longest::            How much text matches.
* Computed Regexps::            Using Dynamic Regexps.
@end menu

@node Regexp Usage, Escape Sequences, Regexp, Regexp
@section How to Use Regular Expressions

A regular expression can be used as a pattern by enclosing it in
slashes.  Then the regular expression is tested against the
entire text of each record.  (Normally, it only needs
to match some part of the text in order to succeed.)  For example, the
following prints the second field of each record that contains the string
@samp{foo} anywhere in it:

@example
$ awk '/foo/ @{ print $2 @}' BBS-list
@print{} 555-1234
@print{} 555-6699
@print{} 555-6480
@print{} 555-2127
@end example

@cindex regexp operators
@cindex string-matching operators
@cindex operators, string-matching
@cindex operators, regexp matching
@cindex @code{~} operator
@cindex @code{!~} operator
Regular expressions can also be used in matching expressions.  These
expressions allow you to specify the string to match against; it need
not be the entire current input record.  The two operators @samp{~}
and @samp{!~} perform regular expression comparisons.  Expressions
using these operators can be used as patterns, or in @code{if},
@code{while}, @code{for}, and @code{do} statements.
(@xref{Statements, ,Control Statements in Actions}.)
For example:

@example
@var{exp} ~ /@var{regexp}/
@end example

@noindent
is true if the expression @var{exp} (taken as a string)
matches @var{regexp}.  The following example matches, or selects,
all input records with the uppercase letter @samp{J} somewhere in the
first field:

@example
$ awk '$1 ~ /J/' inventory-shipped
@print{} Jan  13  25  15 115
@print{} Jun  31  42  75 492
@print{} Jul  24  34  67 436
@print{} Jan  21  36  64 620
@end example

So does this:

@example
awk '@{ if ($1 ~ /J/) print @}' inventory-shipped
@end example

This next example is true if the expression @var{exp}
(taken as a character string)
does @emph{not} match @var{regexp}:

@example
@var{exp} !~ /@var{regexp}/
@end example

The following example matches,
or selects, all input records whose first field @emph{does not} contain
the uppercase letter @samp{J}:

@example
$ awk '$1 !~ /J/' inventory-shipped
@print{} Feb  15  32  24 226
@print{} Mar  15  24  34 228
@print{} Apr  31  52  63 420
@print{} May  16  34  29 208
@dots{}
@end example

@cindex regexp constant
When a regexp is enclosed in slashes, such as @code{/foo/}, we call it
a @dfn{regexp constant}, much like @code{5.27} is a numeric constant and
@code{"foo"} is a string constant.

@node Escape Sequences, Regexp Operators, Regexp Usage, Regexp
@section Escape Sequences

@cindex escape sequence notation
Some characters cannot be included literally in string constants
(@code{"foo"}) or regexp constants (@code{/foo/}).
Instead, they should be represented with @dfn{escape sequences},
which are character sequences beginning with a backslash (@samp{\}).
One use of an escape sequence is to include a double quote character in
a string constant.  Because a plain double quote ends the string, you
must use @samp{\"} to represent an actual double quote character as a
part of the string.  For example:

@example
$ awk 'BEGIN @{ print "He said \"hi!\" to her." @}'
@print{} He said "hi!" to her.
@end example

The  backslash character itself is another character that cannot be
included normally; you must write @samp{\\} to put one backslash in the
string or regexp.  Thus, the string whose contents are the two characters
@samp{"} and @samp{\} must be written @code{"\"\\"}.

Another use of backslash is to represent unprintable characters
such as tab or newline.  While there is nothing to stop you from entering most
unprintable characters directly in a string constant or regexp constant,
they may look ugly.

The following table lists
all the escape sequences used in @command{awk} and
what they represent. Unless noted otherwise, all these escape
sequences apply to both string constants and regexp constants:

@table @code
@item \\
A literal backslash, @samp{\}.

@cindex @command{awk} language, V.4 version
@cindex @code{\a} escape sequence
@item \a
The ``alert'' character, @kbd{Ctrl-g}, ASCII code 7 (BEL).
(This usually makes some sort of audible noise.)

@cindex @code{\b} escape sequence
@item \b
Backspace, @kbd{Ctrl-h}, ASCII code 8 (BS).

@cindex @code{\f} escape sequence
@item \f
Formfeed, @kbd{Ctrl-l}, ASCII code 12 (FF).

@cindex @code{\n} escape sequence
@item \n
Newline, @kbd{Ctrl-j}, ASCII code 10 (LF).

@cindex @code{\r} escape sequence
@item \r
Carriage return, @kbd{Ctrl-m}, ASCII code 13 (CR).

@cindex @code{\t} escape sequence
@item \t
Horizontal tab, @kbd{Ctrl-i}, ASCII code 9 (HT).

@cindex @command{awk} language, V.4 version
@cindex @code{\v} escape sequence
@item \v
Vertical tab, @kbd{Ctrl-k}, ASCII code 11 (VT).

@cindex @code{\}@var{nnn} escape sequence (octal)
@item \@var{nnn}
The octal value @var{nnn}, where @var{nnn} stands for 1 to 3 digits
between @samp{0} and @samp{7}.  For example, the code for the ASCII ESC
(escape) character is @samp{\033}.

@cindex @code{\x} escape sequence
@cindex @command{awk} language, V.4 version
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@item \x@var{hh}@dots{}
The hexadecimal value @var{hh}, where @var{hh} stands for a sequence
of hexadecimal digits (@samp{0} through @samp{9}, and either @samp{A}
through @samp{F} or @samp{a} through @samp{f}).  Like the same construct
in ISO C, the escape sequence continues until the first non-hexadecimal
digit is seen.  However, using more than two hexadecimal digits produces
undefined results. (The @samp{\x} escape sequence is not allowed in
POSIX @command{awk}.)

@cindex @code{\/} escape sequence
@item \/
A literal slash (necessary for regexp constants only).
This expression is used when you want to write a regexp
constant that contains a slash. Because the regexp is delimited by
slashes, you need to escape the slash that is part of the pattern,
in order to tell @command{awk} to keep processing the rest of the regexp.

@cindex @code{\"} escape sequence
@item \"
A literal double quote (necessary for string constants only).
This expression is used when you want to write a string
constant that contains a double quote. Because the string is delimited by
double quotes, you need to escape the quote that is part of the string,
in order to tell @command{awk} to keep processing the rest of the string.
@end table

In @command{gawk}, a number of additional two-character sequences that begin
with a backslash have special meaning in regexps.
@xref{GNU Regexp Operators, ,@command{gawk}-Specific Regexp Operators}.

In a regexp, a backslash before any character that is not in the above table
and not listed in
@ref{GNU Regexp Operators, ,@command{gawk}-Specific Regexp Operators},
means that the next character should be taken literally, even if it would
normally be a regexp operator.  For example, @code{/a\+b/} matches the three
characters @samp{a+b}.

@cindex portability issues
For complete portability, do not use a backslash before any character not
shown in the table above.

To summarize:

@itemize @bullet
@item
The escape sequences in the table above are always processed first,
for both string constants and regexp constants. This happens very early,
as soon as @command{awk} reads your program.

@item
@command{gawk} processes both regexp constants and dynamic regexps
(@pxref{Computed Regexps, ,Using Dynamic Regexps}),
for the special operators listed in
@ref{GNU Regexp Operators, ,@command{gawk}-Specific Regexp Operators}.

@item
A backslash before any other character means to treat that character
literally.
@end itemize

@c fakenode --- for prepinfo
@subheading Advanced Notes: Backslash Before Regular Characters
@cindex advanced notes

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
If you place a backslash in a string constant before something that is
not one of the characters listed above, POSIX @command{awk} purposely
leaves what happens as undefined.  There are two choices:

@cindex automatic warnings
@cindex warnings, automatic
@table @asis
@item Strip the backslash out
This is what Unix @command{awk} and @command{gawk} both do.
For example, @code{"a\qc"} is the same as @code{"aqc"}.
(Because this is such an easy bug to both introduce and to miss,
@command{gawk} warns you about it.)
Consider @samp{FS = @w{"[ \t]+\|[ \t]+"}} to use vertical bars
surrounded by whitespace as the field separator. There should be
two backslashes in the string, @samp{FS = @w{"[ \t]+\\|[ \t]+"}}.)
@c I did this!  This is why I added the warning.

@item Leave the backslash alone
Some other @command{awk} implementations do this.
In such implementations, @code{"a\qc"} is the same as if you had typed
@code{"a\\qc"}.
@end table

@c fakenode --- for prepinfo
@subheading Advanced Notes: Escape Sequences for Metacharacters
@cindex advanced notes

Suppose you use an octal or hexadecimal
escape to represent a regexp metacharacter
(@pxref{Regexp Operators, ,  Regular Expression Operators}).
Does @command{awk} treat the character as a literal character or as a regexp
operator?

@cindex dark corner
Historically, such characters were taken literally.
@value{DARKCORNER}
However, the POSIX standard indicates that they should be treated
as real metacharacters, which is what @command{gawk} does.
In compatibility mode (@pxref{Options, ,Command-Line Options}),
@command{gawk} treats the characters represented by octal and hexadecimal
escape sequences literally when used in regexp constants. Thus,
@code{/a\52b/} is equivalent to @code{/a\*b/}.

@node Regexp Operators, Character Lists, Escape Sequences, Regexp
@section Regular Expression Operators
@cindex metacharacters
@cindex regular expression metacharacters
@cindex regexp operators

You can combine regular expressions with special characters,
called @dfn{regular expression operators} or @dfn{metacharacters}, to
increase the power and versatility of regular expressions.

The escape sequences described
@ifnotinfo
earlier
@end ifnotinfo
in @ref{Escape Sequences},
are valid inside a regexp.  They are introduced by a @samp{\}, and
are recognized and converted into the corresponding real characters as
the very first step in processing regexps.

Here is a list of metacharacters.  All characters that are not escape
sequences and that are not listed in the table stand for themselves:

@table @code
@item \
This is used to suppress the special meaning of a character when
matching.  For example, @samp{\$}
matches the character @samp{$}.

@cindex anchors in regexps
@cindex regexp, anchors
@cindex Texinfo
@item ^
This matches the beginning of a string.  For example, @samp{^@@chapter}
matches @samp{@@chapter} at the beginning of a string, and can be used
to identify chapter beginnings in Texinfo source files.
The @samp{^} is known as an @dfn{anchor}, because it anchors the pattern to
match only at the beginning of the string.

It is important to realize that @samp{^} does not match the beginning of
a line embedded in a string.
The condition is not true in the following example:

@example
if ("line1\nLINE 2" ~ /^L/) @dots{}
@end example

@item $
This is similar to @samp{^} but it matches only at the end of a string.
For example, @samp{p$}
matches a record that ends with a @samp{p}.  The @samp{$} is an anchor
and does not match the end of a line embedded in a string.
The condition is not true in the following example:

@example
if ("line1\nLINE 2" ~ /1$/) @dots{}
@end example

@item .
This matches any single character,
@emph{including} the newline character.  For example, @samp{.P}
matches any single character followed by a @samp{P} in a string.  Using
concatenation, we can make a regular expression such as @samp{U.A}, that
matches any three-character sequence that begins with @samp{U} and ends
with @samp{A}.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
In strict POSIX mode (@pxref{Options, ,Command-Line Options}),
@samp{.} does not match the @sc{nul}
character, which is a character with all bits equal to zero.
Otherwise, @sc{nul} is just another character. Other versions of @command{awk}
may not be able to match the @sc{nul} character.

@cindex character list
@cindex character set (regexp component)
@cindex character class
@cindex bracket expression
@item [@dots{}]
This is called a @dfn{character list}.@footnote{In other literature,
you may see a character list referred to as either a
@dfn{character set}, a @dfn{character class} or a @dfn{bracket expression}.}
It matches any @emph{one} of the characters that are enclosed in
the square brackets.  For example, @samp{[MVX]} matches any one of
the characters @samp{M}, @samp{V}, or @samp{X}, in a string.  A full
discussion of what can be inside the square brackets of a character list
is given in
@ref{Character Lists, ,Using Character Lists}.

@cindex complemented character list
@cindex character list, complemented
@item [^ @dots{}]
This is a @dfn{complemented character list}.  The first character after
the @samp{[} @emph{must} be a @samp{^}.  It matches any characters
@emph{except} those in the square brackets.  For example, @samp{[^awk]}
matches any character that is not an @samp{a}, a @samp{w},
or a @samp{k}.

@item |
This is the @dfn{alternation operator} and it is used to specify
alternatives.
The @samp{|} has the lowest precedence of all the regular
expression operators.
For example, @samp{^P|[[:digit:]]}
matches any string that matches either @samp{^P} or @samp{[[:digit:]]}.  This
means it matches any string that starts with @samp{P} or contains a digit.

The alternation applies to the largest possible regexps on either side.

@cindex Texinfo
@item (@dots{})
Parentheses are used for grouping in regular expressions, similar to
arithmetic.  They can be used to concatenate regular expressions
containing the alternation operator, @samp{|}.  For example,
@samp{@@(samp|code)\@{[^@}]+\@}} matches both @samp{@@code@{foo@}} and
@samp{@@samp@{bar@}}.
(These are Texinfo formatting control sequences.)

@item *
This symbol means that the preceding regular expression should be
repeated as many times as necessary to find a match.  For example, @samp{ph*}
applies the @samp{*} symbol to the preceding @samp{h} and looks for matches
of one @samp{p} followed by any number of @samp{h}s.  This also matches
just @samp{p} if no @samp{h}s are present.

The @samp{*} repeats the @emph{smallest} possible preceding expression.
(Use parentheses if you want to repeat a larger expression.)  It finds
as many repetitions as possible.  For example,
@samp{awk '/\(c[ad][ad]*r x\)/ @{ print @}' sample}
prints every record in @file{sample} containing a string of the form
@samp{(car x)}, @samp{(cdr x)}, @samp{(cadr x)}, and so on.
Notice the escaping of the parentheses by preceding them
with backslashes.

@item +
This symbol is similar to @samp{*} except that the preceding expression must be
matched at least once.  This means that @samp{wh+y}
would match @samp{why} and @samp{whhy}, but not @samp{wy}, whereas
@samp{wh*y} would match all three of these strings.
The following is a simpler
way of writing the last @samp{*} example:

@example
awk '/\(c[ad]+r x\)/ @{ print @}' sample
@end example

@item ?
This symbol is similar to @samp{*} except that the preceding expression can be
matched either once or not at all.  For example, @samp{fe?d}
matches @samp{fed} and @samp{fd}, but nothing else.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@cindex interval expressions
@item @{@var{n}@}
@itemx @{@var{n},@}
@itemx @{@var{n},@var{m}@}
One or two numbers inside braces denote an @dfn{interval expression}.
If there is one number in the braces, the preceding regexp is repeated
@var{n} times.
If there are two numbers separated by a comma, the preceding regexp is
repeated @var{n} to @var{m} times.
If there is one number followed by a comma, then the preceding regexp
is repeated at least @var{n} times:

@table @code
@item wh@{3@}y
Matches @samp{whhhy}, but not @samp{why} or @samp{whhhhy}.

@item wh@{3,5@}y
Matches @samp{whhhy}, @samp{whhhhy}, or @samp{whhhhhy}, only.

@item wh@{2,@}y
Matches @samp{whhy} or @samp{whhhy}, and so on.
@end table

Interval expressions were not traditionally available in @command{awk}.
They were added as part of the POSIX standard to make @command{awk}
and @command{egrep} consistent with each other.

However, because old programs may use @samp{@{} and @samp{@}} in regexp
constants, by default @command{gawk} does @emph{not} match interval expressions
in regexps.  If either @option{--posix} or @option{--re-interval} are specified
(@pxref{Options, , Command-Line Options}), then interval expressions
are allowed in regexps.

For new programs that use @samp{@{} and @samp{@}} in regexp constants,
it is good practice to always escape them with a backslash.  Then the
regexp constants are valid and work the way you want them to, using
any version of @command{awk}.@footnote{Use two backslashes if you're
using a string constant with a regexp operator or function.}
@end table

@cindex precedence, regexp operators
@cindex regexp operators, precedence of
In regular expressions, the @samp{*}, @samp{+}, and @samp{?} operators,
as well as the braces @samp{@{} and @samp{@}},
have
the highest precedence, followed by concatenation, and finally by @samp{|}.
As in arithmetic, parentheses can change how operators are grouped.

In POSIX @command{awk} and @command{gawk}, the @samp{*}, @samp{+}, and @samp{?} operators
stand for themselves when there is nothing in the regexp that precedes them.
For example, @samp{/+/} matches a literal plus sign.  However, many other versions of
@command{awk} treat such a usage as a syntax error.

If @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
POSIX character classes and interval expressions are not available in
regular expressions.

@node Character Lists, GNU Regexp Operators, Regexp Operators, Regexp
@section Using Character Lists

Within a character list, a @dfn{range expression} consists of two
characters separated by a hyphen.  It matches any single character that
sorts between the two characters, using the locale's
collating sequence and character set.  For example, in the default C
locale, @samp{[a-dx-z]} is equivalent to @samp{[abcdxyz]}.  Many locales
sort characters in dictionary order, and in these locales,
@samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]}; instead it
might be equivalent to @samp{[aBbCcDdxXyYz]}, for example.  To obtain
the traditional interpretation of bracket expressions, you can use the C
locale by setting the @env{LC_ALL} environment variable to the value
@samp{C}.

To include one of the characters @samp{\}, @samp{]}, @samp{-}, or @samp{^} in a
character list, put a @samp{\} in front of it.  For example:

@example
[d\]]
@end example

@noindent
matches either @samp{d} or @samp{]}.

@cindex @command{egrep} utility
This treatment of @samp{\} in character lists
is compatible with other @command{awk}
implementations and is also mandated by POSIX.
The regular expressions in @command{awk} are a superset
of the POSIX specification for Extended Regular Expressions (EREs).
POSIX EREs are based on the regular expressions accepted by the
traditional @command{egrep} utility.

@cindex character class
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@dfn{Character classes} are a new feature introduced in the POSIX standard.
A character class is a special notation for describing
lists of characters that have a specific attribute, but the
actual characters can vary from country to country and/or
from character set to character set.  For example, the notion of what
is an alphabetic character differs between the United States and France.

A character class is only valid in a regexp @emph{inside} the
brackets of a character list.  Character classes consist of @samp{[:},
a keyword denoting the class, and @samp{:]}.  Here are the character
classes defined by the POSIX standard:

@c the regular table is commented out while trying out the multitable.
@c leave it here in case we need to go back, but make sure the text
@c still corresponds!

@ignore
@table @code
@item [:alnum:]
Alphanumeric characters.

@item [:alpha:]
Alphabetic characters.

@item [:blank:]
Space and tab characters.

@item [:cntrl:]
Control characters.

@item [:digit:]
Numeric characters.

@item [:graph:]
Characters that are printable and visible.
(A space is printable but not visible, whereas an @samp{a} is both.)

@item [:lower:]
Lowercase alphabetic characters.

@item [:print:]
Printable characters (characters that are not control characters).

@item [:punct:]
Punctuation characters (characters that are not letters, digits,
control characters, or space characters).

@item [:space:]
Space characters (such as space, tab, and formfeed, to name a few).

@item [:upper:]
Uppercase alphabetic characters.

@item [:xdigit:]
Characters that are hexadecimal digits.
@end table
@end ignore

@multitable {@code{[:xdigit:]}} {Characters that are both printable and visible.  (A space is}
@item @code{[:alnum:]} @tab Alphanumeric characters.
@item @code{[:alpha:]} @tab Alphabetic characters.
@item @code{[:blank:]} @tab Space and tab characters.
@item @code{[:cntrl:]} @tab Control characters.
@item @code{[:digit:]} @tab Numeric characters.
@item @code{[:graph:]} @tab Characters that are both printable and visible.
(A space is printable but not visible, whereas an @samp{a} is both.)
@item @code{[:lower:]} @tab Lowercase alphabetic characters.
@item @code{[:print:]} @tab Printable characters (characters that are not control characters).
@item @code{[:punct:]} @tab Punctuation characters (characters that are not letters, digits,
control characters, or space characters).
@item @code{[:space:]} @tab Space characters (such as space, tab, and formfeed, to name a few).
@item @code{[:upper:]} @tab Uppercase alphabetic characters.
@item @code{[:xdigit:]} @tab Characters that are hexadecimal digits.
@end multitable

For example, before the POSIX standard, you had to write @code{/[A-Za-z0-9]/}
to match alphanumeric characters.  If your
character set had other alphabetic characters in it, this would not
match them, and if your character set collated differently from
ASCII, this might not even match the ASCII alphanumeric characters.
With the POSIX character classes, you can write
@code{/[[:alnum:]]/} to match the alphabetic
and numeric characters in your character set.

@cindex collating elements
Two additional special sequences can appear in character lists.
These apply to non-ASCII character sets, which can have single symbols
(called @dfn{collating elements}) that are represented with more than one
character. They can also have several characters that are equivalent for
@dfn{collating}, or sorting, purposes.  (For example, in French, a plain ``e''
and a grave-accented ``@`e'' are equivalent.)

@table @asis
@cindex collating symbols
@item Collating Symbols
A @dfn{collating symbol} is a multicharacter collating element enclosed between
@samp{[.} and @samp{.]}.  For example, if @samp{ch} is a collating element,
then @code{[[.ch.]]} is a regexp that matches this collating element, whereas
@code{[ch]} is a regexp that matches either @samp{c} or @samp{h}.

@cindex equivalence classes
@item Equivalence Classes
An @dfn{equivalence class} is a locale-specific name for a list of
characters that are equal. The name is enclosed between
@samp{[=} and @samp{=]}.
For example, the name @samp{e} might be used to represent all of
``e,'' ``@`e,'' and ``@'e.'' In this case, @code{[[=e=]]} is a regexp
that matches any of @samp{e}, @samp{@'e}, or @samp{@`e}.
@end table

These features are very valuable in non-English speaking locales.

@strong{Caution:} The library functions that @command{gawk} uses for regular
expression matching currently only recognize POSIX character classes;
they do not recognize collating symbols or equivalence classes.
@c maybe one day ...

@node GNU Regexp Operators, Case-sensitivity, Character Lists, Regexp
@section @command{gawk}-Specific Regexp Operators

@c This section adapted (long ago) from the regex-0.12 manual

@cindex regexp operators, GNU specific
@cindex word, regexp definition of
GNU software that deals with regular expressions provides a number of
additional regexp operators.  These operators are described in this
@value{SECTION} and are specific to @command{gawk};
they are not available in other @command{awk} implementations.
Most of the additional operators deal with word matching.
For our purposes, a @dfn{word} is a sequence of one or more letters, digits,
or underscores (@samp{_}):

@table @code
@cindex @code{\w} regexp operator
@item \w
Matches any word-constituent character---that is, it matches any
letter, digit, or underscore. Think of it as short-hand for
@w{@code{[[:alnum:]_]}}.

@cindex @code{\W} regexp operator
@item \W
Matches any character that is not word-constituent.
Think of it as short-hand for
@w{@code{[^[:alnum:]_]}}.

@cindex @code{\<} regexp operator
@item \<
Matches the empty string at the beginning of a word.
For example, @code{/\<away/} matches @samp{away} but not
@samp{stowaway}.

@cindex @code{\>} regexp operator
@item \>
Matches the empty string at the end of a word.
For example, @code{/stow\>/} matches @samp{stow} but not @samp{stowaway}.

@cindex @code{\y} regexp operator
@cindex word boundaries, matching
@item \y
Matches the empty string at either the beginning or the
end of a word (i.e., the word boundar@strong{y}).  For example, @samp{\yballs?\y}
matches either @samp{ball} or @samp{balls}, as a separate word.

@cindex @code{\B} regexp operator
@item \B
Matches the empty string that occurs between two
word-constituent characters. For example,
@code{/\Brat\B/} matches @samp{crate} but it does not match @samp{dirty rat}.
@samp{\B} is essentially the opposite of @samp{\y}.
@end table

@cindex buffer matching operators
There are two other operators that work on buffers.  In Emacs, a
@dfn{buffer} is, naturally, an Emacs buffer.  For other programs,
@command{gawk}'s regexp library routines consider the entire
string to match as the buffer.

@table @code
@item \`
@cindex @code{\`} regexp operator
Matches the empty string at the
beginning of a buffer (string).

@cindex @code{\'} regexp operator
@item \'
Matches the empty string at the
end of a buffer (string).
@end table

Because @samp{^} and @samp{$} always work in terms of the beginning
and end of strings, these operators don't add any new capabilities
for @command{awk}.  They are provided for compatibility with other
GNU software.

In other GNU software, the word-boundary operator is @samp{\b}. However,
that conflicts with the @command{awk} language's definition of @samp{\b}
as backspace, so @command{gawk} uses a different letter.
An alternative method would have been to require two backslashes in the
GNU operators, but this was deemed too confusing. The current
method of using @samp{\y} for the GNU @samp{\b} appears to be the
lesser of two evils.

@c NOTE!!! Keep this in sync with the same table in the summary appendix!
@c
@c Should really do this with file inclusion.
@cindex regexp, effect of command-line options
The various command-line options
(@pxref{Options, ,Command-Line Options})
control how @command{gawk} interprets characters in regexps:

@table @asis
@item No options
In the default case, @command{gawk} provides all the facilities of
POSIX regexps and the
@ifnotinfo
previously described
GNU regexp operators.
@end ifnotinfo
@ifnottex
GNU regexp operators described
in @ref{Regexp Operators, ,Regular Expression Operators}.
@end ifnottex
However, interval expressions are not supported.

@item @code{--posix}
Only POSIX regexps are supported; the GNU operators are not special
(e.g., @samp{\w} matches a literal @samp{w}).  Interval expressions
are allowed.

@item @code{--traditional}
Traditional Unix @command{awk} regexps are matched. The GNU operators
are not special, interval expressions are not available, nor
are the POSIX character classes (@code{[[:alnum:]]} and so on).
Characters described by octal and hexadecimal escape sequences are
treated literally, even if they represent regexp metacharacters.

@item @code{--re-interval}
Allow interval expressions in regexps, even if @option{--traditional}
has been provided.
@end table

@node Case-sensitivity, Leftmost Longest, GNU Regexp Operators, Regexp
@section Case Sensitivity in Matching

@cindex case sensitivity
@cindex ignoring case
Case is normally significant in regular expressions, both when matching
ordinary characters (i.e., not metacharacters) and inside character
sets.  Thus, a @samp{w} in a regular expression matches only a lowercase
@samp{w} and not an uppercase @samp{W}.

The simplest way to do a case-independent match is to use a character
list---for example, @samp{[Ww]}.  However, this can be cumbersome if
you need to use it often and it can make the regular expressions harder
to read.  There are two alternatives that you might prefer.

One way to perform a case-insensitive match at a particular point in the
program is to convert the data to a single case, using the
@code{tolower} or @code{toupper} built-in string functions (which we
haven't discussed yet;
@pxref{String Functions, ,String Manipulation Functions}).
For example:

@example
tolower($1) ~ /foo/  @{ @dots{} @}
@end example

@noindent
converts the first field to lowercase before matching against it.
This works in any POSIX-compliant @command{awk}.

@cindex differences between @command{gawk} and @command{awk}
@cindex @code{~} operator
@cindex @code{!~} operator
@cindex @code{IGNORECASE} variable
Another method, specific to @command{gawk}, is to set the variable
@code{IGNORECASE} to a nonzero value (@pxref{Built-in Variables}).
When @code{IGNORECASE} is not zero, @emph{all} regexp and string
operations ignore case.  Changing the value of
@code{IGNORECASE} dynamically controls the case sensitivity of the
program as it runs.  Case is significant by default because
@code{IGNORECASE} (like most variables) is initialized to zero:

@example
x = "aB"
if (x ~ /ab/) @dots{}   # this test will fail

IGNORECASE = 1
if (x ~ /ab/) @dots{}   # now it will succeed
@end example

In general, you cannot use @code{IGNORECASE} to make certain rules
case-insensitive and other rules case-sensitive, because there is no
straightforward way
to set @code{IGNORECASE} just for the pattern of
a particular rule.@footnote{Experienced C and C++ programmers will note
that it is possible, using something like
@samp{IGNORECASE = 1 && /foObAr/ @{ @dots{} @}}
and
@samp{IGNORECASE = 0 || /foobar/ @{ @dots{} @}}.
However, this is somewhat obscure and we don't recommend it.}
To do this, use either character lists or @code{tolower}.  However, one
thing you can do with @code{IGNORECASE} only is dynamically turn
case-sensitivity on or off for all the rules at once.

@code{IGNORECASE} can be set on the command line or in a @code{BEGIN} rule
(@pxref{Other Arguments, ,Other Command-Line Arguments}; also
@pxref{Using BEGIN/END, ,Startup and Cleanup Actions}).
Setting @code{IGNORECASE} from the command line is a way to make
a program case-insensitive without having to edit it.

Prior to @command{gawk} 3.0, the value of @code{IGNORECASE}
affected regexp operations only. It did not affect string comparison
with @samp{==}, @samp{!=}, and so on.
Beginning with @value{PVERSION} 3.0, both regexp and string comparison
operations are also affected by @code{IGNORECASE}.

@cindex ISO 8859-1
@cindex ISO Latin-1
Beginning with @command{gawk} 3.0,
the equivalences between upper-
and lowercase characters are based on the ISO-8859-1 (ISO Latin-1)
character set. This character set is a superset of the traditional 128
ASCII characters, that also provides a number of characters suitable
for use with European languages.

The value of @code{IGNORECASE} has no effect if @command{gawk} is in
compatibility mode (@pxref{Options, ,Command-Line Options}).
Case is always significant in compatibility mode.

@node Leftmost Longest, Computed Regexps, Case-sensitivity, Regexp
@section How Much Text Matches?

@cindex leftmost longest match
@cindex matching, leftmost longest
Consider the following:

@example
echo aaaabcd | awk '@{ sub(/a+/, "<A>"); print @}'
@end example

This example uses the @code{sub} function (which we haven't discussed yet;
@pxref{String Functions, ,String Manipulation Functions})
to make a change to the input record. Here, the regexp @code{/a+/}
indicates ``one or more @samp{a} characters,'' and the replacement
text is @samp{<A>}.

The input contains four @samp{a} characters.
@command{awk} (and POSIX) regular expressions always match
the leftmost, @emph{longest} sequence of input characters that can
match.  Thus, all four @samp{a} characters are
replaced with @samp{<A>} in this example:

@example
$ echo aaaabcd | awk '@{ sub(/a+/, "<A>"); print @}'
@print{} <A>bcd
@end example

For simple match/no-match tests, this is not so important. But when doing
text matching and substitutions with the @code{match}, @code{sub}, @code{gsub},
and @code{gensub} functions, it is very important.
@ifinfo
@xref{String Functions, ,String Manipulation Functions},
for more information on these functions.
@end ifinfo
Understanding this principle is also important for regexp-based record
and field splitting (@pxref{Records, ,How Input Is Split into Records},
and also @pxref{Field Separators, ,Specifying How Fields Are Separated}).

@node Computed Regexps, , Leftmost Longest, Regexp
@section Using Dynamic Regexps

@cindex computed regular expressions
@cindex regular expressions, computed
@cindex dynamic regular expressions
@cindex regexp, dynamic
@cindex @code{~} operator
@cindex @code{!~} operator
The righthand side of a @samp{~} or @samp{!~} operator need not be a
regexp constant (i.e., a string of characters between slashes).  It may
be any expression.  The expression is evaluated and converted to a string
if necessary; the contents of the string are used as the
regexp.  A regexp that is computed in this way is called a @dfn{dynamic
regexp}:

@example
BEGIN @{ digits_regexp = "[[:digit:]]+" @}
$0 ~ digits_regexp    @{ print @}
@end example

@noindent
This sets @code{digits_regexp} to a regexp that describes one or more digits,
and tests whether the input record matches this regexp.

@c @strong{Caution:}
When using the @samp{~} and @samp{!~}
@strong{Caution:} When using the @samp{~} and @samp{!~}
operators, there is a difference between a regexp constant
enclosed in slashes and a string constant enclosed in double quotes.
If you are going to use a string constant, you have to understand that
the string is, in essence, scanned @emph{twice}: the first time when
@command{awk} reads your program, and the second time when it goes to
match the string on the lefthand side of the operator with the pattern
on the right.  This is true of any string valued expression (such as
@code{digits_regexp} shown previously), not just string constants.

@cindex regexp constants, difference between slashes and quotes
What difference does it make if the string is
scanned twice? The answer has to do with escape sequences, and particularly
with backslashes.  To get a backslash into a regular expression inside a
string, you have to type two backslashes.

For example, @code{/\*/} is a regexp constant for a literal @samp{*}.
Only one backslash is needed.  To do the same thing with a string,
you have to type @code{"\\*"}.  The first backslash escapes the
second one so that the string actually contains the
two characters @samp{\} and @samp{*}.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
Given that you can use both regexp and string constants to describe
regular expressions, which should you use?  The answer is ``regexp
constants,'' for several reasons:

@itemize @bullet
@item
String constants are more complicated to write and
more difficult to read. Using regexp constants makes your programs
less error-prone.  Not understanding the difference between the two
kinds of constants is a common source of errors.

@item
It is more efficient to use regexp constants. @command{awk} can note
that you have supplied a regexp, and store it internally in a form that
makes pattern matching more efficient.  When using a string constant,
@command{awk} must first convert the string into this internal form and
then perform the pattern matching.

@item
Using regexp constants is better form; it shows clearly that you
intend a regexp match.
@end itemize

@c fakenode --- for prepinfo
@subheading Advanced Notes: Using @code{\n} in Character Lists of Dynamic Regexps
@cindex advanced notes
@cindex dynamic regular expressions with embedded newlines
@cindex regexp, dynamic, with embedded newlines
@cindex newlines, embedded in dynamic regexps
@cindex embedded newlines, in dynamic regexps

Some commercial versions of @command{awk} do not allow the newline
character to be used inside a character list for a dynamic regexp:

@example
$ awk '$0 ~ "[ \t\n]"'
@error{} awk: newline in character class [
@error{} ]...
@error{}  source line number 1
@error{}  context is
@error{}          >>>  <<<
@end example

But a newline in a regexp constant works with no problem:

@example
$ awk '$0 ~ /[ \t\n]/'
here is a sample line
@print{} here is a sample line
@kbd{Ctrl-d}
@end example

@command{gawk} does not have this problem, and it isn't likely to
occur often in practice, but it's worth noting for future reference.

@node Reading Files, Printing, Regexp, Top
@chapter Reading Input Files

@cindex reading files
@cindex input
@cindex standard input
@cindex @code{FILENAME} variable
In the typical @command{awk} program, all input is read either from the
standard input (by default, this is the keyboard but often it is a pipe from another
command), or from files whose names you specify on the @command{awk}
command line.  If you specify input files, @command{awk} reads them
in order, processing all the data from one before going on to the next.
The name of the current input file can be found in the built-in variable
@code{FILENAME}
(@pxref{Built-in Variables}).

The input is read in units called @dfn{records}, and is processed by the
rules of your program one record at a time.
By default, each record is one line.  Each
record is automatically split into chunks called @dfn{fields}.
This makes it more convenient for programs to work on the parts of a record.

On rare occasions, you may need to use the @code{getline} command.
The  @code{getline} command is valuable, both because it
can do explicit input from any number of files, and because the files
used with it do not have to be named on the @command{awk} command line
(@pxref{Getline, ,Explicit Input with @code{getline}}).

@menu
* Records::                     Controlling how data is split into records.
* Fields::                      An introduction to fields.
* Non-Constant Fields::         Non-constant Field Numbers.
* Changing Fields::             Changing the Contents of a Field.
* Field Separators::            The field separator and how to change it.
* Constant Size::               Reading constant width data.
* Multiple Line::               Reading multi-line records.
* Getline::                     Reading files under explicit program control
                                using the @code{getline} function.
@end menu

@node Records, Fields, Reading Files, Reading Files
@section How Input Is Split into Records

@cindex number of records, @code{NR}, @code{FNR}
@cindex @code{NR} variable
@cindex @code{FNR} variable
The @command{awk} utility divides the input for your @command{awk}
program into records and fields.
@command{awk} keeps track of the number of records that have
been read
so far
from the current input file.  This value is stored in a
built-in variable called @code{FNR}.  It is reset to zero when a new
file is started.  Another built-in variable, @code{NR}, is the total
number of input records read so far from all @value{DF}s.  It starts at zero,
but is never automatically reset to zero.

@cindex record separator, @code{RS}
@cindex changing the record separator
@cindex record, definition of
@cindex @code{RS} variable
Records are separated by a character called the @dfn{record separator}.
By default, the record separator is the newline character.
This is why records are, by default, single lines.
A different character can be used for the record separator by
assigning the character to the built-in variable @code{RS}.

Like any other variable,
the value of @code{RS} can be changed in the @command{awk} program
with the assignment operator, @samp{=}
(@pxref{Assignment Ops, ,Assignment Expressions}).
The new record-separator character should be enclosed in quotation marks,
which indicate a string constant.  Often the right time to do this is
at the beginning of execution, before any input is processed,
so that the very first record is read with the proper separator.
To do this, use the special @code{BEGIN} pattern
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).
For example:

@example
awk 'BEGIN @{ RS = "/" @}
     @{ print $0 @}' BBS-list
@end example

@noindent
changes the value of @code{RS} to @code{"/"}, before reading any input.
This is a string whose first character is a slash; as a result, records
are separated by slashes.  Then the input file is read, and the second
rule in the @command{awk} program (the action with no pattern) prints each
record.  Because each @code{print} statement adds a newline at the end of
its output, the effect of this @command{awk} program is to copy the input
with each slash changed to a newline.  Here are the results of running
the program on @file{BBS-list}:

@example
$ awk 'BEGIN @{ RS = "/" @}
>      @{ print $0 @}' BBS-list
@print{} aardvark     555-5553     1200
@print{} 300          B
@print{} alpo-net     555-3412     2400
@print{} 1200
@print{} 300     A
@print{} barfly       555-7685     1200
@print{} 300          A
@print{} bites        555-1675     2400
@print{} 1200
@print{} 300     A
@print{} camelot      555-0542     300               C
@print{} core         555-2912     1200
@print{} 300          C
@print{} fooey        555-1234     2400
@print{} 1200
@print{} 300     B
@print{} foot         555-6699     1200
@print{} 300          B
@print{} macfoo       555-6480     1200
@print{} 300          A
@print{} sdace        555-3430     2400
@print{} 1200
@print{} 300     A
@print{} sabafoo      555-2127     1200
@print{} 300          C
@print{}
@end example

@noindent
Note that the entry for the @samp{camelot} BBS is not split.
In the original @value{DF}
(@pxref{Sample Data Files, ,@value{DDF}s for the Examples}),
the line looks like this:

@example
camelot      555-0542     300               C
@end example

@noindent
It has one baud rate only, so there are no slashes in the record,
unlike the others which have two or more baud rates.
In fact, this record is treated as part of the record
for the @samp{core} BBS; the newline separating them in the output
is the original newline in the @value{DF}, not the one added by
@command{awk} when it printed the record!

Another way to change the record separator is on the command line,
using the variable-assignment feature
(@pxref{Other Arguments, ,Other Command-Line Arguments}):

@example
awk '@{ print $0 @}' RS="/" BBS-list
@end example

@noindent
This sets @code{RS} to @samp{/} before processing @file{BBS-list}.

Using an unusual character such as @samp{/} for the record separator
produces correct behavior in the vast majority of cases.  However,
the following (extreme) pipeline prints a surprising @samp{1}:

@example
$ echo | awk 'BEGIN @{ RS = "a" @} ; @{ print NF @}'
@print{} 1
@end example

There is one field, consisting of a newline.  The value of the built-in
variable @code{NF} is the number of fields in the current record.

@cindex dark corner
Reaching the end of an input file terminates the current input record,
even if the last character in the file is not the character in @code{RS}.
@value{DARKCORNER}

@cindex empty string
The empty string @code{""} (a string without any characters)
has a special meaning
as the value of @code{RS}. It means that records are separated
by one or more blank lines and nothing else.
@xref{Multiple Line, ,Multiple-Line Records}, for more details.

If you change the value of @code{RS} in the middle of an @command{awk} run,
the new value is used to delimit subsequent records, but the record
currently being processed, as well as records already processed, are not
affected.

@cindex @code{RT} variable
@cindex record terminator, @code{RT}
@cindex terminator, record
@cindex differences between @command{gawk} and @command{awk}
@cindex regular expressions as record separators
After the end of the record has been determined, @command{gawk}
sets the variable @code{RT} to the text in the input that matched
@code{RS}.
When using @command{gawk},
the value of @code{RS} is not limited to a one-character
string.  It can be any regular expression
(@pxref{Regexp, ,Regular Expressions}).
In general, each record
ends at the next string that matches the regular expression; the next
record starts at the end of the matching string.  This general rule is
actually at work in the usual case, where @code{RS} contains just a
newline: a record ends at the beginning of the next matching string (the
next newline in the input) and the following record starts just after
the end of this string (at the first character of the following line).
The newline, because it matches @code{RS}, is not part of either record.

When @code{RS} is a single character, @code{RT}
contains the same single character. However, when @code{RS} is a
regular expression, @code{RT} contains
the actual input text that matched the regular expression.

The following example illustrates both of these features.
It sets @code{RS} equal to a regular expression that
matches either a newline or a series of one or more uppercase letters
with optional leading and/or trailing whitespace:

@example
$ echo record 1 AAAA record 2 BBBB record 3 |
> gawk 'BEGIN @{ RS = "\n|( *[[:upper:]]+ *)" @}
>             @{ print "Record =", $0, "and RT =", RT @}'
@print{} Record = record 1 and RT =  AAAA 
@print{} Record = record 2 and RT =  BBBB 
@print{} Record = record 3 and RT = 
@print{}
@end example

@noindent
The final line of output has an extra blank line. This is because the
value of @code{RT} is a newline, and the @code{print} statement
supplies its own terminating newline.
@xref{Simple Sed, ,A Simple Stream Editor}, for a more useful example
of @code{RS} as a regexp and @code{RT}.

@cindex differences between @command{gawk} and @command{awk}
The use of @code{RS} as a regular expression and the @code{RT}
variable are @command{gawk} extensions; they are not available in
compatibility mode
(@pxref{Options, ,Command-Line Options}).
In compatibility mode, only the first character of the value of
@code{RS} is used to determine the end of the record.

@c fakenode --- for prepinfo
@subheading Advanced Notes: @code{RS = "\0"} Is Not Portable
@cindex advanced notes
@cindex portability issues

There are times when you might want to treat an entire @value{DF} as a
single record.  The only way to make this happen is to give @code{RS}
a value that you know doesn't occur in the input file.  This is hard
to do in a general way, such that a program always works for arbitrary
input files.
@c can you say `understatement' boys and girls?

You might think that for text files, the @sc{nul} character, which
consists of a character with all bits equal to zero, is a good
value to use for @code{RS} in this case:

@example
BEGIN @{ RS = "\0" @}  # whole file becomes one record?
@end example

@cindex differences between @command{gawk} and @command{awk}
@command{gawk} in fact accepts this, and uses the @sc{nul}
character for the record separator.
However, this usage is @emph{not} portable
to other @command{awk} implementations.

@cindex dark corner
All other @command{awk} implementations@footnote{At least that we know
about.} store strings internally as C-style strings.  C strings use the
@sc{nul} character as the string terminator.  In effect, this means that
@samp{RS = "\0"} is the same as @samp{RS = ""}.
@value{DARKCORNER}

The best way to treat a whole file as a single record is to
simply read the file in, one record at a time, concatenating each
record onto the end of the previous ones.

@node Fields, Non-Constant Fields, Records, Reading Files
@section Examining Fields

@cindex examining fields
@cindex fields
@cindex accessing fields
When @command{awk} reads an input record, the record is
automatically separated or @dfn{parsed} by the interpreter into chunks
called @dfn{fields}.  By default, fields are separated by @dfn{whitespace},
like words in a line.
Whitespace in @command{awk} means any string of one or more spaces,
tabs, or newlines;@footnote{In POSIX @command{awk}, newlines are not
considered whitespace for separating fields.} other characters, such as
formfeed, vertical tab, etc.@: that are
considered whitespace by other languages, are @emph{not} considered
whitespace by @command{awk}.

The purpose of fields is to make it more convenient for you to refer to
these pieces of the record.  You don't have to use them---you can
operate on the whole record if you want---but fields are what make
simple @command{awk} programs so powerful.

@cindex @code{$} field operator
@cindex field operator @code{$}
A dollar-sign (@samp{$}) is used
to refer to a field in an @command{awk} program,
followed by the number of the field you want.  Thus, @code{$1}
refers to the first field, @code{$2} to the second, and so on.
(Unlike the Unix shells, the field numbers are not limited to single digits.
@code{$127} is the one hundred and twenty-seventh field in the record.)
For example, suppose the following is a line of input:

@example
This seems like a pretty nice example.
@end example

@noindent
Here the first field, or @code{$1}, is @samp{This}, the second field, or
@code{$2}, is @samp{seems}, and so on.  Note that the last field,
@code{$7}, is @samp{example.}.  Because there is no space between the
@samp{e} and the @samp{.}, the period is considered part of the seventh
field.

@cindex @code{NF} variable
@cindex number of fields, @code{NF}
@code{NF} is a built-in variable whose value is the number of fields
in the current record.  @command{awk} automatically updates the value
of @code{NF} each time it reads a record.  No matter how many fields
there are, the last field in a record can be represented by @code{$NF}.
So, @code{$NF} is the same as @code{$7}, which is @samp{example.}.
If you try to reference a field beyond the last
one (such as @code{$8} when the record has only seven fields), you get
the empty string.  (If used in a numeric operation, you get zero.)

The use of @code{$0}, which looks like a reference to the ``zeroth'' field, is
a special case: it represents the whole input record
when you are not interested in specific fields.
Here are some more examples:

@example
$ awk '$1 ~ /foo/ @{ print $0 @}' BBS-list
@print{} fooey        555-1234     2400/1200/300     B
@print{} foot         555-6699     1200/300          B
@print{} macfoo       555-6480     1200/300          A
@print{} sabafoo      555-2127     1200/300          C
@end example

@noindent
This example prints each record in the file @file{BBS-list} whose first
field contains the string @samp{foo}.  The operator @samp{~} is called a
@dfn{matching operator}
(@pxref{Regexp Usage, , How to Use Regular Expressions});
it tests whether a string (here, the field @code{$1}) matches a given regular
expression.

By contrast, the following example
looks for @samp{foo} in @emph{the entire record} and prints the first
field and the last field for each matching input record:

@example
$ awk '/foo/ @{ print $1, $NF @}' BBS-list
@print{} fooey B
@print{} foot B
@print{} macfoo A
@print{} sabafoo C
@end example

@node Non-Constant Fields, Changing Fields, Fields, Reading Files
@section Non-Constant Field Numbers

The number of a field does not need to be a constant.  Any expression in
the @command{awk} language can be used after a @samp{$} to refer to a
field.  The value of the expression specifies the field number.  If the
value is a string, rather than a number, it is converted to a number.
Consider this example:

@example
awk '@{ print $NR @}'
@end example

@noindent
Recall that @code{NR} is the number of records read so far: one in the
first record, two in the second, etc.  So this example prints the first
field of the first record, the second field of the second record, and so
on.  For the twentieth record, field number 20 is printed; most likely,
the record has fewer than 20 fields, so this prints a blank line.
Here is another example of using expressions as field numbers:

@example
awk '@{ print $(2*2) @}' BBS-list
@end example

@command{awk} evaluates the expression @samp{(2*2)} and uses
its value as the number of the field to print.  The @samp{*} sign
represents multiplication, so the expression @samp{2*2} evaluates to four.
The parentheses are used so that the multiplication is done before the
@samp{$} operation; they are necessary whenever there is a binary
operator in the field-number expression.  This example, then, prints the
hours of operation (the fourth field) for every line of the file
@file{BBS-list}.  (All of the @command{awk} operators are listed, in
order of decreasing precedence, in
@ref{Precedence,  , Operator Precedence (How Operators Nest)}.)

If the field number you compute is zero, you get the entire record.
Thus, @samp{$(2-2)} has the same value as @code{$0}.  Negative field
numbers are not allowed; trying to reference one usually terminates
the program.  (The POSIX standard does not define
what happens when you reference a negative field number.  @command{gawk}
notices this and terminates your program.  Other @command{awk}
implementations may behave differently.)

As mentioned in @ref{Fields, ,Examining Fields},
@command{awk} stores the current record's number of fields in the built-in
variable @code{NF} (also @pxref{Built-in Variables}).  The expression
@code{$NF} is not a special feature---it is the direct consequence of
evaluating @code{NF} and using its value as a field number.

@node Changing Fields, Field Separators, Non-Constant Fields, Reading Files
@section Changing the Contents of a Field

@cindex fields, changing contents of
@cindex changing contents of a field
@cindex assignment to fields
The contents of a field, as seen by @command{awk}, can be changed within an
@command{awk} program; this changes what @command{awk} perceives as the
current input record.  (The actual input is untouched; @command{awk} @emph{never}
modifies the input file.)
Consider this example and its output:

@example
$ awk '@{ nboxes = $3 ; $3 = $3 - 10
>        print nboxes, $3 @}' inventory-shipped
@print{} 13 3
@print{} 15 5
@print{} 15 5
@dots{}
@end example

@noindent
The program first saves the original value of field three in the variable
@code{nboxes}.
The @samp{-} sign represents subtraction, so this program reassigns
field three, @code{$3}, as the original value of field three minus ten:
@samp{$3 - 10}.  (@xref{Arithmetic Ops, ,Arithmetic Operators}.)
Then it prints the original and new values for field three.
(Someone in the warehouse made a consistent mistake while inventorying
the red boxes.)

For this to work, the text in field @code{$2} must make sense
as a number; the string of characters must be converted to a number
for the computer to do arithmetic on it.  The number resulting
from the subtraction is converted back to a string of characters that
then becomes field three.
@xref{Conversion, ,Conversion of Strings and Numbers}.

When the value of a field is changed (as perceived by @command{awk}), the
text of the input record is recalculated to contain the new field where
the old one was.  In other words, @code{$0} changes to reflect the altered
field.  Thus, this program
prints a copy of the input file, with 10 subtracted from the second
field of each line:

@example
$ awk '@{ $2 = $2 - 10; print $0 @}' inventory-shipped
@print{} Jan 3 25 15 115
@print{} Feb 5 32 24 226
@print{} Mar 5 24 34 228
@dots{}
@end example

It is also possible to also assign contents to fields that are out
of range.  For example:

@example
$ awk '@{ $6 = ($5 + $4 + $3 + $2)
>        print $6 @}' inventory-shipped
@print{} 168
@print{} 297
@print{} 301
@dots{}
@end example

@noindent
We've just created @code{$6}, whose value is the sum of fields
@code{$2}, @code{$3}, @code{$4}, and @code{$5}.  The @samp{+} sign
represents addition.  For the file @file{inventory-shipped}, @code{$6}
represents the total number of parcels shipped for a particular month.

Creating a new field changes @command{awk}'s internal copy of the current
input record, which is the value of @code{$0}.  Thus, if you do @samp{print $0}
after adding a field, the record printed includes the new field, with
the appropriate number of field separators between it and the previously
existing fields.

This recomputation affects and is affected by
@code{NF} (the number of fields; @pxref{Fields, ,Examining Fields}).
It is also affected by a feature that has not been discussed yet:
the @dfn{output field separator}, @code{OFS},
used to separate the fields (@pxref{Output Separators}).
For example, the value of @code{NF} is set to the number of the highest
field you create.

Note, however, that merely @emph{referencing} an out-of-range field
does @emph{not} change the value of either @code{$0} or @code{NF}.
Referencing an out-of-range field only produces an empty string.  For
example:

@example
if ($(NF+1) != "")
    print "can't happen"
else
    print "everything is normal"
@end example

@noindent
should print @samp{everything is normal}, because @code{NF+1} is certain
to be out of range.  (@xref{If Statement, ,The @code{if}-@code{else} Statement},
for more information about @command{awk}'s @code{if-else} statements.
@xref{Typing and Comparison, ,Variable Typing and Comparison Expressions},
for more information about the @samp{!=} operator.)

It is important to note that making an assignment to an existing field
changes the
value of @code{$0} but does not change the value of @code{NF},
even when you assign the empty string to a field.  For example:

@example
$ echo a b c d | awk '@{ OFS = ":"; $2 = ""
>                       print $0; print NF @}'
@print{} a::c:d
@print{} 4
@end example

@noindent
The field is still there; it just has an empty value, denoted by
the two colons between @samp{a} and @samp{c}.
This example shows what happens if you create a new field:

@example
$ echo a b c d | awk '@{ OFS = ":"; $2 = ""; $6 = "new"
>                       print $0; print NF @}'
@print{} a::c:d::new
@print{} 6
@end example

@noindent
The intervening field, @code{$5}, is created with an empty value
(indicated by the second pair of adjacent colons),
and @code{NF} is updated with the value six.

@c FIXME: Verify that this is in POSIX
@cindex dark corner
Decrementing @code{NF} throws away the values of the fields
after the new value of @code{NF} and recomputes @code{$0}.
@value{DARKCORNER}
Here is an example:

@example
$ echo a b c d e f | awk '@{ print "NF =", NF;
>                            NF = 3; print $0 @}'
@print{} NF = 6
@print{} a b c
@end example

@cindex portability issues
@strong{Caution:} Some versions of @command{awk} don't
rebuild @code{$0} when @code{NF} is decremented. Caveat emptor.

@node Field Separators, Constant Size, Changing Fields, Reading Files
@section Specifying How Fields Are Separated

@menu
* Regexp Field Splitting::       Using regexps as the field separator.
* Single Character Fields::      Making each character a separate field.
* Command Line Field Separator:: Setting @code{FS} from the command-line.
* Field Splitting Summary::      Some final points and a summary table.
@end menu

@cindex @code{FS} variable
@cindex fields, separating
@cindex field separator, @code{FS}
The @dfn{field separator}, which is either a single character or a regular
expression, controls the way @command{awk} splits an input record into fields.
@command{awk} scans the input record for character sequences that
match the separator; the fields themselves are the text between the matches.

In the examples that follow, we use the bullet symbol (@bullet{}) to
represent spaces in the output.
If the field separator is @samp{oo}, then the following line:

@example
moo goo gai pan
@end example

@noindent
is split into three fields: @samp{m}, @samp{@bullet{}g}, and
@samp{@bullet{}gai@bullet{}pan}.
Note the leading spaces in the values of the second and third fields.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
The field separator is represented by the built-in variable @code{FS}.
Shell programmers take note:  @command{awk} does @emph{not} use the
name @code{IFS} that is used by the POSIX-compliant shells (such as
the Unix Bourne shell, @command{sh}, or @command{bash}).

The value of @code{FS} can be changed in the @command{awk} program with the
assignment operator, @samp{=} (@pxref{Assignment Ops, ,Assignment Expressions}).
Often the right time to do this is at the beginning of execution
before any input has been processed, so that the very first record
is read with the proper separator.  To do this, use the special
@code{BEGIN} pattern
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).
For example, here we set the value of @code{FS} to the string
@code{","}:

@example
awk 'BEGIN @{ FS = "," @} ; @{ print $2 @}'
@end example

@noindent
Given the input line:

@example
John Q. Smith, 29 Oak St., Walamazoo, MI 42139
@end example

@noindent
this @command{awk} program extracts and prints the string
@samp{@bullet{}29@bullet{}Oak@bullet{}St.}.

@cindex field separator, choice of
@cindex regular expressions as field separators
Sometimes the input data contains separator characters that don't
separate fields the way you thought they would.  For instance, the
person's name in the example we just used might have a title or
suffix attached, such as:

@example
John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139
@end example

@noindent
The same program would extract @samp{@bullet{}LXIX}, instead of
@samp{@bullet{}29@bullet{}Oak@bullet{}St.}.
If you were expecting the program to print the
address, you would be surprised.  The moral is to choose your data layout and
separator characters carefully to prevent such problems.
(If the data is not in a form that is easy to process, perhaps you
can massage it first with a separate @command{awk} program.)

Fields are normally separated by whitespace sequences
(spaces, tabs, and newlines), not by single spaces.  Two spaces in a row do not
delimit an empty field.  The default value of the field separator @code{FS}
is a string containing a single space, @w{@code{" "}}.  If @command{awk}
interpreted this value in the usual way, each space character would separate
fields, so two spaces in a row would make an empty field between them.
The reason this does not happen is that a single space as the value of
@code{FS} is a special case---it is taken to specify the default manner
of delimiting fields.

If @code{FS} is any other single character, such as @code{","}, then
each occurrence of that character separates two fields.  Two consecutive
occurrences delimit an empty field.  If the character occurs at the
beginning or the end of the line, that too delimits an empty field.  The
space character is the only single character that does not follow these
rules.

@node Regexp Field Splitting, Single Character Fields, Field Separators, Field Separators
@subsection Using Regular Expressions to Separate Fields

The previous @value{SUBSECTION}
discussed the use of single characters or simple strings as the
value of @code{FS}.
More generally, the value of @code{FS} may be a string containing any
regular expression.  In this case, each match in the record for the regular
expression separates fields.  For example, the assignment:

@example
FS = ", \t"
@end example

@noindent
makes every area of an input line that consists of a comma followed by a
space and a tab into a field separator.
@ifinfo
(@samp{\t}
is an @dfn{escape sequence} that stands for a tab;
@pxref{Escape Sequences},
for the complete list of similar escape sequences.)
@end ifinfo

For a less trivial example of a regular expression, try using
single spaces to separate fields the way single commas are used.
@code{FS} can be set to @w{@code{"[@ ]"}} (left bracket, space, right
bracket).  This regular expression matches a single space and nothing else
(@pxref{Regexp, ,Regular Expressions}).

There is an important difference between the two cases of @samp{FS = @w{" "}}
(a single space) and @samp{FS = @w{"[ \t\n]+"}}
(a regular expression matching one or more spaces, tabs, or newlines).
For both values of @code{FS}, fields are separated by @dfn{runs}
(multiple adjacent occurrences) of spaces, tabs,
and/or newlines.  However, when the value of @code{FS} is @w{@code{" "}},
@command{awk} first strips leading and trailing whitespace from
the record and then decides where the fields are.
For example, the following pipeline prints @samp{b}:

@example
$ echo ' a b c d ' | awk '@{ print $2 @}'
@print{} b
@end example

@noindent
However, this pipeline prints @samp{a} (note the extra spaces around
each letter):

@example
$ echo ' a  b  c  d ' | awk 'BEGIN @{ FS = "[ \t\n]+" @}
>                                  @{ print $2 @}'
@print{} a
@end example

@noindent
@cindex null string
@cindex empty string
In this case, the first field is @dfn{null} or empty.

The stripping of leading and trailing whitespace also comes into
play whenever @code{$0} is recomputed.  For instance, study this pipeline:

@example
$ echo '   a b c d' | awk '@{ print; $2 = $2; print @}'
@print{}    a b c d
@print{} a b c d
@end example

@noindent
The first @code{print} statement prints the record as it was read,
with leading whitespace intact.  The assignment to @code{$2} rebuilds
@code{$0} by concatenating @code{$1} through @code{$NF} together,
separated by the value of @code{OFS}.  Because the leading whitespace
was ignored when finding @code{$1}, it is not part of the new @code{$0}.
Finally, the last @code{print} statement prints the new @code{$0}.

@node Single Character Fields, Command Line Field Separator, Regexp Field Splitting, Field Separators
@subsection Making Each Character a Separate Field

@cindex differences between @command{gawk} and @command{awk}
@cindex single-character fields
There are times when you may want to examine each character
of a record separately.  This can be done in @command{gawk} by
simply assigning the null string (@code{""}) to @code{FS}. In this case,
each individual character in the record becomes a separate field.
For example:

@example
$ echo a b | gawk 'BEGIN @{ FS = "" @}
>                  @{
>                      for (i = 1; i <= NF; i = i + 1)
>                          print "Field", i, "is", $i
>                  @}'
@print{} Field 1 is a
@print{} Field 2 is
@print{} Field 3 is b
@end example

@cindex dark corner
Traditionally, the behavior of @code{FS} equal to @code{""} was not defined.
In this case, most versions of Unix @command{awk} simply treat the entire record
as only having one field.
@value{DARKCORNER}
In compatibility mode
(@pxref{Options, ,Command-Line Options}),
if @code{FS} is the null string, then @command{gawk} also
behaves this way.

@node Command Line Field Separator, Field Splitting Summary, Single Character Fields, Field Separators
@subsection Setting @code{FS} from the Command Line
@cindex @code{-F} option
@cindex command-line option, @code{-F}
@cindex field separator, on command line
@cindex command line, setting @code{FS} on

@code{FS} can be set on the command line.  Use the @option{-F} option to
do so.  For example:

@example
awk -F, '@var{program}' @var{input-files}
@end example

@noindent
sets @code{FS} to the @samp{,} character.  Notice that the option uses
a capital @samp{F} instead of a lowercase @option{-f}, which specifies a file
containing an @command{awk} program.  Case is significant in command-line
options:
the @option{-F} and @option{-f} options have nothing to do with each other.
You can use both options at the same time to set the @code{FS} variable
@emph{and} get an @command{awk} program from a file.

The value used for the argument to @option{-F} is processed in exactly the
same way as assignments to the built-in variable @code{FS}.
Any special characters in the field separator must be escaped
appropriately.  For example, to use a @samp{\} as the field separator
on the command line, you would have to type:

@example
# same as FS = "\\"
awk -F\\\\ '@dots{}' files @dots{}
@end example

@noindent
Because @samp{\} is used for quoting in the shell, @command{awk} sees
@samp{-F\\}.  Then @command{awk} processes the @samp{\\} for escape
characters (@pxref{Escape Sequences}), finally yielding
a single @samp{\} to use for the field separator.

@cindex historical features
As a special case, in compatibility mode
(@pxref{Options, ,Command-Line Options}),
if the argument to @option{-F} is @samp{t}, then @code{FS} is set to
the tab character.  If you type @samp{-F\t} at the
shell, without any quotes, the @samp{\} gets deleted, so @command{awk}
figures that you really want your fields to be separated with tabs and
not @samp{t}s.  Use @samp{-v FS="t"} or @samp{-F"[t]"} on the command line
if you really do want to separate your fields with @samp{t}s.

For example, let's use an @command{awk} program file called @file{baud.awk}
that contains the pattern @code{/300/} and the action @samp{print $1}:

@example
/300/   @{ print $1 @}
@end example

Let's also set @code{FS} to be the @samp{-} character and run the
program on the file @file{BBS-list}.  The following command prints a
list of the names of the bulletin boards that operate at 300 baud and
the first three digits of their phone numbers:

@c tweaked to make the tex output look better in @smallbook
@example
$ awk -F- -f baud.awk BBS-list
@print{} aardvark     555
@print{} alpo
@print{} barfly       555
@print{} bites        555
@print{} camelot      555
@print{} core         555
@print{} fooey        555
@print{} foot         555
@print{} macfoo       555
@print{} sdace        555
@print{} sabafoo      555
@end example

@noindent
Note the second line of output.  The second line
in the original file looked like this:

@example
alpo-net     555-3412     2400/1200/300     A
@end example

The @samp{-} as part of the system's name was used as the field
separator, instead of the @samp{-} in the phone number that was
originally intended.  This demonstrates why you have to be careful in
choosing your field and record separators.

Perhaps the most common use of a single character as the field
separator occurs when processing the Unix system password file.
On many Unix systems, each user has a separate entry in the system password
file, one line per user.  The information in these lines is separated
by colons.  The first field is the user's logon name and the second is
the user's (encrypted or shadow) password.  A password file entry might look
like this:

@cindex Robbins, Arnold
@example
arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/bash
@end example

The following program searches the system password file and prints
the entries for users who have no password:

@example
awk -F: '$2 == ""' /etc/passwd
@end example

@node Field Splitting Summary, , Command Line Field Separator, Field Separators
@subsection Field Splitting Summary

The following
table
summarizes how fields are split, based on the
value of @code{FS}. (@samp{==} means ``is equal to.'')

@table @code
@item FS == " "
Fields are separated by runs of whitespace.  Leading and trailing
whitespace are ignored.  This is the default.

@item FS == @var{any other single character}
Fields are separated by each occurrence of the character.  Multiple
successive occurrences delimit empty fields, as do leading and
trailing occurrences.
The character can even be a regexp metacharacter; it does not need
to be escaped.

@item FS == @var{regexp}
Fields are separated by occurrences of characters that match @var{regexp}.
Leading and trailing matches of @var{regexp} delimit empty fields.

@item FS == ""
Each individual character in the record becomes a separate field.
(This is a @command{gawk} extension; it is not specified by the
POSIX standard.)
@end table

@c fakenode --- for prepinfo
@subheading Advanced Notes: Changing @code{FS} Does Not Affect the Fields

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
According to the POSIX standard, @command{awk} is supposed to behave
as if each record is split into fields at the time it is read.
In particular, this means that if you change the value of @code{FS}
after a record is read, the value of the fields (i.e., how they were split)
should reflect the old value of @code{FS}, not the new one.

@cindex dark corner
@cindex @command{sed} utility
@cindex stream editor
However, many implementations of @command{awk} do not work this way.  Instead,
they defer splitting the fields until a field is actually
referenced.  The fields are split
using the @emph{current} value of @code{FS}!
@value{DARKCORNER}
This behavior can be difficult
to diagnose. The following example illustrates the difference
between the two methods.
(The @command{sed}@footnote{The @command{sed} utility is a ``stream editor.''
Its behavior is also defined by the POSIX standard.}
command prints just the first line of @file{/etc/passwd}.)

@example
sed 1q /etc/passwd | awk '@{ FS = ":" ; print $1 @}'
@end example

@noindent
which usually prints:

@example
root
@end example

@noindent
on an incorrect implementation of @command{awk}, while @command{gawk}
prints something like:

@example
root:nSijPlPhZZwgE:0:0:Root:/:
@end example

@node Constant Size, Multiple Line, Field Separators, Reading Files
@section Reading Fixed-Width Data

@ifnotinfo
@strong{Note:} This @value{SECTION} discusses an advanced
feature of @command{gawk}.  If you are a novice @command{awk} user,
you might want to skip it on the first reading.
@end ifnotinfo

@ifinfo
(This @value{SECTION} discusses an advanced feature of @command{awk}.
If you are a novice @command{awk} user, you might want to skip it on
the first reading.)
@end ifinfo

@command{gawk} @value{PVERSION} 2.13 introduced a facility for dealing with
fixed-width fields with no distinctive field separator.  For example,
data of this nature arises in the input for old Fortran programs where
numbers are run together, or in the output of programs that did not
anticipate the use of their output as input for other programs.

An example of the latter is a table where all the columns are lined up by
the use of a variable number of spaces and @emph{empty fields are just
spaces}.  Clearly, @command{awk}'s normal field splitting based on @code{FS}
does not work well in this case.  Although a portable @command{awk} program
can use a series of @code{substr} calls on @code{$0}
(@pxref{String Functions, ,String Manipulation Functions}),
this is awkward and inefficient for a large number of fields.

@cindex fatal errors
@cindex @command{w} utility
The splitting of an input record into fixed-width fields is specified by
assigning a string containing space-separated numbers to the built-in
variable @code{FIELDWIDTHS}.  Each number specifies the width of the field,
@emph{including} columns between fields.  If you want to ignore the columns
between fields, you can specify the width as a separate field that is
subsequently ignored.
It is a fatal error to supply a field width that is not a positive number.
The following data is the output of the Unix @command{w} utility.  It is useful
to illustrate the use of @code{FIELDWIDTHS}:

@example
@group
 10:06pm  up 21 days, 14:04,  23 users
User     tty       login@  idle   JCPU   PCPU  what
hzuo     ttyV0     8:58pm            9      5  vi p24.tex
hzang    ttyV3     6:37pm    50                -csh
eklye    ttyV5     9:53pm            7      1  em thes.tex
dportein ttyV6     8:17pm  1:47                -csh
gierd    ttyD3    10:00pm     1                elm
dave     ttyD4     9:47pm            4      4  w
brent    ttyp0    26Jun91  4:46  26:46   4:41  bash
dave     ttyq4    26Jun9115days     46     46  wnewmail
@end group
@end example

The following program takes the above input, converts the idle time to
number of seconds, and prints out the first two fields and the calculated
idle time.

@strong{Note:}
This program uses a number of @command{awk} features that
haven't been introduced yet.

@example
BEGIN  @{ FIELDWIDTHS = "9 6 10 6 7 7 35" @}
NR > 2 @{
    idle = $4
    sub(/^  */, "", idle)   # strip leading spaces
    if (idle == "")
        idle = 0
    if (idle ~ /:/) @{
        split(idle, t, ":")
        idle = t[1] * 60 + t[2]
    @}
    if (idle ~ /days/)
        idle *= 24 * 60 * 60

    print $1, $2, idle
@}
@end example

Running the program on the data produces the following results:

@example
hzuo      ttyV0  0
hzang     ttyV3  50
eklye     ttyV5  0
dportein  ttyV6  107
gierd     ttyD3  1
dave      ttyD4  0
brent     ttyp0  286
dave      ttyq4  1296000
@end example

Another (possibly more practical) example of fixed-width input data
is the input from a deck of balloting cards.  In some parts of
the United States, voters mark their choices by punching holes in computer
cards.  These cards are then processed to count the votes for any particular
candidate or on any particular issue.  Because a voter may choose not to
vote on some issue, any column on the card may be empty.  An @command{awk}
program for processing such data could use the @code{FIELDWIDTHS} feature
to simplify reading the data.  (Of course, getting @command{gawk} to run on
a system with card readers is another story!)

@ignore
Exercise: Write a ballot card reading program
@end ignore

Assigning a value to @code{FS} causes @command{gawk} to return to using
@code{FS} for field splitting.  Use @samp{FS = FS} to make this happen,
without having to know the current value of @code{FS}.
In order to tell which kind of field splitting is in effect,
use @code{PROCINFO["FS"]}
(@pxref{Auto-set, ,Built-in Variables That Convey Information}).
The value is @code{"FS"} if regular field splitting is being used,
or it is @code{"FIELDWIDTHS"} if fixed-width field splitting is being used:

@example
if (PROCINFO["FS"] == "FS")
    @var{regular field splitting} @dots{}
else
    @var{fixed-width field splitting} @dots{}
@end example

This information is useful when writing a function
that needs to temporarily change @code{FS} or @code{FIELDWIDTHS},
read some records, and then restore the original settings
(@pxref{Passwd Functions, ,Reading the User Database},
for an example of such a function).

@node Multiple Line, Getline, Constant Size, Reading Files
@section Multiple-Line Records

@cindex multiple line records
@cindex input, multiple line records
@cindex reading files, multiple line records
@cindex records, multiple line
In some databases, a single line cannot conveniently hold all the
information in one entry.  In such cases, you can use multiline
records.  The first step in doing this is to choose your data format.

One technique is to use an unusual character or string to separate
records.  For example, you could use the formfeed character (written
@samp{\f} in @command{awk}, as in C) to separate them, making each record
a page of the file.  To do this, just set the variable @code{RS} to
@code{"\f"} (a string containing the formfeed character).  Any
other character could equally well be used, as long as it won't be part
of the data in a record.

Another technique is to have blank lines separate records.  By a special
dispensation, an empty string as the value of @code{RS} indicates that
records are separated by one or more blank lines.  When @code{RS} is set
to the empty string, each record always ends at the first blank line
encountered.  The next record doesn't start until the first non-blank
line that follows.  No matter how many blank lines appear in a row, they
all act as one record separator.
(Blank lines must be completely empty; lines that contain only
whitespace do not count.)

@cindex leftmost longest match
@cindex matching, leftmost longest
You can achieve the same effect as @samp{RS = ""} by assigning the
string @code{"\n\n+"} to @code{RS}. This regexp matches the newline
at the end of the record and one or more blank lines after the record.
In addition, a regular expression always matches the longest possible
sequence when there is a choice
(@pxref{Leftmost Longest, ,How Much Text Matches?}).
So the next record doesn't start until
the first non-blank line that follows---no matter how many blank lines
appear in a row, they are considered one record separator.

@cindex dark corner
There is an important difference between @samp{RS = ""} and
@samp{RS = "\n\n+"}. In the first case, leading newlines in the input
@value{DF} are ignored, and if a file ends without extra blank lines
after the last record, the final newline is removed from the record.
In the second case, this special processing is not done.
@value{DARKCORNER}

Now that the input is separated into records, the second step is to
separate the fields in the record.  One way to do this is to divide each
of the lines into fields in the normal manner.  This happens by default
as the result of a special feature.  When @code{RS} is set to the empty
string, the newline character @emph{always} acts as a field separator.
This is in addition to whatever field separations result from @code{FS}.

The original motivation for this special exception was probably to provide
useful behavior in the default case (i.e., @code{FS} is equal
to @w{@code{" "}}).  This feature can be a problem if you really don't
want the newline character to separate fields, because there is no way to
prevent it.  However, you can work around this by using the @code{split}
function to break up the record manually
(@pxref{String Functions, ,String Manipulation Functions}).

Another way to separate fields is to
put each field on a separate line: to do this, just set the
variable @code{FS} to the string @code{"\n"}.  (This simple regular
expression matches a single newline.)
A practical example of a @value{DF} organized this way might be a mailing
list, where each entry is separated by blank lines.  Consider a mailing
list in a file named @file{addresses}, that looks like this:

@example
Jane Doe
123 Main Street
Anywhere, SE 12345-6789

John Smith
456 Tree-lined Avenue
Smallville, MW 98765-4321
@dots{}
@end example

@noindent
A simple program to process this file is as follows:

@example
# addrs.awk --- simple mailing list program

# Records are separated by blank lines.
# Each line is one field.
BEGIN @{ RS = "" ; FS = "\n" @}

@{
      print "Name is:", $1
      print "Address is:", $2
      print "City and State are:", $3
      print ""
@}
@end example

Running the program produces the following output:

@example
$ awk -f addrs.awk addresses
@print{} Name is: Jane Doe
@print{} Address is: 123 Main Street
@print{} City and State are: Anywhere, SE 12345-6789
@print{} 
@print{} Name is: John Smith
@print{} Address is: 456 Tree-lined Avenue
@print{} City and State are: Smallville, MW 98765-4321
@print{} 
@dots{}
@end example

@xref{Labels Program, ,Printing Mailing Labels}, for a more realistic
program that deals with address lists.
The following
table
summarizes how records are split, based on the
value of
@ifinfo
@code{RS}.
(@samp{==} means ``is equal to.'')
@end ifinfo
@ifnotinfo
@code{RS}:
@end ifnotinfo

@table @code
@item RS == "\n"
Records are separated by the newline character (@samp{\n}).  In effect,
every line in the @value{DF} is a separate record, including blank lines.
This is the default.

@item RS == @var{any single character}
Records are separated by each occurrence of the character.  Multiple
successive occurrences delimit empty records.

@item RS == ""
Records are separated by runs of blank lines.  The newline character
always serves as a field separator, in addition to whatever value
@code{FS} may have. Leading and trailing newlines in a file are ignored.

@item RS == @var{regexp}
Records are separated by occurrences of characters that match @var{regexp}.
Leading and trailing matches of @var{regexp} delimit empty records.
(This is a @command{gawk} extension, it is not specified by the
POSIX standard.)
@end table

@cindex @code{RT} variable
In all cases, @command{gawk} sets @code{RT} to the input text that matched the
value specified by @code{RS}.

@node Getline, , Multiple Line, Reading Files
@section Explicit Input with @code{getline}

@cindex @code{getline} built-in function
@cindex input, explicit
@cindex explicit input
@cindex input, @code{getline} command
@cindex reading files, @code{getline} command
So far we have been getting our input data from @command{awk}'s main
input stream---either the standard input (usually your terminal, sometimes
the output from another program) or from the
files specified on the command line.  The @command{awk} language has a
special built-in command called @code{getline} that
can be used to read input under your explicit control.

The @code{getline} command is used in several different ways and should
@emph{not} be used by beginners.
The examples that follow the explanation of the @code{getline} command
include material that has not been covered yet.  Therefore, come back
and study the @code{getline} command @emph{after} you have reviewed the
rest of this @value{DOCUMENT} and have a good knowledge of how @command{awk} works.

@cindex @code{ERRNO} variable
@cindex differences between @command{gawk} and @command{awk}
@cindex @code{getline}, return values
The @code{getline} command returns one if it finds a record and zero if
the end of the file is encountered.  If there is some error in getting
a record, such as a file that cannot be opened, then @code{getline}
returns @minus{}1.  In this case, @command{gawk} sets the variable
@code{ERRNO} to a string describing the error that occurred.

In the following examples, @var{command} stands for a string value that
represents a shell command.

@menu
* Plain Getline::               Using @code{getline} with no arguments.
* Getline/Variable::            Using @code{getline} into a variable.
* Getline/File::                Using @code{getline} from a file.
* Getline/Variable/File::       Using @code{getline} into a variable from a
                                file.
* Getline/Pipe::                Using @code{getline} from a pipe.
* Getline/Variable/Pipe::       Using @code{getline} into a variable from a
                                pipe.
* Getline/Coprocess::           Using @code{getline} from a coprocess.
* Getline/Variable/Coprocess::  Using @code{getline} into a variable from a
                                coprocess.
* Getline Notes::               Important things to know about @code{getline}.
* Getline Summary::             Summary of @code{getline} Variants.
@end menu

@node Plain Getline, Getline/Variable, Getline, Getline
@subsection Using @code{getline} with No Arguments

The @code{getline} command can be used without arguments to read input
from the current input file.  All it does in this case is read the next
input record and split it up into fields.  This is useful if you've
finished processing the current record, but want to do some special
processing @emph{right now} on the next record.  Here's an
example:

@example
@{
     if ((t = index($0, "/*")) != 0) @{
          # value of `tmp' will be "" if t is 1
          tmp = substr($0, 1, t - 1)
          u = index(substr($0, t + 2), "*/")
          while (u == 0) @{
               if (getline <= 0) @{
                    m = "unexpected EOF or error"
                    m = (m ": " ERRNO)
                    print m > "/dev/stderr"
                    exit
               @}
               t = -1
               u = index($0, "*/")
          @}
          # substr expression will be "" if */
          # occurred at end of line
          $0 = tmp substr($0, u + 2)
     @}
     print $0
@}
@end example

This @command{awk} program deletes all C-style comments (@samp{/* @dots{}
*/}) from the input.  By replacing the @samp{print $0} with other
statements, you could perform more complicated processing on the
decommented input, such as searching for matches of a regular
expression.  (This program has a subtle problem---it does not work if one
comment ends and another begins on the same line.)

@ignore
Exercise,
write a program that does handle multiple comments on the line.
@end ignore

This form of the @code{getline} command sets @code{NF},
@code{NR}, @code{FNR}, and the value of @code{$0}.

@strong{Note:} The new value of @code{$0} is used to test
the patterns of any subsequent rules.  The original value
of @code{$0} that triggered the rule that executed @code{getline}
is lost.
By contrast, the @code{next} statement reads a new record
but immediately begins processing it normally, starting with the first
rule in the program.  @xref{Next Statement, ,The @code{next} Statement}.

@node Getline/Variable, Getline/File, Plain Getline, Getline
@subsection Using @code{getline} into a Variable

You can use @samp{getline @var{var}} to read the next record from
@command{awk}'s input into the variable @var{var}.  No other processing is
done.
For example, suppose the next line is a comment or a special string,
and you want to read it without triggering
any rules.  This form of @code{getline} allows you to read that line
and store it in a variable so that the main
read-a-line-and-check-each-rule loop of @command{awk} never sees it.
The following example swaps every two lines of input.
The program is as follows:

@example
@{
     if ((getline tmp) > 0) @{
          print tmp
          print $0
     @} else
          print $0
@}
@end example

@noindent
It takes the following list:

@example
wan
tew
free
phore
@end example

@noindent
and produces these results:

@example
tew
wan
phore
free
@end example

The @code{getline} command used in this way sets only the variables
@code{NR} and @code{FNR} (and of course, @var{var}).  The record is not
split into fields, so the values of the fields (including @code{$0}) and
the value of @code{NF} do not change.

@node Getline/File, Getline/Variable/File, Getline/Variable, Getline
@subsection Using @code{getline} from a File

@cindex input redirection
@cindex redirection of input
@cindex @code{<} I/O operator
Use @samp{getline < @var{file}} to read the next record from @var{file}.
Here @var{file} is a string-valued expression that
specifies the @value{FN}.  @samp{< @var{file}} is called a @dfn{redirection}
because it directs input to come from a different place.
For example, the following
program reads its input record from the file @file{secondary.input} when it
encounters a first field with a value equal to 10 in the current input
file:

@example
@{
    if ($1 == 10) @{
         getline < "secondary.input"
         print
    @} else
         print
@}
@end example

Because the main input stream is not used, the values of @code{NR} and
@code{FNR} are not changed. However, the record it reads is split into fields in
the normal manner, so the values of @code{$0} and the other fields are
changed, resulting in a new value of @code{NF}.

@c Thanks to Paul Eggert for initial wording here
According to POSIX, @samp{getline < @var{expression}} is ambiguous if
@var{expression} contains unparenthesized operators other than
@samp{$}; for example, @samp{getline < dir "/" file} is ambiguous
because the concatenation operator is not parenthesized.  You should
write it as @samp{getline < (dir "/" file)} if you want your program
to be portable to other @command{awk} implementations.
(It happens that @command{gawk} gets it right, but you should not
rely on this. Parentheses make it easier to read.)

@node Getline/Variable/File, Getline/Pipe, Getline/File, Getline
@subsection Using @code{getline} into a Variable from a File

Use @samp{getline @var{var} < @var{file}} to read input
from the file
@var{file}, and put it in the variable @var{var}.  As above, @var{file}
is a string-valued expression that specifies the file from which to read.

In this version of @code{getline}, none of the built-in variables are
changed and the record is not split into fields.  The only variable
changed is @var{var}.
For example, the following program copies all the input files to the
output, except for records that say @w{@samp{@@include @var{filename}}}.
Such a record is replaced by the contents of the file
@var{filename}:

@example
@{
     if (NF == 2 && $1 == "@@include") @{
          while ((getline line < $2) > 0)
               print line
          close($2)
     @} else
          print
@}
@end example

Note here how the name of the extra input file is not built into
the program; it is taken directly from the data, from the second field on
the @samp{@@include} line.

The @code{close} function is called to ensure that if two identical
@samp{@@include} lines appear in the input, the entire specified file is
included twice.
@xref{Close Files And Pipes, ,Closing Input and Output Redirections}.

One deficiency of this program is that it does not process nested
@samp{@@include} statements
(i.e., @samp{@@include} statements in included files)
the way a true macro preprocessor would.
@xref{Igawk Program, ,An Easy Way to Use Library Functions}, for a program
that does handle nested @samp{@@include} statements.

@node Getline/Pipe, Getline/Variable/Pipe, Getline/Variable/File, Getline
@subsection Using @code{getline} from a Pipe

@cindex @code{|} I/O operator
@cindex input pipeline
@cindex pipeline, input
The output of a command can also be piped into @code{getline}, using
@samp{@var{command} | getline}.  In
this case, the string @var{command} is run as a shell command and its output
is piped into @command{awk} to be used as input.  This form of @code{getline}
reads one record at a time from the pipe.
For example, the following program copies its input to its output, except for
lines that begin with @samp{@@execute}, which are replaced by the output
produced by running the rest of the line as a shell command:

@example
@{
     if ($1 == "@@execute") @{
          tmp = substr($0, 10)
          while ((tmp | getline) > 0)
               print
          close(tmp)
     @} else
          print
@}
@end example

@noindent
The @code{close} function is called to ensure that if two identical
@samp{@@execute} lines appear in the input, the command is run for
each one.
@ifnottex
@xref{Close Files And Pipes, ,Closing Input and Output Redirections}.
@end ifnottex
@c Exercise!!
@c This example is unrealistic, since you could just use system
Given the input:

@example
foo
bar
baz
@@execute who
bletch
@end example

@noindent
the program might produce:

@cindex Robbins, Bill
@cindex Robbins, Miriam
@cindex Robbins, Arnold
@example
foo
bar
baz
arnold     ttyv0   Jul 13 14:22
miriam     ttyp0   Jul 13 14:23     (murphy:0)
bill       ttyp1   Jul 13 14:23     (murphy:0)
bletch
@end example

@noindent
Notice that this program ran the command @command{who} and printed the result.
(If you try this program yourself, you will of course get different results,
depending upon who is logged in on your system.)

This variation of @code{getline} splits the record into fields, sets the
value of @code{NF} and recomputes the value of @code{$0}.  The values of
@code{NR} and @code{FNR} are not changed.

@c Thanks to Paul Eggert for initial wording here
According to POSIX, @samp{@var{expression} | getline} is ambiguous if
@var{expression} contains unparenthesized operators other than
@samp{$}---for example, @samp{@w{"echo "} "date" | getline} is ambiguous
because the concatenation operator is not parenthesized.  You should
write it as @samp{(@w{"echo "} "date") | getline} if you want your program
to be portable to other @command{awk} implementations.
@ifinfo
(It happens that @command{gawk} gets it right, but you should not
rely on this. Parentheses make it easier to read, anyway.)
@end ifinfo

@node Getline/Variable/Pipe, Getline/Coprocess, Getline/Pipe, Getline
@subsection Using @code{getline} into a Variable from a Pipe

When you use @samp{@var{command} | getline @var{var}}, the
output of @var{command} is sent through a pipe to
@code{getline} and into the variable @var{var}.  For example, the
following program reads the current date and time into the variable
@code{current_time}, using the @command{date} utility, and then
prints it:

@example
BEGIN @{
     "date" | getline current_time
     close("date")
     print "Report printed on " current_time
@}
@end example

In this version of @code{getline}, none of the built-in variables are
changed and the record is not split into fields.

@ifinfo
@c Thanks to Paul Eggert for initial wording here
According to POSIX, @samp{@var{expression} | getline @var{var}} is ambiguous if
@var{expression} contains unparenthesized operators other than
@samp{$}; for example, @samp{@w{"echo "} "date" | getline @var{var}} is ambiguous
because the concatenation operator is not parenthesized. You should
write it as @samp{(@w{"echo "} "date") | getline @var{var}} if you want your
program to be portable to other @command{awk} implementations.
(It happens that @command{gawk} gets it right, but you should not
rely on this. Parentheses make it easier to read, anyway.)
@end ifinfo

@node Getline/Coprocess, Getline/Variable/Coprocess, Getline/Variable/Pipe, Getline
@subsection Using @code{getline} from a Coprocess
@cindex coprocess
@cindex @code{|&} I/O operator
@cindex differences between @command{gawk} and @command{awk}

Input into @code{getline} from a pipe is a one-way operation.
The command that is started with @samp{@var{command} | getline} only
sends data @emph{to} your @command{awk} program.

On occasion, you might want to send data to another program
for processing and then read the results back.
@command{gawk} allows you start a @dfn{coprocess}, with which two-way
communications are possible.  This is done with the @samp{|&}
operator.
Typically, you write data to the coprocess first, and then
read results back, as shown in the following:

@example
print "@var{some query}" |& "db_server"
"db_server" |& getline
@end example

@noindent
which sends a query to @command{db_server} and then reads the results.

The values of @code{NR} and
@code{FNR} are not changed,
because the main input stream is not used.
However, the record is split into fields in
the normal manner, thus changing the values of @code{$0}, the other fields,
and of @code{NF}.

Coprocesses are an advanced feature. They are discussed here only because
this is the @value{SECTION} on @code{getline}.
@xref{Two-way I/O, ,Two-Way Communications with Another Process},
where coprocesses are discussed in more detail.

@node Getline/Variable/Coprocess, Getline Notes, Getline/Coprocess, Getline
@subsection Using @code{getline} into a Variable from a Coprocess

When you use @samp{@var{command} |& getline @var{var}}, the output from
the coprocess @var{command} is sent through a two-way pipe to @code{getline}
and into the variable @var{var}.

In this version of @code{getline}, none of the built-in variables are
changed and the record is not split into fields.  The only variable
changed is @var{var}.

@ifinfo
Coprocesses are an advanced feature. They are discussed here only because
this is the @value{SECTION} on @code{getline}.
@xref{Two-way I/O, ,Two-Way Communications with Another Process},
where coprocesses are discussed in more detail.
@end ifinfo

@node Getline Notes, Getline Summary, Getline/Variable/Coprocess, Getline
@subsection Points About @code{getline} to Remember
Here are some miscellaneous points about @code{getline} that
you should bear in mind:

@itemize @bullet
@item
When @code{getline} changes the value of @code{$0} and @code{NF},
@command{awk} does @emph{not} automatically jump to the start of the
program and start testing the new record against every pattern.
However, the new record is tested against any subsequent rules.

@cindex differences between @command{gawk} and @command{awk}
@cindex limitations
@cindex implementation limits
@item
Many @command{awk} implementations limit the number of pipelines that an @command{awk}
program may have open to just one.  In @command{gawk}, there is no such limit.
You can open as many pipelines (and coprocesses) as the underlying operating
system permits.

@cindex side effects
@cindex @code{FILENAME} variable
@cindex dark corner
@cindex @code{getline}, setting @code{FILENAME}
@cindex @code{FILENAME}, being set by @code{getline}
@item
An interesting side effect occurs if you use @code{getline} without a
redirection inside a @code{BEGIN} rule. Because an unredirected @code{getline}
reads from the command-line @value{DF}s, the first @code{getline} command
causes @command{awk} to set the value of @code{FILENAME}. Normally,
@code{FILENAME} does not have a value inside @code{BEGIN} rules, because you
have not yet started to process the command-line @value{DF}s.
@value{DARKCORNER}
(@xref{BEGIN/END, , The @code{BEGIN} and @code{END} Special Patterns},
also @pxref{Auto-set, ,Built-in Variables That Convey Information}.)
@end itemize

@node Getline Summary, , Getline Notes, Getline
@subsection Summary of @code{getline} Variants

The following table summarizes the eight variants of @code{getline},
listing which built-in variables are set by each one.

@multitable {@var{command} @code{|& getline} @var{var}} {1234567890123456789012345678901234567890}
@item @code{getline} @tab Sets @code{$0}, @code{NF}, @code{FNR} and @code{NR}

@item @code{getline} @var{var} @tab Sets @var{var}, @code{FNR} and @code{NR}

@item @code{getline <} @var{file} @tab Sets @code{$0} and @code{NF}

@item @code{getline @var{var} < @var{file}} @tab Sets @var{var}

@item @var{command} @code{| getline} @tab Sets @code{$0} and @code{NF}

@item @var{command} @code{| getline} @var{var} @tab Sets @var{var}

@item @var{command} @code{|& getline} @tab Sets @code{$0} and @code{NF}
(this is a @command{gawk} extension)

@item @var{command} @code{|& getline} @var{var} @tab Sets @var{var}
(this is a @command{gawk} extension)
@end multitable

@node Printing, Expressions, Reading Files, Top
@chapter Printing Output

@cindex printing
@cindex output
One of the most common programming actions is to @dfn{print} or output,
some or all of the input.  Use the @code{print} statement
for simple output, and the @code{printf} statement
for fancier formatting.
The @code{print} statement is not limited when
computing @emph{which} values to print. However, with two exceptions,
you cannot specify @emph{how} to print them---how many
columns, whether to use exponential notation or not, and so on.
(For the exceptions, @pxref{Output Separators}, and
@ref{OFMT, ,Controlling Numeric Output with @code{print}}.)
For that, you need the @code{printf} statement
(@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}).

Besides basic and formatted printing, this @value{CHAPTER}
also covers I/O redirections to files and pipes, introduces
the special @value{FN}s that @command{gawk} processes internally,
and discusses the @code{close} built-in function.

@menu
* Print::                       The @code{print} statement.
* Print Examples::              Simple examples of @code{print} statements.
* Output Separators::           The output separators and how to change them.
* OFMT::                        Controlling Numeric Output With @code{print}.
* Printf::                      The @code{printf} statement.
* Redirection::                 How to redirect output to multiple files and
                                pipes.
* Special Files::               File name interpretation in @command{gawk}.
                                @command{gawk} allows access to inherited file
                                descriptors.
* Close Files And Pipes::       Closing Input and Output Files and Pipes.
@end menu

@node Print, Print Examples, Printing, Printing
@section The @code{print} Statement
@cindex @code{print} statement

The @code{print} statement is used to produce output with simple, standardized
formatting.  Specify only the strings or numbers to print, in a
list separated by commas.  They are output, separated by single spaces,
followed by a newline.  The statement looks like this:

@example
print @var{item1}, @var{item2}, @dots{}
@end example

@noindent
The entire list of items may be optionally enclosed in parentheses.  The
parentheses are necessary if any of the item expressions uses the @samp{>}
relational operator; otherwise it could be confused with a redirection
(@pxref{Redirection, ,Redirecting Output of @code{print} and @code{printf}}).

The items to print can be constant strings or numbers, fields of the
current record (such as @code{$1}), variables, or any @command{awk}
expression.  Numeric values are converted to strings and then printed.

The simple statement @samp{print} with no items is equivalent to
@samp{print $0}: it prints the entire current record.  To print a blank
line, use @samp{print ""}, where @code{""} is the empty string.
To print a fixed piece of text, use a string constant, such as
@w{@code{"Don't Panic"}}, as one item.  If you forget to use the
double quote characters, your text is taken as an @command{awk}
expression and you will probably get an error.  Keep in mind that a
space is printed between any two items.

@node Print Examples, Output Separators, Print, Printing
@section Examples of @code{print} Statements

Each @code{print} statement makes at least one line of output.  However, it
isn't limited to only one line.  If an item value is a string that contains a
newline, the newline is output along with the rest of the string.  A
single @code{print} statement can make any number of lines this way.

The following is an example of printing a string that contains embedded newlines
(the @samp{\n} is an escape sequence, used to represent the newline
character; @pxref{Escape Sequences}):

@example
$ awk 'BEGIN @{ print "line one\nline two\nline three" @}'
@print{} line one
@print{} line two
@print{} line three
@end example

The next example, which is run on the @file{inventory-shipped} file,
prints the first two fields of each input record, with a space between
them:

@example
$ awk '@{ print $1, $2 @}' inventory-shipped
@print{} Jan 13
@print{} Feb 15
@print{} Mar 15
@dots{}
@end example

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
A common mistake in using the @code{print} statement is to omit the comma
between two items.  This often has the effect of making the items run
together in the output, with no space.  The reason for this is that
juxtaposing two string expressions in @command{awk} means to concatenate
them.  Here is the same program, without the comma:

@example
$ awk '@{ print $1 $2 @}' inventory-shipped
@print{} Jan13
@print{} Feb15
@print{} Mar15
@dots{}
@end example

To someone unfamiliar with the @file{inventory-shipped} file, neither
example's output makes much sense.  A heading line at the beginning
would make it clearer.  Let's add some headings to our table of months
(@code{$1}) and green crates shipped (@code{$2}).  We do this using the
@code{BEGIN} pattern
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns})
so that the headings are only printed once:

@example
awk 'BEGIN @{  print "Month Crates"
              print "----- ------" @}
           @{  print $1, $2 @}' inventory-shipped
@end example

@noindent
When run, the program prints the following:

@example
Month Crates
----- ------
Jan 13
Feb 15
Mar 15
@dots{}
@end example

@noindent
The only problem, however, is that the headings and the table data
don't line up!  We can fix this by printing some spaces between the
two fields:

@example
@group
awk 'BEGIN @{ print "Month Crates"
             print "----- ------" @}
           @{ print $1, "     ", $2 @}' inventory-shipped
@end group
@end example

Lining up columns this way can get pretty
complicated when there are many columns to fix.  Counting spaces for two
or three columns is simple, but any more than this can take up
a lot of time. This is why the @code{printf} statement was
created (@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing});
one of its specialties is lining up columns of data.

@cindex line continuation
@strong{Note:} You can continue either a @code{print} or
@code{printf} statement simply by putting a newline after any comma
(@pxref{Statements/Lines, ,@command{awk} Statements Versus Lines}).

@node Output Separators, OFMT, Print Examples, Printing
@section Output Separators

@cindex output field separator, @code{OFS}
@cindex output record separator, @code{ORS}
@cindex @code{OFS} variable
@cindex @code{ORS} variable
As mentioned previously, a @code{print} statement contains a list
of items separated by commas.  In the output, the items are normally
separated by single spaces.  However, this doesn't need to be the case;
a single space is only the default.  Any string of
characters may be used as the @dfn{output field separator} by setting the
built-in variable @code{OFS}.  The initial value of this variable
is the string @w{@code{" "}}---that is, a single space.

The output from an entire @code{print} statement is called an
@dfn{output record}.  Each @code{print} statement outputs one output
record, and then outputs a string called the @dfn{output record separator}
(or @code{ORS}).  The initial
value of @code{ORS} is the string @code{"\n"}; i.e., a newline
character.  Thus, each @code{print} statement normally makes a separate line.

In order to change how output fields and records are separated, assign
new values to the variables @code{OFS} and @code{ORS}.  The usual
place to do this is in the @code{BEGIN} rule
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}), so
that it happens before any input is processed.  It can also be done
with assignments on the command line, before the names of the input
files, or using the @option{-v} command-line option
(@pxref{Options, ,Command-Line Options}).
The following example prints the first and second fields of each input
record, separated by a semicolon, with a blank line added after each
newline:

@ignore
Exercise,
Rewrite the
@example
awk 'BEGIN @{ print "Month Crates"
             print "----- ------" @}
           @{ print $1, "     ", $2 @}' inventory-shipped
@end example
program by using a new value of @code{OFS}.
@end ignore

@example
$ awk 'BEGIN @{ OFS = ";"; ORS = "\n\n" @}
>            @{ print $1, $2 @}' BBS-list
@print{} aardvark;555-5553
@print{} 
@print{} alpo-net;555-3412
@print{} 
@print{} barfly;555-7685
@dots{}
@end example

If the value of @code{ORS} does not contain a newline, the program's output
is run together on a single line.

@node OFMT, Printf, Output Separators, Printing
@section Controlling Numeric Output with @code{print}
@cindex @code{OFMT} variable
@cindex numeric output format
@cindex format, numeric output
@cindex output format specifier, @code{OFMT}
When the @code{print} statement is used to print numeric values,
@command{awk} internally converts the number to a string of characters
and prints that string.  @command{awk} uses the @code{sprintf} function
to do this conversion
(@pxref{String Functions, ,String Manipulation Functions}).
For now, it suffices to say that the @code{sprintf}
function accepts a @dfn{format specification} that tells it how to format
numbers (or strings), and that there are a number of different ways in which
numbers can be formatted.  The different format specifications are discussed
more fully in
@ref{Control Letters, , Format-Control Letters}.

The built-in variable @code{OFMT} contains the default format specification
that @code{print} uses with @code{sprintf} when it wants to convert a
number to a string for printing.
The default value of @code{OFMT} is @code{"%.6g"}.
The way @code{print} prints numbers can be changed
by supplying different format specifications
as the value of @code{OFMT}, as shown in the following example:

@example
$ awk 'BEGIN @{
>   OFMT = "%.0f"  # print numbers as integers (rounds)
>   print 17.23, 17.54 @}'
@print{} 17 18
@end example

@noindent
@cindex dark corner
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
According to the POSIX standard, @command{awk}'s behavior is undefined
if @code{OFMT} contains anything but a floating-point conversion specification.
@value{DARKCORNER}

@node Printf, Redirection, OFMT, Printing
@section Using @code{printf} Statements for Fancier Printing
@cindex formatted output
@cindex output, formatted
@cindex @code{printf} statement

For more precise control over the output format than what is
normally provided by @code{print}, use @code{printf}.
@code{printf} can be used to
specify the width to use for each item, as well as various
formatting choices for numbers (such as what output base to use, whether to
print an exponent, whether to print a sign, and how many digits to print
after the decimal point).  This is done by supplying a string, called
the @dfn{format string}, that controls how and where to print the other
arguments.

@menu
* Basic Printf::                Syntax of the @code{printf} statement.
* Control Letters::             Format-control letters.
* Format Modifiers::            Format-specification modifiers.
* Printf Examples::             Several examples.
@end menu

@node Basic Printf, Control Letters, Printf, Printf
@subsection Introduction to the @code{printf} Statement

@cindex @code{printf} statement, syntax of
A simple @code{printf} statement looks like this:

@example
printf @var{format}, @var{item1}, @var{item2}, @dots{}
@end example

@noindent
The entire list of arguments may optionally be enclosed in parentheses.  The
parentheses are necessary if any of the item expressions use the @samp{>}
relational operator; otherwise it can be confused with a redirection
(@pxref{Redirection, ,Redirecting Output of @code{print} and @code{printf}}).

@cindex format string
The difference between @code{printf} and @code{print} is the @var{format}
argument.  This is an expression whose value is taken as a string; it
specifies how to output each of the other arguments.  It is called the
@dfn{format string}.

The format string is very similar to that in the ISO C library function
@code{printf}.  Most of @var{format} is text to output verbatim.
Scattered among this text are @dfn{format specifiers}---one per item.
Each format specifier says to output the next item in the argument list
at that place in the format.

The @code{printf} statement does not automatically append a newline
to its output.  It outputs only what the format string specifies.
So if a newline is needed, you must include one in the format string.
The output separator variables @code{OFS} and @code{ORS} have no effect
on @code{printf} statements. For example:

@example
$ awk 'BEGIN @{
>    ORS = "\nOUCH!\n"; OFS = "+"
>    msg = "Dont Panic!"
>    printf "%s\n", msg
> @}'
@print{} Dont Panic!
@end example

@noindent
Here, neither the @samp{+} nor the @samp{OUCH} appear when
the message is printed.

@node Control Letters, Format Modifiers, Basic Printf, Printf
@subsection Format-Control Letters
@cindex @code{printf}, format-control characters
@cindex format specifier, @code{printf}

A format specifier starts with the character @samp{%} and ends with
a @dfn{format-control letter}---it tells the @code{printf} statement
how to output one item.  The format-control letter specifies what @emph{kind}
of value to print.  The rest of the format specifier is made up of
optional @dfn{modifiers} that control @emph{how} to print the value, such as
the field width.  Here is a list of the format-control letters:

@table @code
@item %c
This prints a number as an ASCII character; thus, @samp{printf "%c",
65} outputs the letter @samp{A}. (The output for a string value is
the first character of the string.)

@item %d@r{,} %i
These are equivalent; they both print a decimal integer.
(The @samp{%i} specification is for compatibility with ISO C.)

@item %e@r{,} %E
These print a number in scientific (exponential) notation;
for example:

@example
printf "%4.3e\n", 1950
@end example

@noindent
prints @samp{1.950e+03}, with a total of four significant figures, three of
which follow the decimal point.
(The @samp{4.3} represents two modifiers,
discussed in the next @value{SUBSECTION}.)
@samp{%E} uses @samp{E} instead of @samp{e} in the output.

@item %f
This prints a number in floating-point notation.
For example:

@example
printf "%4.3f", 1950
@end example

@noindent
prints @samp{1950.000}, with a total of four significant figures, three of
which follow the decimal point.
(The @samp{4.3} represents two modifiers,
discussed in the next @value{SUBSECTION}.)

@item %g@r{,} %G
These print a number in either scientific notation or in floating-point
notation, whichever uses fewer characters; if the result is printed in
scientific notation, @samp{%G} uses @samp{E} instead of @samp{e}.

@item %o
This prints an unsigned octal integer.

@item %s
This prints a string.

@item %u
This prints an unsigned decimal integer.
(This format is of marginal use, because all numbers in @command{awk}
are floating-point; it is provided primarily for compatibility with C.)

@item %x@r{,} %X
These print an unsigned hexadecimal integer;
@samp{%X} uses the letters @samp{A} through @samp{F}
instead of @samp{a} through @samp{f}.

@item %%
This isn't a format-control letter but it does have meaning---the
sequence @samp{%%} outputs one @samp{%}; it does not consume an
argument and it ignores any modifiers.
@end table

@cindex dark corner
@strong{Note:}
When using the integer format-control letters for values that are outside
the range of a C @code{long} integer, @command{gawk} switches to the
@samp{%g} format specifier. Other versions of @command{awk} may print
invalid values or do something else entirely.
@value{DARKCORNER}

@node Format Modifiers, Printf Examples, Control Letters, Printf
@subsection Modifiers for @code{printf} Formats

@cindex @code{printf}, modifiers
@cindex modifiers (in format specifiers)
A format specification can also include @dfn{modifiers} that can control
how much of the item's value is printed, as well as how much space it gets.
The modifiers come between the @samp{%} and the format-control letter.
We will use the bullet symbol ``@bullet{}'' in the following examples to
represent
spaces in the output. Here are the possible modifiers, in the order in
which they may appear:

@table @code
@cindex differences between @command{gawk} and @command{awk}
@cindex @code{printf}, positional specifier
@cindex positional specifier, @code{printf}
@item @var{N}$
An integer constant followed by a @samp{$} is a @dfn{positional specifier}.
Normally, format specifications are applied to arguments in the order
given in the format string.  With a positional specifier, the format
specification is applied to a specific argument, instead of what
would be the next argument in the list.  Positional specifiers begin
counting with one:

@example
printf "%s %s\n", "don't", "panic"
printf "%2$s %1$s\n", "panic", "don't"
@end example

@noindent
prints the famous friendly message twice.

At first glance, this feature doesn't seem to be of much use.
It is in fact a @command{gawk} extension, intended for use in translating
messages at runtime.
@xref{Printf Ordering, , Rearranging @code{printf} Arguments},
which describes how and why to use positional specifiers.
For now, we will not use them.

@item -
The minus sign, used before the width modifier (see further on in
this table),
says to left-justify
the argument within its specified width.  Normally, the argument
is printed right-justified in the specified width.  Thus:

@example
printf "%-4s", "foo"
@end example

@noindent
prints @samp{foo@bullet{}}.

@item @var{space}
For numeric conversions, prefix positive values with a space and
negative values with a minus sign.

@item +
The plus sign, used before the width modifier (see further on in
this table),
says to always supply a sign for numeric conversions, even if the data
to format is positive. The @samp{+} overrides the space modifier.

@item #
Use an ``alternate form'' for certain control letters.
For @samp{%o}, supply a leading zero.
For @samp{%x} and @samp{%X}, supply a leading @samp{0x} or @samp{0X} for
a nonzero result.
For @samp{%e}, @samp{%E}, and @samp{%f}, the result always contains a
decimal point.
For @samp{%g} and @samp{%G}, trailing zeros are not removed from the result.

@cindex dark corner
@item 0
A leading @samp{0} (zero) acts as a flag that indicates that output should be
padded with zeros instead of spaces.
This applies even to non-numeric output formats.
@value{DARKCORNER}
This flag only has an effect when the field width is wider than the
value to print.

@item @var{width}
This is a number specifying the desired minimum width of a field.  Inserting any
number between the @samp{%} sign and the format-control character forces the
field to expand to this width.  The default way to do this is to
pad with spaces on the left.  For example:

@example
printf "%4s", "foo"
@end example

@noindent
prints @samp{@bullet{}foo}.

The value of @var{width} is a minimum width, not a maximum.  If the item
value requires more than @var{width} characters, it can be as wide as
necessary.  Thus, the following:

@example
printf "%4s", "foobar"
@end example

@noindent
prints @samp{foobar}.

Preceding the @var{width} with a minus sign causes the output to be
padded with spaces on the right, instead of on the left.

@item .@var{prec}
A period followed by an integer constant
specifies the precision to use when printing.
The meaning of the precision varies by control letter:

@table @asis
@item @code{%e}, @code{%E}, @code{%f}
Number of digits to the right of the decimal point.

@item @code{%g}, @code{%G}
Maximum number of significant digits.

@item @code{%d}, @code{%i}, @code{%o}, @code{%u}, @code{%x}, @code{%X}
Minimum number of digits to print.

@item @code{%s}
Maximum number of characters from the string that should print.
@end table

Thus, the following:

@example
printf "%.4s", "foobar"
@end example

@noindent
prints @samp{foob}.
@end table

The C library @code{printf}'s dynamic @var{width} and @var{prec}
capability (for example, @code{"%*.*s"}) is supported.  Instead of
supplying explicit @var{width} and/or @var{prec} values in the format
string, they are passed in the argument list.  For example:

@example
w = 5
p = 3
s = "abcdefg"
printf "%*.*s\n", w, p, s
@end example

@noindent
is exactly equivalent to:

@example
s = "abcdefg"
printf "%5.3s\n", s
@end example

@noindent
Both programs output @samp{@w{@bullet{}@bullet{}abc}}.
Earlier versions of @command{awk} did not support this capability.
If you must use such a version, you may simulate this feature by using
concatenation to build up the format string, like so:

@example
w = 5
p = 3
s = "abcdefg"
printf "%" w "." p "s\n", s
@end example

@noindent
This is not particularly easy to read but it does work.

@cindex fatal errors
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@cindex lint checks
C programmers may be used to supplying additional
@samp{l}, @samp{L}, and @samp{h}
modifiers in @code{printf} format strings. These are not valid in @command{awk}.
Most @command{awk} implementations silently ignore these modifiers.
If @option{--lint} is provided on the command line
(@pxref{Options, ,Command-Line Options}),
@command{gawk} warns about their use. If @option{--posix} is supplied,
their use is a fatal error.

@node Printf Examples, , Format Modifiers, Printf
@subsection Examples Using @code{printf}

The following is a simple example of
how to use @code{printf} to make an aligned table:

@example
awk '@{ printf "%-10s %s\n", $1, $2 @}' BBS-list
@end example

@noindent
This command
prints the names of the bulletin boards (@code{$1}) in the file
@file{BBS-list} as a string of 10 characters that are left-justified.  It also
prints the phone numbers (@code{$2}) next on the line.  This
produces an aligned two-column table of names and phone numbers,
as shown here:

@example
$ awk '@{ printf "%-10s %s\n", $1, $2 @}' BBS-list
@print{} aardvark   555-5553
@print{} alpo-net   555-3412
@print{} barfly     555-7685
@print{} bites      555-1675
@print{} camelot    555-0542
@print{} core       555-2912
@print{} fooey      555-1234
@print{} foot       555-6699
@print{} macfoo     555-6480
@print{} sdace      555-3430
@print{} sabafoo    555-2127
@end example

In this case, the phone numbers had to be printed as strings because
the numbers are separated by a dash.  Printing the phone numbers as
numbers would have produced just the first three digits: @samp{555}.
This would have been pretty confusing.

It wasn't necessary to specify a width for the phone numbers because
they are last on their lines.  They don't need to have spaces
after them.

The table could be made to look even nicer by adding headings to the
tops of the columns.  This is done using the @code{BEGIN} pattern
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns})
so that the headers are only printed once, at the beginning of
the @command{awk} program:

@example
awk 'BEGIN @{ print "Name      Number"
             print "----      ------" @}
     @{ printf "%-10s %s\n", $1, $2 @}' BBS-list
@end example

The above example mixed @code{print} and @code{printf} statements in
the same program.  Using just @code{printf} statements can produce the
same results:

@example
awk 'BEGIN @{ printf "%-10s %s\n", "Name", "Number"
             printf "%-10s %s\n", "----", "------" @}
     @{ printf "%-10s %s\n", $1, $2 @}' BBS-list
@end example

@noindent
Printing each column heading with the same format specification
used for the column elements ensures that the headings
are aligned just like the columns.

The fact that the same format specification is used three times can be
emphasized by storing it in a variable, like this:

@example
awk 'BEGIN @{ format = "%-10s %s\n"
             printf format, "Name", "Number"
             printf format, "----", "------" @}
     @{ printf format, $1, $2 @}' BBS-list
@end example

@c !!! exercise
At this point, it would be a worthwhile exercise to use the
@code{printf} statement to line up the headings and table data for the
@file{inventory-shipped} example that was covered earlier in the @value{SECTION}
on the @code{print} statement
(@pxref{Print, ,The @code{print} Statement}).

@node Redirection, Special Files, Printf, Printing
@section Redirecting Output of @code{print} and @code{printf}

@cindex output redirection
@cindex redirection of output
So far, the output from @code{print} and @code{printf} has gone
to the standard
output, usually the terminal.  Both @code{print} and @code{printf} can
also send their output to other places.
This is called @dfn{redirection}.

A redirection appears after the @code{print} or @code{printf} statement.
Redirections in @command{awk} are written just like redirections in shell
commands, except that they are written inside the @command{awk} program.

There are four forms of output redirection: output to a file, output
appended to a file, output through a pipe to another command, and output
to a coprocess.  They are all shown for the @code{print} statement,
but they work identically for @code{printf}:

@table @code
@cindex @code{>} I/O operator
@item print @var{items} > @var{output-file}
This type of redirection prints the items into the output file named
@var{output-file}.  The @value{FN} @var{output-file} can be any
expression.  Its value is changed to a string and then used as a
@value{FN} (@pxref{Expressions}).

When this type of redirection is used, the @var{output-file} is erased
before the first output is written to it.  Subsequent writes to the same
@var{output-file} do not erase @var{output-file}, but append to it.
(This is different from how you use redirections in shell scripts.)
If @var{output-file} does not exist, it is created.  For example, here
is how an @command{awk} program can write a list of BBS names to one
file named @file{name-list}, and a list of phone numbers to another file
named @file{phone-list}:

@example
$ awk '@{ print $2 > "phone-list"
>        print $1 > "name-list" @}' BBS-list
$ cat phone-list
@print{} 555-5553
@print{} 555-3412
@dots{}
$ cat name-list
@print{} aardvark
@print{} alpo-net
@dots{}
@end example

@noindent
Each output file contains one name or number per line.

@cindex @code{>>} I/O operator
@item print @var{items} >> @var{output-file}
This type of redirection prints the items into the pre-existing output file
named @var{output-file}.  The difference between this and the
single-@samp{>} redirection is that the old contents (if any) of
@var{output-file} are not erased.  Instead, the @command{awk} output is
appended to the file.
If @var{output-file} does not exist, then it is created.

@cindex @code{|} I/O operator
@cindex pipes for output
@cindex output, piping
@item print @var{items} | @var{command}
It is also possible to send output to another program through a pipe
instead of into a file.   This type of redirection opens a pipe to
@var{command}, and writes the values of @var{items} through this pipe
to another process created to execute @var{command}.

The redirection argument @var{command} is actually an @command{awk}
expression.  Its value is converted to a string whose contents give
the shell command to be run.  For example, the following produces two
files, one unsorted list of BBS names, and one list sorted in reverse
alphabetical order:

@ignore
10/2000:
This isn't the best style, since COMMAND is assigned for each
record.  It's done to avoid overfull hboxes in TeX.  Leave it
alone for now and let's hope no-one notices.
@end ignore

@example
awk '@{ print $1 > "names.unsorted"
       command = "sort -r > names.sorted"
       print $1 | command @}' BBS-list
@end example

The unsorted list is written with an ordinary redirection, while
the sorted list is written by piping through the @command{sort} utility.

The next example uses redirection to mail a message to the mailing
list @samp{bug-system}.  This might be useful when trouble is encountered
in an @command{awk} script run periodically for system maintenance:

@example
report = "mail bug-system"
print "Awk script failed:", $0 | report
m = ("at record number " FNR " of " FILENAME)
print m | report
close(report)
@end example

The message is built using string concatenation and saved in the variable
@code{m}.  It is then sent down the pipeline to the @command{mail} program.
(The parentheses group the items to concatenate---see
@ref{Concatenation, ,String Concatenation}.)

The @code{close} function is called here because it's a good idea to close
the pipe as soon as all the intended output has been sent to it.
@xref{Close Files And Pipes, ,Closing Input and Output Redirections},
for more information on this.

This example also illustrates the use of a variable to represent
a @var{file} or @var{command}---it is not necessary to always
use a string constant.  Using a variable is generally a good idea,
because @command{awk} requires that the string value be spelled identically
every time.

@cindex coprocess
@cindex @code{|&} I/O operator
@cindex differences between @command{gawk} and @command{awk}
@item print @var{items} |& @var{command}
This type of redirection prints the items to the input of @var{command}.
The difference between this and the
single-@samp{|} redirection is that the output from @var{command}
can be read with @code{getline}.
Thus @var{command} is a @dfn{coprocess}, that works together with,
but subsidiary to, the @command{awk} program.

This feature is a @command{gawk} extension, and is not available in
POSIX @command{awk}.
@xref{Two-way I/O, ,Two-Way Communications with Another Process},
for a more complete discussion.
@end table

Redirecting output using @samp{>}, @samp{>>}, @samp{|}, or @samp{|&}
asks the system to open a file, pipe, or coprocess, only if the particular
@var{file} or @var{command} you specify has not already been written
to by your program or if it has been closed since it was last written to.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
It is a common error to use @samp{>} redirection for the first @code{print}
to a file, and then to use @samp{>>} for subsequent output:

@example
# clear the file
print "Don't panic" > "guide.txt"
@dots{}
# append
print "Avoid improbability generators" >> "guide.txt"
@end example

@noindent
This is indeed how redirections must be used from the shell.  But in
@command{awk}, it isn't necessary.  In this kind of case, a program should
use @samp{>} for all the @code{print} statements, since the output file
is only opened once.

@cindex differences between @command{gawk} and @command{awk}
@cindex limitations
@cindex implementation limits
@ifnotinfo
As mentioned earlier
(@pxref{Getline Notes, ,Points About @code{getline} to Remember}),
many
@end ifnotinfo
@ifnottex
Many
@end ifnottex
@command{awk} implementations limit the number of pipelines that an @command{awk}
program may have open to just one!  In @command{gawk}, there is no such limit.
@command{gawk} allows a program to
open as many pipelines as the underlying operating system permits.

@c fakenode --- for prepinfo
@subheading Advanced Notes: Piping into @command{sh}
@cindex advanced notes
@cindex shell, piping commands into
@cindex piping commands into the shell

A particularly powerful way to use redirection is to build command lines,
and pipe them into the shell, @command{sh}.  For example, suppose you
have a list of files brought over from a system where all the @value{FN}s
are stored in uppercase, and you wish to rename them to have names in
all lowercase.  The following program is both simple and efficient:

@cindex @command{mv} utility
@example
@{ printf("mv %s %s\n", $0, tolower($0)) | "sh" @}

END @{ close("sh") @}
@end example

The @code{tolower} function returns its argument string with all
uppercase characters converted to lowercase
(@pxref{String Functions, ,String Manipulation Functions}).
The program builds up a list of command lines,
using the @command{mv} utility to rename the files.
It then sends the list to the shell for execution.

@node Special Files, Close Files And Pipes, Redirection, Printing
@section Special @value{FFN}s in @command{gawk}

@command{gawk} provides a number of special @value{FN}s that it interprets
internally.  These @value{FN}s provide access to standard file descriptors,
process-related information, and TCP/IP networking.

@menu
* Special FD::                  Special files for I/O.
* Special Process::             Special files for process information.
* Special Network::             Special files for network communications.
* Special Caveats::             Things to watch out for.
@end menu

@node Special FD, Special Process, Special Files, Special Files
@subsection Special Files for Standard Descriptors
@cindex standard input
@cindex standard output
@cindex standard error output
@cindex file descriptors

Running programs conventionally have three input and output streams
already available to them for reading and writing.  These are known as
the @dfn{standard input}, @dfn{standard output}, and @dfn{standard error
output}.  These streams are, by default, connected to your terminal, but
they are often redirected with the shell, via the @samp{<}, @samp{<<},
@samp{>}, @samp{>>}, @samp{>&}, and @samp{|} operators.  Standard error
is typically used for writing error messages; the reason there are two separate
streams, standard output, and standard error, is so that they can be
redirected separately.

@cindex differences between @command{gawk} and @command{awk}
In other implementations of @command{awk}, the only way to write an error
message to standard error in an @command{awk} program is as follows:

@example
print "Serious error detected!" | "cat 1>&2"
@end example

@noindent
This works by opening a pipeline to a shell command that can access the
standard error stream that it inherits from the @command{awk} process.
This is far from elegant, and it is also inefficient, because it requires a
separate process.  So people writing @command{awk} programs often
don't do this.  Instead, they send the error messages to the
terminal, like this:

@example
print "Serious error detected!" > "/dev/tty"
@end example

@noindent
This usually has the same effect but not always: although the
standard error stream is usually the terminal, it can be redirected; when
that happens, writing to the terminal is not correct.  In fact, if
@command{awk} is run from a background job, it may not have a terminal at all.
Then opening @file{/dev/tty} fails.

@command{gawk} provides special @value{FN}s for accessing the three standard
streams, as well as any other inherited open files.  If the @value{FN} matches
one of these special names when @command{gawk} redirects input or output,
then it directly uses the stream that the @value{FN} stands for.
(These special @value{FN}s work for all operating systems that @command{gawk}
has been ported to, not just those that are POSIX-compliant.):

@cindex @file{/dev/stdin} special file
@cindex @file{/dev/stdout} special file
@cindex @file{/dev/stderr} special file
@cindex @file{/dev/fd} special files
@table @file
@item /dev/stdin
The standard input (file descriptor 0).

@item /dev/stdout
The standard output (file descriptor 1).

@item /dev/stderr
The standard error output (file descriptor 2).

@item /dev/fd/@var{N}
The file associated with file descriptor @var{N}.  Such a file must
be opened by the program initiating the @command{awk} execution (typically
the shell).  Unless special pains are taken in the shell from which
@command{gawk} is invoked, only descriptors 0, 1, and 2 are available.
@end table

The @value{FN}s @file{/dev/stdin}, @file{/dev/stdout}, and @file{/dev/stderr}
are aliases for @file{/dev/fd/0}, @file{/dev/fd/1}, and @file{/dev/fd/2},
respectively. However, they are more self-explanatory.
The proper way to write an error message in a @command{gawk} program
is to use @file{/dev/stderr}, like this:

@example
print "Serious error detected!" > "/dev/stderr"
@end example

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
Note the use of quotes around the @value{FN}.
Like any other redirection, the value must be a string.
It is a common error to omit the quotes, which leads
to confusing results.
@c Exercise: What does it do?  :-)

@node Special Process, Special Network, Special FD, Special Files
@subsection Special Files for Process-Related Information

@command{gawk} also provides special @value{FN}s that give access to information
about the running @command{gawk} process.  Each of these ``files'' provides
a single record of information.  To read them more than once, they must
first be closed with the @code{close} function
(@pxref{Close Files And Pipes, ,Closing Input and Output Redirections}).
The @value{FN}s are:

@cindex process information
@cindex @file{/dev/pid} special file
@cindex @file{/dev/pgrpid} special file
@cindex @file{/dev/ppid} special file
@cindex @file{/dev/user} special file
@table @file
@item /dev/pid
Reading this file returns the process ID of the current process,
in decimal form, terminated with a newline.

@item /dev/ppid
Reading this file returns the parent process ID of the current process,
in decimal form, terminated with a newline.

@item /dev/pgrpid
Reading this file returns the process group ID of the current process,
in decimal form, terminated with a newline.

@item /dev/user
Reading this file returns a single record terminated with a newline.
The fields are separated with spaces.  The fields represent the
following information:

@table @code
@item $1
The return value of the @code{getuid} system call
(the real user ID number).

@item $2
The return value of the @code{geteuid} system call
(the effective user ID number).

@item $3
The return value of the @code{getgid} system call
(the real group ID number).

@item $4
The return value of the @code{getegid} system call
(the effective group ID number).
@end table

If there are any additional fields, they are the group IDs returned by
the @code{getgroups} system call.
(Multiple groups may not be supported on all systems.)
@end table

These special @value{FN}s may be used on the command line as @value{DF}s,
as well as for I/O redirections within an @command{awk} program.
They may not be used as source files with the @option{-f} option.

@cindex automatic warnings
@cindex warnings, automatic
@strong{Note:}
The special files that provide process-related information are now considered
obsolete and will disappear entirely
in the next release of @command{gawk}.
@command{gawk} prints a warning message every time you use one of
these files.
To obtain process-related information, use the @code{PROCINFO} array.
@xref{Auto-set, ,Built-in Variables That Convey Information}.

@node Special Network, Special Caveats, Special Process, Special Files
@subsection Special Files for Network Communications

Starting with @value{PVERSION} 3.1 of @command{gawk}, @command{awk} programs
can open a two-way
TCP/IP connection, acting as either a client or server.
This is done using a special @value{FN} of the form:

@example
@file{/inet/@var{protocol}/@var{local-port}/@var{remote-host}/@var{remote-port}}
@end example

The @var{protocol} is one of @samp{tcp}, @samp{udp}, or @samp{raw},
and the other fields represent the other essential pieces of information
for making a networking connection.
These @value{FN}s are used with the @samp{|&} operator for communicating
with a coprocess
(@pxref{Two-way I/O, ,Two-Way Communications with Another Process}).
This is an advanced feature, mentioned here only for completeness.
Full discussion is delayed until
@ref{TCP/IP Networking, ,Using @command{gawk} for Network Programming}.

@node Special Caveats, , Special Network, Special Files
@subsection Special @value{FFN} Caveats

Here is a list of things to bear in mind when using the
special @value{FN}s that @command{gawk} provides.

@itemize @bullet
@item
Recognition of these special @value{FN}s is disabled if @command{gawk} is in
compatibility mode (@pxref{Options, ,Command-Line Options}).

@cindex automatic warnings
@cindex warnings, automatic
@item
@ifnottex
The
@end ifnottex
@ifnotinfo
As mentioned earlier, the
@end ifnotinfo
special files that provide process-related information are now considered
obsolete and will disappear entirely
in the next release of @command{gawk}.
@command{gawk} prints a warning message every time you use one of
these files.
@ifnottex
To obtain process-related information, use the @code{PROCINFO} array.
@xref{Built-in Variables}.
@end ifnottex

@item
Starting with @value{PVERSION} 3.1, @command{gawk} @emph{always}
interprets these special @value{FN}s.@footnote{Older versions of
@command{gawk} would only interpret these names internally if the system
did not actually have a a @file{/dev/fd} directory or any of the other
above listed special files.  Usually this didn't make a difference,
but sometimes it did; thus, it was decided to make @command{gawk}'s
behavior consistent on all systems and to have it always interpret
the special @value{FN}s itself.}
For example, using @samp{/dev/fd/4}
for output actually writes on file descriptor 4, and not on a new
file descriptor that is @code{dup}'ed from file descriptor 4.  Most of
the time this does not matter; however, it is important to @emph{not}
close any of the files related to file descriptors 0, 1, and 2.
Doing so results in unpredictable behavior.
@end itemize

@node Close Files And Pipes, , Special Files, Printing
@section Closing Input and Output Redirections
@cindex closing input files and pipes
@cindex closing output files and pipes
@cindex closing coprocesses
@cindex coprocess
@cindex @code{close} built-in function

If the same @value{FN} or the same shell command is used with @code{getline}
more than once during the execution of an @command{awk} program
(@pxref{Getline, ,Explicit Input with @code{getline}}),
the file is opened (or the command is executed) the first time only.
At that time, the first record of input is read from that file or command.
The next time the same file or command is used with @code{getline},
another record is read from it, and so on.

Similarly, when a file or pipe is opened for output, the @value{FN} or
command associated with it is remembered by @command{awk}, and subsequent
writes to the same file or command are appended to the previous writes.
The file or pipe stays open until @command{awk} exits.

This implies that special steps are necessary in order to read the same
file again from the beginning, or to rerun a shell command (rather than
reading more output from the same command).  The @code{close} function
makes these things possible:

@example
close(@var{filename})
@end example

@noindent
or:

@example
close(@var{command})
@end example

The argument @var{filename} or @var{command} can be any expression.  Its
value must @emph{exactly} match the string that was used to open the file or
start the command (spaces and other ``irrelevant'' characters
included). For example, if you open a pipe with this:

@example
"sort -r names" | getline foo
@end example

@noindent
then you must close it with this:

@example
close("sort -r names")
@end example

Once this function call is executed, the next @code{getline} from that
file or command, or the next @code{print} or @code{printf} to that
file or command, reopens the file or reruns the command.
Because the expression that you use to close a file or pipeline must
exactly match the expression used to open the file or run the command,
it is good practice to use a variable to store the @value{FN} or command.
The previous example becomes the following:

@example
sortcom = "sort -r names"
sortcom | getline foo
@dots{}
close(sortcom)
@end example

@noindent
This helps avoid hard-to-find typographical errors in your @command{awk}
programs.  Here are some of the reasons for closing an output file:

@itemize @bullet
@item
To write a file and read it back later on in the same @command{awk}
program.  Close the file after writing it, then
begin reading it with @code{getline}.

@item
To write numerous files, successively, in the same @command{awk}
program.  If the files aren't closed, eventually @command{awk} may exceed a
system limit on the number of open files in one process.  It is best to
close each one when the program has finished writing it.

@item
To make a command finish.  When output is redirected through a pipe,
the command reading the pipe normally continues to try to read input
as long as the pipe is open.  Often this means the command cannot
really do its work until the pipe is closed.  For example, if
output is redirected to the @command{mail} program, the message is not
actually sent until the pipe is closed.

@item
To run the same program a second time, with the same arguments.
This is not the same thing as giving more input to the first run!

For example, suppose a program pipes output to the @command{mail} program.
If it outputs several lines redirected to this pipe without closing
it, they make a single message of several lines.  By contrast, if the
program closes the pipe after each line of output, then each line makes
a separate message.
@end itemize

@cindex differences between @command{gawk} and @command{awk}
@cindex portability issues
If you use more files than the system allows you to have open,
@command{gawk} attempts to multiplex the available open files among
your @value{DF}s.  @command{gawk}'s ability to do this depends upon the
facilities of your operating system, so it may not always work.  It is
therefore both good practice and good portability advice to always
use @code{close} on your files when you are done with them.
In fact, if you are using a lot of pipes, it is essential that
you close commands when done. For example, consider something like this:

@example
@{
    @dots{}
    command = ("grep " $1 " /some/file | my_prog -q " $3)
    while ((command | getline) > 0) @{
        @var{process output of} command
    @}
    # need close(command) here
@}
@end example

This example creates a new pipeline based on data in @emph{each} record.
Without the call to @code{close} indicated in the comment, @command{awk}
creates child processes to run the commands, until it eventually
runs out of file descriptors for more pipelines.

Even though each command has finished (as indicated by the end-of-file
return status from @code{getline}), the child process is not
terminated;@footnote{The technical terminology is rather morbid.
The finished child is called a ``zombie,'' and cleaning up after
it is referred to as ``reaping.''}
@c Good old UNIX: give the marketing guys fits, that's the ticket
more importantly, the file descriptor for the pipe
is not closed and released until @code{close} is called or
@command{awk} exits.

@code{close} will silently do nothing if given an argument that
does not represent a file, pipe or coprocess that was opened with
a redirection.

When using the @samp{|&} operator to communicate with a coprocess,
it is occasionally useful to be able to close one end of the two-way
pipe without closing the other.
This is done by supplying a second argument to @code{close}.
As in any other call to @code{close},
the first argument is the name of the command or special file used
to start the coprocess.
The second argument should be a string, with either of the values
@code{"to"} or @code{"from"}.  Case does not matter.
As this is an advanced feature, a more complete discussion is
delayed until
@ref{Two-way I/O, ,Two-Way Communications with Another Process},
which discusses it in more detail and gives an example.

@c fakenode --- for prepinfo
@subheading Advanced Notes: Using @code{close}'s Return Value
@cindex advanced notes
@cindex dark corner
@cindex differences between @command{gawk} and @command{awk}
@cindex @code{close}, return value
@cindex return value from @code{close}

In many versions of Unix @command{awk}, the @code{close} function
is actually a statement.  It is a syntax error to try and use the return
value from @code{close}:
@value{DARKCORNER}

@example
command = "@dots{}"
command | getline info
retval = close(command)  # syntax error in most Unix awks
@end example

@command{gawk} treats @code{close} as a function.
The return value is @minus{}1 if the argument names something
that was never opened with a redirection, or if there is
a system problem closing the file or process.
In these cases, @command{gawk} sets the built-in variable
@code{ERRNO} to a string describing the problem.

In @command{gawk},
when closing a pipe or coprocess,
the return value is the exit status of the command.
Otherwise, it is the return value from the system's @code{close} or
@code{fclose} C functions when closing input or output
files, respectively.
This value is zero if the close succeeds, or @minus{}1 if
it fails.

The return value for closing a pipeline is particularly useful.
It allows you to get the output from a command as well as its
exit status.

For POSIX-compliant systems,
if the exit status is a number above 128, then the program
was terminated by a signal.  Subtract 128 to get the signal number:

@example
exit_val = close(command)
if (exit_val > 128)
    print command, "died with signal", exit_val - 128
else
    print command, "exited with code", exit_val
@end example

Currently, in @command{gawk}, this only works for commands
piping into @code{getline}.  For commands piped into
from @code{print} or @code{printf}, the
return value from @code{close} is that of the library's
@code{pclose} function.

@node Expressions, Patterns and Actions, Printing, Top
@chapter Expressions
@cindex expression

Expressions are the basic building blocks of @command{awk} patterns
and actions.  An expression evaluates to a value that you can print, test,
or pass to a function.  Additionally, an expression
can assign a new value to a variable or a field by using an assignment operator.

An expression can serve as a pattern or action statement on its own.
Most other kinds of
statements contain one or more expressions that specify the data on which to
operate.  As in other languages, expressions in @command{awk} include
variables, array references, constants, and function calls, as well as
combinations of these with various operators.

@menu
* Constants::                   String, numeric and regexp constants.
* Using Constant Regexps::      When and how to use a regexp constant.
* Variables::                   Variables give names to values for later use.
* Conversion::                  The conversion of strings to numbers and vice
                                versa.
* Arithmetic Ops::              Arithmetic operations (@samp{+}, @samp{-},
                                etc.)
* Concatenation::               Concatenating strings.
* Assignment Ops::              Changing the value of a variable or a field.
* Increment Ops::               Incrementing the numeric value of a variable.
* Truth Values::                What is ``true'' and what is ``false''.
* Typing and Comparison::       How variables acquire types and how this
                                affects comparison of numbers and strings with
                                @samp{<}, etc.
* Boolean Ops::                 Combining comparison expressions using boolean
                                operators @samp{||} (``or''), @samp{&&}
                                (``and'') and @samp{!} (``not'').
* Conditional Exp::             Conditional expressions select between two
                                subexpressions under control of a third
                                subexpression.
* Function Calls::              A function call is an expression.
* Precedence::                  How various operators nest.
@end menu

@node Constants, Using Constant Regexps, Expressions, Expressions
@section Constant Expressions
@cindex constants, types of

The simplest type of expression is the @dfn{constant}, which always has
the same value.  There are three types of constants: numeric,
string, and regular expression.

Each is used in the appropriate context when you need a data
value that isn't going to change.  Numeric constants can
have different forms, but are stored identically internally.

@menu
* Scalar Constants::            Numeric and string constants.
* Non-decimal-numbers::         What are octal and hex numbers.
* Regexp Constants::            Regular Expression constants.
@end menu

@node Scalar Constants, Non-decimal-numbers, Constants, Constants
@subsection Numeric and String Constants

@cindex numeric constant
@cindex numeric value
A @dfn{numeric constant} stands for a number.  This number can be an
integer, a decimal fraction, or a number in scientific (exponential)
notation.@footnote{The internal representation of all numbers,
including integers, uses double-precision
floating-point numbers.
On most modern systems, these are in IEEE 754 standard format.}
Here are some examples of numeric constants that all
have the same value:

@example
105
1.05e+2
1050e-1
@end example

@cindex string constants
A string constant consists of a sequence of characters enclosed in
double quote marks.  For example:

@example
"parrot"
@end example

@noindent
@cindex differences between @command{gawk} and @command{awk}
represents the string whose contents are @samp{parrot}.  Strings in
@command{gawk} can be of any length, and they can contain any of the possible
eight-bit ASCII characters including ASCII @sc{nul} (character code zero).
Other @command{awk}
implementations may have difficulty with some character codes.

@node Non-decimal-numbers, Regexp Constants, Scalar Constants, Constants
@subsection Octal and Hexadecimal Numbers
@cindex octal numbers
@cindex hexadecimal numbers
@cindex numbers, octal
@cindex numbers, hexadecimal

In @command{awk}, all numbers are in decimal; i.e., base 10.  Many other
programming languages allow you to specify numbers in other bases, often
octal (base 8) and hexadecimal (base 16).
In octal, the numbers go 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, etc..
Just as @samp{11} in decimal is 1 times 10 plus 1, so
@samp{11} in octal is 1 times 8, plus 1. This equals nine in decimal.
In hexadecimal, there are 16 digits. Since the everyday decimal
number system only has ten digits (@samp{0}---@samp{9}), the letters
@samp{a} through @samp{f} are used to represent the rest.
(Case in the letters is usually irrelevant; hexadecimal @samp{a} and @samp{A}
have the same value.)
Thus, @samp{11} in
hexadecimal is 1 times 16 plus 1, which equals 17 in decimal.

Just by looking at plain @samp{11}, you can't tell what base it's in.
So, in C, C++, and other languages derived from C,
@c such as PERL, but we won't mention that....
there is a special notation to help signify the base.
Octal numbers start with a leading @samp{0},
and hexadecimal numbers start with a leading @samp{0x} or @samp{0X}:

@table @code
@item 11
Decimal 11.

@item 011
Octal 11, decimal value 9.

@item 0x11
Hexadecimal 11, decimal value 17.
@end table

This example shows the difference:

@example
$ gawk 'BEGIN @{ printf "%d, %d, %d\n", 011, 11, 0x11 @}'
@print{} 9, 11, 17
@end example

Being able to use octal and hexadecimal constants in your programs is most
useful when working with data that cannot be represented conveniently as
characters or as regular numbers, such as binary data of various sorts.

@command{gawk} allows the use of octal and hexadecimal
constants in your program text.  However, such numbers in the input data
are not treated differently; doing so by default would break old
programs.
(If you really need to do this, use the @option{--non-decimal-data}
command-line option,
@pxref{Non-decimal Data, ,Allowing Non-Decimal Input Data}.)
If you have octal or hexadecimal data,
you can use the @code{strtonum} function
(@pxref{String Functions, ,String Manipulation Functions})
to convert the data into a number.
Most of the time, you will want to use octal or hexadecimal constants
when working with the built-in bit manipulation functions;
see @ref{Bitwise Functions, ,Using @command{gawk}'s Bit Manipulation Functions},
for more information.

Unlike some early C implementations, @samp{8} and @samp{9} are not valid
in octal constants; e.g., @command{gawk} treats @samp{018} as decimal 18.

@example
$ gawk 'BEGIN @{ print "021 is", 021 ; print 018 @}'
@print{} 021 is 17
@print{} 18
@end example

Octal and hexadecimal source code constants are a @command{gawk} extension.
If @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
they are not available.

@c fakenode --- for prepinfo
@subheading Advanced Notes: A Constant's Base Does Not Affect Its Value
@cindex advanced notes

Once a numeric constant has
been converted internally into a number,
@command{gawk} no longer remembers
what the original form of the constant was; the internal value is
always used.  This has particular consequences for conversion of
numbers to strings:

@example
$ gawk 'BEGIN @{ printf "0x11 is <%s>\n", 0x11 @}'
@print{} 0x11 is <17>
@end example

@node Regexp Constants, , Non-decimal-numbers, Constants
@subsection Regular Expression Constants

@cindex @code{~} operator
@cindex @code{!~} operator
A regexp constant is a regular expression description enclosed in
slashes, such as @code{@w{/^beginning and end$/}}.  Most regexps used in
@command{awk} programs are constant, but the @samp{~} and @samp{!~}
matching operators can also match computed or ``dynamic'' regexps
(which are just ordinary strings or variables that contain a regexp).

@node Using Constant Regexps, Variables, Constants, Expressions
@section Using Regular Expression Constants

@cindex dark corner
When used on the righthand side of the @samp{~} or @samp{!~}
operators, a regexp constant merely stands for the regexp that is to be
matched.
However, regexp constants (such as @code{/foo/}) may be used like simple expressions.
When a
regexp constant appears by itself, it has the same meaning as if it appeared
in a pattern, i.e.; @samp{($0 ~ /foo/)}
@value{DARKCORNER}
@xref{Expression Patterns, ,Expressions as Patterns}.
This means that the following two code segments:

@example
if ($0 ~ /barfly/ || $0 ~ /camelot/)
    print "found"
@end example

@noindent
and:

@example
if (/barfly/ || /camelot/)
    print "found"
@end example

@noindent
are exactly equivalent.
One rather bizarre consequence of this rule is that the following
Boolean expression is valid, but does not do what the user probably
intended:

@example
# note that /foo/ is on the left of the ~
if (/foo/ ~ $1) print "found foo"
@end example

@cindex automatic warnings
@cindex warnings, automatic
@noindent
This code is ``obviously'' testing @code{$1} for a match against the regexp
@code{/foo/}.  But in fact, the expression @samp{/foo/ ~ $1} actually means
@samp{($0 ~ /foo/) ~ $1}.  In other words, first match the input record
against the regexp @code{/foo/}.  The result is either zero or one,
depending upon the success or failure of the match.  That result
is then matched against the first field in the record.
Because it is unlikely that you would ever really want to make this kind of
test, @command{gawk} issues a warning when it sees this construct in
a program.
Another consequence of this rule is that the assignment statement:

@example
matches = /foo/
@end example

@noindent
assigns either zero or one to the variable @code{matches}, depending
upon the contents of the current input record.
This feature of the language has never been well documented until the
POSIX specification.

@cindex differences between @command{gawk} and @command{awk}
@cindex dark corner
Constant regular expressions are also used as the first argument for
the @code{gensub}, @code{sub}, and @code{gsub} functions, and as the
second argument of the @code{match} function
(@pxref{String Functions, ,String Manipulation Functions}).
Modern implementations of @command{awk}, including @command{gawk}, allow
the third argument of @code{split} to be a regexp constant, but some
older implementations do not.
@value{DARKCORNER}
This can lead to confusion when attempting to use regexp constants
as arguments to user defined functions
(@pxref{User-defined, ,User-Defined Functions}).
For example:

@example
function mysub(pat, repl, str, global)
@{
    if (global)
        gsub(pat, repl, str)
    else
        sub(pat, repl, str)
    return str
@}

@{
    @dots{}
    text = "hi! hi yourself!"
    mysub(/hi/, "howdy", text, 1)
    @dots{}
@}
@end example

@cindex automatic warnings
@cindex warnings, automatic
In this example, the programmer wants to pass a regexp constant to the
user-defined function @code{mysub}, which in turn passes it on to
either @code{sub} or @code{gsub}.  However, what really happens is that
the @code{pat} parameter is either one or zero, depending upon whether
or not @code{$0} matches @code{/hi/}.
@command{gawk} issues a warning when it sees a regexp constant used as
a parameter to a user-defined function, since passing a truth value in
this way is probably not what was intended.

@node Variables, Conversion, Using Constant Regexps, Expressions
@section Variables

Variables are ways of storing values at one point in your program for
use later in another part of your program.  They can be manipulated
entirely within the program text, and they can also be assigned values
on the @command{awk} command line.

@menu
* Using Variables::             Using variables in your programs.
* Assignment Options::          Setting variables on the command-line and a
                                summary of command-line syntax. This is an
                                advanced method of input.
@end menu

@node Using Variables, Assignment Options, Variables, Variables
@subsection Using Variables in a Program

@cindex variables, user-defined
@cindex user-defined variables
Variables let you give names to values and refer to them later.  Variables
have already been used in many of the examples.  The name of a variable
must be a sequence of letters, digits, or underscores, and it may not begin
with a digit.  Case is significant in variable names; @code{a} and @code{A}
are distinct variables.

A variable name is a valid expression by itself; it represents the
variable's current value.  Variables are given new values with
@dfn{assignment operators}, @dfn{increment operators}, and
@dfn{decrement operators}.
@xref{Assignment Ops, ,Assignment Expressions}.
@c NEXT ED: Can also be changed by sub, gsub, split

A few variables have special built-in meanings, such as @code{FS} (the
field separator), and @code{NF} (the number of fields in the current input
record).  @xref{Built-in Variables}, for a list of the built-in variables.
These built-in variables can be used and assigned just like all other
variables, but their values are also used or changed automatically by
@command{awk}.  All built-in variables' names are entirely uppercase.

Variables in @command{awk} can be assigned either numeric or string values.
The kind of value a variable holds can change over the life of a program.
By default, variables are initialized to the empty string, which
is zero if converted to a number.  There is no need to
``initialize'' each variable explicitly in @command{awk},
which is what you would do in C and in most other traditional languages.

@node Assignment Options, , Using Variables, Variables
@subsection Assigning Variables on the Command Line

Any @command{awk} variable can be set by including a @dfn{variable assignment}
among the arguments on the command line when @command{awk} is invoked
(@pxref{Other Arguments, ,Other Command-Line Arguments}).
Such an assignment has the following form:

@example
@var{variable}=@var{text}
@end example

@noindent
With it, a variable is set either at the beginning of the
@command{awk} run or in between input files.
When the assignment is preceded with the @option{-v} option,
as in the following:

@example
-v @var{variable}=@var{text}
@end example

@noindent
the variable is set at the very beginning, even before the
@code{BEGIN} rules are run.  The @option{-v} option and its assignment
must precede all the @value{FN} arguments, as well as the program text.
(@xref{Options, ,Command-Line Options}, for more information about
the @option{-v} option.)
Otherwise, the variable assignment is performed at a time determined by
its position among the input file arguments---after the processing of the
preceding input file argument.  For example:

@example
awk '@{ print $n @}' n=4 inventory-shipped n=2 BBS-list
@end example

@noindent
prints the value of field number @code{n} for all input records.  Before
the first file is read, the command line sets the variable @code{n}
equal to four.  This causes the fourth field to be printed in lines from
the file @file{inventory-shipped}.  After the first file has finished,
but before the second file is started, @code{n} is set to two, so that the
second field is printed in lines from @file{BBS-list}:

@example
$ awk '@{ print $n @}' n=4 inventory-shipped n=2 BBS-list
@print{} 15
@print{} 24
@dots{}
@print{} 555-5553
@print{} 555-3412
@dots{}
@end example

@cindex dark corner
Command-line arguments are made available for explicit examination by
the @command{awk} program in an array named @code{ARGV}
(@pxref{ARGC and ARGV, ,Using @code{ARGC} and @code{ARGV}}).
@command{awk} processes the values of command-line assignments for escape
sequences
@value{DARKCORNER}
(@pxref{Escape Sequences}).

@node Conversion, Arithmetic Ops, Variables, Expressions
@section Conversion of Strings and Numbers

@cindex conversion of strings and numbers
Strings are converted to numbers and numbers are converted to strings, if the context
of the @command{awk} program demands it.  For example, if the value of
either @code{foo} or @code{bar} in the expression @samp{foo + bar}
happens to be a string, it is converted to a number before the addition
is performed.  If numeric values appear in string concatenation, they
are converted to strings.  Consider the following:

@example
two = 2; three = 3
print (two three) + 4
@end example

@noindent
This prints the (numeric) value 27.  The numeric values of
the variables @code{two} and @code{three} are converted to strings and
concatenated together.  The resulting string is converted back to the
number 23, to which four is then added.

@cindex null string
@cindex empty string
@cindex type conversion
If, for some reason, you need to force a number to be converted to a
string, concatenate the empty string, @code{""}, with that number.
To force a string to be converted to a number, add zero to that string.
A string is converted to a number by interpreting any numeric prefix
of the string as numerals:
@code{"2.5"} converts to 2.5, @code{"1e3"} converts to 1000, and @code{"25fix"}
has a numeric value of 25.
Strings that can't be interpreted as valid numbers convert to zero.

@cindex @code{CONVFMT} variable
The exact manner in which numbers are converted into strings is controlled
by the @command{awk} built-in variable @code{CONVFMT} (@pxref{Built-in Variables}).
Numbers are converted using the @code{sprintf} function
with @code{CONVFMT} as the format
specifier
(@pxref{String Functions, ,String Manipulation Functions}).

@code{CONVFMT}'s default value is @code{"%.6g"}, which prints a value with
at least six significant digits.  For some applications, you might want to
change it to specify more precision.
On most modern machines,
17 digits is enough to capture a floating-point number's
value exactly,
most of the time.@footnote{Pathological cases can require up to
752 digits (!), but we doubt that you need to worry about this.}

@cindex dark corner
Strange results can occur if you set @code{CONVFMT} to a string that doesn't
tell @code{sprintf} how to format floating-point numbers in a useful way.
For example, if you forget the @samp{%} in the format, @command{awk} converts
all numbers to the same constant string.
As a special case, if a number is an integer, then the result of converting
it to a string is @emph{always} an integer, no matter what the value of
@code{CONVFMT} may be.  Given the following code fragment:

@example
CONVFMT = "%2.2f"
a = 12
b = a ""
@end example

@noindent
@code{b} has the value @code{"12"}, not @code{"12.00"}.
@value{DARKCORNER}

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@cindex @code{OFMT} variable
Prior to the POSIX standard, @command{awk} used the value
of @code{OFMT} for converting numbers to strings.  @code{OFMT}
specifies the output format to use when printing numbers with @code{print}.
@code{CONVFMT} was introduced in order to separate the semantics of
conversion from the semantics of printing.  Both @code{CONVFMT} and
@code{OFMT} have the same default value: @code{"%.6g"}.  In the vast majority
of cases, old @command{awk} programs do not change their behavior.
However, these semantics for @code{OFMT} are something to keep in mind if you must
port your new style program to older implementations of @command{awk}.
We recommend
that instead of changing your programs, just port @command{gawk} itself.
@xref{Print, ,The @code{print} Statement},
for more information on the @code{print} statement.

@node Arithmetic Ops, Concatenation, Conversion, Expressions
@section Arithmetic Operators
@cindex arithmetic operators
@cindex operators, arithmetic
@cindex addition
@cindex subtraction
@cindex multiplication
@cindex division
@cindex remainder
@cindex quotient
@cindex exponentiation

The @command{awk} language uses the common arithmetic operators when
evaluating expressions.  All of these arithmetic operators follow normal
precedence rules and work as you would expect them to.

The following example uses a file named @file{grades}, which contains
a list of student names as well as three test scores per student (it's
a small class):

@example
Pat   100 97 58
Sandy  84 72 93
Chris  72 92 89
@end example

@noindent
This programs takes the file @file{grades} and prints the average
of the scores:

@example
$ awk '@{ sum = $2 + $3 + $4 ; avg = sum / 3
>        print $1, avg @}' grades
@print{} Pat 85
@print{} Sandy 83
@print{} Chris 84.3333
@end example

The following list provides the arithmetic operators in @command{awk}, in order from
the highest precedence to the lowest:

@table @code
@item - @var{x}
Negation.

@item + @var{x}
Unary plus; the expression is converted to a number.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@item @var{x} ^ @var{y}
@itemx @var{x} ** @var{y}
Exponentiation; @var{x} raised to the @var{y} power.  @samp{2 ^ 3} has
the value eight; the character sequence @samp{**} is equivalent to
@samp{^}.

@item @var{x} * @var{y}
Multiplication.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
@item @var{x} / @var{y}
Division;  because all numbers in @command{awk} are floating-point
numbers, the result is @emph{not} rounded to an integer---@samp{3 / 4} has
the value 0.75.  (It is a common mistake, especially for C programmers,
to forget that @emph{all} numbers in @command{awk} are floating-point,
and that division of integer-looking constants produces a real number,
not an integer.)

@item @var{x} % @var{y}
Remainder; further discussion is provided in the text, just
after this list.

@item @var{x} + @var{y}
Addition.

@item @var{x} - @var{y}
Subtraction.
@end table

Unary plus and minus have the same precedence,
the multiplication operators all have the same precedence, and
addition and subtraction have the same precedence.

@cindex differences between @command{gawk} and @command{awk}
When computing the remainder of @code{@var{x} % @var{y}},
the quotient is rounded toward zero to an integer and
multiplied by @var{y}. This result is subtracted from @var{x};
this operation is sometimes known as ``trunc-mod.''  The following
relation always holds:

@example
b * int(a / b) + (a % b) == a
@end example

One possibly undesirable effect of this definition of remainder is that
@code{@var{x} % @var{y}} is negative if @var{x} is negative.  Thus:

@example
-17 % 8 = -1
@end example

In other @command{awk} implementations, the signedness of the remainder
may be machine dependent.
@c !!! what does posix say?

@cindex portability issues
@strong{Note:}
The POSIX standard only specifies the use of @samp{^}
for exponentiation.
For maximum portability, do not use the @samp{**} operator.

@node Concatenation, Assignment Ops, Arithmetic Ops, Expressions
@section String Concatenation
@cindex Kernighan, Brian
@quotation
@i{It seemed like a good idea at the time.}@*
Brian Kernighan
@end quotation

@cindex string operators
@cindex operators, string
@cindex concatenation
There is only one string operation: concatenation.  It does not have a
specific operator to represent it.  Instead, concatenation is performed by
writing expressions next to one another, with no operator.  For example:

@example
$ awk '@{ print "Field number one: " $1 @}' BBS-list
@print{} Field number one: aardvark
@print{} Field number one: alpo-net
@dots{}
@end example

Without the space in the string constant after the @samp{:}, the line
runs together.  For example:

@example
$ awk '@{ print "Field number one:" $1 @}' BBS-list
@print{} Field number one:aardvark
@print{} Field number one:alpo-net
@dots{}
@end example

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
Because string concatenation does not have an explicit operator, it is
often necessary to insure that it happens at the right time by using
parentheses to enclose the items to concatenate.  For example, the
following code fragment does not concatenate @code{file} and @code{name}
as you might expect:

@example
file = "file"
name = "name"
print "something meaningful" > file name
@end example

@noindent
It is necessary to use the following:

@example
print "something meaningful" > (file name)
@end example

@cindex order of evaluation, concatenation
@cindex concatenation evaluation order
@cindex evaluation, order of
@cindex side effects
Parentheses should be used around concatenation in all but the
most common contexts, such as on the righthand side of @samp{=}.
Be careful about the kinds of expressions used in string concatenation.
In particular, the order of evaluation of expressions used for concatenation
is undefined in the @command{awk} language.  Consider this example:

@example
BEGIN @{
    a = "don't"
    print (a " " (a = "panic"))
@}
@end example

@noindent
It is not defined whether the assignment to @code{a} happens
before or after the value of @code{a} is retrieved for producing the
concatenated value.  The result could be either @samp{don't panic},
or @samp{panic panic}.
@c see test/nasty.awk for a worse example
The precedence of concatenation, when mixed with other operators, is often
counter-intuitive.  Consider this example:

@ignore
> To: bug-gnu-utils@@gnu.org
> CC: arnold@gnu.org
> Subject: gawk 3.0.4 bug with {print -12 " " -24}
> From: Russell Schulz <Russell_Schulz@locutus.ofB.ORG>
> Date: Tue, 8 Feb 2000 19:56:08 -0700
>
> gawk 3.0.4 on NT gives me:
>
> prompt> cat bad.awk
> BEGIN { print -12 " " -24; }
>
> prompt> gawk -f bad.awk
> -12-24
>
> when I would expect
>
> -12 -24
>
> I have not investigated the source, or other implementations.  The
> bug is there on my NT and DOS versions 2.15.6 .
@end ignore

@example
$ awk 'BEGIN @{ print -12 " " -24 @}'
@print{} -12-24
@end example

This ``obviously'' is concatenating @minus{}12, a space, and @minus{}24.
But where did the space disappear to?
The answer lies in the combination of operator precedences and
@command{awk}'s automatic conversion rules.  To get the desired result,
write the program in the following manner:

@example
$ awk 'BEGIN @{ print -12 " " (-24) @}'
@print{} -12 -24
@end example

This forces @command{awk} to treat the @samp{-} on the @samp{-24} as unary.
Otherwise, it's parsed as follows:

@display
    @minus{}12 (@code{"@ "} @minus{} 24)
@result{} @minus{}12 (0 @minus{} 24)
@result{} @minus{}12 (@minus{}24)
@result{} @minus{}12@minus{}24
@end display

As mentioned earlier,
when doing concatenation, @emph{parenthesize}.  Otherwise,
you're never quite sure what you'll get.

@node Assignment Ops, Increment Ops, Concatenation, Expressions
@section Assignment Expressions
@cindex assignment operators
@cindex operators, assignment
@cindex expression, assignment

@cindex @code{=} operator
An @dfn{assignment} is an expression that stores a (usually different)
value into a variable.  For example, let's assign the value one to the variable
@code{z}:

@example
z = 1
@end example

After this expression is executed, the variable @code{z} has the value one.
Whatever old value @code{z} had before the assignment is forgotten.

Assignments can also store string values.  For example, the
following stores
the value @code{"this food is good"} in the variable @code{message}:

@example
thing = "food"
predicate = "good"
message = "this " thing " is " predicate
@end example

@noindent
@cindex side effects
This also illustrates string concatenation.
The @samp{=} sign is called an @dfn{assignment operator}.  It is the
simplest assignment operator because the value of the righthand
operand is stored unchanged.
Most operators (addition, concatenation, and so on) have no effect
except to compute a value.  If the value isn't used, there's no reason to
use the operator.  An assignment operator is different; it does
produce a value, but even if you ignore it, the assignment still
makes itself felt through the alteration of the variable.  We call this
a @dfn{side effect}.

@cindex lvalue
@cindex rvalue
The lefthand operand of an assignment need not be a variable
(@pxref{Variables}); it can also be a field
(@pxref{Changing Fields, ,Changing the Contents of a Field}) or
an array element (@pxref{Arrays, ,Arrays in @command{awk}}).
These are all called @dfn{lvalues},
which means they can appear on the lefthand side of an assignment operator.
The righthand operand may be any expression; it produces the new value
that the assignment stores in the specified variable, field, or array
element. (Such values are called @dfn{rvalues}).

@cindex types of variables
It is important to note that variables do @emph{not} have permanent types.
A variable's type is simply the type of whatever value it happens
to hold at the moment.  In the following program fragment, the variable
@code{foo} has a numeric value at first, and a string value later on:

@example
foo = 1
print foo
foo = "bar"
print foo
@end example

@noindent
When the second assignment gives @code{foo} a string value, the fact that
it previously had a numeric value is forgotten.

String values that do not begin with a digit have a numeric value of
zero. After executing the following code, the value of @code{foo} is five:

@example
foo = "a string"
foo = foo + 5
@end example

@noindent
@strong{Note:} Using a variable as a number and then later as a string
can be confusing and is poor programming style.  The previous two examples
illustrate how @command{awk} works, @emph{not} how you should write your
own programs!

An assignment is an expression, so it has a value---the same value that
is assigned.  Thus, @samp{z = 1} is an expression with the value one.
One consequence of this is that you can write multiple assignments together,
such as:

@example
x = y = z = 5
@end example

@noindent
This example stores the value five in all three variables
(@code{x}, @code{y}, and @code{z}).
It does so because the
value of @samp{z = 5}, which is five, is stored into @code{y} and then
the value of @samp{y = z = 5}, which is five, is stored into @code{x}.

Assignments may be used anywhere an expression is called for.  For
example, it is valid to write @samp{x != (y = 1)} to set @code{y} to one,
and then test whether @code{x} equals one.  But this style tends to make
programs hard to read; such nesting of assignments should be avoided,
except perhaps in a one-shot program.

Aside from @samp{=}, there are several other assignment operators that
do arithmetic with the old value of the variable.  For example, the
operator @samp{+=} computes a new value by adding the righthand value
to the old value of the variable.  Thus, the following assignment adds
five to the value of @code{foo}:

@example
foo += 5
@end example

@noindent
This is equivalent to the following:

@example
foo = foo + 5
@end example

@noindent
Use whichever makes the meaning of your program clearer.

There are situations where using @samp{+=} (or any assignment operator)
is @emph{not} the same as simply repeating the lefthand operand in the
righthand expression.  For example:

@cindex Rankin, Pat
@example
# Thanks to Pat Rankin for this example
BEGIN  @{
    foo[rand()] += 5
    for (x in foo)
       print x, foo[x]

    bar[rand()] = bar[rand()] + 5
    for (x in bar)
       print x, bar[x]
@}
@end example

@noindent
The indices of @code{bar} are practically guaranteed to be different, because
@code{rand} returns different values each time it is called.
(Arrays and the @code{rand} function haven't been covered yet.
@xref{Arrays, ,Arrays in @command{awk}},
and see @ref{Numeric Functions}, for more information).
This example illustrates an important fact about assignment
operators: the lefthand expression is only evaluated @emph{once}.
It is up to the implementation as to which expression is evaluated
first, the lefthand or the righthand.
Consider this example:

@example
i = 1
a[i += 2] = i + 1
@end example

@noindent
The value of @code{a[3]} could be either two or four.

Here is a table of the arithmetic assignment operators.  In each
case, the righthand operand is an expression whose value is converted
to a number.

@ignore
@table @code
@item @var{lvalue} += @var{increment}
Adds @var{increment} to the value of @var{lvalue}.

@item @var{lvalue} -= @var{decrement}
Subtracts @var{decrement} from the value of @var{lvalue}.

@item @var{lvalue} *= @var{coefficient}
Multiplies the value of @var{lvalue} by @var{coefficient}.

@item @var{lvalue} /= @var{divisor}
Divides the value of @var{lvalue} by @var{divisor}.

@item @var{lvalue} %= @var{modulus}
Sets @var{lvalue} to its remainder by @var{modulus}.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@item @var{lvalue} ^= @var{power}
@itemx @var{lvalue} **= @var{power}
Raises @var{lvalue} to the power @var{power}.
(Only the @samp{^=} operator is specified by POSIX.)
@end table
@end ignore

@cindex @code{+=} operator
@cindex @code{-=} operator
@cindex @code{*=} operator
@cindex @code{/=} operator
@cindex @code{%=} operator
@cindex @code{^=} operator
@cindex @code{**=} operator
@multitable {@var{lvalue} *= @var{coefficient}} {Subtracts @var{decrement} from the value of @var{lvalue}.}
@item @var{lvalue} @code{+=} @var{increment} @tab Adds @var{increment} to the value of @var{lvalue}.

@item @var{lvalue} @code{-=} @var{decrement} @tab Subtracts @var{decrement} from the value of @var{lvalue}.

@item @var{lvalue} @code{*=} @var{coefficient} @tab Multiplies the value of @var{lvalue} by @var{coefficient}.

@item @var{lvalue} @code{/=} @var{divisor} @tab Divides the value of @var{lvalue} by @var{divisor}.

@item @var{lvalue} @code{%=} @var{modulus} @tab Sets @var{lvalue} to its remainder by @var{modulus}.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@item @var{lvalue} @code{^=} @var{power} @tab
@item @var{lvalue} @code{**=} @var{power} @tab Raises @var{lvalue} to the power @var{power}.
@end multitable

@cindex portability issues
@strong{Note:}
Only the @samp{^=} operator is specified by POSIX.
For maximum portability, do not use the @samp{**=} operator.

@c fakenode --- for prepinfo
@subheading Advanced Notes: Syntactic Ambiguities Between @samp{/=} and Regular Expressions
@cindex advanced notes

@c derived from email from  "Nelson H. F. Beebe" <beebe@math.utah.edu>
@c Date: Mon, 1 Sep 1997 13:38:35 -0600 (MDT)

@cindex dark corner
@cindex ambiguity, syntactic: @code{/=} operator vs. @code{/=@dots{}/} regexp constant
@cindex syntactic ambiguity: @code{/=} operator vs. @code{/=@dots{}/} regexp constant
@cindex @code{/=} operator vs. @code{/=@dots{}/} regexp constant
There is a syntactic ambiguity between the @samp{/=} assignment
operator and regexp constants whose first character is an @samp{=}.
@value{DARKCORNER}
This is most notable in commercial @command{awk} versions.
For example:

@example
$ awk /==/ /dev/null
@error{} awk: syntax error at source line 1
@error{}  context is
@error{}         >>> /= <<<
@error{} awk: bailing out at source line 1
@end example

@noindent
A workaround is:

@example
awk '/[=]=/' /dev/null
@end example

@command{gawk} does not have this problem,
nor do the other
freely-available versions described in
@ref{Other Versions, , Other Freely Available @command{awk} Implementations}.

@node Increment Ops, Truth Values, Assignment Ops, Expressions
@section Increment and Decrement Operators

@cindex increment operators
@cindex operators, increment
@dfn{Increment} and @dfn{decrement operators} increase or decrease the value of
a variable by one.  An assignment operator can do the same thing, so
the increment operators add no power to the @command{awk} language; however they
are convenient abbreviations for very common operations.

@cindex side effects
The operator used for adding one is written @samp{++}.  It can be used to increment
a variable either before or after taking its value.
To pre-increment a variable @code{v}, write @samp{++v}.  This adds
one to the value of @code{v}---that new value is also the value of the
expression. (The assignment expression @samp{v += 1} is completely
equivalent.)
Writing the @samp{++} after the variable specifies post-increment.  This
increments the variable value just the same; the difference is that the
value of the increment expression itself is the variable's @emph{old}
value.  Thus, if @code{foo} has the value four, then the expression @samp{foo++}
has the value four, but it changes the value of @code{foo} to five.
In other words, the operator returns the old value of the variable,
but with the side effect of incrementing it.

The post-increment @samp{foo++} is nearly the same as writing @samp{(foo
+= 1) - 1}.  It is not perfectly equivalent because all numbers in
@command{awk} are floating-point---in floating-point, @samp{foo + 1 - 1} does
not necessarily equal @code{foo}.  But the difference is minute as
long as you stick to numbers that are fairly small (less than 10e12).

Fields and array elements are incremented
just like variables.  (Use @samp{$(i++)} when you want to do a field reference
and a variable increment at the same time.  The parentheses are necessary
because of the precedence of the field reference operator @samp{$}.)

@cindex decrement operators
@cindex operators, decrement
The decrement operator @samp{--} works just like @samp{++}, except that
it subtracts one instead of adding it.  As with @samp{++}, it can be used before
the lvalue to pre-decrement or after it to post-decrement.
Following is a summary of increment and decrement expressions:

@table @code
@cindex @code{++} operator
@item ++@var{lvalue}
This expression increments @var{lvalue}, and the new value becomes the
value of the expression.

@item @var{lvalue}++
This expression increments @var{lvalue}, but
the value of the expression is the @emph{old} value of @var{lvalue}.

@cindex @code{--} operator
@item --@var{lvalue}
This expression is
like @samp{++@var{lvalue}}, but instead of adding, it subtracts.  It
decrements @var{lvalue} and delivers the value that is the result.

@item @var{lvalue}--
This expression is
like @samp{@var{lvalue}++}, but instead of adding, it subtracts.  It
decrements @var{lvalue}.  The value of the expression is the @emph{old}
value of @var{lvalue}.
@end table

@c fakenode --- for prepinfo
@subheading Advanced Notes: Operator Evaluation Order
@cindex advanced notes
@cindex precedence
@cindex operator precedence
@cindex portability issues
@cindex evaluation, order of
@cindex Marx, Groucho
@quotation
@i{Doctor, doctor!  It hurts when I do this!@*
So don't do that!}@*
Groucho Marx
@end quotation

@noindent
What happens for something like the following?

@example
b = 6
print b += b++
@end example

@noindent
Or something even stranger?

@example
b = 6
b += ++b + b++
print b
@end example

@cindex side effects
In other words, when do the various side effects prescribed by the
postfix operators (@samp{b++}) take effect?
When side effects happen is @dfn{implementation defined}.
In other words, it is up to the particular version of @command{awk}.
The result for the first example may be 12 or 13, and for the second, it
may be 22 or 23.

In short, doing things like this is not recommended and definitely
not anything that you can rely upon for portability.
You should avoid such things in your own programs.
@c You'll sleep better at night and be able to look at yourself
@c in the mirror in the morning.

@node Truth Values, Typing and Comparison, Increment Ops, Expressions
@section True and False in @command{awk}
@cindex truth values
@cindex logical true
@cindex logical false

@cindex null string
@cindex empty string
Many programming languages have a special representation for the concepts
of ``true'' and ``false.''  Such languages usually use the special
constants @code{true} and @code{false}, or perhaps their uppercase
equivalents.
However, @command{awk} is different.
It borrows a very simple concept of true and
false from C.  In @command{awk}, any nonzero numeric value @emph{or} any
non-empty string value is true.  Any other value (zero or the null
string @code{""}) is false.  The following program prints @samp{A strange
truth value} three times:

@example
BEGIN @{
   if (3.1415927)
       print "A strange truth value"
   if ("Four Score And Seven Years Ago")
       print "A strange truth value"
   if (j = 57)
       print "A strange truth value"
@}
@end example

@cindex dark corner
There is a surprising consequence of the ``nonzero or non-null'' rule:
the string constant @code{"0"} is actually true, because it is non-null.
@value{DARKCORNER}

@node Typing and Comparison, Boolean Ops, Truth Values, Expressions
@section Variable Typing and Comparison Expressions
@cindex comparison expressions
@cindex expression, comparison
@cindex expression, matching
@cindex relational operators
@cindex operators, relational
@cindex regexp operators
@cindex variable typing
@cindex types of variables
@quotation
@i{The Guide is definitive. Reality is frequently inaccurate.}@*
The Hitchhiker's Guide to the Galaxy
@end quotation

Unlike other programming languages, @command{awk} variables do not have a
fixed type. Instead, they can be either a number or a string, depending
upon the value that is assigned to them.

@cindex numeric string
The 1992 POSIX standard introduced
the concept of a @dfn{numeric string}, which is simply a string that looks
like a number---for example, @code{@w{" +2"}}.  This concept is used
for determining the type of a variable.
The type of the variable is important because the types of two variables
determine how they are compared.
In @command{gawk}, variable typing follows these rules:

@itemize @bullet
@item
A numeric constant or the result of a numeric operation has the @var{numeric}
attribute.

@item
A string constant or the result of a string operation has the @var{string}
attribute.

@item
Fields, @code{getline} input, @code{FILENAME}, @code{ARGV} elements,
@code{ENVIRON} elements, and the
elements of an array created by @code{split} that are numeric strings
have the @var{strnum} attribute.  Otherwise, they have the @var{string}
attribute.
Uninitialized variables also have the @var{strnum} attribute.

@item
Attributes propagate across assignments but are not changed by
any use.
@c (Although a use may cause the entity to acquire an additional
@c value such that it has both a numeric and string value, this leaves the
@c attribute unchanged.)
@c This is important but not relevant
@end itemize

The last rule is particularly important. In the following program,
@code{a} has numeric type, even though it is later used in a string
operation:

@example
BEGIN @{
         a = 12.345
         b = a " is a cute number"
         print b
@}
@end example

When two operands are compared, either string comparison or numeric comparison
may be used. This depends upon the attributes of the operands, according to the
following symmetric matrix:

@c thanks to Karl Berry, kb@cs.umb.edu, for major help with TeX tables
@tex
\centerline{
\vbox{\bigskip % space above the table (about 1 linespace)
% Because we have vertical rules, we can't let TeX insert interline space
% in its usual way.
\offinterlineskip
%
% Define the table template. & separates columns, and \cr ends the
% template (and each row). # is replaced by the text of that entry on
% each row. The template for the first column breaks down like this:
%   \strut -- a way to make each line have the height and depth
%             of a normal line of type, since we turned off interline spacing.
%   \hfil -- infinite glue; has the effect of right-justifying in this case.
%   #     -- replaced by the text (for instance, `STRNUM', in the last row).
%   \quad -- about the width of an `M'. Just separates the columns.
% 
% The second column (\vrule#) is what generates the vertical rule that
% spans table rows.
% 
% The doubled && before the next entry means `repeat the following
% template as many times as necessary on each line' -- in our case, twice.
% 
% The template itself, \quad#\hfil, left-justifies with a little space before.
% 
\halign{\strut\hfil#\quad&\vrule#&&\quad#\hfil\cr
	&&STRING	&NUMERIC	&STRNUM\cr
% The \omit tells TeX to skip inserting the template for this column on
% this particular row. In this case, we only want a little extra space
% to separate the heading row from the rule below it.  the depth 2pt --
% `\vrule depth 2pt' is that little space.
\omit	&depth 2pt\cr
% This is the horizontal rule below the heading. Since it has nothing to
% do with the columns of the table, we use \noalign to get it in there.
\noalign{\hrule}
% Like above, this time a little more space.
\omit	&depth 4pt\cr
% The remaining rows have nothing special about them.
STRING	&&string	&string		&string\cr
NUMERIC	&&string	&numeric	&numeric\cr
STRNUM  &&string	&numeric	&numeric\cr
}}}
@end tex
@ifnottex
@display
        +----------------------------------------------
        |       STRING          NUMERIC         STRNUM
--------+----------------------------------------------
        |
STRING  |       string          string          string
        |
NUMERIC |       string          numeric         numeric
        |
STRNUM  |       string          numeric         numeric
--------+----------------------------------------------
@end display
@end ifnottex

The basic idea is that user input that looks numeric---and @emph{only}
user input---should be treated as numeric, even though it is actually
made of characters and is therefore also a string.
Thus, for example, the string constant @w{@code{" +3.14"}}
is a string, even though it looks numeric,
and is @emph{never} treated as number for comparison
purposes.

In short, when one operand is a ``pure'' string, such as a string
constant, then a string comparison is performed.  Otherwise, a
numeric comparison is performed.@footnote{The POSIX standard is under
revision.  The revised standard's rules for typing and comparison are
the same as just described for @command{gawk}.}

@dfn{Comparison expressions} compare strings or numbers for
relationships such as equality.  They are written using @dfn{relational
operators}, which are a superset of those in C.  Here is a table of
them:

@cindex relational operators
@cindex operators, relational
@cindex @code{<} operator
@cindex @code{<=} operator
@cindex @code{>} operator
@cindex @code{>=} operator
@cindex @code{==} operator
@cindex @code{!=} operator
@cindex @code{~} operator
@cindex @code{!~} operator
@cindex @code{in} operator
@table @code
@item @var{x} < @var{y}
True if @var{x} is less than @var{y}.

@item @var{x} <= @var{y}
True if @var{x} is less than or equal to @var{y}.

@item @var{x} > @var{y}
True if @var{x} is greater than @var{y}.

@item @var{x} >= @var{y}
True if @var{x} is greater than or equal to @var{y}.

@item @var{x} == @var{y}
True if @var{x} is equal to @var{y}.

@item @var{x} != @var{y}
True if @var{x} is not equal to @var{y}.

@item @var{x} ~ @var{y}
True if the string @var{x} matches the regexp denoted by @var{y}.

@item @var{x} !~ @var{y}
True if the string @var{x} does not match the regexp denoted by @var{y}.

@item @var{subscript} in @var{array}
True if the array @var{array} has an element with the subscript @var{subscript}.
@end table

Comparison expressions have the value one if true and zero if false.
When comparing operands of mixed types, numeric operands are converted
to strings using the value of @code{CONVFMT}
(@pxref{Conversion, ,Conversion of Strings and Numbers}).

Strings are compared
by comparing the first character of each, then the second character of each,
and so on.  Thus, @code{"10"} is less than @code{"9"}.  If there are two
strings where one is a prefix of the other, the shorter string is less than
the longer one.  Thus, @code{"abc"} is less than @code{"abcd"}.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
It is very easy to accidentally mistype the @samp{==} operator and
leave off one of the @samp{=} characters.  The result is still valid @command{awk}
code, but the program does not do what is intended:

@example
if (a = b)   # oops! should be a == b
   @dots{}
else
   @dots{}
@end example

@noindent
Unless @code{b} happens to be zero or the null string, the @code{if}
part of the test always succeeds.  Because the operators are
so similar, this kind of error is very difficult to spot when
scanning the source code.

The following table of expressions illustrates the kind of comparison
@command{gawk} performs, as well as what the result of the comparison is:

@table @code
@item 1.5 <= 2.0
numeric comparison (true)

@item "abc" >= "xyz"
string comparison (false)

@item 1.5 != " +2"
string comparison (true)

@item "1e2" < "3"
string comparison (true)

@item a = 2; b = "2"
@itemx a == b
string comparison (true)

@item a = 2; b = " +2"
@item a == b
string comparison (false)
@end table

In the next example:

@example
$ echo 1e2 3 | awk '@{ print ($1 < $2) ? "true" : "false" @}'
@print{} false
@end example

@cindex comparisons, string vs. regexp
@cindex string comparison vs. regexp comparison
@cindex regexp comparison vs. string comparison
@noindent
the result is @samp{false} because both @code{$1} and @code{$2}
are user input.  They are numeric strings---therefore both have
the @var{strnum} attribute, dictating a numeric comparison.
The purpose of the comparison rules and the use of numeric strings is
to attempt to produce the behavior that is ``least surprising,'' while
still ``doing the right thing.''
String comparisons and regular expression comparisons are very different.
For example:

@example
x == "foo"
@end example

@noindent
has the value one, or is true if the variable @code{x}
is precisely @samp{foo}.  By contrast:

@example
x ~ /foo/
@end example

@noindent
has the value one if @code{x} contains @samp{foo}, such as
@code{"Oh, what a fool am I!"}.

The righthand operand of the @samp{~} and @samp{!~} operators may be
either a regexp constant (@code{/@dots{}/}) or an ordinary
expression. In the latter case, the value of the expression as a string is used as a
dynamic regexp (@pxref{Regexp Usage, ,How to Use Regular Expressions}; also
@pxref{Computed Regexps, ,Using Dynamic Regexps}).

@cindex regexp as expression
In modern implementations of @command{awk}, a constant regular
expression in slashes by itself is also an expression.  The regexp
@code{/@var{regexp}/} is an abbreviation for the following comparison expression:

@example
$0 ~ /@var{regexp}/
@end example

One special place where @code{/foo/} is @emph{not} an abbreviation for
@samp{$0 ~ /foo/} is when it is the righthand operand of @samp{~} or
@samp{!~}.
@xref{Using Constant Regexps, ,Using Regular Expression Constants},
where this is discussed in more detail.

@node Boolean Ops, Conditional Exp, Typing and Comparison, Expressions
@section Boolean Expressions
@cindex expression, boolean
@cindex boolean expressions
@cindex operators, boolean
@cindex boolean operators
@cindex logical operators
@cindex operators, logical
@cindex short-circuit operators
@cindex operators, short-circuit
@cindex AND logical operator
@cindex OR logical operator
@cindex NOT logical operator
@cindex @code{&&} operator
@cindex @code{||} operator
@cindex @code{!} operator

A @dfn{Boolean expression} is a combination of comparison expressions or
matching expressions, using the Boolean operators ``or''
(@samp{||}), ``and'' (@samp{&&}), and ``not'' (@samp{!}), along with
parentheses to control nesting.  The truth value of the Boolean expression is
computed by combining the truth values of the component expressions.
Boolean expressions are also referred to as @dfn{logical expressions}.
The terms are equivalent.

Boolean expressions can be used wherever comparison and matching
expressions can be used.  They can be used in @code{if}, @code{while},
@code{do}, and @code{for} statements
(@pxref{Statements, ,Control Statements in Actions}).
They have numeric values (one if true, zero if false), that come into play
if the result of the Boolean expression is stored in a variable or
used in arithmetic.

In addition, every Boolean expression is also a valid pattern, so
you can use one as a pattern to control the execution of rules.
The Boolean operators are:

@table @code
@item @var{boolean1} && @var{boolean2}
True if both @var{boolean1} and @var{boolean2} are true.  For example,
the following statement prints the current input record if it contains
both @samp{2400} and @samp{foo}:

@example
if ($0 ~ /2400/ && $0 ~ /foo/) print
@end example

@cindex side effects
The subexpression @var{boolean2} is evaluated only if @var{boolean1}
is true.  This can make a difference when @var{boolean2} contains
expressions that have side effects. In the case of @samp{$0 ~ /foo/ &&
($2 == bar++)}, the variable @code{bar} is not incremented if there is
no substring @samp{foo} in the record.

@item @var{boolean1} || @var{boolean2}
True if at least one of @var{boolean1} or @var{boolean2} is true.
For example, the following statement prints all records in the input
that contain @emph{either} @samp{2400} or
@samp{foo} or both:

@example
if ($0 ~ /2400/ || $0 ~ /foo/) print
@end example

The subexpression @var{boolean2} is evaluated only if @var{boolean1}
is false.  This can make a difference when @var{boolean2} contains
expressions that have side effects.

@item ! @var{boolean}
True if @var{boolean} is false.  For example,
the following program prints @samp{no home!} in
the unusual event that the @env{HOME} environment
variable is not defined:

@example
BEGIN @{ if (! ("HOME" in ENVIRON))
               print "no home!" @}
@end example

(The @code{in} operator is described in
@ref{Reference to Elements, ,Referring to an Array Element}.)
@end table

The @samp{&&} and @samp{||} operators are called @dfn{short-circuit}
operators because of the way they work.  Evaluation of the full expression
is ``short-circuited'' if the result can be determined part way through
its evaluation.

@cindex line continuation
Statements that use @samp{&&} or @samp{||} can be continued simply
by putting a newline after them.  But you cannot put a newline in front
of either of these operators without using backslash continuation
(@pxref{Statements/Lines, ,@command{awk} Statements Versus Lines}).

@cindex flag variables
The actual value of an expression using the @samp{!} operator is
either one or zero, depending upon the truth value of the expression it
is applied to.
The @samp{!} operator is often useful for changing the sense of a flag
variable from false to true and back again. For example, the following
program is one way to print lines in between special bracketing lines:

@example
$1 == "START"   @{ interested = ! interested; next @}
interested == 1 @{ print @}
$1 == "END"     @{ interested = ! interested; next @}
@end example

@noindent
The variable @code{interested}, as with all @command{awk} variables, starts
out initialized to zero, which is also false.  When a line is seen whose
first field is @samp{START}, the value of @code{interested} is toggled
to true, using @samp{!}. The next rule prints lines as long as
@code{interested} is true.  When a line is seen whose first field is
@samp{END}, @code{interested} is toggled back to false.

@ignore
Scott Deifik points out that this program isn't robust against
bogus input data, but the point is to illustrate the use of `!',
so we'll leave well enough alone.
@end ignore

@strong{Note:} The @code{next} statement is discussed in
@ref{Next Statement, ,The @code{next} Statement}.
@code{next} tells @command{awk} to skip the rest of the rules, get the
next record, and start processing the rules over again at the top.
The reason it's there is to avoid printing the bracketing
@samp{START} and @samp{END} lines.

@node Conditional Exp, Function Calls, Boolean Ops, Expressions
@section Conditional Expressions
@cindex conditional expression
@cindex expression, conditional

A @dfn{conditional expression} is a special kind of expression that has
three operands.  It allows you to use one expression's value to select
one of two other expressions.
The conditional expression is the same as in the C language,
as shown here:

@example
@var{selector} ? @var{if-true-exp} : @var{if-false-exp}
@end example

@noindent
There are three subexpressions.  The first, @var{selector}, is always
computed first.  If it is ``true'' (not zero or not null), then
@var{if-true-exp} is computed next and its value becomes the value of
the whole expression.  Otherwise, @var{if-false-exp} is computed next
and its value becomes the value of the whole expression.
For example, the following expression produces the absolute value of @code{x}:

@example
x >= 0 ? x : -x
@end example

@cindex side effects
Each time the conditional expression is computed, only one of
@var{if-true-exp} and @var{if-false-exp} is used; the other is ignored.
This is important when the expressions have side effects.  For example,
this conditional expression examines element @code{i} of either array
@code{a} or array @code{b}, and increments @code{i}:

@example
x == y ? a[i++] : b[i++]
@end example

@noindent
This is guaranteed to increment @code{i} exactly once, because each time
only one of the two increment expressions is executed
and the other is not.
@xref{Arrays, ,Arrays in @command{awk}},
for more information about arrays.

@cindex differences between @command{gawk} and @command{awk}
@cindex line continuation
As a minor @command{gawk} extension,
a statement that uses @samp{?:} can be continued simply
by putting a newline after either character.
However, putting a newline in front
of either character does not work without using backslash continuation
(@pxref{Statements/Lines, ,@command{awk} Statements Versus Lines}).
If @option{--posix} is specified
(@pxref{Options, , Command-Line Options}), then this extension is disabled.

@node Function Calls, Precedence, Conditional Exp, Expressions
@section Function Calls
@cindex function call
@cindex calling a function

A @dfn{function} is a name for a particular calculation.
This enables you to
ask for it by name at any point in the program.  For
example, the function @code{sqrt} computes the square root of a number.

A fixed set of functions are @dfn{built-in}, which means they are
available in every @command{awk} program.  The @code{sqrt} function is one
of these.  @xref{Built-in, ,Built-in Functions}, for a list of built-in
functions and their descriptions.  In addition, you can define
functions for use in your program.
@xref{User-defined, ,User-Defined Functions},
for instructions on how to do this.

@cindex arguments in function call
The way to use a function is with a @dfn{function call} expression,
which consists of the function name followed immediately by a list of
@dfn{arguments} in parentheses.  The arguments are expressions that
provide the raw materials for the function's calculations.
When there is more than one argument, they are separated by commas.  If
there are no arguments, just write @samp{()} after the function name.
The following examples show function calls with and without arguments:

@example
sqrt(x^2 + y^2)        @i{one argument}
atan2(y, x)            @i{two arguments}
rand()                 @i{no arguments}
@end example

@strong{Caution:}
Do not put any space between the function name and the open-parenthesis!
A user-defined function name looks just like the name of a
variable---a space would make the expression look like concatenation of
a variable with an expression inside parentheses.

With built-in functions, space before the parenthesis is harmless, but
it is best not to get into the habit of using space to avoid mistakes
with user-defined functions.  Each function expects a particular number
of arguments.  For example, the @code{sqrt} function must be called with
a single argument: the number to take the square root of:

@example
sqrt(@var{argument})
@end example

Some of the built-in functions have one or
more optional arguments.
If those arguments are not supplied, the functions
use a reasonable default value.
@xref{Built-in, ,Built-in Functions}, for full details.  If arguments
are omitted in calls to user-defined functions, then those arguments are
treated as local variables and initialized to the empty string
(@pxref{User-defined, ,User-Defined Functions}).

@cindex side effects
Like every other expression, the function call has a value, which is
computed by the function based on the arguments you give it.  In this
example, the value of @samp{sqrt(@var{argument})} is the square root of
@var{argument}.  A function can also have side effects, such as assigning
values to certain variables or doing I/O.
The following program reads numbers, one number per line, and prints the
square root of each one:

@example
$ awk '@{ print "The square root of", $1, "is", sqrt($1) @}'
1
@print{} The square root of 1 is 1
3
@print{} The square root of 3 is 1.73205
5
@print{} The square root of 5 is 2.23607
@kbd{Ctrl-d}
@end example

@node Precedence, , Function Calls, Expressions
@section Operator Precedence (How Operators Nest)
@cindex precedence
@cindex operator precedence

@dfn{Operator precedence} determines how operators are grouped when
different operators appear close by in one expression.  For example,
@samp{*} has higher precedence than @samp{+}; thus, @samp{a + b * c}
means to multiply @code{b} and @code{c}, and then add @code{a} to the
product (i.e., @samp{a + (b * c)}).

The normal precedence of the operators can be overruled by using parentheses.
Think of the precedence rules as saying where the
parentheses are assumed to be.  In
fact, it is wise to always use parentheses whenever there is an unusual
combination of operators, because other people who read the program may
not remember what the precedence is in this case.
Even experienced programmers occasionally forget the exact rules,
which leads to mistakes.
Explicit parentheses help prevent
any such mistakes.

When operators of equal precedence are used together, the leftmost
operator groups first, except for the assignment, conditional, and
exponentiation operators, which group in the opposite order.
Thus, @samp{a - b + c} groups as @samp{(a - b) + c} and
@samp{a = b = c} groups as @samp{a = (b = c)}.

The precedence of prefix unary operators does not matter as long as only
unary operators are involved, because there is only one way to interpret
them: innermost first.  Thus, @samp{$++i} means @samp{$(++i)} and
@samp{++$x} means @samp{++($x)}.  However, when another operator follows
the operand, then the precedence of the unary operators can matter.
@samp{$x^2} means @samp{($x)^2}, but @samp{-x^2} means
@samp{-(x^2)}, because @samp{-} has lower precedence than @samp{^},
whereas @samp{$} has higher precedence.
This table presents @command{awk}'s operators, in order of highest
precedence to lowest:

@page

@cindex @code{$} field operator
@cindex @code{+} operator
@cindex @code{-} operator
@cindex @code{!} operator
@cindex @code{*} operator
@cindex @code{/} operator
@cindex @code{%} operator
@cindex @code{^} operator
@cindex @code{**} operator
@cindex @code{++} operator
@cindex @code{--} operator
@cindex @code{<} operator
@cindex @code{<=} operator
@cindex @code{==} operator
@cindex @code{!=} operator
@cindex @code{>} operator
@cindex @code{>=} operator
@cindex @code{>>} I/O operator
@cindex @code{|} I/O operator
@cindex @code{|&} I/O operator
@cindex @code{~} operator
@cindex @code{!~} operator
@cindex @code{in} operator
@cindex @code{&&} operator
@cindex @code{||} operator
@cindex @code{?:} operator
@cindex @code{+=} operator
@cindex @code{-=} operator
@cindex @code{*=} operator
@cindex @code{/=} operator
@cindex @code{%=} operator
@cindex @code{^=} operator
@cindex @code{**=} operator
@c use @code in the items, looks better in TeX w/o all the quotes
@table @code
@item (@dots{})
Grouping.

@item $
Field.

@item ++ --
Increment, decrement.

@item ^ **
Exponentiation.  These operators group right-to-left.

@item + - !
Unary plus, minus, logical ``not.''

@item * / %
Multiplication, division, modulus.

@item + -
Addition, subtraction.

@item @r{String Concatenation}
No special symbol is used to indicate concatenation.
The operands are simply written side by side
(@pxref{Concatenation, ,String Concatenation}).

@item < <= == !=
@itemx > >= >> | |&
Relational and redirection.
The relational operators and the redirections have the same precedence
level.  Characters such as @samp{>} serve both as relationals and as
redirections; the context distinguishes between the two meanings.

Note that the I/O redirection operators in @code{print} and @code{printf}
statements belong to the statement level, not to expressions.  The
redirection does not produce an expression that could be the operand of
another operator.  As a result, it does not make sense to use a
redirection operator near another operator of lower precedence without
parentheses.  Such combinations (for example @samp{print foo > a ? b : c}),
result in syntax errors.
The correct way to write this statement is @samp{print foo > (a ? b : c)}.

@item ~ !~
Matching, non-matching.

@item in
Array membership.

@item &&
Logical ``and''.

@item ||
Logical ``or''.

@item ?:
Conditional.  This operator groups right-to-left.

@item = += -= *=
@itemx /= %= ^= **=
Assignment.  These operators group right-to-left.
@end table

@cindex portability issues
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@strong{Note:}
The @samp{|&}, @samp{**}, and @samp{**=} operators are not specified by POSIX.
For maximum portability, do not use them.

@node Patterns and Actions, Arrays, Expressions, Top
@chapter Patterns, Actions, and Variables
@cindex pattern, definition of

As you have already seen, each @command{awk} statement consists of
a pattern with an associated action.  This @value{CHAPTER} describes how
you build patterns and actions, what kinds of things you can do within
actions, and @command{awk}'s built-in variables.

The pattern-action rules and the statements available for use
within actions form the core of @command{awk} programming.
In a sense, everything covered
up to here has been the foundation
that programs are built on top of.  Now it's time to start
building something useful.

@menu
* Pattern Overview::            What goes into a pattern.
* Using Shell Variables::       How to use shell variables with @command{awk}.
* Action Overview::             What goes into an action.
* Statements::                  Describes the various control statements in
                                detail.
* Built-in Variables::          Summarizes the built-in variables.
@end menu

@node Pattern Overview, Using Shell Variables, Patterns and Actions, Patterns and Actions
@section Pattern Elements

@menu
* Regexp Patterns::             Using regexps as patterns.
* Expression Patterns::         Any expression can be used as a pattern.
* Ranges::                      Pairs of patterns specify record ranges.
* BEGIN/END::                   Specifying initialization and cleanup rules.
* Empty::                       The empty pattern, which matches every record.
@end menu

@cindex patterns, types of
Patterns in @command{awk} control the execution of rules---a rule is
executed when its pattern matches the current input record.
The following is a summary of the types of patterns in @command{awk}:

@table @code
@item /@var{regular expression}/
A regular expression. It matches when the text of the
input record fits the regular expression.
(@xref{Regexp, ,Regular Expressions}.)

@item @var{expression}
A single expression.  It matches when its value
is nonzero (if a number) or non-null (if a string).
(@xref{Expression Patterns, ,Expressions as Patterns}.)

@item @var{pat1}, @var{pat2}
A pair of patterns separated by a comma, specifying a range of records.
The range includes both the initial record that matches @var{pat1} and
the final record that matches @var{pat2}.
(@xref{Ranges, ,Specifying Record Ranges with Patterns}.)

@item BEGIN
@itemx END
Special patterns for you to supply startup or cleanup actions for your
@command{awk} program.
(@xref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}.)

@item @var{empty}
The empty pattern matches every input record.
(@xref{Empty, ,The Empty Pattern}.)
@end table

@node Regexp Patterns, Expression Patterns, Pattern Overview, Pattern Overview
@subsection Regular Expressions as Patterns

Regular expressions are one of the first kinds of patterns presented
in this book.
This kind of pattern is simply a regexp constant in the pattern part of
a rule.  Its  meaning is @samp{$0 ~ /@var{pattern}/}.
The pattern matches when the input record matches the regexp.
For example:

@example
/foo|bar|baz/  @{ buzzwords++ @}
END            @{ print buzzwords, "buzzwords seen" @}
@end example

@node Expression Patterns, Ranges, Regexp Patterns, Pattern Overview
@subsection Expressions as Patterns

Any @command{awk} expression is valid as an @command{awk} pattern.
The pattern matches if the expression's value is nonzero (if a
number) or non-null (if a string).
The expression is reevaluated each time the rule is tested against a new
input record.  If the expression uses fields such as @code{$1}, the
value depends directly on the new input record's text; otherwise it
depends on only what has happened so far in the execution of the
@command{awk} program.

Comparison expressions, using the comparison operators described in
@ref{Typing and Comparison, ,Variable Typing and Comparison Expressions},
are a very common kind of pattern.  
Regexp matching and non-matching are also very common expressions.
The left operand of the @samp{~} and @samp{!~} operators is a string.
The right operand is either a constant regular expression enclosed in
slashes (@code{/@var{regexp}/}), or any expression whose string value
is used as a dynamic regular expression
(@pxref{Computed Regexps, , Using Dynamic Regexps}).
The following example prints the second field of each input record
whose first field is precisely @samp{foo}:

@example
$ awk '$1 == "foo" @{ print $2 @}' BBS-list
@end example

@noindent
(There is no output, because there is no BBS site with the exact name @samp{foo}.)
Contrast this with the following regular expression match, which
accepts any record with a first field that contains @samp{foo}:

@example
$ awk '$1 ~ /foo/ @{ print $2 @}' BBS-list
@print{} 555-1234
@print{} 555-6699
@print{} 555-6480
@print{} 555-2127
@end example

A regexp constant as a pattern is also a special case of an expression
pattern.  The expression @code{/foo/} has the value one if @samp{foo}
appears in the current input record. Thus, as a pattern, @code{/foo/}
matches any record containing @samp{foo}.

Boolean expressions are also commonly used as patterns.
Whether the pattern
matches an input record depends on whether its subexpressions match.
For example, the following command prints all the records in
@file{BBS-list} that contain both @samp{2400} and @samp{foo}:

@example
$ awk '/2400/ && /foo/' BBS-list
@print{} fooey        555-1234     2400/1200/300     B
@end example

The following command prints all records in
@file{BBS-list} that contain @emph{either} @samp{2400} or @samp{foo}
(or both, of course):

@example
$ awk '/2400/ || /foo/' BBS-list
@print{} alpo-net     555-3412     2400/1200/300     A
@print{} bites        555-1675     2400/1200/300     A
@print{} fooey        555-1234     2400/1200/300     B
@print{} foot         555-6699     1200/300          B
@print{} macfoo       555-6480     1200/300          A
@print{} sdace        555-3430     2400/1200/300     A
@print{} sabafoo      555-2127     1200/300          C
@end example

The following command prints all records in
@file{BBS-list} that do @emph{not} contain the string @samp{foo}:

@example
$ awk '! /foo/' BBS-list
@print{} aardvark     555-5553     1200/300          B
@print{} alpo-net     555-3412     2400/1200/300     A
@print{} barfly       555-7685     1200/300          A
@print{} bites        555-1675     2400/1200/300     A
@print{} camelot      555-0542     300               C
@print{} core         555-2912     1200/300          C
@print{} sdace        555-3430     2400/1200/300     A
@end example

The subexpressions of a Boolean operator in a pattern can be constant regular
expressions, comparisons, or any other @command{awk} expressions.  Range
patterns are not expressions, so they cannot appear inside Boolean
patterns.  Likewise, the special patterns @code{BEGIN} and @code{END},
which never match any input record, are not expressions and cannot
appear inside Boolean patterns.

@node Ranges, BEGIN/END, Expression Patterns, Pattern Overview
@subsection Specifying Record Ranges with Patterns

@cindex range pattern
@cindex pattern, range
@cindex matching ranges of lines
A @dfn{range pattern} is made of two patterns separated by a comma, in
the form @samp{@var{begpat}, @var{endpat}}.  It is used to match ranges of
consecutive input records.  The first pattern, @var{begpat}, controls
where the range begins, while @var{endpat} controls where
the pattern ends.  For example, the following:

@example
awk '$1 == "on", $1 == "off"' myfile
@end example

@noindent
prints every record in @file{myfile} between @samp{on}/@samp{off} pairs, inclusive.

A range pattern starts out by matching @var{begpat} against every
input record.  When a record matches @var{begpat}, the range pattern is
@dfn{turned on} and the range pattern matches this record as well.  As long as
the range pattern stays turned on, it automatically matches every input
record read.  The range pattern also matches @var{endpat} against every
input record; when this succeeds, the range pattern is turned off again
for the following record.  Then the range pattern goes back to checking
@var{begpat} against each record.

The record that turns on the range pattern and the one that turns it
off both match the range pattern.  If you don't want to operate on
these records, you can write @code{if} statements in the rule's action
to distinguish them from the records you are interested in.

It is possible for a pattern to be turned on and off by the same
record. If the record satisfies both conditions, then the action is
executed for just that record.
For example, suppose there is text between two identical markers (say
the @samp{%} symbol), each on its own line, that should be ignored.
A first attempt would be to
combine a range pattern that describes the delimited text with the
@code{next} statement
(not discussed yet, @pxref{Next Statement, , The @code{next} Statement}).
This causes @command{awk} to skip any further processing of the current
record and start over again with the next input record. Such a program
looks like this:

@example
/^%$/,/^%$/    @{ next @}
               @{ print @}
@end example

@noindent
@cindex skipping lines between markers
@cindex flag variables
This program fails because the range pattern is both turned on and turned off
by the first line, which just has a @samp{%} on it.  To accomplish this task,
write the program in the following manner, using a flag:

@cindex @code{!} operator
@example
/^%$/     @{ skip = ! skip; next @}
skip == 1 @{ next @} # skip lines with `skip' set
@end example

In a range pattern, the comma (@samp{,}) has the lowest precedence of
all the operators (i.e., it is evaluated last).  Thus, the following
program attempts to combine a range pattern with another simpler test:

@example
echo Yes | awk '/1/,/2/ || /Yes/'
@end example

The intent of this program is @samp{(/1/,/2/) || /Yes/}.
However, @command{awk} interprets this as @samp{/1/, (/2/ || /Yes/)}.
This cannot be changed or worked around; range patterns do not combine
with other patterns:

@example
$ echo yes | gawk '(/1/,/2/) || /Yes/'
@error{} gawk: cmd. line:1: (/1/,/2/) || /Yes/
@error{} gawk: cmd. line:1:           ^ parse error
@error{} gawk: cmd. line:2: (/1/,/2/) || /Yes/
@error{} gawk: cmd. line:2:                   ^ unexpected newline
@end example

@node BEGIN/END, Empty, Ranges, Pattern Overview
@subsection The @code{BEGIN} and @code{END} Special Patterns

@cindex @code{BEGIN} special pattern
@cindex pattern, @code{BEGIN}
@cindex @code{END} special pattern
@cindex pattern, @code{END}
@cindex blocks, @code{BEGIN} and @code{END}
All the patterns described so far are for matching input records.
The @code{BEGIN} and @code{END} special patterns are different.
They supply startup and cleanup actions for @command{awk} programs.
@code{BEGIN} and @code{END} rules must have actions; there is no default
action for these rules because there is no current record when they run.
@code{BEGIN} and @code{END} rules are often referred to as
``@code{BEGIN} and @code{END} blocks'' by long-time @command{awk}
programmers.

@menu
* Using BEGIN/END::             How and why to use BEGIN/END rules.
* I/O And BEGIN/END::           I/O issues in BEGIN/END rules.
@end menu

@node Using BEGIN/END, I/O And BEGIN/END, BEGIN/END, BEGIN/END
@subsubsection Startup and Cleanup Actions

A @code{BEGIN} rule is executed once only, before the first input record
is read. Likewise, an @code{END} rule is executed once only, after all the
input is read.  For example:

@example
$ awk '
> BEGIN @{ print "Analysis of \"foo\"" @}
> /foo/ @{ ++n @}
> END   @{ print "\"foo\" appears", n, "times." @}' BBS-list
@print{} Analysis of "foo"
@print{} "foo" appears 4 times.
@end example

This program finds the number of records in the input file @file{BBS-list}
that contain the string @samp{foo}.  The @code{BEGIN} rule prints a title
for the report.  There is no need to use the @code{BEGIN} rule to
initialize the counter @code{n} to zero, since @command{awk} does this
automatically (@pxref{Variables}).
The second rule increments the variable @code{n} every time a
record containing the pattern @samp{foo} is read.  The @code{END} rule
prints the value of @code{n} at the end of the run.

The special patterns @code{BEGIN} and @code{END} cannot be used in ranges
or with Boolean operators (indeed, they cannot be used with any operators).
An @command{awk} program may have multiple @code{BEGIN} and/or @code{END}
rules.  They are executed in the order in which they appear: all the @code{BEGIN}
rules at startup and all the @code{END} rules at termination.
@code{BEGIN} and @code{END} rules may be intermixed with other rules.
This feature was added in the 1987 version of @command{awk} and is included
in the POSIX standard.
The original (1978) version of @command{awk}
required the @code{BEGIN} rule to be placed at the beginning of the
program, the @code{END} rule to be placed at the end, and only allowed one of
each.
This is no longer required, but it is a good idea to follow this template
in terms of program organization and readability.

Multiple @code{BEGIN} and @code{END} rules are useful for writing
library functions, because each library file can have its own @code{BEGIN} and/or
@code{END} rule to do its own initialization and/or cleanup. 
The order in which library functions are named on the command line
controls the order in which their @code{BEGIN} and @code{END} rules are
executed.  Therefore you have to be careful when writing such rules in
library files so that the order in which they are executed doesn't matter.
@xref{Options, ,Command-Line Options}, for more information on
using library functions.
@xref{Library Functions, ,A Library of @command{awk} Functions},
for a number of useful library functions.

If an @command{awk} program only has a @code{BEGIN} rule and no
other rules, then the program exits after the @code{BEGIN} rule is
run.@footnote{The original version of @command{awk} used to keep
reading and ignoring input until end of file was seen.}  However, if an
@code{END} rule exists, then the input is read, even if there are
no other rules in the program.  This is necessary in case the @code{END}
rule checks the @code{FNR} and @code{NR} variables.

@node I/O And BEGIN/END, , Using BEGIN/END, BEGIN/END
@subsubsection Input/Output from @code{BEGIN} and @code{END} Rules

@cindex I/O, from @code{BEGIN} and @code{END}
There are several (sometimes subtle) points to remember when doing I/O
from a @code{BEGIN} or @code{END} rule.
The first has to do with the value of @code{$0} in a @code{BEGIN}
rule.  Because @code{BEGIN} rules are executed before any input is read,
there simply is no input record, and therefore no fields, when
executing @code{BEGIN} rules.  References to @code{$0} and the fields
yield a null string or zero, depending upon the context.  One way
to give @code{$0} a real value is to execute a @code{getline} command
without a variable (@pxref{Getline, ,Explicit Input with @code{getline}}).
Another way is to simply assign a value to @code{$0}.

@cindex differences between @command{gawk} and @command{awk}
The second point is similar to the first but from the other direction.
Traditionally, due largely to implementation issues, @code{$0} and
@code{NF} were @emph{undefined} inside an @code{END} rule.
The POSIX standard specifies that @code{NF} is available in an @code{END}
rule. It contains the number of fields from the last input record.
Most probably due to an oversight, the standard does not say that @code{$0}
is also preserved, although logically one would think that it should be.
In fact, @command{gawk} does preserve the value of @code{$0} for use in
@code{END} rules.  Be aware, however, that Unix @command{awk}, and possibly
other implementations, do not.

The third point follows from the first two.  The meaning of @samp{print}
inside a @code{BEGIN} or @code{END} rule is the same as always:
@samp{print $0}.  If @code{$0} is the null string, then this prints an
empty line.  Many long time @command{awk} programmers use an unadorned
@samp{print} in @code{BEGIN} and @code{END} rules, to mean @samp{@w{print ""}},
relying on @code{$0} being null.  Although one might generally get away with
this in @code{BEGIN} rules, it is a very bad idea in @code{END} rules,
at least in @command{gawk}.  It is also poor style, since if an empty
line is needed in the output, the program should print one explicitly.

Finally, the @code{next} and @code{nextfile} statements are not allowed
in a @code{BEGIN} rule, because the implicit
read-a-record-and-match-against-the-rules loop has not started yet.  Similarly, those statements
are not valid in an @code{END} rule, since all the input has been read.
(@xref{Next Statement, ,The @code{next} Statement}, and see
@ref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}.)

@node Empty, , BEGIN/END, Pattern Overview
@subsection The Empty Pattern

@cindex empty pattern
@cindex pattern, empty
An empty (i.e., non-existent) pattern is considered to match @emph{every}
input record.  For example, the program:

@example
awk '@{ print $1 @}' BBS-list
@end example

@noindent
prints the first field of every record.

@node Using Shell Variables, Action Overview, Pattern Overview, Patterns and Actions
@section Using Shell Variables in Programs
@cindex shell varibles, using in @command{awk} programs
@cindex using shell variables in @command{awk} programs
@cindex shell and @command{awk} interaction

@command{awk} programs are often used as components in larger
programs written in shell.
For example, it is very common to use a shell variable to
hold a pattern that the @command{awk} program searches for.
There are two ways to get the value of the shell variable
into the body of the @command{awk} program.

The most common method is to use shell quoting to substitute
the variable's value into the program inside the script.
For example, in the following program:

@example
echo -n "Enter search pattern: "
read pattern
awk "/$pattern/ "'@{ nmatches++ @}
     END @{ print nmatches, "found" @}' /path/to/data
@end example

@noindent
the @command{awk} program consists of two pieces of quoted text
that are concatenated together to form the program.
The first part is double-quoted, which allows substitution of
the @code{pattern} variable inside the quotes.
The second part is single-quoted.

Variable substitution via quoting works, but can be potentially
messy.  It requires a good understanding of the shell's quoting rules
(@pxref{Quoting, ,Shell Quoting Issues}),
and it's often difficult to correctly
match up the quotes when reading the program.

A better method is to use @command{awk}'s variable assignment feature
(@pxref{Assignment Options, ,Assigning Variables on the Command Line})
to assign the shell variable's value to an @command{awk} variable's
value.  Then use dynamic regexps to match the pattern
(@pxref{Computed Regexps, ,Using Dynamic Regexps}).
The following shows how to redo the
previous example using this technique:

@example
echo -n "Enter search pattern: "
read pattern
awk -v pat="$pattern" '$0 ~ pat @{ nmatches++ @}
       END @{ print nmatches, "found" @}' /path/to/data
@end example

@noindent
Now, the @command{awk} program is just one single-quoted string.
The assignment @samp{-v pat="$pattern"} still requires double quotes,
in case there is whitespace in the value of @code{$pattern}.
The @command{awk} variable @code{pat} could be named @code{pattern}
too, but that would be more confusing.  Using a variable also
provides more flexibility, since the variable can be used anywhere inside
the program---for printing, as an array subscript, or for any other
use---without requiring the quoting tricks at every point in the program.

@node Action Overview, Statements, Using Shell Variables, Patterns and Actions
@section Actions
@cindex action, definition of
@cindex curly braces
@cindex action, curly braces
@cindex action, separating statements

An @command{awk} program or script consists of a series of
rules and function definitions interspersed.  (Functions are
described later.  @xref{User-defined, ,User-Defined Functions}.)
A rule contains a pattern and an action, either of which (but not
both) may be omitted.  The purpose of the @dfn{action} is to tell
@command{awk} what to do once a match for the pattern is found.  Thus,
in outline, an @command{awk} program generally looks like this:

@example
@r{[}@var{pattern}@r{]} @r{[}@{ @var{action} @}@r{]}
@r{[}@var{pattern}@r{]} @r{[}@{ @var{action} @}@r{]}
@dots{}
function @var{name}(@var{args}) @{ @dots{} @}
@dots{}
@end example

An action consists of one or more @command{awk} @dfn{statements}, enclosed
in curly braces (@samp{@{} and @samp{@}}).  Each statement specifies one
thing to do.  The statements are separated by newlines or semicolons.
The curly braces around an action must be used even if the action
contains only one statement, or if it contains no statements at
all.  However, if you omit the action entirely, omit the curly braces as
well.  An omitted action is equivalent to @samp{@{ print $0 @}}:

@example
/foo/  @{ @}     @i{match @code{foo}, do nothing --- empty action}
/foo/          @i{match @code{foo}, print the record --- omitted action}
@end example

The following types of statements are supported in @command{awk}:

@itemize @bullet
@cindex side effects
@item
Expressions, which can call functions or assign values to variables
(@pxref{Expressions}).  Executing
this kind of statement simply computes the value of the expression.
This is useful when the expression has side effects
(@pxref{Assignment Ops, ,Assignment Expressions}).

@item
Control statements, which specify the control flow of @command{awk}
programs.  The @command{awk} language gives you C-like constructs
(@code{if}, @code{for}, @code{while}, and @code{do}) as well as a few
special ones (@pxref{Statements, ,Control Statements in Actions}).

@item
Compound statements, which consist of one or more statements enclosed in
curly braces.  A compound statement is used in order to put several
statements together in the body of an @code{if}, @code{while}, @code{do},
or @code{for} statement.

@item
Input statements using the @code{getline} command
(@pxref{Getline, ,Explicit Input with @code{getline}}), the @code{next}
statement (@pxref{Next Statement, ,The @code{next} Statement}),
and the @code{nextfile} statement
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).

@item
Output statements, such as @code{print} and @code{printf}.
@xref{Printing, ,Printing Output}.

@item
Deletion statements for deleting array elements.
@xref{Delete, ,The @code{delete} Statement}.
@end itemize

@node Statements, Built-in Variables, Action Overview, Patterns and Actions
@section Control Statements in Actions
@cindex control statement

@dfn{Control statements}, such as @code{if}, @code{while}, and so on,
control the flow of execution in @command{awk} programs.  Most of the
control statements in @command{awk} are patterned on similar statements in C.

@cindex compound statement
@cindex statement, compound
All the control statements start with special keywords, such as @code{if}
and @code{while}, to distinguish them from simple expressions.
Many control statements contain other statements.  For example, the
@code{if} statement contains another statement that may or may not be
executed.  The contained statement is called the @dfn{body}.
To include more than one statement in the body, group them into a
single @dfn{compound statement} with curly braces, separating them with
newlines or semicolons.

@menu
* If Statement::                Conditionally execute some @command{awk}
                                statements.
* While Statement::             Loop until some condition is satisfied.
* Do Statement::                Do specified action while looping until some
                                condition is satisfied.
* For Statement::               Another looping statement, that provides
                                initialization and increment clauses.
* Break Statement::             Immediately exit the innermost enclosing loop.
* Continue Statement::          Skip to the end of the innermost enclosing
                                loop.
* Next Statement::              Stop processing the current input record.
* Nextfile Statement::          Stop processing the current file.
* Exit Statement::              Stop execution of @command{awk}.
@end menu

@node If Statement, While Statement, Statements, Statements
@subsection The @code{if}-@code{else} Statement

@cindex @code{if}-@code{else} statement
The @code{if}-@code{else} statement is @command{awk}'s decision-making
statement.  It looks like this:

@example
if (@var{condition}) @var{then-body} @r{[}else @var{else-body}@r{]}
@end example

@noindent
The @var{condition} is an expression that controls what the rest of the
statement does.  If the @var{condition} is true, @var{then-body} is
executed; otherwise, @var{else-body} is executed.
The @code{else} part of the statement is
optional.  The condition is considered false if its value is zero or
the null string; otherwise the condition is true.
Refer to the following:

@example
if (x % 2 == 0)
    print "x is even"
else
    print "x is odd"
@end example

In this example, if the expression @samp{x % 2 == 0} is true (that is,
if the value of @code{x} is evenly divisible by two), then the first
@code{print} statement is executed; otherwise the second @code{print}
statement is executed.
If the @code{else} keyword appears on the same line as @var{then-body} and
@var{then-body} is not a compound statement (i.e., not surrounded by
curly braces), then a semicolon must separate @var{then-body} from
the @code{else}.
To illustrate this, the previous example can be rewritten as:

@example
if (x % 2 == 0) print "x is even"; else
        print "x is odd"
@end example

@noindent
If the @samp{;} is left out, @command{awk} can't interpret the statement and
it produces a syntax error.  Don't actually write programs this way,
because a human reader might fail to see the @code{else} if it is not
the first thing on its line.

@node While Statement, Do Statement, If Statement, Statements
@subsection The @code{while} Statement
@cindex @code{while} statement
@cindex loop
@cindex body of a loop

In programming, a @dfn{loop} is a part of a program that can
be executed two or more times in succession.
The @code{while} statement is the simplest looping statement in
@command{awk}.  It repeatedly executes a statement as long as a condition is
true.  For example:

@example
while (@var{condition})
  @var{body}
@end example

@noindent
@var{body} is a statement called the @dfn{body} of the loop,
and @var{condition} is an expression that controls how long the loop
keeps running.
The first thing the @code{while} statement does is test the @var{condition}.
If the @var{condition} is true, it executes the statement @var{body}.
@ifinfo
(The @var{condition} is true when the value
is not zero and not a null string.)
@end ifinfo
After @var{body} has been executed,
@var{condition} is tested again, and if it is still true, @var{body} is
executed again.  This process repeats until the @var{condition} is no longer
true.  If the @var{condition} is initially false, the body of the loop is
never executed and @command{awk} continues with the statement following
the loop.
This example prints the first three fields of each record, one per line:

@example
awk '@{ i = 1
       while (i <= 3) @{
           print $i
           i++
       @}
@}' inventory-shipped
@end example

@noindent
The body of this loop is a compound statement enclosed in braces,
containing two statements.
The loop works in the following manner: first, the value of @code{i} is set to one.
Then, the @code{while} statement tests whether @code{i} is less than or equal to
three.  This is true when @code{i} equals one, so the @code{i}-th
field is printed.  Then the @samp{i++} increments the value of @code{i}
and the loop repeats.  The loop terminates when @code{i} reaches four.

A newline is not required between the condition and the
body; however using one makes the program clearer unless the body is a
compound statement or else is very simple.  The newline after the open-brace
that begins the compound statement is not required either, but the
program is harder to read without it.

@node Do Statement, For Statement, While Statement, Statements
@subsection The @code{do}-@code{while} Statement
@cindex @code{do}-@code{while} statement

The @code{do} loop is a variation of the @code{while} looping statement.
The @code{do} loop executes the @var{body} once and then repeats the
@var{body} as long as the @var{condition} is true.  It looks like this:

@example
do
  @var{body}
while (@var{condition})
@end example

Even if the @var{condition} is false at the start, the @var{body} is
executed at least once (and only once, unless executing @var{body}
makes @var{condition} true).  Contrast this with the corresponding
@code{while} statement:

@example
while (@var{condition})
  @var{body}
@end example

@noindent
This statement does not execute @var{body} even once if the @var{condition}
is false to begin with.
The following is an example of a @code{do} statement:

@example
@{      i = 1
       do @{
          print $0
          i++
       @} while (i <= 10)
@}
@end example

@noindent
This program prints each input record ten times.  However, it isn't a very
realistic example, since in this case an ordinary @code{while} would do
just as well.  This situation reflects actual experience; only
occasionally is there a real use for a @code{do} statement.

@node For Statement, Break Statement, Do Statement, Statements
@subsection The @code{for} Statement
@cindex @code{for} statement

The @code{for} statement makes it more convenient to count iterations of a
loop.  The general form of the @code{for} statement looks like this:

@example
for (@var{initialization}; @var{condition}; @var{increment})
  @var{body}
@end example

@noindent
The @var{initialization}, @var{condition}, and @var{increment} parts are
arbitrary @command{awk} expressions, and @var{body} stands for any
@command{awk} statement.

The @code{for} statement starts by executing @var{initialization}.
Then, as long
as the @var{condition} is true, it repeatedly executes @var{body} and then
@var{increment}.  Typically, @var{initialization} sets a variable to
either zero or one, @var{increment} adds one to it, and @var{condition}
compares it against the desired number of iterations.
For example:

@example
awk '@{ for (i = 1; i <= 3; i++)
          print $i
@}' inventory-shipped
@end example

@noindent
This prints the first three fields of each input record, with one field per
line.

It isn't possible to
set more than one variable in the
@var{initialization} part without using a multiple assignment statement
such as @samp{x = y = 0}. This makes sense only if all the initial values
are equal.  (But it is possible to initialize additional variables by writing
their assignments as separate statements preceding the @code{for} loop.)

@cindex comma operator, not supported
The same is true of the @var{increment} part. Incrementing additional
variables requires separate statements at the end of the loop.
The C compound expression, using C's comma operator, is useful in
this context but it is not supported in @command{awk}.

Most often, @var{increment} is an increment expression, as in the previous
example.  But this is not required; it can be any expression
whatsoever.  For example, the following statement prints all the powers of two
between 1 and 100:

@example
for (i = 1; i <= 100; i *= 2)
  print i
@end example

If there is nothing to be done, any of the three expressions in the
parentheses following the @code{for} keyword may be omitted.  Thus,
@w{@samp{for (; x > 0;)}} is equivalent to @w{@samp{while (x > 0)}}.  If the
@var{condition} is omitted, it is treated as true, effectively
yielding an @dfn{infinite loop} (i.e., a loop that never terminates).

In most cases, a @code{for} loop is an abbreviation for a @code{while}
loop, as shown here:

@example
@var{initialization}
while (@var{condition}) @{
  @var{body}
  @var{increment}
@}
@end example

@noindent
The only exception is when the @code{continue} statement
(@pxref{Continue Statement, ,The @code{continue} Statement}) is used
inside the loop. Changing a @code{for} statement to a @code{while}
statement in this way can change the effect of the @code{continue}
statement inside the loop.

The @command{awk} language has a @code{for} statement in addition to a
@code{while} statement because a @code{for} loop is often both less work to
type and more natural to think of.  Counting the number of iterations is
very common in loops.  It can be easier to think of this counting as part
of looping rather than as something to do inside the loop.

@ifinfo
@cindex @code{in} operator
There is an alternate version of the @code{for} loop, for iterating over
all the indices of an array:

@example
for (i in array)
    @var{do something with} array[i]
@end example

@noindent
@xref{Scanning an Array, ,Scanning All Elements of an Array},
for more information on this version of the @code{for} loop.
@end ifinfo

@node Break Statement, Continue Statement, For Statement, Statements
@subsection The @code{break} Statement
@cindex @code{break} statement
@cindex loops, exiting

The @code{break} statement jumps out of the innermost @code{for},
@code{while}, or @code{do} loop that encloses it.  The following example
finds the smallest divisor of any integer, and also identifies prime
numbers:

@example
# find smallest divisor of num
@{
   num = $1
   for (div = 2; div*div <= num; div++)
     if (num % div == 0)
       break
   if (num % div == 0)
     printf "Smallest divisor of %d is %d\n", num, div
   else
     printf "%d is prime\n", num
@}
@end example

When the remainder is zero in the first @code{if} statement, @command{awk}
immediately @dfn{breaks out} of the containing @code{for} loop.  This means
that @command{awk} proceeds immediately to the statement following the loop
and continues processing.  (This is very different from the @code{exit}
statement, which stops the entire @command{awk} program.
@xref{Exit Statement, ,The @code{exit} Statement}.)

Th following program illustrates how the @var{condition} of a @code{for}
or @code{while} statement could be replaced with a @code{break} inside
an @code{if}:

@example
# find smallest divisor of num
@{
  num = $1
  for (div = 2; ; div++) @{
    if (num % div == 0) @{
      printf "Smallest divisor of %d is %d\n", num, div
      break
    @}
    if (div*div > num) @{
      printf "%d is prime\n", num
      break
    @}
  @}
@}
@end example

@cindex @code{break}, outside of loops
@cindex historical features
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@cindex dark corner
The @code{break} statement has no meaning when
used outside the body of a loop.  However, although it was never documented,
historical implementations of @command{awk} treated the @code{break}
statement outside of a loop as if it were a @code{next} statement
(@pxref{Next Statement, ,The @code{next} Statement}).
Recent versions of Unix @command{awk} no longer allow this usage.
@command{gawk} supports this use of @code{break} only
if @option{--traditional}
has been specified on the command line
(@pxref{Options, ,Command-Line Options}).
Otherwise, it is treated as an error, since the POSIX standard
specifies that @code{break} should only be used inside the body of a
loop.
@value{DARKCORNER}

@node Continue Statement, Next Statement, Break Statement, Statements
@subsection The @code{continue} Statement

@cindex @code{continue} statement
As with @code{break}, the @code{continue} statement is used only inside
@code{for}, @code{while}, and @code{do} loops.  It skips
over the rest of the loop body, causing the next cycle around the loop
to begin immediately.  Contrast this with @code{break}, which jumps out
of the loop altogether.

The @code{continue} statement in a @code{for} loop directs @command{awk} to
skip the rest of the body of the loop and resume execution with the
increment-expression of the @code{for} statement.  The following program
illustrates this fact:

@example
BEGIN @{
     for (x = 0; x <= 20; x++) @{
         if (x == 5)
             continue
         printf "%d ", x
     @}
     print ""
@}
@end example

@noindent
This program prints all the numbers from 0 to 20---except for five, for
which the @code{printf} is skipped.  Because the increment @samp{x++}
is not skipped, @code{x} does not remain stuck at five.  Contrast the
@code{for} loop from the previous example with the following @code{while} loop:

@example
BEGIN @{
     x = 0
     while (x <= 20) @{
         if (x == 5)
             continue
         printf "%d ", x
         x++
     @}
     print ""
@}
@end example

@noindent
This program loops forever once @code{x} reaches five.

@cindex @code{continue}, outside of loops
@cindex historical features
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@cindex dark corner
The @code{continue} statement has no meaning when used outside the body of
a loop.  Historical versions of @command{awk} treated a @code{continue}
statement outside a loop the same way they treated a @code{break}
statement outside a loop: as if it were a @code{next}
statement
(@pxref{Next Statement, ,The @code{next} Statement}).
Recent versions of Unix @command{awk} no longer work this way, and
@command{gawk} allows it only if @option{--traditional} is specified on
the command line (@pxref{Options, ,Command-Line Options}).  Just like the
@code{break} statement, the POSIX standard specifies that @code{continue}
should only be used inside the body of a loop.
@value{DARKCORNER}

@node Next Statement, Nextfile Statement, Continue Statement, Statements
@subsection The @code{next} Statement
@cindex @code{next} statement

The @code{next} statement forces @command{awk} to immediately stop processing
the current record and go on to the next record.  This means that no
further rules are executed for the current record, and the rest of the
current rule's action isn't executed.

Contrast this with the effect of the @code{getline} function
(@pxref{Getline, ,Explicit Input with @code{getline}}).  That also causes
@command{awk} to read the next record immediately, but it does not alter the
flow of control in any way (i.e., the rest of the current action executes
with a new input record).

At the highest level, @command{awk} program execution is a loop that reads
an input record and then tests each rule's pattern against it.  If you
think of this loop as a @code{for} statement whose body contains the
rules, then the @code{next} statement is analogous to a @code{continue}
statement. It skips to the end of the body of this implicit loop and
executes the increment (which reads another record).

For example, suppose an @command{awk} program works only on records
with four fields, and it shouldn't fail when given bad input.  To avoid
complicating the rest of the program, write a ``weed out'' rule near
the beginning, in the following manner:

@example
NF != 4 @{
  err = sprintf("%s:%d: skipped: NF != 4\n", FILENAME, FNR)
  print err > "/dev/stderr"
  next
@}
@end example

@noindent
Because of the @code{next} statement,
the program's subsequent rules won't see the bad record.  The error
message is redirected to the standard error output stream, as error
messages should be.
@xref{Special Files, ,Special @value{FFN}s in @command{gawk}}.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@cindex @code{next}, inside a user-defined function
According to the POSIX standard, the behavior is undefined if
the @code{next} statement is used in a @code{BEGIN} or @code{END} rule.
@command{gawk} treats it as a syntax error.
Although POSIX permits it,
some other @command{awk} implementations don't allow the @code{next}
statement inside function bodies
(@pxref{User-defined, ,User-Defined Functions}).
Just as with any other @code{next} statement, a @code{next} statement inside a
function body reads the next record and starts processing it with the
first rule in the program.
If the @code{next} statement causes the end of the input to be reached,
then the code in any @code{END} rules is executed.
@xref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}.

@node Nextfile Statement, Exit Statement, Next Statement, Statements
@subsection Using @command{gawk}'s @code{nextfile} Statement
@cindex @code{nextfile} statement
@cindex differences between @command{gawk} and @command{awk}

@command{gawk} provides the @code{nextfile} statement,
which is similar to the @code{next} statement.
However, instead of abandoning processing of the current record, the
@code{nextfile} statement instructs @command{gawk} to stop processing the
current @value{DF}.

The @code{nextfile} statement is a @command{gawk} extension.
In most other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
@code{nextfile} is not special.

Upon execution of the @code{nextfile} statement, @code{FILENAME} is
updated to the name of the next @value{DF} listed on the command line,
@code{FNR} is reset to one, @code{ARGIND} is incremented, and processing
starts over with the first rule in the program.
(@code{ARGIND} hasn't been introduced yet. @xref{Built-in Variables}.)
If the @code{nextfile} statement causes the end of the input to be reached,
then the code in any @code{END} rules is executed.
@xref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}.

The @code{nextfile} statement is useful when there are many @value{DF}s
to process but it isn't necessary to process every record in every file.
Normally, in order to move on to the next @value{DF}, a program
has to continue scanning the unwanted records.  The @code{nextfile}
statement accomplishes this much more efficiently.

While one might think that @samp{close(FILENAME)} would accomplish
the same as @code{nextfile}, this isn't true.  @code{close} is
reserved for closing files, pipes, and coprocesses that are
opened with redirections.  It is not related to the main processing that
@command{awk} does with the files listed in @code{ARGV}.

If it's necessary to use an @command{awk} version that doesn't support
@code{nextfile}, see
@ref{Nextfile Function, ,Implementing @code{nextfile} as a Function},
for a user-defined function that simulates the @code{nextfile}
statement.

@cindex @code{nextfile}, inside a user-defined function
The current version of the Bell Laboratories @command{awk}
(@pxref{Other Versions, ,Other Freely Available @command{awk} Implementations})
also supports @code{nextfile}.  However, it doesn't allow the @code{nextfile}
statement inside function bodies
(@pxref{User-defined, ,User-Defined Functions}).
@command{gawk} does; a @code{nextfile} inside a
function body reads the next record and starts processing it with the
first rule in the program, just as any other @code{nextfile} statement.

@cindex @code{next file} statement
@strong{Caution:}  Versions of @command{gawk} prior to 3.0 used two
words (@samp{next file}) for the @code{nextfile} statement.
In @value{PVERSION} 3.0, this was changed
to one word, because the treatment of @samp{file} was
inconsistent. When it appeared after @code{next}, @samp{file} was a keyword;
otherwise, it was a regular identifier.  The old usage is no longer
accepted; @samp{next file} generates a syntax error.

@node Exit Statement, , Nextfile Statement, Statements
@subsection The @code{exit} Statement

@cindex @code{exit} statement
The @code{exit} statement causes @command{awk} to immediately stop
executing the current rule and to stop processing input; any remaining input
is ignored.  The @code{exit} statement is written as follows:

@example
exit @r{[}@var{return code}@r{]}
@end example

When an @code{exit} statement is executed from a @code{BEGIN} rule, the
program stops processing everything immediately.  No input records are
read.  However, if an @code{END} rule is present,
as part of executing the @code{exit} statement,
the @code{END} rule is executed
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).
If @code{exit} is used as part of an @code{END} rule, it causes
the program to stop immediately.

An @code{exit} statement that is not part of a @code{BEGIN} or @code{END}
rule stops the execution of any further automatic rules for the current
record, skips reading any remaining input records, and executes the
@code{END} rule if there is one.

In such a case,
if you don't want the @code{END} rule to do its job, set a variable
to nonzero before the @code{exit} statement and check that variable in
the @code{END} rule.
@xref{Assert Function, ,Assertions},
for an example that does this.

@cindex dark corner
If an argument is supplied to @code{exit}, its value is used as the exit
status code for the @command{awk} process.  If no argument is supplied,
@code{exit} returns status zero (success).  In the case where an argument
is supplied to a first @code{exit} statement, and then @code{exit} is
called a second time from an @code{END} rule with no argument,
@command{awk} uses the previously supplied exit value.
@value{DARKCORNER}

@cindex conventions, programming
@cindex programming conventions
For example, suppose an error condition occurs that is difficult or
impossible to handle.  Conventionally, programs report this by
exiting with a nonzero status.  An @command{awk} program can do this
using an @code{exit} statement with a nonzero argument, as shown
in the following example:

@example
BEGIN @{
       if (("date" | getline date_now) <= 0) @{
         print "Can't get system date" > "/dev/stderr"
         exit 1
       @}
       print "current date is", date_now
       close("date")
@}
@end example

@node Built-in Variables, , Statements, Patterns and Actions
@section Built-in Variables
@cindex built-in variables

Most @command{awk} variables are available for you to use for your own
purposes; they never change unless your program assigns values to
them, and they never affect anything unless your program examines them.
However, a few variables in @command{awk} have special built-in meanings.
@command{awk} examines some of these automatically, so that they enable you
to tell @command{awk} how to do certain things.  Others are set
automatically by @command{awk}, so that they carry information from the
internal workings of @command{awk} to your program.

This @value{SECTION} documents all the built-in variables of
@command{gawk}, most of which are also documented in the chapters
describing their areas of activity.

@menu
* User-modified::               Built-in variables that you change to control
                                @command{awk}.
* Auto-set::                    Built-in variables where @command{awk} gives
                                you information.
* ARGC and ARGV::               Ways to use @code{ARGC} and @code{ARGV}.
@end menu

@node User-modified, Auto-set, Built-in Variables, Built-in Variables
@subsection Built-in Variables That Control @command{awk}
@cindex built-in variables, user modifiable

The following is an alphabetical list of variables that you can change to
control how @command{awk} does certain things. The variables that are
specific to @command{gawk} are marked with a pound sign (@samp{#}).

@table @code
@cindex @code{BINMODE} variable
@cindex binary I/O
@cindex I/O, binary
@cindex differences between @command{gawk} and @command{awk}
@item BINMODE #
On non-POSIX systems, this variable specifies use of ``binary'' mode for all I/O.
Numeric values of one, two, or three, specify that input files, output files, or
all files, respectively, should use binary I/O.
Alternatively,
string values of @code{"r"} or @code{"w"} specify that input files and
output files, respectively, should use binary I/O.
A string value of @code{"rw"} or @code{"wr"} indicates that all
files should use binary I/O.
Any other string value is equivalent to @code{"rw"}, but @command{gawk}
generates a warning message.
@code{BINMODE} is described in more detail in
@ref{PC Using, ,Using @command{gawk} on PC Operating Systems}.

This variable is a @command{gawk} extension.
In other @command{awk} implementations
(except @command{mawk},
@pxref{Other Versions, , Other Freely Available @command{awk} Implementations}),
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
it is not special.

@cindex @code{CONVFMT} variable
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@item CONVFMT
This string controls conversion of numbers to
strings (@pxref{Conversion, ,Conversion of Strings and Numbers}).
It works by being passed, in effect, as the first argument to the
@code{sprintf} function
(@pxref{String Functions, ,String Manipulation Functions}).
Its default value is @code{"%.6g"}.
@code{CONVFMT} was introduced by the POSIX standard.

@cindex @code{FIELDWIDTHS} variable
@item FIELDWIDTHS #
This is a space-separated list of columns that tells @command{gawk}
how to split input with fixed columnar boundaries.
Assigning a value to @code{FIELDWIDTHS}
overrides the use of @code{FS} for field splitting.
@xref{Constant Size, ,Reading Fixed-Width Data}, for more information.

If @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}), then @code{FIELDWIDTHS}
has no special meaning, and field-splitting operations occur based
exclusively on the value of @code{FS}.

@cindex @code{FS} variable
@item FS
This is the input field separator
(@pxref{Field Separators, ,Specifying How Fields Are Separated}).
The value is a single-character string or a multi-character regular
expression that matches the separations between fields in an input
record.  If the value is the null string (@code{""}), then each
character in the record becomes a separate field.
(This behavior is a @command{gawk} extension. POSIX @command{awk} does not
specify the behavior when @code{FS} is the null string.)
@c NEXT ED: Mark as common extension

The default value is @w{@code{" "}}, a string consisting of a single
space.  As a special exception, this value means that any
sequence of spaces, tabs, and/or newlines is a single separator.@footnote{In
POSIX @command{awk}, newline does not count as whitespace.}  It also causes
spaces, tabs, and newlines at the beginning and end of a record to be ignored.

You can set the value of @code{FS} on the command line using the
@option{-F} option:

@example
awk -F, '@var{program}' @var{input-files}
@end example

If @command{gawk} is using @code{FIELDWIDTHS} for field splitting,
assigning a value to @code{FS} causes @command{gawk} to return to
the normal, @code{FS}-based field splitting. An easy way to do this
is to simply say @samp{FS = FS}, perhaps with an explanatory comment.

@cindex @code{IGNORECASE} variable
@item IGNORECASE #
If @code{IGNORECASE} is nonzero or non-null, then all string comparisons
and all regular expression matching are case-independent.  Thus, regexp
matching with @samp{~} and @samp{!~}, as well as the @code{gensub},
@code{gsub}, @code{index}, @code{match}, @code{split}, and @code{sub}
functions, record termination with @code{RS}, and field splitting with
@code{FS}, all ignore case when doing their particular regexp operations.
However, the value of @code{IGNORECASE} does @emph{not} affect array subscripting.
@xref{Case-sensitivity, ,Case Sensitivity in Matching}.

If @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
then @code{IGNORECASE} has no special meaning.  Thus, string
and regexp operations are always case-sensitive.

@cindex @code{LINT} variable
@cindex differences between @command{gawk} and @command{awk}
@cindex lint checks
@item LINT #
When this variable is true (nonzero or non-null), @command{gawk}
behaves as if the @option{--lint} command-line option is in effect.
(@pxref{Options, ,Command-Line Options}).
With a value of @code{"fatal"}, lint warnings become fatal errors.
Any other true value prints non-fatal warnings.
Assigning a false value to @code{LINT} turns off the lint warnings.

This variable is a @command{gawk} extension.  It is not special
in other @command{awk} implementations.  Unlike the other special variables,
changing @code{LINT} does affect the production of lint warnings,
even if @command{gawk} is in compatibility mode.  Much as
the @option{--lint} and @option{--traditional} options independently
control different aspects of @command{gawk}'s behavior, the control
of lint warnings during program execution is independent of the flavor
of @command{awk} being executed.

@cindex @code{OFMT} variable
@item OFMT
This string controls conversion of numbers to
strings (@pxref{Conversion, ,Conversion of Strings and Numbers}) for
printing with the @code{print} statement.  It works by being passed
as the first argument to the @code{sprintf} function
(@pxref{String Functions, ,String Manipulation Functions}).
Its default value is @code{"%.6g"}.  Earlier versions of @command{awk}
also used @code{OFMT} to specify the format for converting numbers to
strings in general expressions; this is now done by @code{CONVFMT}.

@cindex @code{OFS} variable
@item OFS
This is the output field separator (@pxref{Output Separators}).  It is
output between the fields printed by a @code{print} statement.  Its
default value is @w{@code{" "}}, a string consisting of a single space.

@cindex @code{ORS} variable
@item ORS
This is the output record separator.  It is output at the end of every
@code{print} statement.  Its default value is @code{"\n"}, the newline
character.  (@xref{Output Separators}.)

@cindex @code{RS} variable
@item RS
This is @command{awk}'s input record separator.  Its default value is a string
containing a single newline character, which means that an input record
consists of a single line of text.
It can also be the null string, in which case records are separated by
runs of blank lines.
If it is a regexp, records are separated by
matches of the regexp in the input text.
(@xref{Records, ,How Input Is Split into Records}.)

The ability for @code{RS} to be a regular expression
is a @command{gawk} extension.
In most other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
just the first character of @code{RS}'s value is used.

@cindex @code{SUBSEP} variable
@item SUBSEP
This is the subscript separator.  It has the default value of
@code{"\034"} and is used to separate the parts of the indices of a
multidimensional array.  Thus, the expression @code{@w{foo["A", "B"]}}
really accesses @code{foo["A\034B"]}
(@pxref{Multi-dimensional, ,Multidimensional Arrays}).

@cindex @code{TEXTDOMAIN} variable
@cindex internationalization
@item TEXTDOMAIN #
This variable is used for internationalization of programs at the
@command{awk} level.  It sets the default text domain for specially
marked string constants in the source text, as well as for the
@code{dcgettext} and @code{bindtextdomain} functions
(@pxref{Internationalization, ,Internationalization with @command{gawk}}).
The default value of @code{TEXTDOMAIN} is @code{"messages"}.

This variable is a @command{gawk} extension.
In other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
it is not special.
@end table

@node Auto-set, ARGC and ARGV, User-modified, Built-in Variables
@subsection Built-in Variables That Convey Information
@cindex built-in variables, convey information

The following is an alphabetical list of variables that @command{awk}
sets automatically on certain occasions in order to provide
information to your program.  The variables that are specific to
@command{gawk} are marked with an asterisk (@samp{*}).

@table @code
@cindex @code{ARGC} variable
@cindex @code{ARGV} variable
@item ARGC@r{,} ARGV
The command-line arguments available to @command{awk} programs are stored in
an array called @code{ARGV}.  @code{ARGC} is the number of command-line
arguments present.  @xref{Other Arguments, ,Other Command-Line Arguments}.
Unlike most @command{awk} arrays,
@code{ARGV} is indexed from 0 to @code{ARGC} @minus{} 1.
In the following example:

@example
$ awk 'BEGIN @{
>         for (i = 0; i < ARGC; i++)
>             print ARGV[i]
>      @}' inventory-shipped BBS-list
@print{} awk
@print{} inventory-shipped
@print{} BBS-list
@end example

@noindent
@code{ARGV[0]} contains @code{"awk"}, @code{ARGV[1]}
contains @code{"inventory-shipped"} and @code{ARGV[2]} contains
@code{"BBS-list"}.  The value of @code{ARGC} is three, one more than the
index of the last element in @code{ARGV}, because the elements are numbered
from zero.

@cindex conventions, programming
@cindex programming conventions
The names @code{ARGC} and @code{ARGV}, as well as the convention of indexing
the array from 0 to @code{ARGC} @minus{} 1, are derived from the C language's
method of accessing command-line arguments.

The value of @code{ARGV[0]} can vary from system to system.
Also, you should note that the program text is @emph{not} included in
@code{ARGV}, nor are any of @command{awk}'s command-line options.
@xref{ARGC and ARGV, , Using @code{ARGC} and @code{ARGV}}, for information
about how @command{awk} uses these variables.

@cindex @code{ARGIND} variable
@item ARGIND #
This is the index in @code{ARGV} of the current file being processed.
Every time @command{gawk} opens a new @value{DF} for processing, it sets
@code{ARGIND} to the index in @code{ARGV} of the @value{FN}.
When @command{gawk} is processing the input files,
@samp{FILENAME == ARGV[ARGIND]} is always true.

This variable is useful in file processing; it allows you to tell how far
along you are in the list of @value{DF}s as well as to distinguish between
successive instances of the same @value{FN} on the command line.

While you can change the value of @code{ARGIND} within your @command{awk}
program, @command{gawk} automatically sets it to a new value when the
next file is opened.

This variable is a @command{gawk} extension.
In other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
it is not special.

@cindex @code{ENVIRON} variable
@item ENVIRON
An associative array that contains the values of the environment.  The array
indices are the environment variable names; the elements are the values of
the particular environment variables.  For example,
@code{ENVIRON["HOME"]} might be @file{/home/arnold}.  Changing this array
does not affect the environment passed on to any programs that
@command{awk} may spawn via redirection or the @code{system} function.
@c (In a future version of @command{gawk}, it may do so.)

Some operating systems may not have environment variables.
On such systems, the @code{ENVIRON} array is empty (except for
@w{@code{ENVIRON["AWKPATH"]}},
@pxref{AWKPATH Variable, ,The @env{AWKPATH} Environment Variable}).

@cindex @code{ERRNO} variable
@item ERRNO #
If a system error occurs during a redirection for @code{getline},
during a read for @code{getline}, or during a @code{close} operation,
then @code{ERRNO} contains a string describing the error.

This variable is a @command{gawk} extension.
In other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
it is not special.

@cindex dark corner
@cindex @code{FILENAME} variable
@item FILENAME
This is the name of the file that @command{awk} is currently reading.
When no @value{DF}s are listed on the command line, @command{awk} reads
from the standard input and @code{FILENAME} is set to @code{"-"}.
@code{FILENAME} is changed each time a new file is read
(@pxref{Reading Files, ,Reading Input Files}).
Inside a @code{BEGIN} rule, the value of @code{FILENAME} is
@code{""}, since there are no input files being processed
yet.@footnote{Some early implementations of Unix @command{awk} initialized
@code{FILENAME} to @code{"-"}, even if there were @value{DF}s to be
processed. This behavior was incorrect and should not be relied
upon in your programs.}
@value{DARKCORNER}
Note though, that using @code{getline}
(@pxref{Getline, ,Explicit Input with @code{getline}})
inside a @code{BEGIN} rule can give
@code{FILENAME} a value.

@cindex @code{FNR} variable
@item FNR
This is the current record number in the current file.  @code{FNR} is
incremented each time a new record is read
(@pxref{Getline, ,Explicit Input with @code{getline}}).  It is reinitialized
to zero each time a new input file is started.

@cindex @code{NF} variable
@item NF
This is the number of fields in the current input record.
@code{NF} is set each time a new record is read, when a new field is
created or when @code{$0} changes (@pxref{Fields, ,Examining Fields}).

@cindex @code{NR} variable
@item NR
This is the number of input records @command{awk} has processed since
the beginning of the program's execution
(@pxref{Records, ,How Input Is Split into Records}).
@code{NR} is incremented each time a new record is read.

@cindex @code{PROCINFO} variable
@item PROCINFO #
The elements of this array provide access to information about the
running @command{awk} program.
The following elements (listed alphabetically)
are guaranteed to be available:

@table @code
@item PROCINFO["egid"]
The value of the @code{getegid} system call.

@item PROCINFO["euid"]
The value of the @code{geteuid} system call.

@item PROCINFO["FS"]
This is
@code{"FS"} if field splitting with @code{FS} is in effect, or it is
@code{"FIELDWIDTHS"} if field splitting with @code{FIELDWIDTHS} is in effect.

@item PROCINFO["gid"]
The value of the @code{getgid} system call.

@item PROCINFO["pgrpid"]
The process group ID of the current process.

@item PROCINFO["pid"]
The process ID of the current process.

@item PROCINFO["ppid"]
The parent process ID of the current process.

@item PROCINFO["uid"]
The value of the @code{getuid} system call.
@end table

On some systems, there may be elements in the array, @code{"group1"}
through @code{"group@var{N}"} for some @var{N}. @var{N} is the number of
supplementary groups that the process has.  Use the @code{in} operator
to test for these elements
(@pxref{Reference to Elements, , Referring to an Array Element}).

This array is a @command{gawk} extension.
In other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
it is not special.

@cindex @code{RLENGTH} variable
@item RLENGTH
This is the length of the substring matched by the
@code{match} function
(@pxref{String Functions, ,String Manipulation Functions}).
@code{RLENGTH} is set by invoking the @code{match} function.  Its value
is the length of the matched string, or @minus{}1 if no match is found.

@cindex @code{RSTART} variable
@item RSTART
This is the start-index in characters of the substring that is matched by the
@code{match} function
(@pxref{String Functions, ,String Manipulation Functions}).
@code{RSTART} is set by invoking the @code{match} function.  Its value
is the position of the string where the matched substring starts, or zero
if no match was found.

@cindex @code{RT} variable
@item RT #
This is set each time a record is read. It contains the input text
that matched the text denoted by @code{RS}, the record separator.

This variable is a @command{gawk} extension.
In other @command{awk} implementations,
or if @command{gawk} is in compatibility mode
(@pxref{Options, ,Command-Line Options}),
it is not special.
@end table

@c fakenode --- for prepinfo
@subheading Advanced Notes: Changing @code{NR} and @code{FNR}
@cindex advanced notes
@cindex dark corner
@command{awk} increments @code{NR} and @code{FNR}
each time it reads a record, instead of setting them to the absolute
value of the number of records read.  This means that a program can
change these variables and their new values are incremented for
each record.
@value{DARKCORNER}
This is demonstrated in the following example:

@example
$ echo '1
> 2
> 3
> 4' | awk 'NR == 2 @{ NR = 17 @}
> @{ print NR @}'
@print{} 1
@print{} 17
@print{} 18
@print{} 19
@end example

@noindent
Before @code{FNR} was added to the @command{awk} language
(@pxref{V7/SVR3.1, ,Major Changes Between V7 and SVR3.1}),
many @command{awk} programs used this feature to track the number of
records in a file by resetting @code{NR} to zero when @code{FILENAME}
changed.

@node ARGC and ARGV, , Auto-set, Built-in Variables
@subsection Using @code{ARGC} and @code{ARGV}

@ref{Auto-set, ,Built-in Variables That Convey Information},
presented the following program describing the information contained in @code{ARGC}
and @code{ARGV}:

@example
$ awk 'BEGIN @{
>        for (i = 0; i < ARGC; i++)
>            print ARGV[i]
>      @}' inventory-shipped BBS-list
@print{} awk
@print{} inventory-shipped
@print{} BBS-list
@end example

@noindent
In this example, @code{ARGV[0]} contains @samp{awk}, @code{ARGV[1]}
contains @samp{inventory-shipped}, and @code{ARGV[2]} contains
@samp{BBS-list}.
Notice that the @command{awk} program is not entered in @code{ARGV}.  The
other special command-line options, with their arguments, are also not
entered.  This includes variable assignments done with the @option{-v}
option (@pxref{Options, ,Command-Line Options}).
Normal variable assignments on the command line @emph{are}
treated as arguments and do show up in the @code{ARGV} array:

@example
$ cat showargs.awk
@print{} BEGIN @{
@print{}     printf "A=%d, B=%d\n", A, B
@print{}     for (i = 0; i < ARGC; i++)
@print{}         printf "\tARGV[%d] = %s\n", i, ARGV[i]
@print{} @}
@print{} END   @{ printf "A=%d, B=%d\n", A, B @}
$ awk -v A=1 -f showargs.awk B=2 /dev/null
@print{} A=1, B=0
@print{}        ARGV[0] = awk
@print{}        ARGV[1] = B=2
@print{}        ARGV[2] = /dev/null
@print{} A=1, B=2
@end example

A program can alter @code{ARGC} and the elements of @code{ARGV}.
Each time @command{awk} reaches the end of an input file, it uses the next
element of @code{ARGV} as the name of the next input file.  By storing a
different string there, a program can change which files are read.
Use @code{"-"} to represent the standard input.  Storing
additional elements and incrementing @code{ARGC} causes
additional files to be read.

If the value of @code{ARGC} is decreased, that eliminates input files
from the end of the list.  By recording the old value of @code{ARGC}
elsewhere, a program can treat the eliminated arguments as
something other than @value{FN}s.

To eliminate a file from the middle of the list, store the null string
(@code{""}) into @code{ARGV} in place of the file's name.  As a
special feature, @command{awk} ignores @value{FN}s that have been
replaced with the null string.
Another option is to
use the @code{delete} statement to remove elements from
@code{ARGV} (@pxref{Delete, ,The @code{delete} Statement}).

All of these actions are typically done in the @code{BEGIN} rule,
before actual processing of the input begins.
@xref{Split Program, ,Splitting a Large File into Pieces}, and see
@ref{Tee Program, ,Duplicating Output into Multiple Files}, for examples
of each way of removing elements from @code{ARGV}.
The following fragment processes @code{ARGV} in order to examine, and
then remove, command-line options:
@c NEXT ED: Add xref to rewind() function

@example
BEGIN @{
    for (i = 1; i < ARGC; i++) @{
        if (ARGV[i] == "-v")
            verbose = 1
        else if (ARGV[i] == "-d")
            debug = 1
        else if (ARGV[i] ~ /^-?/) @{
            e = sprintf("%s: unrecognized option -- %c",
                    ARGV[0], substr(ARGV[i], 1, ,1))
            print e > "/dev/stderr"
        @} else
            break
        delete ARGV[i]
    @}
@}
@end example

To actually get the options into the @command{awk} program,
end the @command{awk} options with @option{--} and then supply
the @command{awk} program's options, in the following manner:

@example
awk -f myprog -- -v -d file1 file2 @dots{}
@end example

@cindex differences between @command{gawk} and @command{awk}
This is not necessary in @command{gawk}. Unless @option{--posix} has
been specified, @command{gawk} silently puts any unrecognized options
into @code{ARGV} for the @command{awk} program to deal with.  As soon
as it sees an unknown option, @command{gawk} stops looking for other
options that it might otherwise recognize.  The previous example with
@command{gawk} would be:

@example
gawk -f myprog -d -v file1 file2 @dots{}
@end example

@noindent
Because @option{-d} is not a valid @command{gawk} option,
it and the following @option{-v}
are passed on to the @command{awk} program.

@node Arrays, Functions, Patterns and Actions, Top
@chapter Arrays in @command{awk}

An @dfn{array} is a table of values called @dfn{elements}.  The
elements of an array are distinguished by their indices.  @dfn{Indices}
may be either numbers or strings.

This @value{CHAPTER} describes how arrays work in @command{awk},
how to use array elements, how to scan through every element in an array,
and how to remove array elements.
It also describes how @command{awk} simulates multidimensional
arrays, as well as some of the less obvious points about array usage.
The @value{CHAPTER} finishes with a discussion of @command{gawk}'s facility
for sorting an array based on its indices.

@cindex names, use of
@cindex namespace issues in @command{awk}
@command{awk} maintains a single set
of names that may be used for naming variables, arrays, and functions
(@pxref{User-defined, ,User-Defined Functions}).
Thus, you cannot have a variable and an array with the same name in the
same @command{awk} program.

@menu
* Array Intro::                 Introduction to Arrays
* Reference to Elements::       How to examine one element of an array.
* Assigning Elements::          How to change an element of an array.
* Array Example::               Basic Example of an Array
* Scanning an Array::           A variation of the @code{for} statement. It
                                loops through the indices of an array's
                                existing elements.
* Delete::                      The @code{delete} statement removes an element
                                from an array.
* Numeric Array Subscripts::    How to use numbers as subscripts in
                                @command{awk}.
* Uninitialized Subscripts::    Using Uninitialized variables as subscripts.
* Multi-dimensional::           Emulating multidimensional arrays in
                                @command{awk}.
* Multi-scanning::              Scanning multidimensional arrays.
* Array Sorting::               Sorting array values and indices.
@end menu

@node Array Intro, Reference to Elements, Arrays, Arrays
@section Introduction to Arrays

@cindex arrays
The @command{awk} language provides one-dimensional arrays
for storing groups of related strings or numbers.
Every @command{awk} array must have a name.  Array names have the same
syntax as variable names; any valid variable name would also be a valid
array name.  But one name cannot be used in both ways (as an array and
as a variable) in the same @command{awk} program.

Arrays in @command{awk} superficially resemble arrays in other programming
languages, but there are fundamental differences.  In @command{awk}, it
isn't necessary to specify the size of an array before starting to use it.
Additionally, any number or string in @command{awk}, not just consecutive integers,
may be used as an array index.

In most other languages, arrays must be @dfn{declared} before use,
including a specification of
how many elements or components they contain.  In such languages, the
declaration causes a contiguous block of memory to be allocated for that
many elements.  Usually, an index in the array must be a positive integer.
For example, the index zero specifies the first element in the array, which is
actually stored at the beginning of the block of memory.  Index one
specifies the second element, which is stored in memory right after the
first element, and so on.  It is impossible to add more elements to the
array, because it has room only for as many elements as given in
the declaration.
(Some languages allow arbitrary starting and ending
indices---e.g., @samp{15 .. 27}---but the size of the array is still fixed when
the array is declared.)

A contiguous array of four elements might look like the following example,
conceptually, if the element values are 8, @code{"foo"},
@code{""}, and 30:

@c NEXT ED: Use real images here
@iftex
@c from Karl Berry, much thanks for the help.
@tex
\bigskip % space above the table (about 1 linespace)
\offinterlineskip
\newdimen\width \width = 1.5cm
\newdimen\hwidth \hwidth = 4\width \advance\hwidth by 2pt % 5 * 0.4pt
\centerline{\vbox{
\halign{\strut\hfil\ignorespaces#&&\vrule#&\hbox to\width{\hfil#\unskip\hfil}\cr
\noalign{\hrule width\hwidth}
	&&{\tt 8} &&{\tt "foo"} &&{\tt ""} &&{\tt 30} &&\quad Value\cr
\noalign{\hrule width\hwidth}
\noalign{\smallskip}
	&\omit&0&\omit &1   &\omit&2 &\omit&3 &\omit&\quad Index\cr
}
}}
@end tex
@end iftex
@ifinfo
@example
+---------+---------+--------+---------+
|    8    |  "foo"  |   ""   |    30   |    @r{Value}
+---------+---------+--------+---------+
     0         1         2         3        @r{Index}
@end example
@end ifinfo

@noindent
Only the values are stored; the indices are implicit from the order of
the values.  8 is the value at index zero, because 8 appears in the
position with zero elements before it.

@cindex arrays, definition of
@cindex associative arrays
@cindex arrays, associative
Arrays in @command{awk} are different---they are @dfn{associative}.  This means
that each array is a collection of pairs: an index, and its corresponding
array element value:

@example
@r{Element} 3     @r{Value} 30
@r{Element} 1     @r{Value} "foo"
@r{Element} 0     @r{Value} 8
@r{Element} 2     @r{Value} ""
@end example

@noindent
The pairs are shown in jumbled order because their order is irrelevant.

One advantage of associative arrays is that new pairs can be added
at any time.  For example, suppose a tenth element is added to the array
whose value is @w{@code{"number ten"}}.  The result is:

@example
@r{Element} 10    @r{Value} "number ten"
@r{Element} 3     @r{Value} 30
@r{Element} 1     @r{Value} "foo"
@r{Element} 0     @r{Value} 8
@r{Element} 2     @r{Value} ""
@end example

@noindent
@cindex sparse arrays
@cindex arrays, sparse
Now the array is @dfn{sparse}, which just means some indices are missing.
It has elements 0--3 and 10, but doesn't have elements 4, 5, 6, 7, 8, or 9.

Another consequence of associative arrays is that the indices don't
have to be positive integers.  Any number, or even a string, can be
an index.  For example, the following is an array that translates words from
English into French:

@example
@r{Element} "dog" @r{Value} "chien"
@r{Element} "cat" @r{Value} "chat"
@r{Element} "one" @r{Value} "un"
@r{Element} 1     @r{Value} "un"
@end example

@noindent
Here we decided to translate the number one in both spelled-out and
numeric form---thus illustrating that a single array can have both
numbers and strings as indices.
In fact, array subscripts are always strings; this is discussed
in more detail in
@ref{Numeric Array Subscripts, ,Using Numbers to Subscript Arrays}.
Here, the number @code{1} isn't double-quoted, since @command{awk}
automatically converts it to a string.

@cindex arrays, subscripts, and @code{IGNORECASE}
@cindex @code{IGNORECASE}, and array subscripts
@cindex @code{IGNORECASE} variable
The value of @code{IGNORECASE} has no effect upon array subscripting.
The identical string value used to store an array element must be used
to retrieve it.
When @command{awk} creates an array (e.g., with the @code{split}
built-in function),
that array's indices are consecutive integers starting at one.
(@xref{String Functions, ,String Manipulation Functions}.)

@command{awk}'s arrays are efficient---the time to access an element
is independent of the number of elements in the array.

@node Reference to Elements, Assigning Elements, Array Intro, Arrays
@section Referring to an Array Element
@cindex array reference
@cindex element of array
@cindex reference to array

The principal way to use an array is to refer to one of its elements.
An array reference is an expression as follows:

@example
@var{array}[@var{index}]
@end example

@noindent
Here, @var{array} is the name of an array.  The expression @var{index} is
the index of the desired element of the array.

The value of the array reference is the current value of that array
element.  For example, @code{foo[4.3]} is an expression for the element
of array @code{foo} at index @samp{4.3}.

A reference to an array element that has no recorded value yields a value of
@code{""}, the null string.  This includes elements
that have not been assigned any value as well as elements that have been
deleted (@pxref{Delete, ,The @code{delete} Statement}).  Such a reference
automatically creates that array element, with the null string as its value.
(In some cases, this is unfortunate, because it might waste memory inside
@command{awk}.)

@cindex arrays, presence of elements
@cindex arrays, the @code{in} operator
To determine whether an element exists in an array at a certain index, use
the following expression:

@example
@var{index} in @var{array}
@end example

@cindex side effects
@noindent
This expression tests whether or not the particular index exists,
without the side effect of creating that element if it is not present.
The expression has the value one (true) if @code{@var{array}[@var{index}]}
exists and zero (false) if it does not exist.
For example, this statement tests whether the array @code{frequencies}
contains the index @samp{2}:

@example
if (2 in frequencies)
    print "Subscript 2 is present."
@end example

Note that this is @emph{not} a test of whether the array
@code{frequencies} contains an element whose @emph{value} is two.
There is no way to do that except to scan all the elements.  Also, this
@emph{does not} create @code{frequencies[2]}, while the following
(incorrect) alternative does:

@example
if (frequencies[2] != "")
    print "Subscript 2 is present."
@end example

@node Assigning Elements, Array Example, Reference to Elements, Arrays
@section Assigning Array Elements
@cindex array assignment
@cindex element assignment

Array elements can be assigned values just like
@command{awk} variables:

@example
@var{array}[@var{subscript}] = @var{value}
@end example

@noindent
@var{array} is the name of an array.  The expression
@var{subscript} is the index of the element of the array that is
assigned a value.  The expression @var{value} is the value to
assign to that element of the array.

@node Array Example, Scanning an Array, Assigning Elements, Arrays
@section Basic Array Example

The following program takes a list of lines, each beginning with a line
number, and prints them out in order of line number.  The line numbers
are not in order when they are first read---instead they
are scrambled.  This program sorts the lines by making an array using
the line numbers as subscripts.  The program then prints out the lines
in sorted order of their numbers.  It is a very simple program and gets
confused upon encountering repeated numbers, gaps, or lines that don't
begin with a number:

@example
@c file eg/misc/arraymax.awk
@{
  if ($1 > max)
    max = $1
  arr[$1] = $0
@}

END @{
  for (x = 1; x <= max; x++)
    print arr[x]
@}
@c endfile
@end example

The first rule keeps track of the largest line number seen so far;
it also stores each line into the array @code{arr}, at an index that
is the line's number.
The second rule runs after all the input has been read, to print out
all the lines.
When this program is run with the following input:

@example
@c file eg/misc/arraymax.data
5  I am the Five man
2  Who are you?  The new number two!
4  . . . And four on the floor
1  Who is number one?
3  I three you.
@c endfile
@end example

@noindent
its output is:

@example
1  Who is number one?
2  Who are you?  The new number two!
3  I three you.
4  . . . And four on the floor
5  I am the Five man
@end example

If a line number is repeated, the last line with a given number overrides
the others.
Gaps in the line numbers can be handled with an easy improvement to the
program's @code{END} rule, as follows:

@example
END @{
  for (x = 1; x <= max; x++)
    if (x in arr)
      print arr[x]
@}
@end example

@node Scanning an Array, Delete, Array Example, Arrays
@section Scanning All Elements of an Array
@cindex @code{for (x in @dots{})} statement
@cindex arrays, special @code{for} statement
@cindex scanning an array
@cindex @code{in} operator

In programs that use arrays, it is often necessary to use a loop that
executes once for each element of an array.  In other languages, where
arrays are contiguous and indices are limited to positive integers,
this is easy: all the valid indices can be found by counting from
the lowest index up to the highest.  This technique won't do the job
in @command{awk}, because any number or string can be an array index.
So @command{awk} has a special kind of @code{for} statement for scanning
an array:

@example
for (@var{var} in @var{array})
  @var{body}
@end example

@noindent
This loop executes @var{body} once for each index in @var{array} that the
program has previously used, with the variable @var{var} set to that index.

The following program uses this form of the @code{for} statement.  The
first rule scans the input records and notes which words appear (at
least once) in the input, by storing a one into the array @code{used} with
the word as index.  The second rule scans the elements of @code{used} to
find all the distinct words that appear in the input.  It prints each
word that is more than 10 characters long and also prints the number of
such words.
@xref{String Functions, ,String Manipulation Functions},
for more information on the built-in function @code{length}.

@example
# Record a 1 for each word that is used at least once
@{
    for (i = 1; i <= NF; i++)
        used[$i] = 1
@}

# Find number of distinct words more than 10 characters long
END @{
    for (x in used)
        if (length(x) > 10) @{
            ++num_long_words
            print x
        @}
    print num_long_words, "words longer than 10 characters"
@}
@end example

@noindent
@xref{Word Sorting, ,Generating Word Usage Counts},
for a more detailed example of this type.

The order in which elements of the array are accessed by this statement
is determined by the internal arrangement of the array elements within
@command{awk} and cannot be controlled or changed.  This can lead to
problems if new elements are added to @var{array} by statements in
the loop body; it is not predictable whether or not the @code{for} loop will
reach them.  Similarly, changing @var{var} inside the loop may produce
strange results.  It is best to avoid such things.

@node Delete, Numeric Array Subscripts, Scanning an Array, Arrays
@section The @code{delete} Statement
@cindex @code{delete} statement
@cindex deleting elements of arrays
@cindex removing elements of arrays
@cindex arrays, deleting an element

To remove an individual element of an array, use the @code{delete}
statement:

@example
delete @var{array}[@var{index}]
@end example

Once an array element has been deleted, any value the element once
had is no longer available. It is as if the element had never 
been referred to or had been given a value.
The following is an example of deleting elements in an array:

@example
for (i in frequencies)
  delete frequencies[i]
@end example

@noindent
This example removes all the elements from the array @code{frequencies}.
Once an element is deleted, a subsequent @code{for} statement to scan the array
does not report that element and the @code{in} operator to check for
the presence of that element returns zero (i.e., false):

@example
delete foo[4]
if (4 in foo)
    print "This will never be printed"
@end example

It is important to note that deleting an element is @emph{not} the
same as assigning it a null value (the empty string, @code{""}).
For example:

@example
foo[4] = ""
if (4 in foo)
  print "This is printed, even though foo[4] is empty"
@end example

@cindex lint checks
It is not an error to delete an element that does not exist.
If @option{--lint} is provided on the command line
(@pxref{Options, ,Command-Line Options}),
@command{gawk} issues a warning message when an element that
is not in the array is deleted.

@cindex arrays, deleting entire contents
@cindex deleting entire arrays
@cindex differences between @command{gawk} and @command{awk}
All the elements of an array may be deleted with a single statement
by leaving off the subscript in the @code{delete} statement,
as follows:

@example
delete @var{array}
@end example

This ability is a @command{gawk} extension; it is not available in
compatibility mode (@pxref{Options, ,Command-Line Options}).

Using this version of the @code{delete} statement is about three times
more efficient than the equivalent loop that deletes each element one
at a time.

@cindex portability issues
@cindex Brennan, Michael
The following statement provides a portable but non-obvious way to clear
out an array:@footnote{Thanks to Michael Brennan for pointing this out.}

@example
split("", array)
@end example

The @code{split} function
(@pxref{String Functions, ,String Manipulation Functions})
clears out the target array first. This call asks it to split
apart the null string. Because there is no data to split out, the
function simply clears the array and then returns.

@strong{Caution:} Deleting an array does not change its type; you cannot
delete an array and then use the array's name as a scalar
(i.e., a regular variable). For example, the following does not work:

@example
a[1] = 3; delete a; a = 3
@end example

@node Numeric Array Subscripts, Uninitialized Subscripts, Delete, Arrays
@section Using Numbers to Subscript Arrays

@cindex conversions, during subscripting
@cindex numbers, used as subscripts
@cindex @code{CONVFMT} variable
An important aspect about arrays to remember is that @emph{array subscripts
are always strings}.  When a numeric value is used as a subscript,
it is converted to a string value before being used for subscripting
(@pxref{Conversion, ,Conversion of Strings and Numbers}).
This means that the value of the built-in variable @code{CONVFMT} can
affect how your program accesses elements of an array.  For example:

@example
xyz = 12.153
data[xyz] = 1
CONVFMT = "%2.2f"
if (xyz in data)
    printf "%s is in data\n", xyz
else
    printf "%s is not in data\n", xyz
@end example

@noindent
This prints @samp{12.15 is not in data}.  The first statement gives
@code{xyz} a numeric value.  Assigning to
@code{data[xyz]} subscripts @code{data} with the string value @code{"12.153"}
(using the default conversion value of @code{CONVFMT}, @code{"%.6g"}).
Thus, the array element @code{data["12.153"]} is assigned the value one.
The program then changes
the value of @code{CONVFMT}.  The test @samp{(xyz in data)} generates a new
string value from @code{xyz}---this time @code{"12.15"}---because the value of
@code{CONVFMT} only allows two significant digits.  This test fails,
since @code{"12.15"} is a different string from @code{"12.153"}.

According to the rules for conversions
(@pxref{Conversion, ,Conversion of Strings and Numbers}), integer
values are always converted to strings as integers, no matter what the
value of @code{CONVFMT} may happen to be.  So the usual case of
the following works:

@example
for (i = 1; i <= maxsub; i++)
    @i{do something with} array[i]
@end example

The ``integer values always convert to strings as integers'' rule
has an additional consequence for array indexing.
Octal and hexadecimal constants
(@pxref{Non-decimal-numbers, ,Octal and Hexadecimal Numbers})
are converted internally into numbers and their original form
is forgotten.
This means, for example, that
@code{array[17]},
@code{array[021]},
and
@code{array[0x11]}
all refer to the same element!

As with many things in @command{awk}, the majority of the time
things work as one would expect them to.  But it is useful to have a precise
knowledge of the actual rules which sometimes can have a subtle
effect on your programs.

@node Uninitialized Subscripts, Multi-dimensional, Numeric Array Subscripts, Arrays
@section Using Uninitialized Variables as Subscripts

@cindex uninitialized variables, as array subscripts
@cindex arrays, subscripts, uninitialized variables
Suppose it's necessary to write a program
to print the input data in reverse order.
A reasonable attempt to do so (with some test
data) might look like this:

@example
$ echo 'line 1
> line 2
> line 3' | awk '@{ l[lines] = $0; ++lines @}
> END @{
>     for (i = lines-1; i >= 0; --i)
>        print l[i]
> @}'
@print{} line 3
@print{} line 2
@end example

Unfortunately, the very first line of input data did not come out in the
output!

At first glance, this program should have worked.  The variable @code{lines}
is uninitialized, and uninitialized variables have the numeric value zero.
So, @command{awk} should have printed the value of @code{l[0]}.

The issue here is that subscripts for @command{awk} arrays are @emph{always}
strings. Uninitialized variables, when used as strings, have the
value @code{""}, not zero.  Thus, @samp{line 1} ends up stored in
@code{l[""]}.
The following version of the program works correctly:

@example
@{ l[lines++] = $0 @}
END @{
    for (i = lines - 1; i >= 0; --i)
       print l[i]
@}
@end example

Here, the @samp{++} forces @code{lines} to be numeric, thus making
the ``old value'' numeric zero. This is then converted to @code{"0"}
as the array subscript.

@cindex null string, as array subscript
@cindex dark corner
@cindex lint checks
Even though it is somewhat unusual, the null string
(@code{""}) is a valid array subscript.
@value{DARKCORNER}
@command{gawk} warns about the use of the null string as a subscript
if @option{--lint} is provided
on the command line (@pxref{Options, ,Command-Line Options}).

@node Multi-dimensional, Multi-scanning, Uninitialized Subscripts, Arrays
@section Multidimensional Arrays

@cindex subscripts in arrays
@cindex arrays, multidimensional subscripts
@cindex multidimensional subscripts
A multidimensional array is an array in which an element is identified
by a sequence of indices instead of a single index.  For example, a
two-dimensional array requires two indices.  The usual way (in most
languages, including @command{awk}) to refer to an element of a
two-dimensional array named @code{grid} is with
@code{grid[@var{x},@var{y}]}.

@cindex @code{SUBSEP} variable
Multidimensional arrays are supported in @command{awk} through
concatenation of indices into one string.
@command{awk} converts the indices into strings
(@pxref{Conversion, ,Conversion of Strings and Numbers}) and
concatenates them together, with a separator between them.  This creates
a single string that describes the values of the separate indices.  The
combined string is used as a single index into an ordinary,
one-dimensional array.  The separator used is the value of the built-in
variable @code{SUBSEP}.

For example, suppose we evaluate the expression @samp{foo[5,12] = "value"}
when the value of @code{SUBSEP} is @code{"@@"}.  The numbers 5 and 12 are
converted to strings and
concatenated with an @samp{@@} between them, yielding @code{"5@@12"}; thus,
the array element @code{foo["5@@12"]} is set to @code{"value"}.

Once the element's value is stored, @command{awk} has no record of whether
it was stored with a single index or a sequence of indices.  The two
expressions @samp{foo[5,12]} and @w{@samp{foo[5 SUBSEP 12]}} are always
equivalent.

The default value of @code{SUBSEP} is the string @code{"\034"},
which contains a non-printing character that is unlikely to appear in an
@command{awk} program or in most input data.
The usefulness of choosing an unlikely character comes from the fact
that index values that contain a string matching @code{SUBSEP} can lead to
combined strings that are ambiguous.  Suppose that @code{SUBSEP} is
@code{"@@"}; then @w{@samp{foo["a@@b", "c"]}} and @w{@samp{foo["a",
"b@@c"]}} are indistinguishable because both are actually
stored as @samp{foo["a@@b@@c"]}.

To test whether a particular index sequence exists in a
``multidimensional'' array, use the same operator (@samp{in}) that is
used for single dimensional arrays.  Write the whole sequence of indices
in parentheses, separated by commas, as the left operand:

@example
(@var{subscript1}, @var{subscript2}, @dots{}) in @var{array}
@end example

The following example treats its input as a two-dimensional array of
fields; it rotates this array 90 degrees clockwise and prints the
result.  It assumes that all lines have the same number of
elements.

@example
@{
     if (max_nf < NF)
          max_nf = NF
     max_nr = NR
     for (x = 1; x <= NF; x++)
          vector[x, NR] = $x
@}

END @{
     for (x = 1; x <= max_nf; x++) @{
          for (y = max_nr; y >= 1; --y)
               printf("%s ", vector[x, y])
          printf("\n")
     @}
@}
@end example

@noindent
When given the input:

@example
1 2 3 4 5 6
2 3 4 5 6 1
3 4 5 6 1 2
4 5 6 1 2 3
@end example

@noindent
the program produces the following output:

@example
4 3 2 1
5 4 3 2
6 5 4 3
1 6 5 4
2 1 6 5
3 2 1 6
@end example

@node Multi-scanning, Array Sorting, Multi-dimensional, Arrays
@section Scanning Multidimensional Arrays

There is no special @code{for} statement for scanning a
``multidimensional'' array. There cannot be one, because in truth there
are no multidimensional arrays or elements---there is only a
multidimensional @emph{way of accessing} an array.

However, if your program has an array that is always accessed as
multidimensional, you can get the effect of scanning it by combining
the scanning @code{for} statement
(@pxref{Scanning an Array, ,Scanning All Elements of an Array}) with the
built-in @code{split} function
(@pxref{String Functions, ,String Manipulation Functions}).
It works in the following manner:

@example
for (combined in array) @{
    split(combined, separate, SUBSEP)
    @dots{}
@}
@end example

@noindent
This sets the variable @code{combined} to
each concatenated combined index in the array, and splits it
into the individual indices by breaking it apart where the value of
@code{SUBSEP} appears.  The individual indices then become the elements of
the array @code{separate}.

Thus, if a value is previously stored in @code{array[1, "foo"]}; then
an element with index @code{"1\034foo"} exists in @code{array}.  (Recall
that the default value of @code{SUBSEP} is the character with code 034.)
Sooner or later, the @code{for} statement finds that index and does an
iteration with the variable @code{combined} set to @code{"1\034foo"}.
Then the @code{split} function is called as follows:

@example
split("1\034foo", separate, "\034")
@end example

@noindent
The result is to set @code{separate[1]} to @code{"1"} and
@code{separate[2]} to @code{"foo"}.  Presto! The original sequence of
separate indices is recovered.

@node Array Sorting, , Multi-scanning, Arrays
@section Sorting Array Values and Indices with @command{gawk}

@cindex arrays, sorting
@cindex @code{asort} built-in function
The order in which an array is scanned with a @samp{for (i in array)}
loop is essentially arbitrary.
In most @command{awk} implementations, sorting an array requires
writing a @code{sort} function.
While this can be educational for exploring different sorting algorithms,
usually that's not the point of the program.
@command{gawk} provides the built-in @code{asort} function
(@pxref{String Functions, ,String Manipulation Functions})
that sorts an array.  For example:

@example
@var{populate the array} data
n = asort(data)
for (i = 1; i <= n; i++)
    @var{do something with} data[i]
@end example

After the call to @code{asort}, the array @code{data} is indexed from 1
to some number @var{n}, the total number of elements in @code{data}.
(This count is @code{asort}'s return value.)
@code{data[1]} @value{LEQ} @code{data[2]} @value{LEQ} @code{data[3]}, and so on.
The comparison of array elements is done
using @command{gawk}'s usual comparison rules
(@pxref{Typing and Comparison, ,Variable Typing and Comparison Expressions}).

@cindex side effects
An important side effect of calling @code{asort} is that
@emph{the array's original indices are irrevocably lost}.
As this isn't always desirable, @code{asort} accepts a
second argument:

@example
@var{populate the array} source
n = asort(source, dest)
for (i = 1; i <= n; i++)
    @var{do something with} dest[i]
@end example

In this case, @command{gawk} copies the @code{source} array into the
@code{dest} array and then sorts @code{dest}, destroying its indices.
However, the @code{source} array is not affected.

Often, what's needed is to sort on the values of the @emph{indices}
instead of the values of the elements.  To do this, use a helper array
to hold the sorted index values, and then access the original array's
elements.  It works in the following way:

@example
@var{populate the array} data
# copy indices
j = 1
for (i in data) @{
    ind[j] = i    # index value becomes element value
    j++
@}
n = asort(ind)    # index values are now sorted
for (i = 1; i <= n; i++)
    @var{do something with} data[ind[i]]
@end example

Sorting the array by replacing the indices provides maximal flexibility.
To traverse the elements in decreasing order, use a loop that goes from
@var{n} down to 1, either over the elements or over the indices.

@cindex reference counting
Copying array indices and elements isn't expensive in terms of memory.
Internally, @command{gawk} maintains @dfn{reference counts} to data.
For example, when @code{asort} copies the first array to the second one,
there is only one copy of the original array elements' data, even though
both arrays use the values.  Similarly, when copying the indices from
@code{data} to @code{ind}, there is only one copy of the actual index
strings.

@cindex arrays, sorting and @code{IGNORECASE}
@cindex @code{IGNORECASE}, and array sorting
@cindex @code{IGNORECASE} variable
As with array subscripts, the value of @code{IGNORECASE}
does not affect array sorting.

@node Functions, Internationalization, Arrays, Top
@chapter Functions

This @value{CHAPTER} describes @command{awk}'s built-in functions,
which fall into three categories: numeric, string, and I/O.
@command{gawk} provides additional groups of functions
to work with values that represent time, do
bit manipulation, and to internationalize and localize programs.

Besides the built-in functions, @command{awk} has provisions for
writing new functions that the rest of a program can use.
The second half of this @value{CHAPTER} describes these
@dfn{user-defined} functions.

@menu
* Built-in::                    Summarizes the built-in functions.
* User-defined::                Describes User-defined functions in detail.
@end menu

@node Built-in, User-defined, Functions, Functions
@section Built-in Functions

@c 2e: USE TEXINFO-2 FUNCTION DEFINITION STUFF!!!!!!!!!!!!!
@cindex built-in functions
@dfn{Built-in} functions are always available for
your @command{awk} program to call.  This @value{SECTION} defines all
the built-in
functions in @command{awk}; some of these are mentioned in other sections
but are summarized here for your convenience.

@menu
* Calling Built-in::            How to call built-in functions.
* Numeric Functions::           Functions that work with numbers, including
                                @code{int}, @code{sin} and @code{rand}.
* String Functions::            Functions for string manipulation, such as
                                @code{split}, @code{match} and @code{sprintf}.
* I/O Functions::               Functions for files and shell commands.
* Time Functions::              Functions for dealing with timestamps.
* Bitwise Functions::           Functions for bitwise operations.
* I18N Functions::              Functions for string translation.
@end menu

@node Calling Built-in, Numeric Functions, Built-in, Built-in
@subsection Calling Built-in Functions

To call one of @command{awk}'s built-in functions, write the name of
the function followed
by arguments in parentheses.  For example, @samp{atan2(y + z, 1)}
is a call to the function @code{atan2}, and has two arguments.

@cindex conventions, programming
@cindex programming conventions
Whitespace is ignored between the built-in function name and the
open parenthesis, and it is good practice to avoid using whitespace
there.  User-defined functions do not permit whitespace in this way, and
it is easier to avoid mistakes by following a simple
convention that always works---no whitespace after a function name.

@cindex fatal errors
@cindex differences between @command{gawk} and @command{awk}
Each built-in function accepts a certain number of arguments.
In some cases, arguments can be omitted. The defaults for omitted
arguments vary from function to function and are described under the
individual functions.  In some @command{awk} implementations, extra
arguments given to built-in functions are ignored.  However, in @command{gawk},
it is a fatal error to give extra arguments to a built-in function.

When a function is called, expressions that create the function's actual
parameters are evaluated completely before the call is performed.
For example, in the following code fragment:

@example
i = 4
j = sqrt(i++)
@end example

@cindex evaluation, order of
@cindex order of evaluation
@noindent
the variable @code{i} is incremented to the value five before @code{sqrt}
is called with a value of four for its actual parameter.
The order of evaluation of the expressions used for the function's
parameters is undefined.  Thus, avoid writing programs that
assume that parameters are evaluated from left to right or from
right to left.  For example:

@example
i = 5
j = atan2(i++, i *= 2)
@end example

If the order of evaluation is left to right, then @code{i} first becomes
six, and then 12, and @code{atan2} is called with the two arguments 6
and 12.  But if the order of evaluation is right to left, @code{i}
first becomes 10, then 11, and @code{atan2} is called with the
two arguments 11 and 10.

@node Numeric Functions, String Functions, Calling Built-in, Built-in
@subsection Numeric Functions

The following list describes all of
the built-in functions that work with numbers.
Optional parameters are enclosed in square brackets ([ and ]):

@table @code
@item int(@var{x})
@cindex @code{int} built-in function
This returns the nearest integer to @var{x}, located between @var{x} and zero and
truncated toward zero.

For example, @code{int(3)} is three, @code{int(3.9)} is three, @code{int(-3.9)}
is @minus{}3, and @code{int(-3)} is @minus{}3 as well.

@item sqrt(@var{x})
@cindex @code{sqrt} built-in function
This returns the positive square root of @var{x}.
@command{gawk} reports an error
if @var{x} is negative.  Thus, @code{sqrt(4)} is two.

@item exp(@var{x})
@cindex @code{exp} built-in function
This returns the exponential of @var{x} (@code{e ^ @var{x}}) or reports
an error if @var{x} is out of range.  The range of values @var{x} can have
depends on your machine's floating-point representation.

@item log(@var{x})
@cindex @code{log} built-in function
This returns the natural logarithm of @var{x}, if @var{x} is positive;
otherwise, it reports an error.

@item sin(@var{x})
@cindex @code{sin} built-in function
This returns the sine of @var{x}, with @var{x} in radians.

@item cos(@var{x})
@cindex @code{cos} built-in function
This returns the cosine of @var{x}, with @var{x} in radians.

@item atan2(@var{y}, @var{x})
@cindex @code{atan2} built-in function
This returns the arctangent of @code{@var{y} / @var{x}} in radians.

@item rand()
@cindex @code{rand} built-in function
This returns a random number.  The values of @code{rand} are
uniformly distributed between zero and one.
The value is never zero and never one.@footnote{The C version of @code{rand}
is known to produce fairly poor sequences of random numbers.
However, nothing requires that an @command{awk} implementation use the C
@code{rand} to implement the @command{awk} version of @code{rand}.
In fact, @command{gawk} uses the BSD @code{random} function, which is
considerably better than @code{rand}, to produce random numbers.}

Often random integers are needed instead.  Following is a user-defined function
that can be used to obtain a random non-negative integer less than @var{n}:

@example
function randint(n) @{
     return int(n * rand())
@}
@end example

@noindent
The multiplication produces a random number greater than zero and less
than @code{n}.  Using @code{int}, this result is made into
an integer between zero and @code{n} @minus{} 1, inclusive.

The following example uses a similar function to produce random integers
between one and @var{n}.  This program prints a new random number for
each input record.

@example
# Function to roll a simulated die.
function roll(n) @{ return 1 + int(rand() * n) @}

# Roll 3 six-sided dice and
# print total number of points.
@{
      printf("%d points\n",
             roll(6)+roll(6)+roll(6))
@}
@end example

@cindex seed for random numbers
@cindex random numbers, seed of
@c MAWK uses a different seed each time.
@strong{Caution:} In most @command{awk} implementations, including @command{gawk},
@code{rand} starts generating numbers from the same
starting number, or @dfn{seed}, each time you run @command{awk}.  Thus,
a program generates the same results each time you run it.
The numbers are random within one @command{awk} run but predictable
from run to run.  This is convenient for debugging, but if you want
a program to do different things each time it is used, you must change
the seed to a value that is different in each run.  To do this,
use @code{srand}.

@item srand(@r{[}@var{x}@r{]})
@cindex @code{srand} built-in function
The function @code{srand} sets the starting point, or seed,
for generating random numbers to the value @var{x}.

Each seed value leads to a particular sequence of random
numbers.@footnote{Computer generated random numbers really are not truly
random.  They are technically known as ``pseudo-random.''  This means
that while the numbers in a sequence appear to be random, you can in
fact generate the same sequence of random numbers over and over again.}
Thus, if the seed is set to the same value a second time,
the same sequence of random numbers is produced again.

Different @command{awk} implementations use different random number
generators internally.  Don't expect the same @command{awk} program
to produce the same series of random numbers when executed by
different versions of @command{awk}.

If the argument @var{x} is omitted, as in @samp{srand()}, then the current
date and time of day are used for a seed.  This is the way to get random
numbers that are truly unpredictable.

The return value of @code{srand} is the previous seed.  This makes it
easy to keep track of the seeds in case you need to consistently reproduce
sequences of random numbers.
@end table

@node String Functions, I/O Functions, Numeric Functions, Built-in
@subsection String Manipulation Functions

The functions in this @value{SECTION} look at or change the text of one or more
strings.
Optional parameters are enclosed in square brackets ([ and ]).
Those functions that are
specific to @command{gawk} are marked with a pound sign (@samp{#}):

@menu
* Gory Details::                More than you want to know about @samp{\} and
                                @samp{&} with @code{sub}, @code{gsub}, and
                                @code{gensub}.
@end menu

@table @code
@item asort(@var{source} @r{[}, @var{dest}@r{]}) #
@cindex @code{asort} built-in function
@code{asort} is a @command{gawk}-specific extension, returning the number of
elements in the array @var{source}.  The contents of @var{source} are
sorted using @command{gawk}'s normal rules for comparing values, and the indices
of the sorted values of @var{source} are replaced with sequential
integers starting with one. If the optional array @var{dest} is specified,
then @var{source} is duplicated into @var{dest}.  @var{dest} is then
sorted, leaving the indices of @var{source} unchanged.
For example, if the contents of @code{a} are as follows:

@example
a["last"] = "de"
a["first"] = "sac"
a["middle"] = "cul"
@end example

@noindent
A call to @code{asort}:

@example
asort(a)
@end example

@noindent
results in the following contents of @code{a}:

@example
a[1] = "cul"
a[2] = "de"
a[3] = "sac"
@end example

@cindex differences between @command{gawk} and @command{awk}
The @code{asort} function is described in more detail in
@ref{Array Sorting, ,Sorting Array Values and Indices with @command{gawk}}.
@code{asort} is a @command{gawk} extension; it is not available
in compatibility mode (@pxref{Options, ,Command-Line Options}).

@item index(@var{in}, @var{find})
@cindex @code{index} built-in function
This searches the string @var{in} for the first occurrence of the string
@var{find}, and returns the position in characters where that occurrence
begins in the string @var{in}.  Consider the following example:

@example
$ awk 'BEGIN @{ print index("peanut", "an") @}'
@print{} 3
@end example

@noindent
If @var{find} is not found, @code{index} returns zero.
(Remember that string indices in @command{awk} start at one.)

@item length(@r{[}@var{string}@r{]})
@cindex @code{length} built-in function
This returns the number of characters in @var{string}.  If
@var{string} is a number, the length of the digit string representing
that number is returned.  For example, @code{length("abcde")} is 5.  By
contrast, @code{length(15 * 35)} works out to 3. In this example, 15 * 35 =
525, and 525 is then converted to the string @code{"525"}, which has
three characters.

If no argument is supplied, @code{length} returns the length of @code{$0}.

@cindex historical features
@cindex portability issues
@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
@strong{Note:}
In older versions of @command{awk}, the @code{length} function could
be called
without any parentheses.  Doing so is marked as ``deprecated'' in the
POSIX standard.  This means that while a program can do this,
it is a feature that can eventually be removed from a future
version of the standard.  Therefore, for programs to be maximally portable,
always supply the parentheses.

@item match(@var{string}, @var{regexp} @r{[}, @var{array}@r{]})
@cindex @code{match} built-in function
The @code{match} function searches @var{string} for the
longest leftmost substring matched by the regular expression,
@var{regexp}.  It returns the character position, or @dfn{index},
where that substring begins (one, if it starts at the beginning of
@var{string}).  If no match is found, it returns zero.

The order of the first two arguments is backwards from most other string
functions that work with regular expressions, such as
@code{sub} and @code{gsub}.  It might help to remember that
for @code{match}, the order is the same as for the @samp{~} operator:
@samp{@var{string} ~ @var{regexp}}.

@cindex @code{RSTART} variable
@cindex @code{RLENGTH} variable
The @code{match} function sets the built-in variable @code{RSTART} to
the index.  It also sets the built-in variable @code{RLENGTH} to the
length in characters of the matched substring.  If no match is found,
@code{RSTART} is set to zero, and @code{RLENGTH} to @minus{}1.

For example:

@example
@c file eg/misc/findpat.awk
@{
       if ($1 == "FIND")
         regex = $2
       else @{
         where = match($0, regex)
         if (where != 0)
           print "Match of", regex, "found at",
                     where, "in", $0
       @}
@}
@c endfile
@end example

@noindent
This program looks for lines that match the regular expression stored in
the variable @code{regex}.  This regular expression can be changed.  If the
first word on a line is @samp{FIND}, @code{regex} is changed to be the
second word on that line.  Therefore, if given:

@example
@c file eg/misc/findpat.data
FIND ru+n
My program runs
but not very quickly
FIND Melvin
JF+KM
This line is property of Reality Engineering Co.
Melvin was here.
@c endfile
@end example

@noindent
@command{awk} prints:

@example
Match of ru+n found at 12 in My program runs
Match of Melvin found at 1 in Melvin was here.
@end example

@cindex differences between @command{gawk} and @command{awk}
If @var{array} is present, it is cleared, and then the 0'th element
of @var{array} is set to the entire portion of @var{string}
matched by @var{regexp}.  If @var{regexp} contains parentheses,
the integer-indexed elements of @var{array} are set to contain the
portion of @var{string} matching the corresponding parenthesized
sub-expression.
For example:

@example
$ echo foooobazbarrrrr |
> gawk '@{ match($0, /(fo+).+(ba*r)/, arr)
>           print arr[1], arr[2] @}'
@print{} foooo barrrrr
@end example

@cindex fatal errors
The @var{array} argument to @code{match} is a
@command{gawk} extension.  In compatibility mode
(@pxref{Options, ,Command-Line Options}),
using a third argument is a fatal error.

@item split(@var{string}, @var{array} @r{[}, @var{fieldsep}@r{]})
@cindex @code{split} built-in function
This function divides @var{string} into pieces separated by @var{fieldsep},
and stores the pieces in @var{array}.  The first piece is stored in
@code{@var{array}[1]}, the second piece in @code{@var{array}[2]}, and so
forth.  The string value of the third argument, @var{fieldsep}, is
a regexp describing where to split @var{string} (much as @code{FS} can
be a regexp describing where to split input records).  If
the @var{fieldsep} is omitted, the value of @code{FS} is used.
@code{split} returns the number of elements created.
If @var{string} does not match @var{fieldsep}, @var{array} is empty
and @code{split} returns zero.

The @code{split} function splits strings into pieces in a
manner similar to the way input lines are split into fields.  For example:

@example
split("cul-de-sac", a, "-")
@end example

@noindent
splits the string @samp{cul-de-sac} into three fields using @samp{-} as the
separator.  It sets the contents of the array @code{a} as follows:

@example
a[1] = "cul"
a[2] = "de"
a[3] = "sac"
@end example

@noindent
The value returned by this call to @code{split} is three.

@cindex differences between @command{gawk} and @command{awk}
As with input field-splitting, when the value of @var{fieldsep} is
@w{@code{" "}}, leading and trailing whitespace is ignored and the elements
are separated by runs of whitespace.
Also as with input field-splitting, if @var{fieldsep} is the null string, each
individual character in the string is split into its own array element.
(This is a @command{gawk}-specific extension.)

@cindex dark corner
Modern implementations of @command{awk}, including @command{gawk}, allow
the third argument to be a regexp constant (@code{/abc/}) as well as a
string.
@value{DARKCORNER}
The POSIX standard allows this as well.

Before splitting the string, @code{split} deletes any previously existing
elements in the array @var{array}.
If @var{string} does not match @var{fieldsep} at all, @var{array} has
one element only. The value of that element is the original @var{string}.

@item sprintf(@var{format}, @var{expression1}, @dots{})
@cindex @code{sprintf} built-in function
This returns (without printing) the string that @code{printf} would
have printed out with the same arguments
(@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}).
For example:

@example
pival = sprintf("pi = %.2f (approx.)", 22/7)
@end example

@noindent
assigns the string @w{@code{"pi = 3.14 (approx.)"}} to the variable @code{pival}.

@cindex @code{strtonum} built-in function
@item strtonum(@var{str}) #
Examines @var{str} and returns its numeric value.  If @var{str}
begins with a leading @samp{0}, @code{strtonum} assumes that @var{str}
is an octal number.  If @var{str} begins with a leading @samp{0x} or
@samp{0X}, @code{strtonum} assumes that @var{str} is a hexadecimal number.
For example:

@example
$ echo 0x11 |
> gawk '@{ printf "%d\n", strtonum($1) @}'
@print{} 17
@end example

Using the @code{strtonum} function is @emph{not} the same as adding zero
to a string value; the automatic coercion of strings to numbers
works only for decimal data, not for octal or hexadecimal.@footnote{Unless
you use the @option{--non-decimal-data} option, which isn't recommended.
@xref{Non-decimal Data, ,Allowing Non-Decimal Input Data}, for more information.}

@cindex differences between @command{gawk} and @command{awk}
@code{strtonum} is a @command{gawk} extension; it is not available
in compatibility mode (@pxref{Options, ,Command-Line Options}).

@item sub(@var{regexp}, @var{replacement} @r{[}, @var{target}@r{]})
@cindex @code{sub} built-in function
The @code{sub} function alters the value of @var{target}.
It searches this value, which is treated as a string, for the
leftmost longest substring matched by the regular expression @var{regexp}.
Then the entire string is
changed by replacing the matched text with @var{replacement}.
The modified string becomes the new value of @var{target}.

This function is peculiar because @var{target} is not simply
used to compute a value, and not just any expression will do---it
must be a variable, field, or array element so that @code{sub} can
store a modified value there.  If this argument is omitted, then the
default is to use and alter @code{$0}.
For example:

@example
str = "water, water, everywhere"
sub(/at/, "ith", str)
@end example

@noindent
sets @code{str} to @w{@code{"wither, water, everywhere"}}, by replacing the
leftmost longest occurrence of @samp{at} with @samp{ith}.

The @code{sub} function returns the number of substitutions made (either
one or zero).

If the special character @samp{&} appears in @var{replacement}, it
stands for the precise substring that was matched by @var{regexp}.  (If
the regexp can match more than one string, then this precise substring
may vary.)  For example:

@example
@{ sub(/candidate/, "& and his wife"); print @}
@end example

@noindent
changes the first occurrence of @samp{candidate} to @samp{candidate
and his wife} on each input line.
Here is another example:

@example
$ awk 'BEGIN @{
>         str = "daabaaa"
>         sub(/a+/, "C&C", str)
>         print str
> @}'
@print{} dCaaCbaaa
@end example

@noindent
This shows how @samp{&} can represent a non-constant string and also
illustrates the ``leftmost, longest'' rule in regexp matching
(@pxref{Leftmost Longest, ,How Much Text Matches?}).

The effect of this special character (@samp{&}) can be turned off by putting a
backslash before it in the string.  As usual, to insert one backslash in
the string, you must write two backslashes.  Therefore, write @samp{\\&}
in a string constant to include a literal @samp{&} in the replacement.
For example, following is shown how to replace the first @samp{|} on each line with
an @samp{&}:

@example
@{ sub(/\|/, "\\&"); print @}
@end example

@cindex @code{sub}, third argument of
@cindex @code{gsub}, third argument of
As mentioned, the third argument to @code{sub} must
be a variable, field or array reference.
Some versions of @command{awk} allow the third argument to
be an expression that is not an lvalue.  In such a case, @code{sub}
still searches for the pattern and returns zero or one, but the result of
the substitution (if any) is thrown away because there is no place
to put it.  Such versions of @command{awk} accept expressions
such as the following:

@example
sub(/USA/, "United States", "the USA and Canada")
@end example

@noindent
@cindex fatal errors
For historical compatibility, @command{gawk} accepts erroneous code,
such as in the previous example. However, using any other non-changeable
object as the third parameter causes a fatal error and your program
will not run.

Finally, if the @var{regexp} is not a regexp constant, it is converted into a
string, and then the value of that string is treated as the regexp to match.

@item gsub(@var{regexp}, @var{replacement} @r{[}, @var{target}@r{]})
@cindex @code{gsub} built-in function
This is similar to the @code{sub} function, except @code{gsub} replaces
@emph{all} of the longest, leftmost, @emph{non-overlapping} matching
substrings it can find.  The @samp{g} in @code{gsub} stands for
``global,'' which means replace everywhere.  For example:

@example
@{ gsub(/Britain/, "United Kingdom"); print @}
@end example

@noindent
replaces all occurrences of the string @samp{Britain} with @samp{United
Kingdom} for all input records.

The @code{gsub} function returns the number of substitutions made.  If
the variable to search and alter (@var{target}) is
omitted, then the entire input record (@code{$0}) is used.
As in @code{sub}, the characters @samp{&} and @samp{\} are special,
and the third argument must be assignable.

@item gensub(@var{regexp}, @var{replacement}, @var{how} @r{[}, @var{target}@r{]}) #
@cindex @code{gensub} built-in function
@code{gensub} is a general substitution function.  Like @code{sub} and
@code{gsub}, it searches the target string @var{target} for matches of
the regular expression @var{regexp}.  Unlike @code{sub} and @code{gsub},
the modified string is returned as the result of the function and the
original target string is @emph{not} changed.  If @var{how} is a string
beginning with @samp{g} or @samp{G}, then it replaces all matches of
@var{regexp} with @var{replacement}.  Otherwise, @var{how} is treated
as a number that indicates which match of @var{regexp} to replace. If
no @var{target} is supplied, @code{$0} is used.

@code{gensub} provides an additional feature that is not available
in @code{sub} or @code{gsub}: the ability to specify components of a
regexp in the replacement text.  This is done by using parentheses in
the regexp to mark the components and then specifying @samp{\@var{N}}
in the replacement text, where @var{N} is a digit from 1 to 9.
For example:

@example
$ gawk '
> BEGIN @{
>      a = "abc def"
>      b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
>      print b
> @}'
@print{} def abc
@end example

@noindent
As with @code{sub}, you must type two backslashes in order
to get one into the string.

In the replacement text, the sequence @samp{\0} represents the entire
matched text, as does the character @samp{&}.

The following example shows how you can use the third argument to control
which match of the regexp should be changed:

@example
$ echo a b c a b c |
> gawk '@{ print gensub(/a/, "AA", 2) @}'
@print{} a b c AA b c
@end example

In this case, @code{$0} is used as the default target string.
@code{gensub} returns the new string as its result, which is
passed directly to @code{print} for printing.

@cindex automatic warnings
@cindex warnings, automatic
If the @var{how} argument is a string that does not begin with @samp{g} or
@samp{G}, or if it is a number that is less than or equal to zero, only one
substitution is performed.  If @var{how} is zero, @command{gawk} issues
a warning message.

If @var{regexp} does not match @var{target}, @code{gensub}'s return value
is the original unchanged value of @var{target}.

@cindex differences between @command{gawk} and @command{awk}
@code{gensub} is a @command{gawk} extension; it is not available
in compatibility mode (@pxref{Options, ,Command-Line Options}).

@item substr(@var{string}, @var{start} @r{[}, @var{length}@r{]})
@cindex @code{substr} built-in function
This returns a @var{length}-character-long substring of @var{string},
starting at character number @var{start}.  The first character of a
string is character number one.@footnote{This is different from
C and C++, where the first character is number zero.}
For example, @code{substr("washington", 5, 3)} returns @code{"ing"}.

If @var{length} is not present, this function returns the whole suffix of
@var{string} that begins at character number @var{start}.  For example,
@code{substr("washington", 5)} returns @code{"ington"}.  The whole
suffix is also returned
if @var{length} is greater than the number of characters remaining
in the string, counting from character number @var{start}.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
The string returned by @code{substr} @emph{cannot} be
assigned.  Thus, it is a mistake to attempt to change a portion of
a string, as shown in the following example:

@example
string = "abcdef"
# try to get "abCDEf", won't work
substr(string, 3, 3) = "CDE"
@end example

@noindent
It is also a mistake to use @code{substr} as the third argument
of @code{sub} or @code{gsub}:

@example
gsub(/xyz/, "pdq", substr($0, 5, 20))  # WRONG
@end example

@cindex portability issues
(Some commercial versions of @command{awk} do in fact let you use
@code{substr} this way, but doing so is not portable.)

If you need to replace bits and pieces of a string, combine @code{substr}
with string concatenation, in the following manner:

@example
string = "abcdef"
@dots{}
string = substr(string, 1, 2) "CDE" substr(string, 6)
@end example

@cindex case conversion
@cindex conversion of case
@item tolower(@var{string})
@cindex @code{tolower} built-in function
This returns a copy of @var{string}, with each uppercase character
in the string replaced with its corresponding lowercase character.
Non-alphabetic characters are left unchanged.  For example,
@code{tolower("MiXeD cAsE 123")} returns @code{"mixed case 123"}.

@item toupper(@var{string})
@cindex @code{toupper} built-in function
This returns a copy of @var{string}, with each lowercase character
in the string replaced with its corresponding uppercase character.
Non-alphabetic characters are left unchanged.  For example,
@code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}.
@end table

@node Gory Details, , String Functions, String Functions
@subsubsection More About @samp{\} and @samp{&} with @code{sub}, @code{gsub}, and @code{gensub}

@cindex escape processing, @code{sub} et. al.
@cindex @code{sub}, escape processing
@cindex @code{gsub}, escape processing
@cindex @code{gensub}, escape processing
When using @code{sub}, @code{gsub}, or @code{gensub}, and trying to get literal
backslashes and ampersands into the replacement text, you need to remember
that there are several levels of @dfn{escape processing} going on.

First, there is the @dfn{lexical} level, which is when @command{awk} reads
your program
and builds an internal copy of it that can be executed.
Then there is the runtime level, which is when @command{awk} actually scans the
replacement string to determine what to generate.

At both levels, @command{awk} looks for a defined set of characters that
can come after a backslash.  At the lexical level, it looks for the
escape sequences listed in @ref{Escape Sequences}.
Thus, for every @samp{\} that @command{awk} processes at the runtime
level, type two backslashes at the lexical level.
When a character that is not valid for an escape sequence follows the
@samp{\}, Unix @command{awk} and @command{gawk} both simply remove the initial
@samp{\} and put the next character into the string. Thus, for
example, @code{"a\qb"} is treated as @code{"aqb"}.

At the runtime level, the various functions handle sequences of
@samp{\} and @samp{&} differently.  The situation is (sadly) somewhat complex.
Historically, the @code{sub} and @code{gsub} functions treated the two
character sequence @samp{\&} specially; this sequence was replaced in
the generated text with a single @samp{&}.  Any other @samp{\} within
the @var{replacement} string that did not precede an @samp{&} was passed
through unchanged.  To illustrate with a table:

@c Thank to Karl Berry for help with the TeX stuff.
@tex
\vbox{\bigskip
% This table has lots of &'s and \'s, so unspecialize them.
\catcode`\& = \other \catcode`\\ = \other
% But then we need character for escape and tab.
@catcode`! = 4
@halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr
    You type!@code{sub} sees!@code{sub} generates@cr
@hrulefill!@hrulefill!@hrulefill@cr
   @code{\&}!       @code{&}!the matched text@cr
  @code{\\&}!      @code{\&}!a literal @samp{&}@cr
 @code{\\\&}!      @code{\&}!a literal @samp{&}@cr
@code{\\\\&}!     @code{\\&}!a literal @samp{\&}@cr
@code{\\\\\&}!     @code{\\&}!a literal @samp{\&}@cr
@code{\\\\\\&}!     @code{\\\&}!a literal @samp{\\&}@cr
  @code{\\q}!      @code{\q}!a literal @samp{\q}@cr
}
@bigskip}
@end tex
@ifnottex
@display
 You type         @code{sub} sees          @code{sub} generates
 --------         ----------          ---------------
     @code{\&}              @code{&}            the matched text
    @code{\\&}             @code{\&}            a literal @samp{&}
   @code{\\\&}             @code{\&}            a literal @samp{&}
  @code{\\\\&}            @code{\\&}            a literal @samp{\&}
 @code{\\\\\&}            @code{\\&}            a literal @samp{\&}
@code{\\\\\\&}           @code{\\\&}            a literal @samp{\\&}
    @code{\\q}             @code{\q}            a literal @samp{\q}
@end display
@end ifnottex

@noindent
This table shows both the lexical-level processing, where
an odd number of backslashes becomes an even number at the runtime level,
as well as the runtime processing done by @code{sub}.
(For the sake of simplicity, the rest of the tables below only show the
case of even numbers of backslashes entered at the lexical level.)

The problem with the historical approach is that there is no way to get
a literal @samp{\} followed by the matched text.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
The 1992 POSIX standard attempted to fix this problem. The standard
says that @code{sub} and @code{gsub} look for either a @samp{\} or an @samp{&}
after the @samp{\}. If either one follows a @samp{\}, that character is
output literally.  The interpretation of @samp{\} and @samp{&} then becomes:

@c thanks to Karl Berry for formatting this table
@tex
\vbox{\bigskip
% This table has lots of &'s and \'s, so unspecialize them.
\catcode`\& = \other \catcode`\\ = \other
% But then we need character for escape and tab.
@catcode`! = 4
@halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr
    You type!@code{sub} sees!@code{sub} generates@cr
@hrulefill!@hrulefill!@hrulefill@cr
    @code{&}!       @code{&}!the matched text@cr
  @code{\\&}!      @code{\&}!a literal @samp{&}@cr
@code{\\\\&}!     @code{\\&}!a literal @samp{\}, then the matched text@cr
@code{\\\\\\&}!  @code{\\\&}!a literal @samp{\&}@cr
}
@bigskip}
@end tex
@ifnottex
@display
 You type         @code{sub} sees          @code{sub} generates
 --------         ----------          ---------------
      @code{&}              @code{&}            the matched text
    @code{\\&}             @code{\&}            a literal @samp{&}
  @code{\\\\&}            @code{\\&}            a literal @samp{\}, then the matched text
@code{\\\\\\&}           @code{\\\&}            a literal @samp{\&}
@end display
@end ifnottex

@noindent
This appears to solve the problem.
Unfortunately, the phrasing of the standard is unusual. It
says, in effect, that @samp{\} turns off the special meaning of any
following character, but for anything other than @samp{\} and @samp{&},
such special meaning is undefined.  This wording leads to two problems:

@itemize @bullet
@item
Backslashes must now be doubled in the @var{replacement} string, breaking
historical @command{awk} programs.

@item
To make sure that an @command{awk} program is portable, @emph{every} character
in the @var{replacement} string must be preceded with a
backslash.@footnote{This consequence was certainly unintended.}
@c I can say that, 'cause I was involved in making this change
@end itemize

The POSIX standard is under revision.
Because of the problems just listed, proposed text for the revised standard
reverts to rules that correspond more closely to the original existing
practice. The proposed rules have special cases that make it possible
to produce a @samp{\} preceding the matched text:

@tex
\vbox{\bigskip
% This table has lots of &'s and \'s, so unspecialize them.
\catcode`\& = \other \catcode`\\ = \other
% But then we need character for escape and tab.
@catcode`! = 4
@halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr
    You type!@code{sub} sees!@code{sub} generates@cr
@hrulefill!@hrulefill!@hrulefill@cr
@code{\\\\\\&}!     @code{\\\&}!a literal @samp{\&}@cr
@code{\\\\&}!     @code{\\&}!a literal @samp{\}, followed by the matched text@cr
  @code{\\&}!      @code{\&}!a literal @samp{&}@cr
  @code{\\q}!      @code{\q}!a literal @samp{\q}@cr
}
@bigskip}
@end tex
@ifinfo
@display
 You type         @code{sub} sees         @code{sub} generates
 --------         ----------         ---------------
@code{\\\\\\&}           @code{\\\&}            a literal @samp{\&}
  @code{\\\\&}            @code{\\&}            a literal @samp{\}, followed by the matched text
    @code{\\&}             @code{\&}            a literal @samp{&}
    @code{\\q}             @code{\q}            a literal @samp{\q}
@end display
@end ifinfo

In a nutshell, at the runtime level, there are now three special sequences
of characters (@samp{\\\&}, @samp{\\&} and @samp{\&}) whereas historically
there was only one.  However, as in the historical case, any @samp{\} that
is not part of one of these three sequences is not special and appears
in the output literally.

@command{gawk} 3.0 and 3.1 follow these proposed POSIX rules for @code{sub} and
@code{gsub}.
@c As much as we think it's a lousy idea. You win some, you lose some. Sigh.
Whether these proposed rules will actually become codified into the
standard is unknown at this point. Subsequent @command{gawk} releases will
track the standard and implement whatever the final version specifies;
this @value{DOCUMENT} will be updated as
well.@footnote{As this @value{DOCUMENT} was being finalized,
we learned that the POSIX standard will not use these rules.
However, it was too late to change @command{gawk} for the 3.1 release.
@command{gawk} behaves as described here.}

The rules for @code{gensub} are considerably simpler. At the runtime
level, whenever @command{gawk} sees a @samp{\}, if the following character
is a digit, then the text that matched the corresponding parenthesized
subexpression is placed in the generated output.  Otherwise,
no matter what the character after the @samp{\} is, it
appears in the generated text and the @samp{\} does not:

@tex
\vbox{\bigskip
% This table has lots of &'s and \'s, so unspecialize them.
\catcode`\& = \other \catcode`\\ = \other
% But then we need character for escape and tab.
@catcode`! = 4
@halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr
    You type!@code{gensub} sees!@code{gensub} generates@cr
@hrulefill!@hrulefill!@hrulefill@cr
      @code{&}!           @code{&}!the matched text@cr
    @code{\\&}!          @code{\&}!a literal @samp{&}@cr
   @code{\\\\}!          @code{\\}!a literal @samp{\}@cr
  @code{\\\\&}!         @code{\\&}!a literal @samp{\}, then the matched text@cr
@code{\\\\\\&}!        @code{\\\&}!a literal @samp{\&}@cr
    @code{\\q}!          @code{\q}!a literal @samp{q}@cr
}
@bigskip}
@end tex
@ifnottex
@display
  You type          @code{gensub} sees         @code{gensub} generates
  --------          -------------         ------------------
      @code{&}                    @code{&}            the matched text
    @code{\\&}                   @code{\&}            a literal @samp{&}
   @code{\\\\}                   @code{\\}            a literal @samp{\}
  @code{\\\\&}                  @code{\\&}            a literal @samp{\}, then the matched text
@code{\\\\\\&}                 @code{\\\&}            a literal @samp{\&}
    @code{\\q}                   @code{\q}            a literal @samp{q}
@end display
@end ifnottex

Because of the complexity of the lexical and runtime level processing
and the special cases for @code{sub} and @code{gsub},
we recommend the use of @command{gawk} and @code{gensub} when you have
to do substitutions.

@c fakenode --- for prepinfo
@subheading Advanced Notes: Matching the Null String
@cindex advanced notes
@cindex matching, the null string

In @command{awk}, the @samp{*} operator can match the null string.
This is particularly important for the @code{sub}, @code{gsub},
and @code{gensub} functions.  For example:

@example
$ echo abc | awk '@{ gsub(/m*/, "X"); print @}'
@print{} XaXbXcX
@end example

@noindent
Although this makes a certain amount of sense, it can be surprising.

@node I/O Functions, Time Functions, String Functions, Built-in
@subsection Input/Output Functions

The following functions relate to Input/Output (I/O).
Optional parameters are enclosed in square brackets ([ and ]):

@table @code
@item close(@var{filename} @r{[}, @var{how}@r{]})
@cindex @code{close} built-in function
Close the file @var{filename} for input or output. Alternatively, the
argument may be a shell command that was used for creating a coprocess, or
for redirecting to or from a pipe; then the coprocess or pipe is closed.
@xref{Close Files And Pipes, ,Closing Input and Output Redirections},
for more information.

When closing a coprocess, it is occasionally useful to first close
one end of the two-way pipe, and then to close the other.  This is done
by providing a second argument to @code{close}.  This second argument
should be one of the two string values @code{"to"} or @code{"from"},
indicating which end of the pipe to close.  Case in the string does
not matter.
@xref{Two-way I/O, ,Two-Way Communications with Another Process},
which discusses this feature in more detail and gives an example.

@item fflush(@r{[}@var{filename}@r{]})
@cindex @code{fflush} built-in function
@cindex portability issues
@cindex flushing buffers
@cindex buffers, flushing
@cindex buffering output
@cindex output, buffering
Flush any buffered output associated with @var{filename}, which is either a
file opened for writing or a shell command for redirecting output to
a pipe or coprocess.

Many utility programs @dfn{buffer} their output; i.e., they save information
to write to a disk file or terminal in memory, until there is enough
for it to be worthwhile to send the data to the output device.
This is often more efficient than writing
every little bit of information as soon as it is ready.  However, sometimes
it is necessary to force a program to @dfn{flush} its buffers; that is,
write the information to its destination, even if a buffer is not full.
This is the purpose of the @code{fflush} function---@command{gawk} also
buffers its output and the @code{fflush} function forces
@command{gawk} to flush its buffers.

@code{fflush} was added to the Bell Laboratories research
version of @command{awk} in 1994; it is not part of the POSIX standard and is
not available if @option{--posix} has been specified on the
command line (@pxref{Options, ,Command-Line Options}).

@command{gawk} extends the @code{fflush} function in two ways.  The first
is to allow no argument at all. In this case, the buffer for the
standard output is flushed.  The second is to allow the null string
(@w{@code{""}}) as the argument. In this case, the buffers for
@emph{all} open output files and pipes are flushed.

@cindex automatic warnings
@cindex warnings, automatic
@code{fflush} returns zero if the buffer is successfully flushed;
otherwise it returns @minus{}1.
In the case where all buffers are flushed, the return value is zero
only if all buffers were flushed successfully.  Otherwise, it is
@minus{}1, and @command{gawk} warns about the @var{filename} that had the problem.

@command{gawk} also issues a warning message if you attempt to flush
a file or pipe that was opened for reading (such as with @code{getline}),
or if @var{filename} is not an open file, pipe, or coprocess.
In such a case, @code{fflush} returns @minus{}1 as well.

@item system(@var{command})
@cindex @code{system} built-in function
@cindex interaction, @command{awk} and other programs
The @code{system} function allows the user to execute operating system
commands and then return to the @command{awk} program.  The @code{system}
function executes the command given by the string @var{command}.
It returns the status returned by the command that was executed as
its value.

For example, if the following fragment of code is put in your @command{awk}
program:

@example
END @{
     system("date | mail -s 'awk run done' root")
@}
@end example

@noindent
the system administrator is sent mail when the @command{awk} program
finishes processing input and begins its end-of-input processing.

Note that redirecting @code{print} or @code{printf} into a pipe is often
enough to accomplish your task.  If you need to run many commands, it
is more efficient to simply print them down a pipeline to the shell:

@example
while (@var{more stuff to do})
    print @var{command} | "/bin/sh"
close("/bin/sh")
@end example

@noindent
@cindex fatal errors
However, if your @command{awk}
program is interactive, @code{system} is useful for cranking up large
self-contained programs, such as a shell or an editor.
Some operating systems cannot implement the @code{system} function.
@code{system} causes a fatal error if it is not supported.
@end table

@c fakenode --- for prepinfo
@subheading Advanced Notes: Interactive Versus Non-Interactive Buffering
@cindex advanced notes
@cindex buffering, interactive vs. non-interactive
@cindex buffering, non-interactive vs. interactive
@cindex interactive buffering vs. non-interactive
@cindex non-interactive buffering vs. interactive

As a side point, buffering issues can be even more confusing, depending
upon whether your program is @dfn{interactive}; i.e., communicating
with a user sitting at a keyboard.@footnote{A program is interactive
if the standard output is connected
to a terminal device.}

@c Thanks to Walter.Mecky@dresdnerbank.de for this example, and for
@c motivating me to write this section.
Interactive programs generally @dfn{line buffer} their output; i.e., they
write out every line.  Non-interactive programs wait until they have
a full buffer, which may be many lines of output.
Here is an example of the difference:

@example
$ awk '@{ print $1 + $2 @}'
1 1
@print{} 2
2 3
@print{} 5
@kbd{Ctrl-d}
@end example

@noindent
Each line of output is printed immediately. Compare that behavior
with this example:

@example
$ awk '@{ print $1 + $2 @}' | cat
1 1
2 3
@kbd{Ctrl-d}
@print{} 2
@print{} 5
@end example

@noindent
Here, no output is printed until after the @kbd{Ctrl-d} is typed, because
it is all buffered and sent down the pipe to @command{cat} in one shot.

@c fakenode --- for prepinfo
@subheading Advanced Notes: Controlling Output Buffering with @code{system}
@cindex advanced notes
@cindex flushing buffers
@cindex buffers, flushing
@cindex buffering output
@cindex output, buffering

The @code{fflush} function provides explicit control over output buffering for
individual files and pipes.  However, its use is not portable to many other
@command{awk} implementations.  An alternative method to flush output
buffers is to call @code{system} with a null string as its argument:

@example
system("")   # flush output
@end example

@noindent
@command{gawk} treats this use of the @code{system} function as a special
case and is smart enough not to run a shell (or other command
interpreter) with the empty command.  Therefore, with @command{gawk}, this
idiom is not only useful, it is also efficient.  While this method should work
with other @command{awk} implementations, it does not necessarily avoid
starting an unnecessary shell.  (Other implementations may only
flush the buffer associated with the standard output and not necessarily
all buffered output.)

If you think about what a programmer expects, it makes sense that
@code{system} should flush any pending output.  The following program:

@example
BEGIN @{
     print "first print"
     system("echo system echo")
     print "second print"
@}
@end example

@noindent
must print:

@example
first print
system echo
second print
@end example

@noindent
and not:

@example
system echo
first print
second print
@end example

If @command{awk} did not flush its buffers before calling @code{system}, the
latter (undesirable) output is what you see.

@node Time Functions, Bitwise Functions, I/O Functions, Built-in
@subsection Using @command{gawk}'s Timestamp Functions

@cindex timestamps
@cindex time of day
A common use for @command{awk} programs is the processing of log files
containing timestamp information, indicating when a
particular log record was written.  Many programs log their timestamp
in the form returned by the @code{time} system call, which is the
number of seconds since a particular epoch.  On POSIX-compliant systems,
it is the number of seconds since
1970-01-01 00:00:00 UTC, not counting leap seconds.@footnote{@xref{Glossary},
especially the entries for ``Epoch'' and ``UTC.''}
All known POSIX-compliant systems support timestamps from 0 through
@math{2^31 - 1}, which is sufficient to represent times through
2038-01-19 03:14:07 UTC.  Many systems support a wider range of timestamps,
including negative timestamps that represent times before the
epoch.

In order to make it easier to process such log files and to produce
useful reports, @command{gawk} provides the following functions for
working with timestamps.  They are @command{gawk} extensions; they are
not specified in the POSIX standard, nor are they in any other known
version of @command{awk}.@footnote{The GNU @command{date} utility can
also do many of the things described here.  It's use may be preferable
for simple time-related operations in shell scripts.}
Optional parameters are enclosed in square brackets ([ and ]):

@table @code
@item systime()
@cindex @code{systime} built-in function
This function returns the current time as the number of seconds since
the system epoch.  On POSIX systems, this is the number of seconds
since 1970-01-01 00:00:00 UTC, not counting leap seconds.
It may be a different number on
other systems.

@item mktime(@var{datespec})
@cindex @code{mktime} built-in function
This function turns @var{datespec} into a timestamp in the same form
as is returned by @code{systime}.  It is similar to the function of the
same name in ISO C.  The argument, @var{datespec}, is a string of the form
@w{@code{"@var{YYYY} @var{MM} @var{DD} @var{HH} @var{MM} @var{SS} [@var{DST}]"}}.
The string consists of six or seven numbers representing, respectively,
the full year including century, the month from 1 to 12, the day of the month
from 1 to 31, the hour of the day from 0 to 23, the minute from 0 to
59, the second from 0 to 60,@footnote{Occasionally there are
minutes in a year with a leap second, which is why the
seconds can go up to 60.}
and an optional daylight savings flag.

The values of these numbers need not be within the ranges specified;
for example, an hour of @minus{}1 means 1 hour before midnight.
The origin-zero Gregorian calendar is assumed, with year 0 preceding
year 1 and year @minus{}1 preceding year 0.
The time is assumed to be in the local timezone.
If the daylight savings flag is positive, the time is assumed to be
daylight savings time; if zero, the time is assumed to be standard
time; and if negative (the default), @code{mktime} attempts to determine
whether daylight savings time is in effect for the specified time.

If @var{datespec} does not contain enough elements or if the resulting time
is out of range, @code{mktime} returns @minus{}1.

@item strftime(@r{[}@var{format} @r{[}, @var{timestamp}@r{]]})
@cindex @code{strftime} built-in function
This function returns a string.  It is similar to the function of the
same name in ISO C.  The time specified by @var{timestamp} is used to
produce a string, based on the contents of the @var{format} string.
The @var{timestamp} is in the same format as the value returned by the
@code{systime} function.  If no @var{timestamp} argument is supplied,
@command{gawk} uses the current time of day as the timestamp.
If no @var{format} argument is supplied, @code{strftime} uses
@code{@w{"%a %b %d %H:%M:%S %Z %Y"}}.  This format string produces
output that is (almost) equivalent to that of the @command{date} utility.
(Versions of @command{gawk} prior to 3.0 require the @var{format} argument.)
@end table

The @code{systime} function allows you to compare a timestamp from a
log file with the current time of day.  In particular, it is easy to
determine how long ago a particular record was logged.  It also allows
you to produce log records using the ``seconds since the epoch'' format.

@cindex converting dates to timestamps
@cindex dates, converting to timestamps
@cindex timestamps, converting from dates
The @code{mktime} function allows you to convert a textual representation
of a date and time into a timestamp.   This makes it easy to do before/after
comparisons of dates and times, particularly when dealing with date and
time data coming from an external source, such as a log file.

The @code{strftime} function allows you to easily turn a timestamp
into human-readable information.  It is similar in nature to the @code{sprintf}
function
(@pxref{String Functions, ,String Manipulation Functions}),
in that it copies non-format specification characters verbatim to the
returned string, while substituting date and time values for format
specifications in the @var{format} string.

@code{strftime} is guaranteed by the 1999 ISO C standard@footnote{As this
is a recent standard, not every system's @code{strftime} necessarily
supports all of the conversions listed here.}
to support the following date format specifications:

@cindex format specifier, @code{strftime}
@table @code
@item %a
The locale's abbreviated weekday name.

@item %A
The locale's full weekday name.

@item %b
The locale's abbreviated month name.

@item %B
The locale's full month name.

@item %c
The locale's ``appropriate'' date and time representation.
(This is @samp{%A %B %d %T %Y} in the @code{"C"} locale.)

@item %C
The century.  This is the year divided by 100 and truncated to the next
lower integer.

@item %d
The day of the month as a decimal number (01--31).

@item %D
Equivalent to specifying @samp{%m/%d/%y}.

@item %e
The day of the month, padded with a space if it is only one digit.

@item %F
Equivalent to specifying @samp{%Y-%m-%d}.
This is the ISO 8601 date format.

@item %g
The year modulo 100 of the ISO week number, as a decimal number (00--99).
For example, January 1, 1993, is in week 53 of 1992. Thus, the year
of its ISO week number is 1992, even though its year is 1993.
Similarly, December 31, 1973, is in week 1 of 1974. Thus, the year
of its ISO week number is 1974, even though its year is 1973.

@item %G
The full year of the ISO week number, as a decimal number.

@item %h
Equivalent to @samp{%b}.

@item %H
The hour (24-hour clock) as a decimal number (00--23).

@item %I
The hour (12-hour clock) as a decimal number (01--12).

@item %j
The day of the year as a decimal number (001--366).

@item %m
The month as a decimal number (01--12).

@item %M
The minute as a decimal number (00--59).

@item %n
A newline character (ASCII LF).

@item %p
The locale's equivalent of the AM/PM designations associated
with a 12-hour clock.

@item %r
The locale's 12-hour clock time.
(This is @samp{%I:%M:%S %p} in the @code{"C"} locale.)

@item %R
Equivalent to specifying @samp{%H:%M}.

@item %S
The second as a decimal number (00--60).

@item %t
A tab character.

@item %T
Equivalent to specifying @samp{%H:%M:%S}.

@item %u
The weekday as a decimal number (1--7).  Monday is day one.

@item %U
The week number of the year (the first Sunday as the first day of week one)
as a decimal number (00--53).

@cindex ISO 8601
@item %V
The week number of the year (the first Monday as the first
day of week one) as a decimal number (01--53).
The method for determining the week number is as specified by ISO 8601.
(To wit: if the week containing January 1 has four or more days in the
new year, then it is week one, otherwise it is week 53 of the previous year
and the next week is week one.)

@item %w
The weekday as a decimal number (0--6).  Sunday is day zero.

@item %W
The week number of the year (the first Monday as the first day of week one)
as a decimal number (00--53).

@item %x
The locale's ``appropriate'' date representation.
(This is @samp{%A %B %d %Y} in the @code{"C"} locale.)

@item %X
The locale's ``appropriate'' time representation.
(This is @samp{%T} in the @code{"C"} locale.)

@item %y
The year modulo 100 as a decimal number (00--99).

@item %Y
The full year as a decimal number (e.g., 1995).

@cindex RFC 822
@cindex RFC 1036
@item %z
The timezone offset in a +HHMM format (e.g., the format necessary to
produce RFC 822/RFC 1036 date headers).

@item %Z
The time zone name or abbreviation; no characters if
no time zone is determinable.

@item %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH
@itemx %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy
These are ``alternate representations'' for the specifications
that use only the second letter (@samp{%c}, @samp{%C},
and so on).@footnote{If you don't understand any of this, don't worry about
it; these facilities are meant to make it easier to ``internationalize''
programs.
Other internationalization features are described in
@ref{Internationalization, ,Internationalization with @command{gawk}}.}
(These facilitate compliance with the POSIX @command{date} utility.)

@item %%
A literal @samp{%}.
@end table

If a conversion specifier is not one of the above, the behavior is
undefined.@footnote{This is because ISO C leaves the
behavior of the C version of @code{strftime} undefined and @command{gawk}
uses the system's version of @code{strftime} if it's there.
Typically, the conversion specifier either does not appear in the
returned string or it appears literally.}

@cindex locale, definition of
Informally, a @dfn{locale} is the geographic place in which a program
is meant to run.  For example, a common way to abbreviate the date
September 4, 1991 in the United States is ``9/4/91.''
In many countries in Europe, however, it is abbreviated ``4.9.91.''
Thus, the @samp{%x} specification in a @code{"US"} locale might produce
@samp{9/4/91}, while in a @code{"EUROPE"} locale, it might produce
@samp{4.9.91}.  The ISO C standard defines a default @code{"C"}
locale, which is an environment that is typical of what most C programmers
are used to.

A public-domain C version of @code{strftime} is supplied with @command{gawk}
for systems that are not yet fully standards-compliant.
It supports all of the just listed format specifications.
If that version is
used to compile @command{gawk} (@pxref{Installation, ,Installing @command{gawk}}),
then the following additional format specifications are available:

@table @code
@item %k
The hour (24-hour clock) as a decimal number (0--23).
Single digit numbers are padded with a space.

@item %l
The hour (12-hour clock) as a decimal number (1--12).
Single digit numbers are padded with a space.

@item %N
The ``Emperor/Era'' name.
Equivalent to @code{%C}.

@item %o
The ``Emperor/Era'' year.
Equivalent to @code{%y}.

@item %s
The time as a decimal timestamp in seconds since the epoch.

@item %v
The date in VMS format (e.g., @samp{20-JUN-1991}).
@end table

Additionally, the alternate representations are recognized but their
normal representations are used.

This example is an @command{awk} implementation of the POSIX
@command{date} utility.  Normally, the @command{date} utility prints the
current date and time of day in a well-known format.  However, if you
provide an argument to it that begins with a @samp{+}, @command{date}
copies non-format specifier characters to the standard output and
interprets the current time according to the format specifiers in
the string.  For example:

@example
$ date '+Today is %A, %B %d, %Y.'
@print{} Today is Thursday, September 14, 2000.
@end example

Here is the @command{gawk} version of the @command{date} utility.
It has a shell ``wrapper'' to handle the @option{-u} option,
which requires that @command{date} run as if the time zone
is set to UTC:

@example
#! /bin/sh
#
# date --- approximate the P1003.2 'date' command

case $1 in
-u)  TZ=UTC0     # use UTC
     export TZ
     shift ;;
esac

@c FIXME: One day, change %d to %e, when C 99 is common.
gawk 'BEGIN  @{
    format = "%a %b %d %H:%M:%S %Z %Y"
    exitval = 0

    if (ARGC > 2)
        exitval = 1
    else if (ARGC == 2) @{
        format = ARGV[1]
        if (format ~ /^\+/)
            format = substr(format, 2)   # remove leading +
    @}
    print strftime(format)
    exit exitval
@}' "$@@"
@end example

@node Bitwise Functions, I18N Functions, Time Functions, Built-in
@subsection Using @command{gawk}'s Bit Manipulation Functions
@cindex bitwise operations
@quotation
@i{I can explain it for you, but I can't understand it for you.}@*
Anonymous
@end quotation

@cindex AND bitwise operation
@cindex OR bitwise operation
@cindex XOR bitwise operation
Many languages provide the ability to perform @dfn{bitwise} operations
on two integer numbers.  In other words, the operation is performed on
each successive pair of bits in the operands.
Three common operations are bitwise AND, OR, and XOR.
The operations are described by the following table:

@ifnottex
@display
                Bit Operator
          |  AND  |   OR  |  XOR
          |---+---+---+---+---+---
Operands  | 0 | 1 | 0 | 1 | 0 | 1
----------+---+---+---+---+---+---
    0     | 0   0 | 0   1 | 0   1
    1     | 0   1 | 1   1 | 1   0
@end display
@end ifnottex
@tex
\centerline{
\vbox{\bigskip % space above the table (about 1 linespace)
% Because we have vertical rules, we can't let TeX insert interline space
% in its usual way.
\offinterlineskip
\halign{\strut\hfil#\quad\hfil  % operands
        &\vrule#&\quad#\quad    % rule, 0 (of and)
        &\vrule#&\quad#\quad    % rule, 1 (of and)
        &\vrule#                % rule between and and or
        &\quad#\quad            % 0 (of or)
        &\vrule#&\quad#\quad    % rule, 1 (of of)
        &\vrule#                % rule between or and xor
        &\quad#\quad            % 0 of xor
        &\vrule#&\quad#\quad    % rule, 1 of xor
        \cr
&\omit&\multispan{11}\hfil\bf Bit operator\hfil\cr
\noalign{\smallskip}
&     &\multispan3\hfil AND\hfil&&\multispan3\hfil  OR\hfil
                           &&\multispan3\hfil XOR\hfil\cr
\bf Operands&&0&&1&&0&&1&&0&&1\cr
\noalign{\hrule}
\omit&height 2pt&&\omit&&&&\omit&&&&\omit\cr
\noalign{\hrule height0pt}% without this the rule does not extend; why?
0&&0&\omit&0&&0&\omit&1&&0&\omit&1\cr
1&&0&\omit&1&&1&\omit&1&&1&\omit&0\cr
}}}
@end tex

@cindex bitwise complement
@cindex complement, bitwise
As you can see, the result of an AND operation is 1 only when @emph{both}
bits are 1.
The result of an OR operation is 1 if @emph{either} bit is 1.
The result of an XOR operation is 1 if either bit is 1,
but not both.
The next operation is the @dfn{complement}; the complement of 1 is 0 and
the complement of 0 is 1. Thus, this operation ``flips'' all the bits
of a given value.

@cindex bitwise shift
@cindex left shift, bitwise
@cindex right shift, bitwise
@cindex shift, bitwise
Finally, two other common operations are to shift the bits left or right.
For example, if you have a bit string @samp{10111001} and you shift it
right by three bits, you end up with @samp{00010111}.@footnote{This example
shows that 0's come in on the left side. For @command{gawk}, this is
always true, but in some languages, it's possible to have the left side
fill with 1's. Caveat emptor.}
@c Purposely decided to use   0's   and   1's   here.  2/2001.
If you start over
again with @samp{10111001} and shift it left by three bits, you end up
with @samp{11001000}.
@command{gawk} provides built-in functions that implement the
bitwise operations just described. They are:

@ignore
@table @code
@cindex @code{and} built-in function
@item and(@var{v1}, @var{v2})
Return the bitwise AND of the values provided by @var{v1} and @var{v2}.

@cindex @code{or} built-in function
@item or(@var{v1}, @var{v2})
Return the bitwise OR of the values provided by @var{v1} and @var{v2}.

@cindex @code{xor} built-in function
@item xor(@var{v1}, @var{v2})
Return the bitwise XOR of the values provided by @var{v1} and @var{v2}.

@cindex @code{compl} built-in function
@item compl(@var{val})
Return the bitwise complement of @var{val}.

@cindex @code{lshift} built-in function
@item lshift(@var{val}, @var{count})
Return the value of @var{val}, shifted left by @var{count} bits.

@cindex @code{rshift} built-in function
@item rshift(@var{val}, @var{count})
Return the value of @var{val}, shifted right by @var{count} bits.
@end table
@end ignore

@multitable {@code{rshift(@var{val}, @var{count})}} {Return the value of @var{val}, shifted right by @var{count} bits.}
@cindex @code{and} built-in function
@item @code{and(@var{v1}, @var{v2})}
@tab Return the bitwise AND of the values provided by @var{v1} and @var{v2}.

@cindex @code{or} built-in function
@item @code{or(@var{v1}, @var{v2})}
@tab Return the bitwise OR of the values provided by @var{v1} and @var{v2}.

@cindex @code{xor} built-in function
@item @code{xor(@var{v1}, @var{v2})}
@tab Return the bitwise XOR of the values provided by @var{v1} and @var{v2}.

@cindex @code{compl} built-in function
@item @code{compl(@var{val})}
@tab Return the bitwise complement of @var{val}.

@cindex @code{lshift} built-in function
@item @code{lshift(@var{val}, @var{count})}
@tab Return the value of @var{val}, shifted left by @var{count} bits.

@cindex @code{rshift} built-in function
@item @code{rshift(@var{val}, @var{count})}
@tab Return the value of @var{val}, shifted right by @var{count} bits.
@end multitable

For all of these functions, first the double-precision floating-point value is
converted to a C @code{unsigned long}, then the bitwise operation is
performed and then the result is converted back into a C @code{double}. (If
you don't understand this paragraph, don't worry about it.)

Here is a user-defined function
(@pxref{User-defined, ,User-Defined Functions})
that illustrates the use of these functions:

@cindex @code{bits2str} user-defined function
@cindex @code{testbits.awk} program
@smallexample
@group
@c file eg/lib/bits2str.awk
# bits2str --- turn a byte into readable 1's and 0's

function bits2str(bits,        data, mask)
@{
    if (bits == 0)
        return "0"

    mask = 1
    for (; bits != 0; bits = rshift(bits, 1))
        data = (and(bits, mask) ? "1" : "0") data

    while ((length(data) % 8) != 0)
        data = "0" data

    return data
@}
@c endfile
@end group

@c this is a hack to make testbits.awk self-contained
@ignore
@c file eg/prog/testbits.awk
# bits2str --- turn a byte into readable 1's and 0's

function bits2str(bits,        data, mask)
@{
    if (bits == 0)
        return "0"

    mask = 1
    for (; bits != 0; bits = rshift(bits, 1))
        data = (and(bits, mask) ? "1" : "0") data

    while ((length(data) % 8) != 0)
        data = "0" data

    return data
@}
@c endfile
@end ignore
@c file eg/prog/testbits.awk
BEGIN @{
    printf "123 = %s\n", bits2str(123)
    printf "0123 = %s\n", bits2str(0123)
    printf "0x99 = %s\n", bits2str(0x99)
    comp = compl(0x99)
    printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp)
    shift = lshift(0x99, 2)
    printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
    shift = rshift(0x99, 2)
    printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
@}
@c endfile
@end smallexample

@noindent
This program produces the following output when run:

@smallexample
$ gawk -f testbits.awk
@print{} 123 = 01111011
@print{} 0123 = 01010011
@print{} 0x99 = 10011001
@print{} compl(0x99) = 0xffffff66 = 11111111111111111111111101100110
@print{} lshift(0x99, 2) = 0x264 = 0000001001100100
@print{} rshift(0x99, 2) = 0x26 = 00100110
@end smallexample

The @code{bits2str} function turns a binary number into a string.
The number @code{1} represents a binary value where the rightmost bit
is set to 1.  Using this mask,
the function repeatedly checks the rightmost bit.
AND-ing the mask with the value indicates whether the
rightmost bit is 1 or not. If so, a @code{"1"} is concatenated onto the front
of the string.
Otherwise, a @code{"0"} is added.
The value is then shifted right by one bit and the loop continues
until there are no more 1 bits.

If the initial value is zero it returns a simple @code{"0"}.
Otherwise, at the end, it pads the value with zeros to represent multiples
of eight-bit quantities. This is typical in modern computers.

The main code in the @code{BEGIN} rule shows the difference between the
decimal and octal values for the same numbers
(@pxref{Non-decimal-numbers, ,Octal and Hexadecimal Numbers}),
and then demonstrates the
results of the @code{compl}, @code{lshift}, and @code{rshift} functions.

@node I18N Functions, , Bitwise Functions, Built-in
@subsection Using @command{gawk}'s String Translation Functions

@command{gawk} provides facilities for internationalizing @command{awk} programs.
These include the functions described in the following list.
The description here is purposely brief.
@xref{Internationalization, ,Internationalization with @command{gawk}},
for the full story.
Optional parameters are enclosed in square brackets ([ and ]):

@table @code
@cindex @code{dcgettext} built-in function
@item dcgettext(@var{string} @r{[}, @var{domain} @r{[}, @var{category}@r{]]})
This function returns the translation of @var{string} in
text domain @var{domain} for locale category @var{category}.
The default value for @var{domain} is the current value of @code{TEXTDOMAIN}.
The default value for @var{category} is @code{"LC_MESSAGES"}.

@cindex @code{bindtextdomain} built-in function
@item bindtextdomain(@var{directory} @r{[}, @var{domain}@r{]})
This function allows you to specify the directory where
@command{gawk} will look for message translation files, in case they
will not or cannot be placed in the ``standard'' locations
(e.g., during testing).
It returns the directory where @var{domain} is ``bound.''

The default @var{domain} is the value of @code{TEXTDOMAIN}.
If @var{directory} is the null string (@code{""}), then
@code{bindtextdomain} returns the current binding for the
given @var{domain}.
@end table

@node User-defined, , Built-in, Functions
@section User-Defined Functions

@cindex user-defined functions
@cindex function, user-defined
Complicated @command{awk} programs can often be simplified by defining
your own functions.  User-defined functions can be called just like
built-in ones (@pxref{Function Calls}), but it is up to you to define
them; i.e., to tell @command{awk} what they should do.

@menu
* Definition Syntax::           How to write definitions and what they mean.
* Function Example::            An example function definition and what it
                                does.
* Function Caveats::            Things to watch out for.
* Return Statement::            Specifying the value a function returns.
* Dynamic Typing::              How variable types can change at runtime.
@end menu

@node Definition Syntax, Function Example, User-defined, User-defined
@subsection Function Definition Syntax
@cindex defining functions
@cindex function definition

Definitions of functions can appear anywhere between the rules of an
@command{awk} program.  Thus, the general form of an @command{awk} program is
extended to include sequences of rules @emph{and} user-defined function
definitions.
There is no need to put the definition of a function
before all uses of the function.  This is because @command{awk} reads the
entire program before starting to execute any of it.

The definition of a function named @var{name} looks like this:
@c NEXT ED: put [ ] around parameter list

@example
function @var{name}(@var{parameter-list})
@{
     @var{body-of-function}
@}
@end example

@cindex names, use of
@cindex namespace issues in @command{awk}
@noindent
@var{name} is the name of the function to define.  A valid function
name is like a valid variable name: a sequence of letters, digits, and
underscores, that doesn't start with a digit.
Within a single @command{awk} program, any particular name can only be
used as a variable, array, or function.

@c NEXT ED: parameter-list is an OPTIONAL list of ...
@var{parameter-list} is a list of the function's arguments and local
variable names, separated by commas.  When the function is called,
the argument names are used to hold the argument values given in
the call.  The local variables are initialized to the empty string.
A function cannot have two parameters with the same name, nor may it
have a parameter with the same name as the function itself.

The @var{body-of-function} consists of @command{awk} statements.  It is the
most important part of the definition, because it says what the function
should actually @emph{do}.  The argument names exist to give the body a
way to talk about the arguments; local variables exist to give the body
places to keep temporary values.

Argument names are not distinguished syntactically from local variable
names. Instead, the number of arguments supplied when the function is
called determines how many argument variables there are.  Thus, if three
argument values are given, the first three names in @var{parameter-list}
are arguments and the rest are local variables.

It follows that if the number of arguments is not the same in all calls
to the function, some of the names in @var{parameter-list} may be
arguments on some occasions and local variables on others.  Another
way to think of this is that omitted arguments default to the
null string.

@cindex conventions, programming
@cindex programming conventions
Usually when you write a function, you know how many names you intend to
use for arguments and how many you intend to use as local variables.  It is
conventional to place some extra space between the arguments and
the local variables, in order to document how your function is supposed to be used.

@cindex variable shadowing
During execution of the function body, the arguments and local variable
values hide or @dfn{shadow} any variables of the same names used in the
rest of the program.  The shadowed variables are not accessible in the
function definition, because there is no way to name them while their
names have been taken away for the local variables.  All other variables
used in the @command{awk} program can be referenced or set normally in the
function's body.

The arguments and local variables last only as long as the function body
is executing.  Once the body finishes, you can once again access the
variables that were shadowed while the function was running.

@cindex recursive function
@cindex function, recursive
The function body can contain expressions that call functions.  They
can even call this function, either directly or by way of another
function.  When this happens, we say the function is @dfn{recursive}.
The act of a function calling itself is called @dfn{recursion}.

@cindex @command{awk} language, POSIX version
@cindex POSIX @command{awk}
In many @command{awk} implementations, including @command{gawk},
the keyword @code{function} may be
abbreviated @code{func}.  However, POSIX only specifies the use of
the keyword @code{function}.  This actually has some practical implications.
If @command{gawk} is in POSIX-compatibility mode
(@pxref{Options, ,Command-Line Options}), then the following
statement does @emph{not} define a function:

@example
func foo() @{ a = sqrt($1) ; print a @}
@end example

@noindent
Instead it defines a rule that, for each record, concatenates the value
of the variable @samp{func} with the return value of the function @samp{foo}.
If the resulting string is non-null, the action is executed.
This is probably not what is desired.  (@command{awk} accepts this input as
syntactically valid, because functions may be used before they are defined
in @command{awk} programs.)
@c NEXT ED: This won't actually run, since foo() is undefined ...

@cindex portability issues
To ensure that your @command{awk} programs are portable, always use the
keyword @code{function} when defining a function.

@node Function Example, Function Caveats, Definition Syntax, User-defined
@subsection Function Definition Examples

Here is an example of a user-defined function, called @code{myprint}, that
takes a number and prints it in a specific format:

@example
function myprint(num)
@{
     printf "%6.3g\n", num
@}
@end example

@noindent
To illustrate, here is an @command{awk} rule that uses our @code{myprint}
function:

@example
$3 > 0     @{ myprint($3) @}
@end example

@noindent
This program prints, in our special format, all the third fields that
contain a positive number in our input.  Therefore, when given the following:

@example
 1.2   3.4    5.6   7.8
 9.10 11.12 -13.14 15.16
17.18 19.20  21.22 23.24
@end example

@noindent
this program, using our function to format the results, prints:

@example
   5.6
  21.2
@end example

@page
This function deletes all the elements in an array:

@example
function delarray(a,    i)
@{
    for (i in a)
       delete a[i]
@}
@end example

When working with arrays, it is often necessary to delete all the elements
in an array and start over with a new list of elements
(@pxref{Delete, ,The @code{delete} Statement}).
Instead of having
to repeat this loop everywhere that you need to clear out
an array, your program can just call @code{delarray}.
(This guarantees portability.  The use of @samp{delete @var{array}} to delete
the contents of an entire array is a non-standard extension.)

The following is an example of a recursive function.  It takes a string
as an input parameter and returns the string in backwards order.
Recursive functions must always have a test that stops the recursion.
In this case, the recursion terminates when the starting position
is zero; i.e., when there are no more characters left in the string.

@example
function rev(str, start)
@{
    if (start == 0)
        return ""

    return (substr(str, start, 1) rev(str, start - 1))
@}
@end example

If this function is in a file named @file{rev.awk}, it can be tested
this way:

@example
$ echo "Don't Panic!" |
> gawk --source '@{ print rev($0, length($0)) @}' -f rev.awk
@print{} !cinaP t'noD
@end example

The C @code{ctime} function takes a timestamp and returns it in a string,
formatted in a well-known fashion.
The following example uses the built-in @code{strftime} function
(@pxref{Time Functions, ,Using @command{gawk}'s Timestamp Functions})
to create an @command{awk} version of @code{ctime}:

@c FIXME: One day, change %d to %e, when C 99 is common.
@example
@c file eg/lib/ctime.awk
# ctime.awk
#
# awk version of C ctime(3) function

function ctime(ts,    format)
@{
    format = "%a %b %d %H:%M:%S %Z %Y"
    if (ts == 0)
        ts = systime()       # use current time as default
    return strftime(format, ts)
@}
@c endfile
@end example

@node Function Caveats, Return Statement, Function Example, User-defined
@subsection Calling User-Defined Functions

@cindex calling a function
@cindex function call
@dfn{Calling a function} means causing the function to run and do its job.
A function call is an expression and its value is the value returned by
the function.

A function call consists of the function name followed by the arguments
in parentheses.  @command{awk} expressions are what you write in the
call for the arguments.  Each time the call is executed, these
expressions are evaluated, and the values are the actual arguments.  For
example, here is a call to @code{foo} with three arguments (the first
being a string concatenation):

@example
foo(x y, "lose", 4 * z)
@end example

@strong{Caution:} Whitespace characters (spaces and tabs) are not allowed
between the function name and the open-parenthesis of the argument list.
If you write whitespace by mistake, @command{awk} might think that you mean
to concatenate a variable with an expression in parentheses.  However, it
notices that you used a function name and not a variable name, and reports
an error.

@cindex call by value
When a function is called, it is given a @emph{copy} of the values of
its arguments.  This is known as @dfn{call by value}.  The caller may use
a variable as the expression for the argument, but the called function
does not know this---it only knows what value the argument had.  For
example, if you write the following code:

@example
foo = "bar"
z = myfunc(foo)
@end example

@noindent
then you should not think of the argument to @code{myfunc} as being
``the variable @code{foo}.''  Instead, think of the argument as the
string value @code{"bar"}.
If the function @code{myfunc} alters the values of its local variables,
this has no effect on any other variables.  Thus, if @code{myfunc}
does this:

@example
function myfunc(str)
@{
  print str
  str = "zzz"
  print str
@}
@end example

@noindent
to change its first argument variable @code{str}, it @emph{does not}
change the value of @code{foo} in the caller.  The role of @code{foo} in
calling @code{myfunc} ended when its value (@code{"bar"}) was computed.
If @code{str} also exists outside of @code{myfunc}, the function body
cannot alter this outer value, because it is shadowed during the
execution of @code{myfunc} and cannot be seen or changed from there.

@cindex call by reference
However, when arrays are the parameters to functions, they are @emph{not}
copied.  Instead, the array itself is made available for direct manipulation
by the function.  This is usually called @dfn{call by reference}.
Changes made to an array parameter inside the body of a function @emph{are}
visible outside that function.

@strong{Note:} Changing an array parameter inside a function
can be very dangerous if you do not watch what you are doing.
For example:

@example
function changeit(array, ind, nvalue)
@{
     array[ind] = nvalue
@}

BEGIN @{
    a[1] = 1; a[2] = 2; a[3] = 3
    changeit(a, 2, "two")
    printf "a[1] = %s, a[2] = %s, a[3] = %s\n",
            a[1], a[2], a[3]
@}
@end example

@noindent
This program prints @samp{a[1] = 1, a[2] = two, a[3] = 3}, because
@code{changeit} stores @code{"two"} in the second element of @code{a}.

@cindex undefined functions
@cindex functions, undefined
Some @command{awk} implementations allow you to call a function that
has not been defined. They only report a problem at runtime when the
program actually tries to call the function. For example:

@example
BEGIN @{
    if (0)
        foo()
    else
        bar()
@}
function bar() @{ @dots{} @}
# note that `foo' is not defined
@end example

@noindent
Because the @samp{if} statement will never be true, it is not really a
problem that @code{foo} has not been defined.  Usually though, it is a
problem if a program calls an undefined function.

@cindex lint checks
If @option{--lint} is specified
(@pxref{Options, ,Command-Line Options}),
@command{gawk} reports calls to undefined functions.

@cindex portability issues
Some @command{awk} implementations generate a runtime
error if you use the @code{next} statement
(@pxref{Next Statement, , The @code{next} Statement})
inside a user-defined function.
@command{gawk} does not have this limitation.

@node Return Statement, Dynamic Typing, Function Caveats, User-defined
@subsection The @code{return} Statement
@cindex @code{return} statement

The body of a user-defined function can contain a @code{return} statement.
This statement returns control to the calling part of the @command{awk} program.  It
can also be used to return a value for use in the rest of the @command{awk}
program.  It looks like this:

@example
return @r{[}@var{expression}@r{]}
@end example

The @var{expression} part is optional.  If it is omitted, then the returned
value is undefined, and therefore, unpredictable.

A @code{return} statement with no value expression is assumed at the end of
every function definition.  So if control reaches the end of the function
body, then the function returns an unpredictable value.  @command{awk}
does @emph{not} warn you if you use the return value of such a function.

Sometimes, you want to write a function for what it does, not for
what it returns.  Such a function corresponds to a @code{void} function
in C or to a @code{procedure} in Pascal.  Thus, it may be appropriate to not
return any value; simply bear in mind that if you use the return
value of such a function, you do so at your own risk.

The following is an example of a user-defined function that returns a value
for the largest number among the elements of an array:

@example
function maxelt(vec,   i, ret)
@{
     for (i in vec) @{
          if (ret == "" || vec[i] > ret)
               ret = vec[i]
     @}
     return ret
@}
@end example

@cindex conventions, programming
@cindex programming conventions
@noindent
You call @code{maxelt} with one argument, which is an array name.  The local
variables @code{i} and @code{ret} are not intended to be arguments;
while there is nothing to stop you from passing two or three arguments
to @code{maxelt}, the results would be strange.  The extra space before
@code{i} in the function parameter list indicates that @code{i} and
@code{ret} are not supposed to be arguments.  This is a convention that
you should follow when you define functions.

The following program uses the @code{maxelt} function.  It loads an
array, calls @code{maxelt}, and then reports the maximum number in that
array:

@example
function maxelt(vec,   i, ret)
@{
     for (i in vec) @{
          if (ret == "" || vec[i] > ret)
               ret = vec[i]
     @}
     return ret
@}

# Load all fields of each record into nums.
@{
     for(i = 1; i <= NF; i++)
          nums[NR, i] = $i
@}

END @{
     print maxelt(nums)
@}
@end example

Given the following input:

@example
 1 5 23 8 16
44 3 5 2 8 26
256 291 1396 2962 100
-6 467 998 1101
99385 11 0 225
@end example

@noindent
the program reports (predictably) that @code{99385} is the largest number
in the array.

@node Dynamic Typing, , Return Statement, User-defined
@subsection Functions and Their Effect on Variable Typing

@command{awk} is a very fluid language.
It is possible that @command{awk} can't tell if an identifier
represents a regular variable or an array until runtime.
Here is an annotated sample program:

@example
function foo(a)
@{
    a[1] = 1   # parameter is an array
@}

BEGIN @{
    b = 1
    foo(b)  # invalid: fatal type mismatch

    foo(x)  # x uninitialized, becomes an array dynamically
    x = 1   # now not allowed, runtime error
@}
@end example

Usually, such things aren't a big issue, but it's worth
being aware of them.

@node Internationalization, Advanced Features, Functions, Top
@chapter Internationalization with @command{gawk}

Once upon a time, computer makers
wrote software that only worked in English.
Eventually, hardware and software vendors noticed that if their
systems worked in the native languages of non-English-speaking
countries, they were able to sell more systems.
As a result, internationalization and localization
of programs and software systems became a common practice.

@cindex internationalization features in @command{gawk}
Until recently, the ability to provide internationalization
was largely restricted to programs written in C and C++.
This @value{CHAPTER} describes the underlying library @command{gawk}
uses for internationalization, as well as how
@command{gawk} makes internationalization
features available at the @command{awk} program level.
Having internationalization available at the @command{awk} level
gives software developers additional flexibility---they are no
longer required to write in C when internationalization is
a requirement.

@menu
* I18N and L10N::               Internationalization and Localization.
* Explaining gettext::          How GNU @code{gettext} works.
* Programmer i18n::             Features for the programmer.
* Translator i18n::             Features for the translator.
* I18N Example::                A simple i18n example.
* Gawk I18N::                   @command{gawk} is also internationalized.
@end menu

@node I18N and L10N, Explaining gettext, Internationalization, Internationalization
@section Internationalization and Localization

@cindex internationalization
@cindex localization
@dfn{Internationalization} means writing (or modifying) a program once,
in such a way that it can use multiple languages without requiring
further source code changes.
@dfn{Localization} means providing the data necessary for an
internationalized program to work in a particular language.
Most typically, these terms refer to features such as the language
used for printing error messages, the language used to read
responses, and information related to how numerical and
monetary values are printed and read.

@node Explaining gettext, Programmer i18n, I18N and L10N, Internationalization
@section GNU @code{gettext}

@cindex @code{gettext}, how it works
@cindex internationalizing a program
The facilities in GNU @code{gettext} focus on messages; strings printed
by a program, either directly or via formatting with @code{printf} or
@code{sprintf}.@footnote{For some operating systems, the @command{gawk}
port doesn't support GNU @code{gettext}.  This applies most notably to
the PC operating systems.  As such, these features are not available
if you are using one of those operating systems.  Sorry.}

When using GNU @code{gettext}, each application has its own
@dfn{text domain}.  This is a unique name such as @samp{kpilot} or @samp{gawk},
that identifies the application.
A complete application may have multiple components---programs written
in C or C++, as well as scripts written in @command{sh} or @command{awk}.
All of the components use the same text domain.

To make the discussion concrete, assume we're writing an application
named @command{guide}.  Internationalization consists of the
following steps, in this order:

@enumerate
@item
The programmer goes
through the source for all of @command{guide}'s components
and marks each string that is a candidate for translation.
For example, @code{"`-F': option required"} is a good candidate for translation.
A table with strings of option names is not (e.g., @command{gawk}'s
@option{--profile} option should remain the same, no matter what the local
language).

@cindex @code{textdomain} C library function
@item
The programmer indicates the application's text domain
(@code{"guide"}) to the @code{gettext} library,
by calling the @code{textdomain} function.

@item
Messages from the application are extracted from the source code and
collected into a Portable Object file (@file{guide.po}),
which lists the strings and their translations.
The translations are initially empty.
The original (usually English) messages serve as the key for
lookup of the translations.

@cindex portable object files (@code{gettext})
@item
For each language with a translator, @file{guide.po}
is copied and translations are created and shipped with the application.

@cindex message object files (@code{gettext})
@item
Each language's @file{.po} file is converted into a binary
message object (@file{.mo}) file.
A message object file contains the original messages and their
translations in a binary format that allows fast lookup of translations
at runtime.

@item
When @command{guide} is built and installed, the binary translation files
are installed in a standard place.

@cindex @code{bindtextdomain} C library function
@item
For testing and development, it is possible to tell @code{gettext}
to use @file{.mo} files in a different directory than the standard
one by using the @code{bindtextdomain} function.

@item
At runtime, @command{guide} looks up each string via a call
to @code{gettext}.  The returned string is the translated string
if available, or the original string if not.

@item
If necessary, it is possible to access messages from a different
text domain than the one belonging to the application, without
having to switch the application's default text domain back
and forth.
@end enumerate

@cindex @code{gettext} C library function
In C (or C++), the string marking and dynamic translation lookup
are accomplished by wrapping each string in a call to @code{gettext}:

@example
printf(gettext("Don't Panic!\n"));
@end example

The tools that extract messages from source code pull out all
strings enclosed in calls to @code{gettext}.

@cindex @code{_} C macro (@code{gettext})
The GNU @code{gettext} developers, recognizing that typing
@samp{gettext} over and over again is both painful and ugly to look
at, use the macro @samp{_} (an underscore) to make things easier:

@example
/* In the standard header file: */
#define _(str) gettext(str)

/* In the program text: */
printf(_("Don't Panic!\n"));
@end example

@cindex locale categories
@noindent
This reduces the typing overhead to just three extra characters per string
and is considerably easier to read as well.
There are locale @dfn{categories}
for different types of locale-related information.
The defined locale categories that @code{gettext} knows about are:

@table @code
@cindex @code{LC_MESSAGES} locale category
@item LC_MESSAGES
Text messages.  This is the default category for @code{gettext}
operations, but it is possible to supply a different one explicitly,
if necessary.  (It is almost never necessary to supply a different category.)

@cindex @code{LC_COLLATE} locale category
@item LC_COLLATE
Text collation information; i.e., how different characters
and/or groups of characters sort in a given language.

@cindex @code{LC_CTYPE} locale category
@item LC_CTYPE
Character type information (alphabetic, digit, upper- or lowercase, and
so on).
This information is accessed via the
POSIX character classes in regular expressions,
such as @code{/[[:alnum:]]/}
(@pxref{Regexp Operators, ,Regular Expression Operators}).

@cindex @code{LC_MONETARY} locale category
@item LC_MONETARY
Monetary information, such as the currency symbol, and whether the
symbol goes before or after a number.

@cindex @code{LC_NUMERIC} locale category
@item LC_NUMERIC
Numeric information, such as which characters to use for the decimal
point and the thousands separator.@footnote{Americans
use a comma every three decimal places and a period for the decimal
point, while many Europeans do exactly the opposite:
@code{1,234.56} vs.@: @code{1.234,56}.}

@cindex @code{LC_RESPONSE} locale category
@item LC_RESPONSE
Response information, such as how ``yes'' and ``no'' appear in the
local language, and possibly other information as well.

@cindex @code{LC_TIME} locale category
@item LC_TIME
Time and date related information, such as 12- or 24-hour clock, month printed
before or after day in a date, local month abbreviations, and so on.

@cindex @code{LC_ALL} locale category
@item LC_ALL
All of the above.  (Not too useful in the context of @code{gettext}.)
@end table

@node Programmer i18n, Translator i18n, Explaining gettext, Internationalization
@section Internationalizing @command{awk} Programs

@command{gawk} provides the following variables and functions for
internationalization:

@table @code
@cindex @code{TEXTDOMAIN} variable
@item TEXTDOMAIN
This variable indicates the application's text domain.
For compatibility with GNU @code{gettext}, the default
value is @code{"messages"}.

@cindex internationalization, marked strings
@cindex marked strings for internationalization
@item _"your message here"
String constants marked with a leading underscore
are candidates for translation at runtime.
String constants without a leading underscore are not translated.

@cindex @code{dcgettext} built-in function
@item dcgettext(@var{string} @r{[}, @var{domain} @r{[}, @var{category}@r{]]})
This built-in function returns the translation of @var{string} in
text domain @var{domain} for locale category @var{category}.
The default value for @var{domain} is the current value of @code{TEXTDOMAIN}.
The default value for @var{category} is @code{"LC_MESSAGES"}.

If you supply a value for @var{category}, it must be a string equal to
one of the known locale categories described in
@ifnotinfo
the previous @value{SECTION}.
@end ifnotinfo
@ifinfo
@ref{Explaining gettext, ,GNU @code{gettext}}.
@end ifinfo
You must also supply a text domain.  Use @code{TEXTDOMAIN} if
you want to use the current domain.

@strong{Caution:} The order of arguments to the @command{awk} version
of the @code{dcgettext} function is purposely different from the order for
the C version.  The @command{awk} version's order was
chosen to be simple and to allow for reasonable @command{awk}-style
default arguments.

@cindex @code{bindtextdomain} built-in function
@item bindtextdomain(@var{directory} @r{[}, @var{domain}@r{]})
This built-in function allows you to specify the directory where
@code{gettext} looks for @file{.mo} files, in case they
will not or cannot be placed in the standard locations
(e.g., during testing).
It returns the directory where @var{domain} is ``bound.''

The default @var{domain} is the value of @code{TEXTDOMAIN}.
If @var{directory} is the null string (@code{""}), then
@code{bindtextdomain} returns the current binding for the
given @var{domain}.
@end table

To use these facilities in your @command{awk} program, follow the steps
outlined in
@ifnotinfo
the previous @value{SECTION},
@end ifnotinfo
@ifinfo
@ref{Explaining gettext, ,GNU @code{gettext}},
@end ifinfo
like so:

@enumerate
@item
Set the variable @code{TEXTDOMAIN} to the text domain of
your program.  This is best done in a @code{BEGIN} rule
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}),
or it can also be done via the @option{-v} command-line
option (@pxref{Options, ,Command-Line Options}):

@example
BEGIN @{
    TEXTDOMAIN = "guide"
    @dots{}
@}
@end example

@item
Mark all translatable strings with a leading underscore (@samp{_})
character.  It @emph{must} be adjacent to the opening
quote of the string.  For example:

@example
print _"hello, world"
x = _"you goofed"
printf(_"Number of users is %d\n", nusers)
@end example

@item
If you are creating strings dynamically, you can
still translate them, using the @code{dcgettext}
built-in function.

@example
message = nusers " users logged in"
message = dcgettext(message, "adminprog")
print message
@end example

Here, the call to @code{dcgettext} supplies a different
text domain (@code{"adminprog"}) in which to find the
message, but it uses the default @code{"LC_MESSAGES"} category.

@item
During development, you might want to put the @file{.mo}
file in a private directory for testing.  This is done
with the @code{bindtextdomain} built-in function:

@example
BEGIN @{
   TEXTDOMAIN = "guide"   # our text domain
   if (Testing) @{
       # where to find our files
       bindtextdomain("testdir")
       # joe is in charge of adminprog
       bindtextdomain("../joe/testdir", "adminprog")
   @}
   @dots{}
@}
@end example

@end enumerate

@xref{I18N Example, ,A Simple Internationalization Example},
for an example program showing the steps necessary to create
and use translations from @command{awk}.

@node Translator i18n, I18N Example, Programmer i18n, Internationalization
@section Translating @command{awk} Programs

Once a program's translatable strings have been marked, they must
be extracted to create the initial @file{.po} file.
As part of translation, it is often helpful to rearrange the order
in which arguments to @code{printf} are output.  

@command{gawk}'s @option{--gen-po} command-line option extracts
the messages and is discussed next.
After that, @code{printf}'s ability to
rearrange the order for @code{printf} arguments at runtime
is covered.

@menu
* String Extraction::           Extracting marked strings.
* Printf Ordering::             Rearranging @code{printf} arguments.
* I18N Portability::            @command{awk}-level portability issues.
@end menu

@node String Extraction, Printf Ordering, Translator i18n, Translator i18n
@subsection Extracting Marked Strings
@cindex string extraction (internationalization)
@cindex marked string extraction (internationalization)
@cindex extraction, of marked strings (internationalization)

@cindex @code{--gen-po} option
@cindex command-line option, @code{--gen-po}
Once your @command{awk} program is working, and all the strings have
been marked and you've set (and perhaps bound) the text domain,
it is time to produce translations.
First, use the @option{--gen-po} command-line option to create
the initial @file{.po} file:

@example
$ gawk --gen-po -f guide.awk > guide.po
@end example

@cindex @code{xgettext} utility
When run with @option{--gen-po}, @command{gawk} does not execute your
program.  Instead, it parses it as usual and prints all marked strings
to standard output in the format of a GNU @code{gettext} Portable Object
file.  Also included in the output are any constant strings that
appear as the first argument to @code{dcgettext}.@footnote{Eventually,
the @command{xgettext} utility that comes with GNU @code{gettext} will be
taught to automatically run @samp{gawk --gen-po} for @file{.awk} files,
freeing the translator from having to do it manually.}
@xref{I18N Example, ,A Simple Internationalization Example},
for the full list of steps to go through to create and test
translations for @command{guide}.

@node Printf Ordering, I18N Portability, String Extraction, Translator i18n
@subsection Rearranging @code{printf} Arguments

@cindex @code{printf}, positional specifier
@cindex positional specifier, @code{printf}
Format strings for @code{printf} and @code{sprintf}
(@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing})
present a special problem for translation.
Consider the following:@footnote{This example is borrowed
from the GNU @code{gettext} manual.}

@c line broken here only for smallbook format
@example
printf(_"String `%s' has %d characters\n",
          string, length(string)))
@end example

A possible German translation for this might be:

@example
"%d Zeichen lang ist die Zeichenkette `%s'\n"
@end example

The problem should be obvious: the order of the format
specifications is different from the original!
Even though @code{gettext} can return the translated string
at runtime,
it cannot change the argument order in the call to @code{printf}.

To solve this problem, @code{printf} format specificiers may have
an additional optional element, which we call a @dfn{positional specifier}.
For example:

@example
"%2$d Zeichen lang ist die Zeichenkette `%1$s'\n"
@end example

Here, the positional specifier consists of an integer count, which indicates which
argument to use, and a @samp{$}. Counts are one-based, and the
format string itself is @emph{not} included.  Thus, in the following
example, @samp{string} is the first argument and @samp{length(string)} is the second.

@example
$ gawk 'BEGIN @{
>     string = "Dont Panic"
>     printf _"%2$d characters live in \"%1$s\"\n",
>                         string, length(string)
> @}'
@print{} 10 characters live in "Dont Panic"
@end example

If present, positional specifiers come first in the format specification,
before the flags, the field width, and/or the precision.

Positional specifiers can be used with the dynamic field width and
precision capability:

@example
$ gawk 'BEGIN @{
>    printf("%*.*s\n", 10, 20, "hello")
>    printf("%3$*2$.*1$s\n", 20, 10, "hello")
> @}'
@print{}      hello
@print{}      hello
@end example

@noindent
@strong{Note:} When using @samp{*} with a positional specifier, the @samp{*}
comes first, then the integer position, and then the @samp{$}.
This is somewhat counter-intutive.

@cindex @code{printf}, mixing positional specifiers with regular formats
@cindex positional specifiers, mixing with regular formats (@code{printf})
@cindex format specifiers, mixing regular with positional specifiers (@code{printf})
@command{gawk} does not allow you to mix regular format specifiers
and those with positional specifiers in the same string:

@smallexample
$ gawk 'BEGIN @{ printf _"%d %3$s\n", 1, 2, "hi" @}'
@error{} gawk: cmd. line:1: fatal: must use `count$' on all formats or none
@end smallexample

@strong{Note:} There are some pathological cases that @command{gawk} may fail to
diagnose.  In such cases, the output may not be what you expect.
It's still a bad idea to try mixing them, even if @command{gawk}
doesn't detect it.

Although positional specifiers can be used directly in @command{awk} programs,
their primary purpose is to help in producing correct translations of
format strings into languages different from the one in which the program
is first written.

@node I18N Portability, , Printf Ordering, Translator i18n
@subsection @command{awk} Portability Issues

@cindex portability issues
@cindex portability issues, internationalization of @command{awk} programs
@cindex internationalization of @command{awk} programs, portability issues
@command{gawk}'s internationalization features were purposely chosen to
have as little impact as possible on the portability of @command{awk}
programs that use them to other versions of @command{awk}.
Consider this program:

@example
BEGIN @{
    TEXTDOMAIN = "guide"
    if (Test_Guide)   # set with -v
        bindtextdomain("/test/guide/messages")
    print _"don't panic!"
@}
@end example

@noindent
As written, it won't work on other versions of @command{awk}.
However, it is actually almost portable, requiring very little
change.

@itemize @bullet
@item
Assignments to @code{TEXTDOMAIN} won't have any effect,
since @code{TEXTDOMAIN} is not special in other @command{awk} implementations.

@item
Non-GNU versions of @command{awk} treat marked strings
as the concatenation of a variable named @code{_} with the string
following it.@footnote{This is good fodder for an ``Obfuscated
@command{awk}'' contest.} Typically, the variable @code{_} has
the null string (@code{""}) as its value, leaving the original string constant as
the result.

@item
By defining ``dummy'' functions to replace @code{dcgettext}
and @code{bindtextdomain}, the @command{awk} program can be made to run, but
all the messages are output in the original language.
For example:

@cindex @code{bindtextdomain} user-defined function
@cindex @code{dcgettext} user-defined function
@example
@c file eg/lib/libintl.awk
function bindtextdomain(dir, domain)
@{
    return dir
@}

function dcgettext(string, domain, category)
@{
    return string
@}
@c endfile
@end example

@item
The use of positional specifications in @code{printf} or
@code{sprintf} is @emph{not} portable.
To support @code{gettext} at the C level, many systems' C versions of
@code{sprintf} do support positional specifiers.  But it works only if
enough arguments are supplied in the function call.  Many versions of
@command{awk} pass @code{printf} formats and arguments unchanged to the
underlying C library version of @code{sprintf}, but only one format and
argument at a time.  What happens if a positional specification is
used is anybody's guess.
However, since the positional specifications are primarily for use in
@emph{translated} format strings, and since non-GNU @command{awk}s never
retrieve the translated string, this should not be a problem in practice.
@end itemize

@node I18N Example, Gawk I18N, Translator i18n, Internationalization
@section A Simple Internationalization Example

Now let's look at a step-by-step example of how to internationalize and
localize a simple @command{awk} program, using @file{guide.awk} as our
original source:

@example
@c file eg/prog/guide.awk
BEGIN @{
    TEXTDOMAIN = "guide"
    bindtextdomain(".")  # for testing
    print _"Don't Panic"
    print _"The Answer Is", 42
    print "Pardon me, Zaphod who?"
@}
@c endfile
@end example

@noindent
Run @samp{gawk --gen-po} to create the @file{.po} file:

@example
$ gawk --gen-po -f guide.awk > guide.po
@end example

@noindent
This produces:

@example
@c file eg/data/guide.po
#: guide.awk:4
msgid "Don't Panic"
msgstr ""

#: guide.awk:5
msgid "The Answer Is"
msgstr ""

@c endfile
@end example

This original portable object file is saved and reused for each language
into which the application is translated.  The @code{msgid}
is the original string and the @code{msgstr} is the translation.

@strong{Note:} Strings not marked with a leading underscore do not
appear in the @file{guide.po} file.

Next, the messages must be translated.
Here is a translation to a hypothetical dialect of English,
called ``Mellow'':@footnote{Perhaps it would be better if it were
called ``Hippy.'' Ah, well.}

@example
@group
$ cp guide.po guide-mellow.po
@var{Add translations to} guide-mellow.po @dots{}
@end group
@end example

@noindent
Following are the translations:

@example
@c file eg/data/guide-mellow.po
#: guide.awk:4
msgid "Don't Panic"
msgstr "Hey man, relax!"

#: guide.awk:5
msgid "The Answer Is"
msgstr "Like, the scoop is"

@c endfile
@end example

@cindex Linux
@cindex GNU/Linux
The next step is to make the directory to hold the binary message object
file and then to create the @file{guide.mo} file.
The directory layout shown here is standard for GNU @code{gettext} on
GNU/Linux systems.  Other versions of @code{gettext} may use a different
layout:

@example
$ mkdir en_US en_US/LC_MESSAGES
@end example

@cindex @command{msgfmt} utility
The @command{msgfmt} utility does the conversion from human-readable
@file{.po} file to machine-readable @file{.mo} file.
By default, @command{msgfmt} creates a file named @file{messages}.
This file must be renamed and placed in the proper directory so that
@command{gawk} can find it:

@example
$ msgfmt guide-mellow.po
$ mv messages en_US/LC_MESSAGES/guide.mo
@end example

Finally, we run the program to test it:

@example
$ gawk -f guide.awk
@print{} Hey man, relax!
@print{} Like, the scoop is 42
@print{} Pardon me, Zaphod who?
@end example

If the two replacement functions for @code{dcgettext}
and @code{bindtextdomain}
(@pxref{I18N Portability, ,@command{awk} Portability Issues})
are in a file named @file{libintl.awk},
then we can run @file{guide.awk} unchanged as follows:

@example
$ gawk --posix -f guide.awk -f libintl.awk
@print{} Don't Panic
@print{} The Answer Is 42
@print{} Pardon me, Zaphod who?
@end example

@node Gawk I18N, , I18N Example, Internationalization
@section @command{gawk} Can Speak Your Language

As of @value{PVERSION} 3.1, @command{gawk} itself has been internationalized
using the GNU @code{gettext} package.
@ifinfo
(GNU @code{gettext} is described in
complete detail in
@ref{Top}.)
@end ifinfo
@ifnotinfo
(GNU @code{gettext} is described in
complete detail in
@cite{GNU gettext tools}.)
@end ifnotinfo
As of this writing, the latest version of GNU @code{gettext} is
@uref{ftp://gnudist.gnu.org/gnu/gettext/gettext-0.10.37.tar.gz, @value{PVERSION} 0.10.37}.

If a translation of @command{gawk}'s messages exists,
then @command{gawk} produces usage messages, warnings,
and fatal errors in the local language.

@cindex @code{--with-included-gettext} configuration option
@cindex configuration option, @code{--with-included-gettext}
On systems that do not use @value{PVERSION} 2 (or later) of the GNU C library, you should
configure @command{gawk} with the @option{--with-included-gettext} option
before compiling and installing it.
@xref{Additional Configuration Options},
for more information.

@node Advanced Features, Invoking Gawk, Internationalization, Top
@chapter Advanced Features of @command{gawk}
@cindex advanced features
@cindex features, advanced
@ignore
Contributed by: Peter Langston <pud!psl@bellcore.bellcore.com>

    Found in Steve English's "signature" line:

"Write documentation as if whoever reads it is a violent psychopath
who knows where you live."
@end ignore
@quotation
@i{Write documentation as if whoever reads it is
a violent psychopath who knows where you live.}@*
Steve English, as quoted by Peter Langston
@end quotation

This @value{CHAPTER} discusses advanced features in @command{gawk}.
It's a bit of a ``grab bag'' of items that are otherwise unrelated
to each other.
First, a command-line option allows @command{gawk} to recognize
non-decimal numbers in input data, not just in @command{awk}
programs.  Next, two-way I/O, discussed briefly in earlier parts of this
@value{DOCUMENT}, is described in full detail, along with the basics
of TCP/IP networking and BSD portal files.  Finally, @command{gawk}
can @dfn{profile} an @command{awk} program, making it possible to tune
it for performance.

@ref{Dynamic Extensions, ,Adding New Built-in Functions to @command{gawk}},
discusses the ability to dynamically add new built-in functions to
@command{gawk}.  As this feature is still immature and likely to change,
its description is relegated to an appendix.

@menu
* Non-decimal Data::            Allowing non-decimal input data.
* Two-way I/O::                 Two-way communications with another process.
* TCP/IP Networking::           Using @command{gawk} for network programming.
* Portal Files::                Using @command{gawk} with BSD portals.
* Profiling::                   Profiling your @command{awk} programs.
@end menu

@node Non-decimal Data, Two-way I/O, Advanced Features, Advanced Features
@section Allowing Non-Decimal Input Data
@cindex @code{--non-decimal-data} option
@cindex command-line option, @code{--non-decimal-data}

If you run @command{gawk} with the @option{--non-decimal-data} option,
you can have non-decimal constants in your input data:

@c line break here for small book format
@example
$ echo 0123 123 0x123 |
> gawk --non-decimal-data '@{ printf "%d, %d, %d\n",
>                                         $1, $2, $3 @}'
@print{} 83, 123, 291
@end example

For this feature to work, write your program so that
@command{gawk} treats your data as numeric:

@example
$ echo 0123 123 0x123 | gawk '@{ print $1, $2, $3 @}'
@print{} 0123 123 0x123
@end example

@noindent
The @code{print} statement treats its expressions as strings.
Although the fields can act as numbers when necessary,
they are still strings, so @code{print} does not try to treat them
numerically.  You may need to add zero to a field to force it to
be treated as a number.  For example:

@example
$ echo 0123 123 0x123 | gawk --non-decimal-data '
> @{ print $1, $2, $3
>   print $1 + 0, $2 + 0, $3 + 0 @}'
@print{} 0123 123 0x123
@print{} 83 123 291
@end example

Because it is common to have decimal data with leading zeros, and because
using it could lead to surprising results, the default is to leave this
facility disabled.  If you want it, you must explicitly request it.

@cindex conventions, programming
@cindex programming conventions
@strong{Caution:}
@emph{Use of this option is not recommended.}
It can break old programs very badly.
Instead, use the @code{strtonum} function to convert your data
(@pxref{Non-decimal-numbers, ,Octal and Hexadecimal Numbers}).
This makes your programs easier to write and easier to read, and
leads to less surprising results.

@node Two-way I/O, TCP/IP Networking, Non-decimal Data, Advanced Features
@section Two-Way Communications with Another Process
@cindex Brennan, Michael
@cindex sex, programmer attractiveness
@smallexample
@c Path: cssun.mathcs.emory.edu!gatech!newsxfer3.itd.umich.edu!news-peer.sprintlink.net!news-sea-19.sprintlink.net!news-in-west.sprintlink.net!news.sprintlink.net!Sprint!204.94.52.5!news.whidbey.com!brennan
From: brennan@@whidbey.com (Mike Brennan)
Newsgroups: comp.lang.awk
Subject: Re: Learn the SECRET to Attract Women Easily
Date: 4 Aug 1997 17:34:46 GMT
@c Organization: WhidbeyNet
@c Lines: 12
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On 3 Aug 1997 13:17:43 GMT, Want More Dates???
<tracy78@@kilgrona.com> wrote:
>Learn the SECRET to Attract Women Easily
>
>The SCENT(tm)  Pheromone Sex Attractant For Men to Attract Women

The scent of awk programmers is a lot more attractive to women than
the scent of perl programmers.
--
Mike Brennan
@c brennan@@whidbey.com
@end smallexample

It is often useful to be able to
send data to a separate program for
processing and then read the result.  This can always be
done with temporary files:

@example
# write the data for processing
tempfile = ("/tmp/mydata." PROCINFO["pid"])
while (@var{not done with data})
    print @var{data} | ("subprogram > " tempfile)
close("subprogram > " tempfile)

# read the results, remove tempfile when done
while ((getline newdata < tempfile) > 0)
    @var{process} newdata @var{appropriately}
close(tempfile)
system("rm " tempfile)
@end example

@noindent
This works, but not elegantly.

@cindex coprocess
@cindex two-way I/O
@cindex I/O, two-way
@cindex @code{|&} I/O operator
@cindex @command{csh} utility
Starting with @value{PVERSION} 3.1 of @command{gawk}, it is possible to
open a @emph{two-way} pipe to another process.  The second process is
termed a @dfn{coprocess}, since it runs in parallel with @command{gawk}.
The two-way connection is created using the new @samp{|&} operator
(borrowed from the Korn Shell, @command{ksh}):@footnote{This is very
different from the same operator in the C shell, @command{csh}.}

@example
do @{
    print @var{data} |& "subprogram"
    "subprogram" |& getline results
@} while (@var{data left to process})
close("subprogram")
@end example

The first time an I/O operation is executed using the @samp{|&}
operator, @command{gawk} creates a two-way pipeline to a child process
that runs the other program.  Output created with @code{print}
or @code{printf} is written to the program's standard input, and
output from the program's standard output can be read by the @command{gawk}
program using @code{getline}.
As is the case with processes started by @samp{|}, the subprogram
can be any program, or pipeline of programs, that can be started by
the shell.

There are some cautionary items to be aware of:

@itemize @bullet
@item
As the code inside @command{gawk} currently stands, the coprocess's
standard error goes to the same place that the parent @command{gawk}'s
standard error goes. It is not possible to read the child's
standard error separately.

@cindex deadlock
@item
I/O buffering may be a problem.  @command{gawk} automatically
flushes all output down the pipe to the child process.
However, if the coprocess does not flush its output,
@command{gawk} may hang when doing a @code{getline} in order to read
the coprocess's results.  This could lead to a situation
known as @dfn{deadlock}, where each process is waiting for the
other one to do something.
@end itemize

It is possible to close just one end of the two-way pipe to
a coprocess, by supplying a second argument to the @code{close}
function of either @code{"to"} or @code{"from"}
(@pxref{Close Files And Pipes, ,Closing Input and Output Redirections}).
These strings tell @command{gawk} to close the end of the pipe
that sends data to the process or the end that reads from it,
respectively.

This is particularly necessary in order to use
the system @command{sort} utility as part of a coprocess;
@command{sort} must read @emph{all} of its input
data before it can produce any output.
The @command{sort} program does not receive an end-of-file indication
until @command{gawk} closes the write end of the pipe.

When you have finished writing data to the @command{sort}
utility, you can close the @code{"to"} end of the pipe, and
then start reading sorted data via @code{getline}.
For example:

@example
BEGIN @{
    command = "LC_ALL=C sort"
    n = split("abcdefghijklmnopqrstuvwxyz", a, "")

    for (i = n; i > 0; i--)
        print a[i] |& command
    close(command, "to")

    while ((command |& getline line) > 0)
        print "got", line
    close(command)
@}
@end example

This program writes the letters of the alphabet in reverse order, one
per line, down the two-way pipe to @command{sort}.  It then closes the
write end of the pipe, so that @command{sort} receives an end-of-file
indication.  This causes @command{sort} to sort the data and write the
sorted data back to the @command{gawk} program.  Once all of the data
has been read, @command{gawk} terminates the coprocess and exits.

As a side note, the assignment @samp{LC_ALL=C} in the @command{sort}
command ensures traditional Unix (ASCII) sorting from @command{sort}.

@node TCP/IP Networking, Portal Files, Two-way I/O, Advanced Features
@section Using @command{gawk} for Network Programming
@cindex networking, TCP/IP
@cindex TCP/IP networking
@cindex @file{/inet} special files
@cindex @code{EMISTERED}
@quotation
@code{EMISTERED}: @i{A host is a host from coast to coast,@*
and no-one can talk to host that's close,@*
unless the host that isn't close@*
is busy hung or dead.}
@end quotation

In addition to being able to open a two-way pipeline to a coprocess
on the same system
(@pxref{Two-way I/O, ,Two-Way Communications with Another Process}),
it is possible to make a two-way connection to
another process on another system across an IP networking connection.

You can think of this as just a @emph{very long} two-way pipeline to
a coprocess.
The way @command{gawk} decides that you want to use TCP/IP networking is
by recognizing special @value{FN}s that begin with @samp{/inet/}.

The full syntax of the special @value{FN} is
@file{/inet/@var{protocol}/@var{local-port}/@var{remote-host}/@var{remote-port}}.
The meaning of the components are:

@table @var
@item protocol
The protocol to use over IP.  This must be either @samp{tcp},
@samp{udp}, or @samp{raw}, for a TCP, UDP, or raw IP connection,
respectively.  The use of TCP is recommended for most applications.

@strong{Caution:} The use of raw sockets is not currently supported
in @value{PVERSION} 3.1 of @command{gawk}.

@item local-port
@cindex @code{getservbyname} C library function
The local TCP or UDP port number to use.  Use a port number of @samp{0}
when you want the system to pick a port. This is what you should do
when writing a TCP or UDP client.
You may also use a well-known service name, such as @samp{smtp}
or @samp{http}, in which case @command{gawk} attempts to determine
the pre-defined port number using the C @code{getservbyname} function.

@item remote-host
The IP address or fully-qualified domain name of the Internet
host to which you want to connect.

@item remote-port
The TCP or UDP port number to use on the given @var{remote-host}.
Again, use @samp{0} if you don't care, or else a well-known
service name.
@end table

Consider the following very simple example:

@example
BEGIN @{
  Service = "/inet/tcp/0/localhost/daytime"
  Service |& getline
  print $0
  close(Service)
@}
@end example

This program reads the current date and time from the local system's
TCP @samp{daytime} server.
It then prints the results and closes the connection.

Because this topic is extensive, the use of @command{gawk} for
TCP/IP programming is documented separately.
@ifinfo
@xref{Top},
@end ifinfo
@ifnotinfo
See @cite{TCP/IP Internetworking with @command{gawk}},
which comes as part of the @command{gawk} distribution,
@end ifnotinfo
for a much more complete introduction and discussion, as well as
extensive examples.

@node Portal Files, Profiling, TCP/IP Networking, Advanced Features
@section Using @command{gawk} with BSD Portals
@cindex portal files
@cindex BSD portal files
@cindex TCP/IP networking
@cindex @file{/p} special files
@cindex @code{--enable-portals} configuration option
@cindex configuration option, @code{--enable-portals}
@cindex BSD-based operating systems

Similar to the @file{/inet} special files, if @command{gawk}
is configured with the @option{--enable-portals} option
(@pxref{Quick Installation, , Compiling @command{gawk} for Unix}),
then @command{gawk} treats
files whose pathnames begin with @code{/p} as 4.4 BSD-style portals.

When used with the @samp{|&} operator, @command{gawk} opens the file
for two-way communications.  The operating system's portal mechanism
then manages creating the process associated with the portal and
the corresponding communications with the portal's process.

@node Profiling, , Portal Files, Advanced Features
@section Profiling Your @command{awk} Programs
@cindex profiling @command{awk} programs
@cindex @command{pgawk} program

Beginning with @value{PVERSION} 3.1 of @command{gawk}, you may produce execution
traces of your @command{awk} programs.
This is done with a specially compiled version of @command{gawk},
called @command{pgawk} (``profiling @command{gawk}'').

@cindex @file{awkprof.out} profiling output file
@cindex profiling output file (@file{awkprof.out})
@command{pgawk} is identical in every way to @command{gawk}, except that when
it has finished running, it creates a profile of your program in a file
named @file{awkprof.out}.
Because it is profiling, it also executes up to 45 percent slower than
@command{gawk} normally does.

As shown in the following example,
the @option{--profile} option can be used to change the name of the file
where @command{pgawk} will write the profile:

@example
$ pgawk --profile=myprog.prof -f myprog.awk data1 data2
@end example

@noindent
In the above example, @command{pgawk} places the profile in
@file{myprog.prof} instead of in @file{awkprof.out}.

Regular @command{gawk} also accepts this option.  When called with just
@option{--profile}, @command{gawk} ``pretty prints'' the program into
@file{awkprof.out}, without any execution counts.  You may supply an
option to @option{--profile} to change the @value{FN}.  Here is a sample
session showing a simple @command{awk} program, its input data, and the
results from running @command{pgawk}.  First, the @command{awk} program:

@example
BEGIN @{ print "First BEGIN rule" @}

END @{ print "First END rule" @}

/foo/ @{
    print "matched /foo/, gosh"
    for (i = 1; i <= 3; i++)
        sing()
@}

@{
    if (/foo/)
        print "if is true"
    else
        print "else is true"
@}

BEGIN @{ print "Second BEGIN rule" @}

END @{ print "Second END rule" @}

function sing(    dummy)
@{
    print "I gotta be me!"
@}
@end example

Following is the input data:

@example
foo
bar
baz
foo
junk
@end example

Here is the @file{awkprof.out} that results from running @command{pgawk}
on this program and data.  (This example also illustrates that @command{awk}
programmers sometimes have to work late.):

@cindex blocks, @code{BEGIN} and @code{END}
@example
        # gawk profile, created Sun Aug 13 00:00:15 2000

        # BEGIN block(s)

        BEGIN @{
     1          print "First BEGIN rule"
     1          print "Second BEGIN rule"
        @}

        # Rule(s)

     5  /foo/   @{ # 2
     2          print "matched /foo/, gosh"
     6          for (i = 1; i <= 3; i++) @{
     6                  sing()
                @}
        @}

     5  @{
     5          if (/foo/) @{ # 2
     2                  print "if is true"
     3          @} else @{
     3                  print "else is true"
                @}
        @}

        # END block(s)

        END @{
     1          print "First END rule"
     1          print "Second END rule"
        @}

        # Functions, listed alphabetically

     6  function sing(dummy)
        @{
     6          print "I gotta be me!"
        @}
@end example

The previous example illustrates many of the basic rules for profiling output.
The rules are as follows:

@itemize @bullet
@item
The program is printed in the order @code{BEGIN} rule,
pattern/action rules, @code{END} rule and functions, listed
alphabetically.
Multiple @code{BEGIN} and @code{END} rules are merged together.

@item
Pattern-action rules have two counts.
The first count, to the left of the rule, shows how many times
the rule's pattern was @emph{tested}.
The second count, to the right of the rule's opening left brace
in a comment,
shows how many times the rule's action was @emph{executed}.
The difference between the two indicates how many times the rule's
pattern evaluated to false.

@item
Similarly,
the count for an @code{if}-@code{else} statement shows how many times
the condition was tested.
To the right of the opening left brace for the @code{if}'s body
is a count showing how many times the condition was true.
The count for the @code{else}
indicates how many times the test failed.

@item
The count for a loop header (such as @code{for}
or @code{while}) shows how many times the loop test was executed.
(Because of this, you can't just look at the count on the first
statement in a rule to determine how many times the rule was executed.
If the first statement is a loop, the count is misleading.)

@item
For user-defined functions, the count next to the @code{function}
keyword indicates how many times the function was called.
The counts next to the statements in the body show how many times
those statements were executed.

@item
The layout uses ``K&R'' style using tabs.
Braces are used everywhere, even when
the body of an @code{if}, @code{else}, or loop is only a single statement.

@item
Parentheses are used only where needed, as indicated by the structure
of the program and the precedence rules.
@c extra verbiage here satisfies the copyeditor. ugh.
For example, @samp{(3 + 5) * 4} means add three plus five, then multiply
the total by four.  However, @samp{3 + 5 * 4} has no parentheses, and
means @samp{3 + (5 * 4)}.

@item
All string concatenations are parenthesized too.
(This could be made a bit smarter.)

@item
Parentheses are used around the arguments to @code{print}
and @code{printf} only when
the @code{print} or @code{printf} statement is followed by a redirection.
Similarly, if
the target of a redirection isn't a scalar, it gets parenthesized.

@item
@command{pgawk} supplies leading comments in
front of the @code{BEGIN} and @code{END} rules,
the pattern/action rules, and the functions.

@end itemize

The profiled version of your program may not look exactly like what you
typed when you wrote it.  This is because @command{pgawk} creates the
profiled version by ``pretty printing'' its internal representation of
the program.  The advantage to this is that @command{pgawk} can produce
a standard representation.  The disadvantage is that all source code
comments are lost, as are the distinctions among multiple @code{BEGIN}
and @code{END} rules.  Also, things such as:

@example
/foo/
@end example

@noindent
come out as:

@example
/foo/   @{
    print $0
@}
@end example

@noindent
which is correct, but possibly surprising.

@cindex dynamic profiling
@cindex profiling, dynamic
Besides creating profiles when a program has completed,
@command{pgawk} can produce a profile while it is running.
This is useful if your @command{awk} program goes into an
infinite loop and you want to see what has been executed.
To use this feature, run @command{pgawk} in the background:

@example
$ pgawk -f myprog &
[1] 13992
@end example

@cindex @command{kill} command
@cindex @code{SIGUSR1} signal
@cindex @code{USR1} signal
@cindex signals, @code{SIGUSR1}
@noindent
The shell prints a job number and process ID number, in this case, 13992.
Use the @command{kill} command to send the @code{USR1} signal
to @command{pgawk}:

@example
$ kill -USR1 13992
@end example

@noindent
As usual, the profiled version of the program is written to
@file{awkprof.out}, or to a different file if you use the @option{--profile}
option.

Along with the regular profile, as shown earlier, the profile
includes a trace of any active functions:

@example
# Function Call Stack:

#   3. baz
#   2. bar
#   1. foo
# -- main --
@end example

You may send @command{pgawk} the @code{USR1} signal as many times as you like.
Each time, the profile and function call trace are appended to the output
profile file.

@cindex @code{SIGHUP} signal
@cindex @code{HUP} signal
@cindex signals, @code{SIGHUP}
If you use the @code{HUP} signal instead of the @code{USR1} signal,
@command{pgawk} produces the profile and the function call trace, and then exits.

@node Invoking Gawk, Library Functions, Advanced Features, Top
@chapter Running @command{awk} and @command{gawk}

This @value{CHAPTER} covers how to run awk, both POSIX-standard
and @command{gawk}-specific command-line options, and what
@command{awk} and
@command{gawk} do with non-option arguments.
It then proceeds to cover how @command{gawk} searches for source files,
obsolete options and/or features, and known bugs in @command{gawk}.
This @value{CHAPTER} rounds out the discussion of @command{awk}
as a program and as a language.

While a number of the options and features described here were
discussed in passing earlier in the book, this @value{CHAPTER} provides the
full details.

@menu
* Command Line::                How to run @command{awk}.
* Options::                     Command-line options and their meanings.
* Other Arguments::             Input file names and variable assignments.
* AWKPATH Variable::            Searching directories for @command{awk}
                                programs.
* Obsolete::                    Obsolete Options and/or features.
* Undocumented::                Undocumented Options and Features.
* Known Bugs::                  Known Bugs in @command{gawk}.
@end menu

@node Command Line, Options, Invoking Gawk, Invoking Gawk
@section Invoking @command{awk}
@cindex command line
@cindex invocation of @command{gawk}
@cindex arguments, command-line
@cindex options, command-line
@cindex long options
@cindex options, long

There are two ways to run @command{awk}---with an explicit program or with
one or more program files.  Here are templates for both of them; items
enclosed in [@dots{}] in these templates are optional:

@example
awk @r{[@var{options}]} -f progfile @r{[@code{--}]} @var{file} @dots{}
awk @r{[@var{options}]} @r{[@code{--}]} '@var{program}' @var{file} @dots{}
@end example

Besides traditional one-letter POSIX-style options, @command{gawk} also
supports GNU long options.

@cindex empty program
@cindex dark corner
@cindex lint checks
It is possible to invoke @command{awk} with an empty program:

@example
awk '' datafile1 datafile2
@end example

@noindent
Doing so makes little sense though; @command{awk} exits
silently when given an empty program.
@value{DARKCORNER}
If @option{--lint} has
been specified on the command-line, @command{gawk} issues a
warning that the program is empty.

@node Options, Other Arguments, Command Line, Invoking Gawk
@section Command-Line Options

Options begin with a dash and consist of a single character.
GNU-style long options consist of two dashes and a keyword.
The keyword can be abbreviated, as long as the abbreviation allows the option
to be uniquely identified.  If the option takes an argument, then the
keyword is either immediately followed by an equals sign (@samp{=}) and the
argument's value, or the keyword and the argument's value are separated
by whitespace.
If a particular option with a value is given more than once, it is the
last value that counts.

Each long option for @command{gawk} has a corresponding
POSIX-style option.
The long and short options are
interchangeable in all contexts.
The options and their meanings are as follows:

@table @code
@item -F @var{fs}
@itemx --field-separator @var{fs}
@cindex @code{-F} option
@cindex command-line option, @code{-F}
@cindex @code{--field-separator} option
@cindex command-line option, @code{--field-separator}
Sets the @code{FS} variable to @var{fs}
(@pxref{Field Separators, ,Specifying How Fields Are Separated}).

@item -f @var{source-file}
@itemx --file @var{source-file}
@cindex @code{-f} option
@cindex command-line option, @code{-f}
@cindex @code{--file} option
@cindex command-line option, @code{--file}
Indicates that the @command{awk} program is to be found in @var{source-file}
instead of in the first non-option argument.

@item -v @var{var}=@var{val}
@itemx --assign @var{var}=@var{val}
@cindex @code{-v} option
@cindex command-line option, @code{-v}
@cindex @code{--assign} option
@cindex command-line option, @code{--assign}
Sets the variable @var{var} to the value @var{val} @emph{before}
execution of the program begins.  Such variable values are available
inside the @code{BEGIN} rule
(@pxref{Other Arguments, ,Other Command-Line Arguments}).

The @option{-v} option can only set one variable, but it can be used
more than once, setting another variable each time, like this:
@samp{awk @w{-v foo=1} @w{-v bar=2} @dots{}}.

@strong{Caution:}  Using @option{-v} to set the values of the built-in
variables may lead to surprising results.  @command{awk} will reset the
values of those variables as it needs to, possibly ignoring any
predefined value you may have given.

@item -mf @var{N}
@itemx -mr @var{N}
@cindex @code{-mf} option
@cindex command-line option, @code{-mf}
@cindex @code{-mr} option
@cindex command-line option, @code{-mr}
Set various memory limits to the value @var{N}.  The @samp{f} flag sets
the maximum number of fields and the @samp{r} flag sets the maximum
record size.  These two flags and the @option{-m} option are from the
Bell Laboratories research version of Unix @command{awk}.  They are provided
for compatibility but otherwise ignored by
@command{gawk}, since @command{gawk} has no predefined limits.
(The Bell Laboratories @command{awk} no longer needs these options;
it continues to accept them to avoid breaking old programs.)

@item -W @var{gawk-opt}
@cindex @code{-W} option
@cindex command-line option, @code{-W}
Following the POSIX standard, implementation-specific
options are supplied as arguments to the @option{-W} option.  These options
also have corresponding GNU-style long options.
Note that the long options may be abbreviated, as long as
the abbreviations remain unique.
The full list of @command{gawk}-specific options is provided next.

@item --
Signals the end of the command-line options.  The following arguments
are not treated as options even if they begin with @samp{-}.  This
interpretation of @option{--} follows the POSIX argument parsing
conventions.

This is useful if you have @value{FN}s that start with @samp{-},
or in shell scripts, if you have @value{FN}s that will be specified
by the user that could start with @samp{-}.
@end table

The previous list described options mandated by the POSIX standard,
as well as options available in the Bell Laboratories version of @command{awk}.
The following list describes @command{gawk}-specific options:

@table @code
@item -W compat
@itemx -W traditional
@itemx --compat
@itemx --traditional
@cindex @code{--compat} option
@cindex command-line option, @code{--compat}
@cindex @code{--traditional} option
@cindex command-line option, @code{--traditional}
@cindex compatibility mode
Specifies @dfn{compatibility mode}, in which the GNU extensions to
the @command{awk} language are disabled, so that @command{gawk} behaves just
like the Bell Laboratories research version of Unix @command{awk}.
@option{--traditional} is the preferred form of this option.
@xref{POSIX/GNU, ,Extensions in @command{gawk} Not in POSIX @command{awk}},
which summarizes the extensions.  Also see
@ref{Compatibility Mode, ,Downward Compatibility and Debugging}.

@item -W copyright
@itemx --copyright
@cindex @code{--copyright} option
@cindex command-line option, @code{--copyright}
Print the short version of the General Public License and then exit.

@item -W copyleft
@itemx --copyleft
@cindex @code{--copyleft} option
@cindex command-line option, @code{--copyleft}
Just like @option{--copyright}.
This option may disappear in a future version of @command{gawk}.

@cindex @code{--dump-variables} option
@cindex command-line option, @code{--dump-variables}
@cindex @file{awkvars.out} global variable list output file
@item -W dump-variables@r{[}=@var{file}@r{]}
@itemx --dump-variables@r{[}=@var{file}@r{]}
Print a sorted list of global variables, their types, and final values
to @var{file}.  If no @var{file} is provided, @command{gawk} prints this
list to a file named @file{awkvars.out} in the current directory.

@cindex common mistakes
@cindex mistakes, common
@cindex errors, common
Having a list of all the global variables is a good way to look for
typographical errors in your programs.
You would also use this option if you have a large program with a lot of
functions, and you want to be sure that your functions don't
inadvertently use global variables that you meant to be local.
(This is a particularly easy mistake to make with simple variable
names like @code{i}, @code{j}, and so on.)

@item -W gen-po
@itemx --gen-po
@cindex @code{--gen-po} option
@cindex command-line option, @code{--gen-po}
Analyze the source program and
generate a GNU @code{gettext} Portable Object file on standard
output for all string constants that have been marked for translation.
@xref{Internationalization, ,Internationalization with @command{gawk}},
for information about this option.

@item -W help
@itemx -W usage
@itemx --help
@itemx --usage
@cindex @code{--help} option
@cindex command-line option, @code{--help}
@cindex @code{--usage} option
@cindex command-line option, @code{--usage}
Print a ``usage'' message summarizing the short and long style options
that @command{gawk} accepts and then exit.

@item -W lint@r{[}=fatal@r{]}
@itemx --lint@r{[}=fatal@r{]}
@cindex @code{--lint} option
@cindex command-line option, @code{--lint}
@cindex lint checks
@cindex fatal errors
Warn about constructs that are dubious or non-portable to
other @command{awk} implementations.
Some warnings are issued when @command{gawk} first reads your program.  Others
are issued at runtime, as your program executes.
With an optional argument of @samp{fatal},
lint warnings become fatal errors.
This may be drastic but its use will certainly encourage the
development of cleaner @command{awk} programs.

@item -W lint-old
@itemx --lint-old
@cindex @code{--lint-old} option
@cindex command-line option, @code{--lint-old}
@cindex lint checks
Warn about constructs that are not available in the original version of
@command{awk} from Version 7 Unix
(@pxref{V7/SVR3.1, ,Major Changes Between V7 and SVR3.1}).

@item -W non-decimal-data
@itemx --non-decimal-data
@cindex @code{--non-decimal-data} option
@cindex command-line option, @code{--non-decimal-data}
Enable automatic interpretation of octal and hexadecimal
values in input data
(@pxref{Non-decimal Data, ,Allowing Non-Decimal Input Data}).

@strong{Caution:} This option can severely break old programs.
Use with care.

@item -W posix
@itemx --posix
@cindex @code{--posix} option
@cindex command-line option, @code{--posix}
@cindex POSIX mode
Operate in strict POSIX mode.  This disables all @command{gawk}
extensions (just like @option{--traditional}) and adds the following additional
restrictions:

@c IMPORTANT! Keep this list in sync with the one in node POSIX

@itemize @bullet
@item
@code{\x} escape sequences are not recognized
(@pxref{Escape Sequences}).

@item
Newlines do not act as whitespace to separate fields when @code{FS} is
equal to a single space
(@pxref{Fields, , Examining Fields}).

@item
Newlines are not allowed after @samp{?} or @samp{:}
(@pxref{Conditional Exp, ,Conditional Expressions}).

@item
The synonym @code{func} for the keyword @code{function} is not
recognized (@pxref{Definition Syntax, ,Function Definition Syntax}).

@item
The @samp{**} and @samp{**=} operators cannot be used in
place of @samp{^} and @samp{^=} (@pxref{Arithmetic Ops, ,Arithmetic Operators},
and also @pxref{Assignment Ops, ,Assignment Expressions}).

@item
Specifying @samp{-Ft} on the command-line does not set the value
of @code{FS} to be a single tab character
(@pxref{Field Separators, ,Specifying How Fields Are Separated}).

@item
The @code{fflush} built-in function is not supported
(@pxref{I/O Functions, ,Input/Output Functions}).
@end itemize

@cindex automatic warnings
@cindex warnings, automatic
If you supply both @option{--traditional} and @option{--posix} on the
command-line, @option{--posix} takes precedence. @command{gawk}
also issues a warning if both options are supplied.

@item -W profile@r{[}=@var{file}@r{]}
@itemx --profile@r{[}=@var{file}@r{]}
@cindex @code{--profile} option
@cindex command-line option, @code{--profile}
Enable profiling of @command{awk} programs
(@pxref{Profiling, ,Profiling Your @command{awk} Programs}).
By default, profiles are created in a file named @file{awkprof.out}.
The optional @var{file} argument allows you to specify a different
@value{FN} for the profile file.

When run with @command{gawk}, the profile is just a ``pretty printed'' version
of the program.  When run with @command{pgawk}, the profile contains execution
counts for each statement in the program in the left margin, and function
call counts for each function.

@item -W re-interval
@itemx --re-interval
@cindex @code{--re-interval} option
@cindex command-line option, @code{--re-interval}
Allow interval expressions
(@pxref{Regexp Operators, , Regular Expression Operators})
in regexps.
Because interval expressions were traditionally not available in @command{awk},
@command{gawk} does not provide them by default. This prevents old @command{awk}
programs from breaking.

@item -W source @var{program-text}
@itemx --source @var{program-text}
@cindex @code{--source} option
@cindex command-line option, @code{--source}
Program source code is taken from the @var{program-text}.  This option
allows you to mix source code in files with source
code that you enter on the command-line. This is particularly useful
when you have library functions that you want to use from your command-line
programs (@pxref{AWKPATH Variable, ,The @env{AWKPATH} Environment Variable}).

@item -W version
@itemx --version
@cindex @code{--version} option
@cindex command-line option, @code{--version}
Print version information for this particular copy of @command{gawk}.
This allows you to determine if your copy of @command{gawk} is up to date
with respect to whatever the Free Software Foundation is currently
distributing.
It is also useful for bug reports
(@pxref{Bugs,  , Reporting Problems and Bugs}).
@end table

As long as program text has been supplied,
any other options are flagged as invalid with a warning message but
are otherwise ignored.

In compatibility mode, as a special case, if the value of @var{fs} supplied
to the @option{-F} option is @samp{t}, then @code{FS} is set to the tab
character (@code{"\t"}).  This is only true for @option{--traditional} and not
for @option{--posix}
(@pxref{Field Separators, ,Specifying How Fields Are Separated}).

The @option{-f} option may be used more than once on the command-line.
If it is, @command{awk} reads its program source from all of the named files, as
if they had been concatenated together into one big file.  This is
useful for creating libraries of @command{awk} functions.  These functions
can be written once and then retrieved from a standard place, instead
of having to be included into each individual program.
(As mentioned in
@ref{Definition Syntax, ,Function Definition Syntax},
function names must be unique.)

Library functions can still be used, even if the program is entered at the terminal,
by specifying @samp{-f /dev/tty}.  After typing your program,
type @kbd{Ctrl-d} (the end-of-file character) to terminate it.
(You may also use @samp{-f -} to read program source from the standard
input but then you will not be able to also use the standard input as a
source of data.)

Because it is clumsy using the standard @command{awk} mechanisms to mix source
file and command-line @command{awk} programs, @command{gawk} provides the
@option{--source} option.  This does not require you to pre-empt the standard
input for your source code; it allows you to easily mix command-line
and library source code
(@pxref{AWKPATH Variable, ,The @env{AWKPATH} Environment Variable}).

If no @option{-f} or @option{--source} option is specified, then @command{gawk}
uses the first non-option command-line argument as the text of the
program source code.

@cindex @code{POSIXLY_CORRECT} environment variable
@cindex environment variable, @code{POSIXLY_CORRECT}
@cindex lint checks
If the environment variable @env{POSIXLY_CORRECT} exists,
then @command{gawk} behaves in strict POSIX mode, exactly as if
you had supplied the @option{--posix} command-line option.
Many GNU programs look for this environment variable to turn on
strict POSIX mode. If @option{--lint} is supplied on the command-line
and @command{gawk} turns on POSIX mode because of @env{POSIXLY_CORRECT},
then it issues a warning message indicating that POSIX
mode is in effect.
You would typically set this variable in your shell's startup file.
For a Bourne-compatible shell (such as @command{bash}), you would add these
lines to the @file{.profile} file in your home directory:

@example
POSIXLY_CORRECT=true
export POSIXLY_CORRECT
@end example

@cindex @command{csh} utility
For a @command{csh} compatible
shell,@footnote{Not recommended.}
you would add this line to the @file{.login} file in your home directory:

@example
setenv POSIXLY_CORRECT true
@end example

Having @env{POSIXLY_CORRECT} set is not recommended for daily use,
but it is good for testing the portability of your programs to other
environments.

@node Other Arguments, AWKPATH Variable, Options, Invoking Gawk
@section Other Command-Line Arguments

Any additional arguments on the command-line are normally treated as
input files to be processed in the order specified.   However, an
argument that has the form @code{@var{var}=@var{value}}, assigns
the value @var{value} to the variable @var{var}---it does not specify a
file at all.
(This was discussed earlier in
@ref{Assignment Options, ,Assigning Variables on the Command Line}.)

@cindex @code{ARGIND} variable
@cindex @code{ARGV} variable
All these arguments are made available to your @command{awk} program in the
@code{ARGV} array (@pxref{Built-in Variables}).  Command-line options
and the program text (if present) are omitted from @code{ARGV}.
All other arguments, including variable assignments, are
included.   As each element of @code{ARGV} is processed, @command{gawk}
sets the variable @code{ARGIND} to the index in @code{ARGV} of the
current element.

The distinction between @value{FN} arguments and variable-assignment
arguments is made when @command{awk} is about to open the next input file.
At that point in execution, it checks the @value{FN} to see whether
it is really a variable assignment; if so, @command{awk} sets the variable
instead of reading a file.

Therefore, the variables actually receive the given values after all
previously specified files have been read.  In particular, the values of
variables assigned in this fashion are @emph{not} available inside a
@code{BEGIN} rule
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}),
because such rules are run before @command{awk} begins scanning the argument list.

@cindex dark corner
The variable values given on the command-line are processed for escape
sequences (@pxref{Escape Sequences}).
@value{DARKCORNER}

In some earlier implementations of @command{awk}, when a variable assignment
occurred before any @value{FN}s, the assignment would happen @emph{before}
the @code{BEGIN} rule was executed.  @command{awk}'s behavior was thus
inconsistent; some command-line assignments were available inside the
@code{BEGIN} rule, while others were not.  Unfortunately,
some applications came to depend
upon this ``feature.''  When @command{awk} was changed to be more consistent,
the @option{-v} option was added to accommodate applications that depended
upon the old behavior.

The variable assignment feature is most useful for assigning to variables
such as @code{RS}, @code{OFS}, and @code{ORS}, which control input and
output formats before scanning the @value{DF}s.  It is also useful for
controlling state if multiple passes are needed over a @value{DF}.  For
example:

@cindex multiple passes over data
@cindex passes, multiple
@example
awk 'pass == 1  @{ @var{pass 1 stuff} @}
     pass == 2  @{ @var{pass 2 stuff} @}' pass=1 mydata pass=2 mydata
@end example

Given the variable assignment feature, the @option{-F} option for setting
the value of @code{FS} is not
strictly necessary.  It remains for historical compatibility.

@node AWKPATH Variable, Obsolete, Other Arguments, Invoking Gawk
@section The @env{AWKPATH} Environment Variable
@cindex @env{AWKPATH} environment variable
@cindex environment variable, @env{AWKPATH}
@cindex search path
@cindex directory search
@cindex path, search
@cindex search path, for source files
@cindex differences between @command{gawk} and @command{awk}
@ifinfo
The previous @value{SECTION} described how @command{awk} program files can be named
on the command-line with the @option{-f} option.
@end ifinfo
In most @command{awk}
implementations, you must supply a precise path name for each program
file, unless the file is in the current directory.
But in @command{gawk}, if the @value{FN} supplied to the @option{-f} option
does not contain a @samp{/}, then @command{gawk} searches a list of
directories (called the @dfn{search path}), one by one, looking for a
file with the specified name.

The search path is a string consisting of directory names
separated by colons.  @command{gawk} gets its search path from the
@env{AWKPATH} environment variable.  If that variable does not exist,
@command{gawk} uses a default path, which is
@samp{.:/usr/local/share/awk}.@footnote{Your version of @command{gawk}
may use a different directory; it
will depend upon how @command{gawk} was built and installed. The actual
directory is the value of @samp{$(datadir)} generated when
@command{gawk} was configured.  You probably don't need to worry about this
though.} (Programs written for use by
system administrators should use an @env{AWKPATH} variable that
does not include the current directory, @file{.}.)

The search path feature is particularly useful for building libraries
of useful @command{awk} functions.  The library files can be placed in a
standard directory in the default path and then specified on
the command-line with a short @value{FN}.  Otherwise, the full @value{FN}
would have to be typed for each file.

By using both the @option{--source} and @option{-f} options, your command-line
@command{awk} programs can use facilities in @command{awk} library files.
@xref{Library Functions, , A Library of @command{awk} Functions}.
Path searching is not done if @command{gawk} is in compatibility mode.
This is true for both @option{--traditional} and @option{--posix}.
@xref{Options, ,Command-Line Options}.

@strong{Note:} If you want files in the current directory to be found,
you must include the current directory in the path, either by including
@file{.} explicitly in the path or by writing a null entry in the
path.  (A null entry is indicated by starting or ending the path with a
colon or by placing two colons next to each other (@samp{::}).)  If the
current directory is not included in the path, then files cannot be
found in the current directory.  This path search mechanism is identical
to the shell's.
@c someday, @cite{The Bourne Again Shell}....

Starting with @value{PVERSION} 3.0, if @env{AWKPATH} is not defined in the
environment, @command{gawk} places its default search path into
@code{ENVIRON["AWKPATH"]}. This makes it easy to determine
the actual search path that @command{gawk} will use
from within an @command{awk} program.

While you can change @code{ENVIRON["AWKPATH"]} within your @command{awk}
program, this has no effect on the running program's behavior.  This makes
sense: the @env{AWKPATH} environment variable is used to find the program
source files.  Once your program is running, all the files have been
found, and @command{gawk} no longer needs to use @env{AWKPATH}.

@node Obsolete, Undocumented, AWKPATH Variable, Invoking Gawk
@section Obsolete Options and/or Features

@cindex deprecated options
@cindex obsolete options
@cindex deprecated features
@cindex obsolete features
This @value{SECTION} describes features and/or command-line options from
previous releases of @command{gawk} that are either not available in the
current version or that are still supported but deprecated (meaning that
they will @emph{not} be in the next release).

@c update this section for each release!

For @value{PVERSION} @value{VERSION} of @command{gawk}, there are no
deprecated command-line options
@c or other deprecated features
from the previous version of @command{gawk}.
The use of @samp{next file} (two words) for @code{nextfile} was deprecated
in @command{gawk} 3.0 but still worked.  Starting with @value{PVERSION} 3.1, the
two word usage is no longer accepted.

The process-related special files described in
@ref{Special Process, ,Special Files for Process-Related Information},
work as described, but
are now considered deprecated.
@command{gawk} prints a warning message every time they are used.
(Use @code{PROCINFO} instead; see
@ref{Auto-set, ,Built-in Variables That Convey Information}.)
They will be removed from the next release of @command{gawk}.

@ignore
This @value{SECTION}
is thus essentially a place holder,
in case some option becomes obsolete in a future version of @command{gawk}.
@end ignore

@node Undocumented, Known Bugs, Obsolete, Invoking Gawk
@section Undocumented Options and Features
@cindex undocumented features
@cindex features, undocumented 
@cindex Skywalker, Luke
@cindex Kenobi, Obi-Wan
@cindex Jedi knights
@cindex Knights, jedi
@quotation
@i{Use the Source, Luke!}@*
Obi-Wan
@end quotation

This @value{SECTION} intentionally left
blank.

@ignore
@c If these came out in the Info file or TeX document, then they wouldn't
@c be undocumented, would they?

@command{gawk} has one undocumented option:

@table @code
@item -W nostalgia
@itemx --nostalgia
Print the message @code{"awk: bailing out near line 1"} and dump core.
This option was inspired by the common behavior of very early versions of
Unix @command{awk} and by a t--shirt.
The message is @emph{not} subject to translation in non-English locales.
@c so there! nyah, nyah.
@end table

Early versions of @command{awk} used to not require any separator (either
a newline or @samp{;}) between the rules in @command{awk} programs.  Thus,
it was common to see one-line programs like:

@example
awk '@{ sum += $1 @} END @{ print sum @}'
@end example

@command{gawk} actually supports this but it is purposely undocumented
because it is considered bad style.  The correct way to write such a program
is either

@example
awk '@{ sum += $1 @} ; END @{ print sum @}'
@end example

@noindent
or

@example
awk '@{ sum += $1 @}
     END @{ print sum @}' data
@end example

@noindent
@xref{Statements/Lines, ,@command{awk} Statements Versus Lines}, for a fuller
explanation.

You can insert newlines after the @samp{;} in @code{for} loops.
This seems to have been a long-undocumented feature in Unix @command{awk}.

If the environment variable @env{WHINY_USERS} exists
when @command{gawk} is run,
then the associative @code{for} loop will go through the array
indices in sorted order.
The comparison used for sorting is simple string comparison;
any non-English or non-ASCII locales are not taken into account.
@code{IGNORECASE} does not affect the comparison either.

@end ignore

@node Known Bugs, , Undocumented, Invoking Gawk
@section Known Bugs in @command{gawk}
@cindex bugs, known in @command{gawk}
@cindex known bugs

@itemize @bullet
@item
The @option{-F} option for changing the value of @code{FS}
(@pxref{Options, ,Command-Line Options})
is not necessary given the command-line variable
assignment feature; it remains only for backwards compatibility.

@item
Syntactically invalid single character programs tend to overflow
the parse stack, generating a rather unhelpful message.  Such programs
are surprisingly difficult to diagnose in the completely general case
and the effort to do so really is not worth it.
@end itemize

@ignore
@c Try this
@iftex
@page
@headings off
@majorheading II@ @ @ Using @command{awk} and @command{gawk}
Part II shows how to use @command{awk} and @command{gawk} for problem solving.
There is lots of code here for you to read and learn from.
It contains the following chapters:

@itemize @bullet
@item
@ref{Library Functions, ,A Library of @command{awk} Functions}.

@item
@ref{Sample Programs, ,Practical @command{awk} Programs}.

@end itemize

@page
@evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @|
@oddheading  @| @| @strong{@thischapter}@ @ @ @thispage
@end iftex
@end ignore

@node Library Functions, Sample Programs, Invoking Gawk, Top
@chapter A Library of @command{awk} Functions

@ref{User-defined, ,User-Defined Functions}, describes how to write
your own @command{awk} functions.  Writing functions is important, because
it allows you to encapsulate algorithms and program tasks in a single
place.  It simplifies programming, making program development more
manageable, and making programs more readable.

One valuable way to learn a new programming language is to @emph{read}
programs in that language.  To that end, this @value{CHAPTER}
and @ref{Sample Programs, ,Practical @command{awk} Programs},
provide a good-sized body of code for you to read,
and hopefully, to learn from.

@c 2e: USE TEXINFO-2 FUNCTION DEFINITION STUFF!!!!!!!!!!!!!
This @value{CHAPTER} presents a library of useful @command{awk} functions.
Many of the sample programs presented later in this @value{DOCUMENT}
use these functions.
The functions are presented here in a progression from simple to complex.

@cindex Texinfo
@ref{Extract Program, ,Extracting Programs from Texinfo Source Files},
presents a program that you can use to extract the source code for
these example library functions and programs from the Texinfo source
for this @value{DOCUMENT}.
(This has already been done as part of the @command{gawk} distribution.)

If you have written one or more useful, general purpose @command{awk} functions
and would like to contribute them to the author's collection of @command{awk}
programs, see
@ref{How To Contribute, ,How to Contribute}, for more information.

@cindex portability issues
The programs in this @value{CHAPTER} and in
@ref{Sample Programs, ,Practical @command{awk} Programs},
freely use features that are @command{gawk}-specific.
It is straightforward to rewrite these programs for
different implementations of @command{awk}.

Diagnostic error messages are sent to @file{/dev/stderr}.
Use @samp{| "cat 1>&2"} instead of @samp{> "/dev/stderr"}, if your system
does not have a @file{/dev/stderr} or if you cannot use @command{gawk}.

A number of programs use @code{nextfile}
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement})
to skip any remaining input in the input file.
@ref{Nextfile Function, ,Implementing @code{nextfile} as a Function},
shows you how to write a function that does the same thing.

@c 12/2000: Thanks to Nelson Beebe for pointing out the output issue.
Finally, some of the programs choose to ignore upper- and lowercase
distinctions in their input. They do so by assigning one to @code{IGNORECASE}.
You can achieve almost the same effect@footnote{The effects are
not identical.  Output of the transformed
record will be in all lowercase, while @code{IGNORECASE} preserves the original
contents of the input record.} by adding the following rule to the
beginning of the program:

@example
# ignore case
@{ $0 = tolower($0) @}
@end example

@noindent
Also, verify that all regexp and string constants used in
comparisons only use lowercase letters.

@menu
* Library Names::               How to best name private global variables in
                                library functions.
* General Functions::           Functions that are of general use.
* Data File Management::        Functions for managing command-line data
                                files.
* Getopt Function::             A function for processing command-line
                                arguments.
* Passwd Functions::            Functions for getting user information.
* Group Functions::             Functions for getting group information.
@end menu

@node Library Names, General Functions, Library Functions, Library Functions
@section Naming Library Function Global Variables

@cindex names, use of
@cindex namespace issues in @command{awk}
@cindex documenting @command{awk} programs
@cindex programs, documenting
Due to the way the @command{awk} language evolved, variables are either
@dfn{global} (usable by the entire program) or @dfn{local} (usable just by
a specific function).  There is no intermediate state analogous to
@code{static} variables in C.

Library functions often need to have global variables that they can use to
preserve state information between calls to the function---for example,
@code{getopt}'s variable @code{_opti}
(@pxref{Getopt Function, ,Processing Command-Line Options}).
Such variables are called @dfn{private}, since the only functions that need to
use them are the ones in the library.

When writing a library function, you should try to choose names for your
private variables that will not conflict with any variables used by
either another library function or a user's main program.  For example, a
name like @samp{i} or @samp{j} is not a good choice, because user programs
often use variable names like these for their own purposes.

@cindex conventions, programming
@cindex programming conventions
The example programs shown in this @value{CHAPTER} all start the names of their
private variables with an underscore (@samp{_}).  Users generally don't use
leading underscores in their variable names, so this convention immediately
decreases the chances that the variable name will be accidentally shared
with the user's program.

In addition, several of the library functions use a prefix that helps
indicate what function or set of functions use the variables---for example,
@code{_pw_byname} in the user database routines
(@pxref{Passwd Functions, ,Reading the User Database}).
This convention is recommended, since it even further decreases the
chance of inadvertent conflict among variable names.  Note that this
convention is used equally well for variable names and for private
function names as well.@footnote{While all the library routines could have
been rewritten to use this convention, this was not done, in order to
show how my own @command{awk} programming style has evolved, and to
provide some basis for this discussion.}

As a final note on variable naming, if a function makes global variables
available for use by a main program, it is a good convention to start that
variable's name with a capital letter---for
example, @code{getopt}'s @code{Opterr} and @code{Optind} variables
(@pxref{Getopt Function, ,Processing Command-Line Options}).
The leading capital letter indicates that it is global, while the fact that
the variable name is not all capital letters indicates that the variable is
not one of @command{awk}'s built-in variables, such as @code{FS}.

It is also important that @emph{all} variables in library
functions that do not need to save state are, in fact, declared
local.@footnote{@command{gawk}'s @option{--dump-variables} command-line
option is useful for verifying this.} If this is not done, the variable
could accidentally be used in the user's program, leading to bugs that
are very difficult to track down:

@example
function lib_func(x, y,    l1, l2)
@{
    @dots{}
    @var{use variable} some_var   # some_var should be local
    @dots{}                   # but is not by oversight
@}
@end example

@cindex Tcl
A different convention, common in the Tcl community, is to use a single
associative array to hold the values needed by the library function(s), or
``package.''  This significantly decreases the number of actual global names
in use.  For example, the functions described in
@ref{Passwd Functions, , Reading the User Database},
might have used array elements @code{@w{PW_data["inited"]}}, @code{@w{PW_data["total"]}},
@code{@w{PW_data["count"]}}, and @code{@w{PW_data["awklib"]}}, instead of
@code{@w{_pw_inited}}, @code{@w{_pw_awklib}}, @code{@w{_pw_total}},
and @code{@w{_pw_count}}.

The conventions presented in this @value{SECTION} are exactly
that: conventions. You are not required to write your programs this
way---we merely recommend that you do so.

@node General Functions, Data File Management, Library Names, Library Functions
@section General Programming

This @value{SECTION} presents a number of functions that are of general
programming use.

@menu
* Nextfile Function::           Two implementations of a @code{nextfile}
                                function.
* Assert Function::             A function for assertions in @command{awk}
                                programs.
* Round Function::              A function for rounding if @code{sprintf} does
                                not do it correctly.
* Cliff Random Function::       The Cliff Random Number Generator.
* Ordinal Functions::           Functions for using characters as numbers and
                                vice versa.
* Join Function::               A function to join an array into a string.
* Gettimeofday Function::       A function to get formatted times.
@end menu

@node Nextfile Function, Assert Function, General Functions, General Functions
@subsection Implementing @code{nextfile} as a Function

@cindex skipping input files
@cindex input files, skipping
The @code{nextfile} statement presented in
@ref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement},
is a @command{gawk}-specific extension---it is not available in most other
implementations of @command{awk}.  This @value{SECTION} shows two versions of a
@code{nextfile} function that you can use to simulate @command{gawk}'s
@code{nextfile} statement if you cannot use @command{gawk}.

A first attempt at writing a @code{nextfile} function is as follows:

@example
# nextfile --- skip remaining records in current file
# this should be read in before the "main" awk program

function nextfile()    @{ _abandon_ = FILENAME; next @}
_abandon_ == FILENAME  @{ next @}
@end example

@cindex conventions, programming
@cindex programming conventions
Because it supplies a rule that must be executed first, this file should
be included before the main program. This rule compares the current
@value{DF}'s name (which is always in the @code{FILENAME} variable) to
a private variable named @code{_abandon_}.  If the @value{FN} matches,
then the action part of the rule executes a @code{next} statement to
go on to the next record.  (The use of @samp{_} in the variable name is
a convention.  It is discussed more fully in
@ref{Library Names,  , Naming Library Function Global Variables}.)

The use of the @code{next} statement effectively creates a loop that reads
all the records from the current @value{DF}.
The end of the file is eventually reached and
a new @value{DF} is opened, changing the value of @code{FILENAME}.
Once this happens, the comparison of @code{_abandon_} to @code{FILENAME}
fails and execution continues with the first rule of the ``real'' program.

The @code{nextfile} function itself simply sets the value of @code{_abandon_}
and then executes a @code{next} statement to start the
loop.
@ignore
@c If the function can't be used on other versions of awk, this whole
@c section is pointless, no?  Sigh.
@footnote{@command{gawk} is the only known @command{awk} implementation
that allows you to
execute @code{next} from within a function body. Some other workaround
is necessary if you are not using @command{gawk}.}
@end ignore

@cindex @code{nextfile} user-defined function
This initial version has a subtle problem.
If the same @value{DF} is listed @emph{twice} on the commandline,
one right after the other
or even with just a variable assignment between them,
this code skips right through the file, a second time, even though
it should stop when it gets to the end of the first occurrence.
A second version of @code{nextfile} that remedies this problem
is shown here:

@example
@c file eg/lib/nextfile.awk
# nextfile --- skip remaining records in current file
# correctly handle successive occurrences of the same file
@c endfile
@ignore
@c file eg/lib/nextfile.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May, 1993

@c endfile
@end ignore
@c file eg/lib/nextfile.awk
# this should be read in before the "main" awk program

function nextfile()   @{ _abandon_ = FILENAME; next @}

_abandon_ == FILENAME @{
      if (FNR == 1)
          _abandon_ = ""
      else
          next
@}
@c endfile
@end example

The @code{nextfile} function has not changed.  It makes @code{_abandon_}
equal to the current @value{FN} and then executes a @code{next} statement.
The @code{next} statement reads the next record and increments @code{FNR}
so that @code{FNR} is guaranteed to have a value of at least two.
However, if @code{nextfile} is called for the last record in the file,
then @command{awk} closes the current @value{DF} and moves on to the next
one.  Upon doing so, @code{FILENAME} is set to the name of the new file
and @code{FNR} is reset to one.  If this next file is the same as
the previous one, @code{_abandon_} is still equal to @code{FILENAME}.
However, @code{FNR} is equal to one, telling us that this is a new
occurrence of the file and not the one we were reading when the
@code{nextfile} function was executed.  In that case, @code{_abandon_}
is reset to the empty string, so that further executions of this rule
fail (until the next time that @code{nextfile} is called).

If @code{FNR} is not one, then we are still in the original @value{DF}
and the program executes a @code{next} statement to skip through it.

An important question to ask at this point is: given that the
functionality of @code{nextfile} can be provided with a library file,
why is it built into @command{gawk}?  Adding
features for little reason leads to larger, slower programs that are
harder to maintain.
The answer is that building @code{nextfile} into @command{gawk} provides
significant gains in efficiency.  If the @code{nextfile} function is executed
at the beginning of a large @value{DF}, @command{awk} still has to scan the entire
file, splitting it up into records,
@c at least conceptually
just to skip over it.  The built-in
@code{nextfile} can simply close the file immediately and proceed to the
next one, which saves a lot of time.  This is particularly important in
@command{awk}, because @command{awk} programs are generally I/O-bound (i.e.,
they spend most of their time doing input and output, instead of performing
computations).

@node Assert Function, Round Function, Nextfile Function, General Functions
@subsection Assertions

@cindex assertions
@cindex @code{assert} C library function
When writing large programs, it is often useful to know
that a condition or set of conditions is true.  Before proceeding with a
particular computation, you make a statement about what you believe to be
the case.  Such a statement is known as an
@dfn{assertion}.  The C language provides an @code{<assert.h>} header file
and corresponding @code{assert} macro that the programmer can use to make
assertions.  If an assertion fails, the @code{assert} macro arranges to
print a diagnostic message describing the condition that should have
been true but was not, and then it kills the program.  In C, using
@code{assert} looks this:

@example
#include <assert.h>

int myfunc(int a, double b)
@{
     assert(a <= 5 && b >= 17.1);
     @dots{}
@}
@end example

If the assertion fails, the program prints a message similar to this:

@example
prog.c:5: assertion failed: a <= 5 && b >= 17.1
@end example

@cindex @code{assert} user-defined function
The C language makes it possible to turn the condition into a string for use
in printing the diagnostic message.  This is not possible in @command{awk}, so
this @code{assert} function also requires a string version of the condition
that is being tested.
Following is the function:

@example
@c file eg/lib/assert.awk
# assert --- assert that a condition is true. Otherwise exit.
@c endfile
@ignore
@c file eg/lib/assert.awk

#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May, 1993

@c endfile
@end ignore
@c file eg/lib/assert.awk
function assert(condition, string)
@{
    if (! condition) @{
        printf("%s:%d: assertion failed: %s\n",
            FILENAME, FNR, string) > "/dev/stderr"
        _assert_exit = 1
        exit 1
    @}
@}

@group
END @{
    if (_assert_exit)
        exit 1
@}
@end group
@c endfile
@end example

The @code{assert} function tests the @code{condition} parameter. If it
is false, it prints a message to standard error, using the @code{string}
parameter to describe the failed condition.  It then sets the variable
@code{_assert_exit} to one and executes the @code{exit} statement.
The @code{exit} statement jumps to the @code{END} rule. If the @code{END}
rules finds @code{_assert_exit} to be true, it then exits immediately.

The purpose of the test in the @code{END} rule is to
keep any other @code{END} rules from running.  When an assertion fails, the
program should exit immediately.
If no assertions fail, then @code{_assert_exit} is still
false when the @code{END} rule is run normally, and the rest of the
program's @code{END} rules execute.
For all of this to work correctly, @file{assert.awk} must be the
first source file read by @command{awk}.
The function can be used in a program in the following way:

@example
function myfunc(a, b)
@{
     assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1")
     @dots{}
@}
@end example

@noindent
If the assertion fails, you see a message similar to the following:

@example
mydata:1357: assertion failed: a <= 5 && b >= 17.1
@end example

There is a small problem with this version of @code{assert}.
An @code{END} rule is automatically added
to the program calling @code{assert}.  Normally, if a program consists
of just a @code{BEGIN} rule, the input files and/or standard input are
not read. However, now that the program has an @code{END} rule, @command{awk}
attempts to read the input @value{DF}s or standard input
(@pxref{Using BEGIN/END, , Startup and Cleanup Actions}),
most likely causing the program to hang as it waits for input.

There is a simple workaround to this:
make sure the @code{BEGIN} rule always ends
with an @code{exit} statement.

@node Round Function, Cliff Random Function, Assert Function, General Functions
@subsection Rounding Numbers

@cindex rounding
The way @code{printf} and @code{sprintf}
(@pxref{Printf, , Using @code{printf} Statements for Fancier Printing})
perform rounding often depends upon the system's C @code{sprintf}
subroutine.  On many machines, @code{sprintf} rounding is ``unbiased,''
which means it doesn't always round a trailing @samp{.5} up, contrary
to naive expectations.  In unbiased rounding, @samp{.5} rounds to even,
rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4.  This means
that if you are using a format that does rounding (e.g., @code{"%.0f"}),
you should check what your system does.  The following function does
traditional rounding; it might be useful if your awk's @code{printf}
does unbiased rounding:

@cindex @code{round} user-defined function
@example
@c file eg/lib/round.awk
# round --- do normal rounding
@c endfile
@ignore
@c file eg/lib/round.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# August, 1996

@c endfile
@end ignore
@c file eg/lib/round.awk
function round(x,   ival, aval, fraction)
@{
   ival = int(x)    # integer part, int() truncates

   # see if fractional part
   if (ival == x)   # no fraction
      return x

   if (x < 0) @{
      aval = -x     # absolute value
      ival = int(aval)
      fraction = aval - ival
      if (fraction >= .5)
         return int(x) - 1   # -2.5 --> -3
      else
         return int(x)       # -2.3 --> -2
   @} else @{
      fraction = x - ival
      if (fraction >= .5)
         return ival + 1
      else
         return ival
   @}
@}

# test harness
@{ print $0, round($0) @}
@c endfile
@end example

@node Cliff Random Function, Ordinal Functions, Round Function, General Functions
@subsection The Cliff Random Number Generator
@cindex random numbers, Cliff
@cindex Cliff random numbers

The Cliff random number
generator@footnote{@uref{http://mathworld.wolfram.com/CliffRandomNumberGenerator.hmtl}}
is a very simple random number generator that ``passes the noise sphere test
for randomness by showing no structure.''
It is easily programmed, in less than 10 lines of @command{awk} code:

@cindex @code{cliff_rand} user-defined function
@example
@c file eg/lib/cliff_rand.awk
# cliff_rand.awk --- generate Cliff random numbers
@c endfile
@ignore
@c file eg/lib/cliff_rand.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# December 2000

@c endfile
@end ignore
@c file eg/lib/cliff_rand.awk
BEGIN @{ _cliff_seed = 0.1 @}

function cliff_rand()
@{
    _cliff_seed = (100 * log(_cliff_seed)) % 1
    if (_cliff_seed < 0)
        _cliff_seed = - _cliff_seed
    return _cliff_seed
@}
@c endfile
@end example

This algorithm requires an initial ``seed'' of 0.1.  Each new value
uses the current seed as input for the calculation.
If the built-in @code{rand} function
(@pxref{Numeric Functions})
isn't random enough, you might try using this function instead.

@node Ordinal Functions, Join Function, Cliff Random Function, General Functions
@subsection Translating Between Characters and Numbers

@cindex numeric character values
@cindex values of characters as numbers
One commercial implementation of @command{awk} supplies a built-in function,
@code{ord}, which takes a character and returns the numeric value for that
character in the machine's character set.  If the string passed to
@code{ord} has more than one character, only the first one is used.

The inverse of this function is @code{chr} (from the function of the same
name in Pascal), which takes a number and returns the corresponding character.
Both functions are written very nicely in @command{awk}; there is no real
reason to build them into the @command{awk} interpreter:

@cindex @code{ord} user-defined function
@cindex @code{chr} user-defined function
@example
@c file eg/lib/ord.awk
# ord.awk --- do ord and chr

# Global identifiers:
#    _ord_:        numerical values indexed by characters
#    _ord_init:    function to initialize _ord_
@c endfile
@ignore
@c file eg/lib/ord.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# 16 January, 1992
# 20 July, 1992, revised

@c endfile
@end ignore
@c file eg/lib/ord.awk
BEGIN    @{ _ord_init() @}

function _ord_init(    low, high, i, t)
@{
    low = sprintf("%c", 7) # BEL is ascii 7
    if (low == "\a") @{    # regular ascii
        low = 0
        high = 127
    @} else if (sprintf("%c", 128 + 7) == "\a") @{
        # ascii, mark parity
        low = 128
        high = 255
    @} else @{        # ebcdic(!)
        low = 0
        high = 255
    @}

    for (i = low; i <= high; i++) @{
        t = sprintf("%c", i)
        _ord_[t] = i
    @}
@}
@c endfile
@end example

@cindex character sets (machine character encodings)
@cindex character encodings
@cindex ASCII
@cindex EBCDIC
@cindex mark parity
Some explanation of the numbers used by @code{chr} is worthwhile.
The most prominent character set in use today is ASCII. Although an
eight-bit byte can hold 256 distinct values (from 0 to 255), ASCII only
defines characters that use the values from 0 to 127.@footnote{ASCII
has been extended in many countries to use the values from 128 to 255
for country-specific characters.  If your  system uses these extensions,
you can simplify @code{_ord_init} to simply loop from 0 to 255.}
In the now distant past,
at least one minicomputer manufacturer
@c Pr1me, blech
used ASCII, but with mark parity, meaning that the leftmost bit in the byte
is always 1.  This means that on those systems, characters
have numeric values from 128 to 255.
Finally, large mainframe systems use the EBCDIC character set, which
uses all 256 values.
While there are other character sets in use on some older systems,
they are not really worth worrying about:

@example
@c file eg/lib/ord.awk
function ord(str,    c)
@{
    # only first character is of interest
    c = substr(str, 1, 1)
    return _ord_[c]
@}

function chr(c)
@{
    # force c to be numeric by adding 0
    return sprintf("%c", c + 0)
@}
@c endfile

#### test code ####
# BEGIN    \
# @{
#    for (;;) @{
#        printf("enter a character: ")
#        if (getline var <= 0)
#            break
#        printf("ord(%s) = %d\n", var, ord(var))
#    @}
# @}
@c endfile
@end example

An obvious improvement to these functions is to move the code for the
@code{@w{_ord_init}} function into the body of the @code{BEGIN} rule.  It was
written this way initially for ease of development.
There is a ``test program'' in a @code{BEGIN} rule, to test the
function.  It is commented out for production use.

@node Join Function, Gettimeofday Function, Ordinal Functions, General Functions
@subsection Merging an Array into a String

@cindex merging strings
When doing string processing, it is often useful to be able to join
all the strings in an array into one long string.  The following function,
@code{join}, accomplishes this task.  It is used later in several of
the application programs
(@pxref{Sample Programs, ,Practical @command{awk} Programs}).

Good function design is important; this function needs to be general but it
should also have a reasonable default behavior.  It is called with an array
as well as the beginning and ending indices of the elements in the array to be
merged.  This assumes that the array indices are numeric---a reasonable
assumption since the array was likely created with @code{split}
(@pxref{String Functions, ,String Manipulation Functions}):

@cindex @code{join} user-defined function
@example
@c file eg/lib/join.awk
# join.awk --- join an array into a string
@c endfile
@ignore
@c file eg/lib/join.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/lib/join.awk
function join(array, start, end, sep,    result, i)
@{
    if (sep == "")
       sep = " "
    else if (sep == SUBSEP) # magic value
       sep = ""
    result = array[start]
    for (i = start + 1; i <= end; i++)
        result = result sep array[i]
    return result
@}
@c endfile
@end example

An optional additional argument is the separator to use when joining the
strings back together.  If the caller supplies a non-empty value,
@code{join} uses it; if it is not supplied, it has a null
value.  In this case, @code{join} uses a single blank as a default
separator for the strings.  If the value is equal to @code{SUBSEP},
then @code{join} joins the strings with no separator between them.
@code{SUBSEP} serves as a ``magic'' value to indicate that there should
be no separation between the component strings.@footnote{It would
be nice if @command{awk} had an assignment operator for concatenation.
The lack of an explicit operator for concatenation makes string operations
more difficult than they really need to be.}

@node Gettimeofday Function, , Join Function, General Functions
@subsection Managing the Time of Day

@cindex formatted timestamps
@cindex timestamps, formatted
The @code{systime} and @code{strftime} functions described in
@ref{Time Functions, ,Using @command{gawk}'s Timestamp Functions},
provide the minimum functionality necessary for dealing with the time of day
in human readable form.  While @code{strftime} is extensive, the control
formats are not necessarily easy to remember or intuitively obvious when
reading a program.

The following function, @code{gettimeofday}, populates a user-supplied array
with preformatted time information.  It returns a string with the current
time formatted in the same way as the @command{date} utility:

@cindex @code{gettimeofday} user-defined function
@example
@c file eg/lib/gettime.awk
# gettimeofday.awk --- get the time of day in a usable format
@c endfile
@ignore
@c file eg/lib/gettime.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain, May 1993
#
@c endfile
@end ignore
@c file eg/lib/gettime.awk

# Returns a string in the format of output of date(1)
# Populates the array argument time with individual values:
#    time["second"]       -- seconds (0 - 59)
#    time["minute"]       -- minutes (0 - 59)
#    time["hour"]         -- hours (0 - 23)
#    time["althour"]      -- hours (0 - 12)
#    time["monthday"]     -- day of month (1 - 31)
#    time["month"]        -- month of year (1 - 12)
#    time["monthname"]    -- name of the month
#    time["shortmonth"]   -- short name of the month
#    time["year"]         -- year modulo 100 (0 - 99)
#    time["fullyear"]     -- full year
#    time["weekday"]      -- day of week (Sunday = 0)
#    time["altweekday"]   -- day of week (Monday = 0)
#    time["dayname"]      -- name of weekday
#    time["shortdayname"] -- short name of weekday
#    time["yearday"]      -- day of year (0 - 365)
#    time["timezone"]     -- abbreviation of timezone name
#    time["ampm"]         -- AM or PM designation
#    time["weeknum"]      -- week number, Sunday first day
#    time["altweeknum"]   -- week number, Monday first day

function gettimeofday(time,    ret, now, i)
@{
    # get time once, avoids unnecessary system calls
    now = systime()

    # return date(1)-style output
    ret = strftime("%a %b %d %H:%M:%S %Z %Y", now)

    # clear out target array
    delete time

    # fill in values, force numeric values to be
    # numeric by adding 0
    time["second"]       = strftime("%S", now) + 0
    time["minute"]       = strftime("%M", now) + 0
    time["hour"]         = strftime("%H", now) + 0
    time["althour"]      = strftime("%I", now) + 0
    time["monthday"]     = strftime("%d", now) + 0
    time["month"]        = strftime("%m", now) + 0
    time["monthname"]    = strftime("%B", now)
    time["shortmonth"]   = strftime("%b", now)
    time["year"]         = strftime("%y", now) + 0
    time["fullyear"]     = strftime("%Y", now) + 0
    time["weekday"]      = strftime("%w", now) + 0
    time["altweekday"]   = strftime("%u", now) + 0
    time["dayname"]      = strftime("%A", now)
    time["shortdayname"] = strftime("%a", now)
    time["yearday"]      = strftime("%j", now) + 0
    time["timezone"]     = strftime("%Z", now)
    time["ampm"]         = strftime("%p", now)
    time["weeknum"]      = strftime("%U", now) + 0
    time["altweeknum"]   = strftime("%W", now) + 0

    return ret
@}
@c endfile
@end example

The string indices are easier to use and read than the various formats
required by @code{strftime}.  The @code{alarm} program presented in
@ref{Alarm Program, ,An Alarm Clock Program},
uses this function.
A more general design for the @code{gettimeofday} function would have
allowed the user to supply an optional timestamp value to use instead
of the current time.

@node Data File Management, Getopt Function, General Functions, Library Functions
@section @value{DDF} Management

This @value{SECTION} presents functions that are useful for managing
command-line datafiles.

@menu
* Filetrans Function::          A function for handling data file transitions.
* Rewind Function::             A function for rereading the current file.
* File Checking::               Checking that data files are readable.
* Ignoring Assigns::            Treating assignments as file names.
@end menu

@node Filetrans Function, Rewind Function, Data File Management, Data File Management
@subsection Noting @value{DDF} Boundaries

@cindex per file initialization and cleanup
The @code{BEGIN} and @code{END} rules are each executed exactly once, at
the beginning and end of your @command{awk} program, respectively 
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).
We (the @command{gawk} authors) once had a user who mistakenly thought that the
@code{BEGIN} rule is executed at the beginning of each @value{DF} and the
@code{END} rule is executed at the end of each @value{DF}.  When informed
that this was not the case, the user requested that we add new special
patterns to @command{gawk}, named @code{BEGIN_FILE} and @code{END_FILE}, that
would have the desired behavior.  He even supplied us the code to do so.

Adding these special patterns to @command{gawk} wasn't necessary;
the job can be done cleanly in @command{awk} itself, as illustrated
by the following library program.
It arranges to call two user-supplied functions, @code{beginfile} and
@code{endfile}, at the beginning and end of each @value{DF}.
Besides solving the problem in only nine(!) lines of code, it does so
@emph{portably}; this works with any implementation of @command{awk}:

@example
# transfile.awk
#
# Give the user a hook for filename transitions
#
# The user must supply functions beginfile() and endfile()
# that each take the name of the file being started or
# finished, respectively.
@c #
@c # Arnold Robbins, arnold@@gnu.org, Public Domain
@c # January 1992

FILENAME != _oldfilename \
@{
    if (_oldfilename != "")
        endfile(_oldfilename)
    _oldfilename = FILENAME
    beginfile(FILENAME)
@}

END   @{ endfile(FILENAME) @}
@end example

This file must be loaded before the user's ``main'' program, so that the
rule it supplies is executed first.

This rule relies on @command{awk}'s @code{FILENAME} variable that
automatically changes for each new @value{DF}.  The current @value{FN} is
saved in a private variable, @code{_oldfilename}.  If @code{FILENAME} does
not equal @code{_oldfilename}, then a new @value{DF} is being processed and
it is necessary to call @code{endfile} for the old file.  Because
@code{endfile} should only be called if a file has been processed, the
program first checks to make sure that @code{_oldfilename} is not the null
string.  The program then assigns the current @value{FN} to
@code{_oldfilename} and calls @code{beginfile} for the file.
Because, like all @command{awk} variables, @code{_oldfilename} is
initialized to the null string, this rule executes correctly even for the
first @value{DF}.

The program also supplies an @code{END} rule to do the final processing for
the last file.  Because this @code{END} rule comes before any @code{END} rules
supplied in the ``main'' program, @code{endfile} is called first.  Once
again the value of multiple @code{BEGIN} and @code{END} rules should be clear.

@cindex @code{beginfile} user-defined function
@cindex @code{endfile} user-defined function
This version has same problem as the first version of @code{nextfile}
(@pxref{Nextfile Function, ,Implementing @code{nextfile} as a Function}).
If the same @value{DF} occurs twice in a row on the command line, then
@code{endfile} and @code{beginfile} are not executed at the end of the
first pass and at the beginning of the second pass.
The following version solves the problem:

@example
@c file eg/lib/ftrans.awk
# ftrans.awk --- handle data file transitions
#
# user supplies beginfile() and endfile() functions
@c endfile
@ignore
@c file eg/lib/ftrans.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# November 1992

@c endfile
@end ignore
@c file eg/lib/ftrans.awk
FNR == 1 @{
    if (_filename_ != "")
        endfile(_filename_)
    _filename_ = FILENAME
    beginfile(FILENAME)
@}

END  @{ endfile(_filename_) @}
@c endfile
@end example

@ref{Wc Program, ,Counting Things},
shows how this library function can be used and
how it simplifies writing the main program.

@node Rewind Function, File Checking, Filetrans Function, Data File Management
@subsection Rereading the Current File

Another request for a new built-in function was for a @code{rewind}
function that would make it possible to reread the current file.
The requesting user didn't want to have to use @code{getline}
(@pxref{Getline, , Explicit Input with @code{getline}})
inside a loop.

However, as long as you are not in the @code{END} rule, it is
quite easy to arrange to immediately close the current input file
and then start over with it from the top.
For lack of a better name, we'll call it @code{rewind}:

@cindex @code{rewind} user-defined function
@example
@c file eg/lib/rewind.awk
# rewind.awk --- rewind the current file and start over
@c endfile
@ignore
@c file eg/lib/rewind.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# September 2000

@c endfile
@end ignore
@c file eg/lib/rewind.awk
function rewind(    i)
@{
    # shift remaining arguments up
    for (i = ARGC; i > ARGIND; i--)
        ARGV[i] = ARGV[i-1]

    # make sure gawk knows to keep going
    ARGC++

    # make current file next to get done
    ARGV[ARGIND+1] = FILENAME

    # do it
    nextfile
@}
@c endfile
@end example

This code relies on the @code{ARGIND} variable
(@pxref{Auto-set, ,Built-in Variables That Convey Information}),
which is specific to @command{gawk}.
If you are not using
@command{gawk}, you can use ideas presented in
@iftex
the previous @value{SECTION}
@end iftex
@ifnottex
@ref{Filetrans Function, ,Noting @value{DDF} Boundaries},
@end ifnottex
to either update @code{ARGIND} on your own
or modify this code as appropriate.

The @code{rewind} function also relies on the @code{nextfile} keyword
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).
@xref{Nextfile Function, ,Implementing @code{nextfile} as a Function},
for a function version of @code{nextfile}.

@node File Checking, Ignoring Assigns, Rewind Function, Data File Management
@subsection Checking for Readable @value{DDF}s

@cindex fatal errors
@cindex readable @value{DF}s, checking
@cindex non-readable @value{DF}s, skipping
@cindex @value{DF}s, non-readable, skipping
@cindex @value{DF}s, readable, checking
Normally, if you give @command{awk} a @value{DF} that isn't readable,
it stops with a fatal error.  There are times when you
might want to just ignore such files and keep going.  You can
do this by prepending the following program to your @command{awk}
program:

@cindex @code{readable.awk} program
@example
@c file eg/lib/readable.awk
# readable.awk --- library file to skip over unreadable files
@c endfile
@ignore
@c file eg/lib/readable.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# October 2000

@c endfile
@end ignore
@c file eg/lib/readable.awk
BEGIN @{
    for (i = 1; i < ARGC; i++) @{
        if (ARGV[i] ~ /^[A-Za-z_][A-Za-z0-9_]*=.*/ \
            || ARGV[i] == "-")
            continue    # assignment or standard input
        else if ((getline junk < ARGV[i]) < 0) # unreadable
            delete ARGV[i]
        else
            close(ARGV[i])
    @}
@}
@c endfile
@end example

@cindex fatal errors
In @command{gawk}, the @code{getline} won't be fatal (unless
@option{--posix} is in force).
Removing the element from @code{ARGV} with @code{delete}
skips the file (since it's no longer in the list).

@c This doesn't handle /dev/stdin etc.  Not worth the hassle to mention or fix.

@node Ignoring Assigns, , File Checking, Data File Management
@subsection Treating Assignments as @value{FFN}s

Occasionally, you might not want @command{awk} to process command-line
variable assignments
(@pxref{Assignment Options, ,Assigning Variables on the Command Line}).
In particular, if you have @value{FN}s that contain an @samp{=} character,
@command{awk} treats the @value{FN} as an assignment, and does not process it.

Some users have suggested an additional command-line option for @command{gawk}
to disable command-line assignments.  However, some simple programming with
a library file does the trick:

@cindex @code{noassign.awk} program
@example
@c file eg/lib/noassign.awk
# noassign.awk --- library file to avoid the need for a
# special option that disables command-line assignments
@c endfile
@ignore
@c file eg/lib/noassign.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# October 1999

@c endfile
@end ignore
@c file eg/lib/noassign.awk
function disable_assigns(argc, argv,    i)
@{
    for (i = 1; i < argc; i++)
        if (argv[i] ~ /^[A-Za-z_][A-Za-z_0-9]*=.*/)
            argv[i] = ("./" argv[i])
@}

BEGIN @{
    if (No_command_assign)
        disable_assigns(ARGC, ARGV)
@}
@c endfile
@end example

You then run your program this way:

@example
awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk *
@end example

The function works by looping through the arguments.
It prepends @samp{./} to 
any argument that matches the form
of a variable assignment, turning that argument into a @value{FN}.

The use of @code{No_command_assign} allows you to disable command-line
assignments at invocation time, by giving the variable a true value.
When not set, it is initially zero (i.e., false), so the command-line arguments
are left alone.

@node Getopt Function, Passwd Functions, Data File Management, Library Functions
@section Processing Command-Line Options

@cindex @code{getopt} C library function
@cindex processing arguments
@cindex argument processing
Most utilities on POSIX compatible systems take options, or ``switches,'' on
the command line that can be used to change the way a program behaves.
@command{awk} is an example of such a program
(@pxref{Options, ,Command-Line Options}).
Often, options take @dfn{arguments}; i.e., data that the program needs to
correctly obey the command-line option.  For example, @command{awk}'s
@option{-F} option requires a string to use as the field separator.
The first occurrence on the command line of either @option{--} or a
string that does not begin with @samp{-} ends the options.

Modern Unix systems provide a C function named @code{getopt} for processing
command-line arguments.  The programmer provides a string describing the
one-letter options. If an option requires an argument, it is followed in the
string with a colon.  @code{getopt} is also passed the
count and values of the command-line arguments and is called in a loop.
@code{getopt} processes the command-line arguments for option letters.
Each time around the loop, it returns a single character representing the
next option letter that it finds, or @samp{?} if it finds an invalid option.
When it returns @minus{}1, there are no options left on the command line.

When using @code{getopt}, options that do not take arguments can be
grouped together.  Furthermore, options that take arguments require that the
argument is present.  The argument can immediately follow the option letter
or it can be a separate command-line argument.

Given a hypothetical program that takes
three command-line options, @option{-a}, @option{-b}, and @option{-c}, where
@option{-b} requires an argument, all of the following are valid ways of
invoking the program:

@example
prog -a -b foo -c data1 data2 data3
prog -ac -bfoo -- data1 data2 data3
prog -acbfoo data1 data2 data3
@end example

Notice that when the argument is grouped with its option, the rest of
the argument is considered to be the option's argument.
In this example, @option{-acbfoo} indicates that all of the
@option{-a}, @option{-b}, and @option{-c} options were supplied,
and that @samp{foo} is the argument to the @option{-b} option.

@code{getopt} provides four external variables that the programmer can use:

@table @code
@item optind
The index in the argument value array (@code{argv}) where the first
non-option command-line argument can be found.

@item optarg
The string value of the argument to an option.

@item opterr
Usually @code{getopt} prints an error message when it finds an invalid
option.  Setting @code{opterr} to zero disables this feature.  (An
application might want to print its own error message.)

@item optopt
The letter representing the command-line option.
@c While not usually documented, most versions supply this variable.
@end table

The following C fragment shows how @code{getopt} might process command-line
arguments for @command{awk}:

@example
int
main(int argc, char *argv[])
@{
    @dots{}
    /* print our own message */
    opterr = 0;
    while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) @{
        switch (c) @{
        case 'f':    /* file */
            @dots{}
            break;
        case 'F':    /* field separator */
            @dots{}
            break;
        case 'v':    /* variable assignment */
            @dots{}
            break;
        case 'W':    /* extension */
            @dots{}
            break;
        case '?':
        default:
            usage();
            break;
        @}
    @}
    @dots{}
@}
@end example

As a side point, @command{gawk} actually uses the GNU @code{getopt_long}
function to process both normal and GNU-style long options
(@pxref{Options, ,Command-Line Options}).

The abstraction provided by @code{getopt} is very useful and is quite
handy in @command{awk} programs as well.  Following is an @command{awk}
version of @code{getopt}.  This function highlights one of the
greatest weaknesses in @command{awk}, which is that it is very poor at
manipulating single characters.  Repeated calls to @code{substr} are
necessary for accessing individual characters
(@pxref{String Functions, ,String Manipulation Functions}).@footnote{This
function was written before @command{gawk} acquired the ability to
split strings into single characters using @code{""} as the separator.
We have left it alone, since using @code{substr} is more portable.}

The discussion that follows walks through the code a bit at a time:

@example
@c file eg/lib/getopt.awk
# getopt.awk --- do C library getopt(3) function in awk
@c endfile
@ignore
@c file eg/lib/getopt.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
#
# Initial version: March, 1991
# Revised: May, 1993

@c endfile
@end ignore
@c file eg/lib/getopt.awk
# External variables:
#    Optind -- index in ARGV of first non-option argument
#    Optarg -- string value of argument to current option
#    Opterr -- if nonzero, print our own diagnostic
#    Optopt -- current option letter

# Returns:
#    -1     at end of options
#    ?      for unrecognized option
#    <c>    a character representing the current option

# Private Data:
#    _opti  -- index in multi-flag option, e.g., -abc
@c endfile
@end example

The function starts out with
a list of the global variables it uses,
what the return values are, what they mean, and any global variables that
are ``private'' to this library function.  Such documentation is essential
for any program, and particularly for library functions.

The @code{getopt} function first checks that it was indeed called with a string of options
(the @code{options} parameter).  If @code{options} has a zero length,
@code{getopt} immediately returns @minus{}1:

@cindex @code{getopt} user-defined function
@example
@c file eg/lib/getopt.awk
function getopt(argc, argv, options,    thisopt, i)
@{
    if (length(options) == 0)    # no options given
        return -1

@group
    if (argv[Optind] == "--") @{  # all done
        Optind++
        _opti = 0
        return -1
@end group
    @} else if (argv[Optind] !~ /^-[^: \t\n\f\r\v\b]/) @{
        _opti = 0
        return -1
    @}
@c endfile
@end example

The next thing to check for is the end of the options.  A @option{--}
ends the command-line options, as does any command-line argument that
does not begin with a @samp{-}.  @code{Optind} is used to step through
the array of command-line arguments; it retains its value across calls
to @code{getopt}, because it is a global variable.

The regular expression that is used, @code{@w{/^-[^: \t\n\f\r\v\b]/}}, is
perhaps a bit of overkill; it checks for a @samp{-} followed by anything
that is not whitespace and not a colon.
If the current command-line argument does not match this pattern,
it is not an option, and it ends option processing.

@example
@c file eg/lib/getopt.awk
    if (_opti == 0)
        _opti = 2
    thisopt = substr(argv[Optind], _opti, 1)
    Optopt = thisopt
    i = index(options, thisopt)
    if (i == 0) @{
        if (Opterr)
            printf("%c -- invalid option\n",
                                  thisopt) > "/dev/stderr"
        if (_opti >= length(argv[Optind])) @{
            Optind++
            _opti = 0
        @} else
            _opti++
        return "?"
    @}
@c endfile
@end example

The @code{_opti} variable tracks the position in the current command-line
argument (@code{argv[Optind]}).  If multiple options are
grouped together with one @samp{-} (e.g., @option{-abx}), it is necessary
to return them to the user one at a time.

If @code{_opti} is equal to zero, it is set to two, which is the index in
the string of the next character to look at (we skip the @samp{-}, which
is at position one).  The variable @code{thisopt} holds the character,
obtained with @code{substr}.  It is saved in @code{Optopt} for the main
program to use.

If @code{thisopt} is not in the @code{options} string, then it is an
invalid option.  If @code{Opterr} is nonzero, @code{getopt} prints an error
message on the standard error that is similar to the message from the C
version of @code{getopt}.

Because the option is invalid, it is necessary to skip it and move on to the
next option character.  If @code{_opti} is greater than or equal to the
length of the current command-line argument, it is necessary to move on
to the next argument, so @code{Optind} is incremented and @code{_opti} is reset
to zero. Otherwise, @code{Optind} is left alone and @code{_opti} is merely
incremented.

In any case, because the option is invalid, @code{getopt} returns @samp{?}.
The main program can examine @code{Optopt} if it needs to know what the
invalid option letter actually is. Continuing on:

@example
@c file eg/lib/getopt.awk
    if (substr(options, i + 1, 1) == ":") @{
        # get option argument
        if (length(substr(argv[Optind], _opti + 1)) > 0)
            Optarg = substr(argv[Optind], _opti + 1)
        else
            Optarg = argv[++Optind]
        _opti = 0
    @} else
        Optarg = ""
@c endfile
@end example

If the option requires an argument, the option letter is followed by a colon
in the @code{options} string.  If there are remaining characters in the
current command-line argument (@code{argv[Optind]}), then the rest of that
string is assigned to @code{Optarg}.  Otherwise, the next command-line
argument is used (@samp{-xFOO} vs.@: @samp{@w{-x FOO}}). In either case,
@code{_opti} is reset to zero, because there are no more characters left to
examine in the current command-line argument. Continuing:

@example
@c file eg/lib/getopt.awk
    if (_opti == 0 || _opti >= length(argv[Optind])) @{
        Optind++
        _opti = 0
    @} else
        _opti++
    return thisopt
@}
@c endfile
@end example

Finally, if @code{_opti} is either zero or greater than the length of the
current command-line argument, it means this element in @code{argv} is
through being processed, so @code{Optind} is incremented to point to the
next element in @code{argv}.  If neither condition is true, then only
@code{_opti} is incremented, so that the next option letter can be processed
on the next call to @code{getopt}.

The @code{BEGIN} rule initializes both @code{Opterr} and @code{Optind} to one.
@code{Opterr} is set to one, since the default behavior is for @code{getopt}
to print a diagnostic message upon seeing an invalid option.  @code{Optind}
is set to one, since there's no reason to look at the program name, which is
in @code{ARGV[0]}:

@example
@c file eg/lib/getopt.awk
BEGIN @{
    Opterr = 1    # default is to diagnose
    Optind = 1    # skip ARGV[0]

    # test program
    if (_getopt_test) @{
        while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1)
            printf("c = <%c>, optarg = <%s>\n",
                                       _go_c, Optarg)
        printf("non-option arguments:\n")
        for (; Optind < ARGC; Optind++)
            printf("\tARGV[%d] = <%s>\n",
                                    Optind, ARGV[Optind])
    @}
@}
@c endfile
@end example

The rest of the @code{BEGIN} rule is a simple test program.  Here is the
result of two sample runs of the test program:

@example
$ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
@print{} c = <a>, optarg = <>
@print{} c = <c>, optarg = <>
@print{} c = <b>, optarg = <ARG>
@print{} non-option arguments:
@print{}         ARGV[3] = <bax>
@print{}         ARGV[4] = <-x>

$ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
@print{} c = <a>, optarg = <>
@error{} x -- invalid option
@print{} c = <?>, optarg = <>
@print{} non-option arguments:
@print{}         ARGV[4] = <xyz>
@print{}         ARGV[5] = <abc>
@end example

In both runs,
the first @option{--} terminates the arguments to @command{awk}, so that it does
not try to interpret the @option{-a}, etc., as its own options.
Several of the sample programs presented in
@ref{Sample Programs, ,Practical @command{awk} Programs},
use @code{getopt} to process their arguments.

@node Passwd Functions, Group Functions, Getopt Function, Library Functions
@section Reading the User Database

The @code{PROCINFO} array
(@pxref{Built-in Variables})
provides access to the current user's real and effective user and group id
numbers, and if available, the user's supplementary group set.
However, because these are numbers, they do not provide very useful
information to the average user.  There needs to be some way to find the
user information associated with the user and group numbers.  This
@value{SECTION} presents a suite of functions for retrieving information from the
user database.  @xref{Group Functions, ,Reading the Group Database},
for a similar suite that retrieves information from the group database.

@cindex @code{getpwent} C library function
@cindex user information
@cindex login information
@cindex account information
@cindex password file
The POSIX standard does not define the file where user information is
kept.  Instead, it provides the @code{<pwd.h>} header file
and several C language subroutines for obtaining user information.
The primary function is @code{getpwent}, for ``get password entry.''
The ``password'' comes from the original user database file,
@file{/etc/passwd}, which stores user information, along with the
encrypted passwords (hence the name).

@cindex @command{pwcat} program
While an @command{awk} program could simply read @file{/etc/passwd}
directly, this file may not contain complete information about the
system's set of users.@footnote{It is often the case that password
information is stored in a network database.} To be sure you are able to
produce a readable and complete version of the user database, it is necessary
to write a small C program that calls @code{getpwent}.  @code{getpwent}
is defined as returning a pointer to a @code{struct passwd}.  Each time it
is called, it returns the next entry in the database.  When there are
no more entries, it returns @code{NULL}, the null pointer.  When this
happens, the C program should call @code{endpwent} to close the database.
Following is @command{pwcat}, a C program that ``cats'' the password database.

@c Use old style function header for portability to old systems (SunOS, HP/UX).

@example
@c file eg/lib/pwcat.c
/*
 * pwcat.c
 *
 * Generate a printable version of the password database
 */
@c endfile
@ignore
@c file eg/lib/pwcat.c
/*
 * Arnold Robbins, arnold@@gnu.org, May 1993
 * Public Domain
 */

@c endfile
@end ignore
@c file eg/lib/pwcat.c
#include <stdio.h>
#include <pwd.h>

int
main(argc, argv)
int argc;
char **argv;
@{
    struct passwd *p;

    while ((p = getpwent()) != NULL)
        printf("%s:%s:%d:%d:%s:%s:%s\n",
            p->pw_name, p->pw_passwd, p->pw_uid,
            p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);

    endpwent();
    exit(0);
@}
@c endfile
@end example

If you don't understand C, don't worry about it.
The output from @command{pwcat} is the user database, in the traditional
@file{/etc/passwd} format of colon-separated fields.  The fields are:

@ignore
@table @asis
@item Login name
The user's login name.

@item Encrypted password
The user's encrypted password.  This may not be available on some systems.

@item User-ID
The user's numeric user-id number.

@item Group-ID
The user's numeric group-id number.

@item Full name
The user's full name, and perhaps other information associated with the
user.

@item Home directory
The user's login (or ``home'') directory (familiar to shell programmers as
@code{$HOME}).

@item Login shell
The program that is run when the user logs in.  This is usually a
shell, such as @command{bash}.
@end table
@end ignore

@multitable {Encrypted password} {1234567890123456789012345678901234567890123456}
@item Login name @tab The user's login name.

@item Encrypted password @tab The user's encrypted password.  This may not be available on some systems.

@item User-ID @tab The user's numeric user-id number.

@item Group-ID @tab The user's numeric group-id number.

@item Full name @tab The user's full name, and perhaps other information associated with the
user.

@item Home directory @tab The user's login (or ``home'') directory (familiar to shell programmers as
@code{$HOME}).

@item Login shell @tab The program that is run when the user logs in.  This is usually a
shell, such as @command{bash}.
@end multitable

A few lines representative of @command{pwcat}'s output are as follows:

@cindex Jacobs, Andrew
@cindex Robbins, Arnold
@cindex Robbins, Miriam
@example
$ pwcat
@print{} root:3Ov02d5VaUPB6:0:1:Operator:/:/bin/sh
@print{} nobody:*:65534:65534::/:
@print{} daemon:*:1:1::/:
@print{} sys:*:2:2::/:/bin/csh
@print{} bin:*:3:3::/bin:
@print{} arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
@print{} miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
@print{} andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
@dots{}
@end example

With that introduction, following is a group of functions for getting user
information.  There are several functions here, corresponding to the C
functions of the same names:

@c Exercise: simplify all these functions that return values.
@c Answer: return foo[key] returns "" if key not there, no need to check with `in'.

@cindex @code{_pw_init} user-defined function
@example
@c file eg/lib/passwdawk.in
# passwd.awk --- access password file information
@c endfile
@ignore
@c file eg/lib/passwdawk.in
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993
# Revised October 2000

@c endfile
@end ignore
@c file eg/lib/passwdawk.in
BEGIN @{
    # tailor this to suit your system
    _pw_awklib = "/usr/local/libexec/awk/"
@}

function _pw_init(    oldfs, oldrs, olddol0, pwcat, using_fw)
@{
    if (_pw_inited)
        return

    oldfs = FS
    oldrs = RS
    olddol0 = $0
    using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
    FS = ":"
    RS = "\n"

    pwcat = _pw_awklib "pwcat"
    while ((pwcat | getline) > 0) @{
        _pw_byname[$1] = $0
        _pw_byuid[$3] = $0
        _pw_bycount[++_pw_total] = $0
    @}
    close(pwcat)
    _pw_count = 0
    _pw_inited = 1
    FS = oldfs
    if (using_fw)
        FIELDWIDTHS = FIELDWIDTHS
    RS = oldrs
    $0 = olddol0
@}
@c endfile
@end example

The @code{BEGIN} rule sets a private variable to the directory where
@command{pwcat} is stored.  Because it is used to help out an @command{awk} library
routine, we have chosen to put it in @file{/usr/local/libexec/awk};
however, you might want it to be in a different directory on your system.

The function @code{_pw_init} keeps three copies of the user information
in three associative arrays.  The arrays are indexed by username
(@code{_pw_byname}), by user-id number (@code{_pw_byuid}), and by order of
occurrence (@code{_pw_bycount}).
The variable @code{_pw_inited} is used for efficiency; @code{_pw_init}
needs only to be called once.

Because this function uses @code{getline} to read information from
@command{pwcat}, it first saves the values of @code{FS}, @code{RS}, and @code{$0}.
It notes in the variable @code{using_fw} whether field splitting
with @code{FIELDWIDTHS} is in effect or not.
Doing so is necessary, since these functions could be called
from anywhere within a user's program, and the user may have his
or her
own way of splitting records and fields.

The @code{using_fw} variable checks @code{PROCINFO["FS"]}, which
is @code{"FIELDWIDTHS"} if field splitting is being done with
@code{FIELDWIDTHS}.  This makes it possible to restore the correct
field-splitting mechanism later.  The test can only be true for
@command{gawk}.  It is false if using @code{FS} or on some other
@command{awk} implementation.

The main part of the function uses a loop to read database lines, split
the line into fields, and then store the line into each array as necessary.
When the loop is done, @code{@w{_pw_init}} cleans up by closing the pipeline,
setting @code{@w{_pw_inited}} to one, and restoring @code{FS} (and @code{FIELDWIDTHS}
if necessary), @code{RS}, and @code{$0}.
The use of @code{@w{_pw_count}} is explained shortly.

@c NEXT ED: All of these functions don't need the ... in ... test.  Just
@c return the array element, which will be "" if not already there.  Duh.
The @code{getpwnam} function takes a username as a string argument. If that
user is in the database, it returns the appropriate line. Otherwise it
returns the null string:

@cindex @code{getpwnam} user-defined function
@example
@group
@c file eg/lib/passwdawk.in
function getpwnam(name)
@{
    _pw_init()
    if (name in _pw_byname)
        return _pw_byname[name]
    return ""
@}
@c endfile
@end group
@end example

Similarly,
the @code{getpwuid} function takes a user-id number argument. If that
user number is in the database, it returns the appropriate line. Otherwise it
returns the null string:

@cindex @code{getpwuid} user-defined function
@example
@c file eg/lib/passwdawk.in
function getpwuid(uid)
@{
    _pw_init()
    if (uid in _pw_byuid)
        return _pw_byuid[uid]
    return ""
@}
@c endfile
@end example

The @code{getpwent} function simply steps through the database, one entry at
a time.  It uses @code{_pw_count} to track its current position in the
@code{_pw_bycount} array:

@cindex @code{getpwent} user-defined function
@example
@c file eg/lib/passwdawk.in
function getpwent()
@{
    _pw_init()
    if (_pw_count < _pw_total)
        return _pw_bycount[++_pw_count]
    return ""
@}
@c endfile
@end example

The @code{@w{endpwent}} function resets @code{@w{_pw_count}} to zero, so that
subsequent calls to @code{getpwent} start over again:

@cindex @code{endpwent} user-defined function
@example
@c file eg/lib/passwdawk.in
function endpwent()
@{
    _pw_count = 0
@}
@c endfile
@end example

A conscious design decision in this suite is that each subroutine calls
@code{@w{_pw_init}} to initialize the database arrays.  The overhead of running
a separate process to generate the user database, and the I/O to scan it,
are only incurred if the user's main program actually calls one of these
functions.  If this library file is loaded along with a user's program, but
none of the routines are ever called, then there is no extra runtime overhead.
(The alternative is move the body of @code{@w{_pw_init}} into a
@code{BEGIN} rule, which always runs @command{pwcat}.  This simplifies the
code but runs an extra process that may never be needed.)

In turn, calling @code{_pw_init} is not too expensive, because the
@code{_pw_inited} variable keeps the program from reading the data more than
once.  If you are worried about squeezing every last cycle out of your
@command{awk} program, the check of @code{_pw_inited} could be moved out of
@code{_pw_init} and duplicated in all the other functions.  In practice,
this is not necessary, since most @command{awk} programs are I/O-bound, and it
clutters up the code.

The @command{id} program in @ref{Id Program, ,Printing out User Information},
uses these functions.

@node Group Functions, , Passwd Functions, Library Functions
@section Reading the Group Database

@cindex @code{getgrent} C library function
@cindex group information
@cindex account information
@cindex group file
Much of the discussion presented in
@ref{Passwd Functions, ,Reading the User Database},
applies to the group database as well.  Although there has traditionally
been a well-known file (@file{/etc/group}) in a well-known format, the POSIX
standard only provides a set of C library routines
(@code{<grp.h>} and @code{getgrent})
for accessing the information.
Even though this file may exist, it likely does not have
complete information.  Therefore, as with the user database, it is necessary
to have a small C program that generates the group database as its output.

@cindex @command{grcat} program
@command{grcat}, a C program that ``cats'' the group database,
is as follows:

@example
@c file eg/lib/grcat.c
/*
 * grcat.c
 *
 * Generate a printable version of the group database
 */
@c endfile
@ignore
@c file eg/lib/grcat.c
/*
 * Arnold Robbins, arnold@@gnu.org, May 1993
 * Public Domain
 */

@c endfile
@end ignore
@c file eg/lib/grcat.c
#include <stdio.h>
#include <grp.h>

int
main(argc, argv)
int argc;
char **argv;
@{
    struct group *g;
    int i;

    while ((g = getgrent()) != NULL) @{
        printf("%s:%s:%d:", g->gr_name, g->gr_passwd,
                                            g->gr_gid);
        for (i = 0; g->gr_mem[i] != NULL; i++) @{
            printf("%s", g->gr_mem[i]);
@group
            if (g->gr_mem[i+1] != NULL)
                putchar(',');
        @}
@end group
        putchar('\n');
    @}
    endgrent();
    exit(0);
@}
@c endfile
@end example

Each line in the group database represents one group.  The fields are
separated with colons and represent the following information:

@ignore
@table @asis
@item Group Name
The name of the group.

@item Group Password
The encrypted group password. In practice, this field is never used. It is
usually empty or set to @samp{*}.

@item Group ID Number
The numeric group-id number. This number should be unique within the file.

@item Group Member List
A comma-separated list of usernames.  These users are members of the group.
Modern Unix systems allow users to be members of several groups
simultaneously.  If your system does, then there are elements
@code{"group1"} through @code{"group@var{N}"} in @code{PROCINFO}
for those group-id numbers.
(Note that @code{PROCINFO} is a @command{gawk} extension;
@pxref{Built-in Variables}.)
@end table
@end ignore

@multitable {Encrypted password} {1234567890123456789012345678901234567890123456}
@item Group name @tab The group's name.

@item Group password @tab The group's encrypted password. In practice, this field is never used;
it is usually empty or set to @samp{*}.

@item Group-ID @tab
The group's numeric group-id number; this number should be unique within the file.

@item Group member list @tab
A comma-separated list of usernames.  These users are members of the group.
Modern Unix systems allow users to be members of several groups
simultaneously.  If your system does, then there are elements
@code{"group1"} through @code{"group@var{N}"} in @code{PROCINFO}
for those group-id numbers.
(Note that @code{PROCINFO} is a @command{gawk} extension;
@pxref{Built-in Variables}.)
@end multitable

Here is what running @command{grcat} might produce:

@example
$ grcat
@print{} wheel:*:0:arnold
@print{} nogroup:*:65534:
@print{} daemon:*:1:
@print{} kmem:*:2:
@print{} staff:*:10:arnold,miriam,andy
@print{} other:*:20:
@dots{}
@end example

Here are the functions for obtaining information from the group database.
There are several, modeled after the C library functions of the same names:

@cindex @code{_gr_init} user-defined function
@example
@c file eg/lib/groupawk.in
# group.awk --- functions for dealing with the group file
@c endfile
@ignore
@c file eg/lib/groupawk.in
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993
# Revised October 2000

@c endfile
@end ignore
@c line break on _gr_init for smallbook
@c file eg/lib/groupawk.in
BEGIN    \
@{
    # Change to suit your system
    _gr_awklib = "/usr/local/libexec/awk/"
@}

function _gr_init(    oldfs, oldrs, olddol0, grcat,
                             using_fw, n, a, i)
@{
    if (_gr_inited)
        return

    oldfs = FS
    oldrs = RS
    olddol0 = $0
    using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
    FS = ":"
    RS = "\n"

    grcat = _gr_awklib "grcat"
    while ((grcat | getline) > 0) @{
        if ($1 in _gr_byname)
            _gr_byname[$1] = _gr_byname[$1] "," $4
        else
            _gr_byname[$1] = $0
        if ($3 in _gr_bygid)
            _gr_bygid[$3] = _gr_bygid[$3] "," $4
        else
            _gr_bygid[$3] = $0

        n = split($4, a, "[ \t]*,[ \t]*")
        for (i = 1; i <= n; i++)
            if (a[i] in _gr_groupsbyuser)
                _gr_groupsbyuser[a[i]] = \
                    _gr_groupsbyuser[a[i]] " " $1
            else
                _gr_groupsbyuser[a[i]] = $1

        _gr_bycount[++_gr_count] = $0
    @}
    close(grcat)
    _gr_count = 0
    _gr_inited++
    FS = oldfs
    if (using_fw)
        FIELDWIDTHS = FIELDWIDTHS
    RS = oldrs
    $0 = olddol0
@}
@c endfile
@end example

The @code{BEGIN} rule sets a private variable to the directory where
@command{grcat} is stored.  Because it is used to help out an @command{awk} library
routine, we have chosen to put it in @file{/usr/local/libexec/awk}.  You might
want it to be in a different directory on your system.

These routines follow the same general outline as the user database routines
(@pxref{Passwd Functions, ,Reading the User Database}).
The @code{@w{_gr_inited}} variable is used to
ensure that the database is scanned no more than once.
The @code{@w{_gr_init}} function first saves @code{FS}, @code{FIELDWIDTHS}, @code{RS}, and
@code{$0}, and then sets @code{FS} and @code{RS} to the correct values for
scanning the group information.

The group information is stored is several associative arrays.
The arrays are indexed by group name (@code{@w{_gr_byname}}), by group-id number
(@code{@w{_gr_bygid}}), and by position in the database (@code{@w{_gr_bycount}}).
There is an additional array indexed by username (@code{@w{_gr_groupsbyuser}}),
which is a space-separated list of groups that each user belongs to.

Unlike the user database, it is possible to have multiple records in the
database for the same group.  This is common when a group has a large number
of members.  A pair of such entries might look like the following:

@example
tvpeople:*:101:johnny,jay,arsenio
tvpeople:*:101:david,conan,tom,joan
@end example

For this reason, @code{_gr_init} looks to see if a group name or
group-id number is already seen.  If it is, then the usernames are
simply concatenated onto the previous list of users.  (There is actually a
subtle problem with the code just presented.  Suppose that
the first time there were no names. This code adds the names with
a leading comma. It also doesn't check that there is a @code{$4}.)

Finally, @code{_gr_init} closes the pipeline to @command{grcat}, restores
@code{FS} (and @code{FIELDWIDTHS} if necessary), @code{RS}, and @code{$0},
initializes @code{_gr_count} to zero
(it is used later), and makes @code{_gr_inited} nonzero.

The @code{getgrnam} function takes a group name as its argument, and if that
group exists, it is returned. Otherwise, @code{getgrnam} returns the null
string:

@cindex @code{getgrnam} user-defined function
@example
@c file eg/lib/groupawk.in
function getgrnam(group)
@{
    _gr_init()
    if (group in _gr_byname)
        return _gr_byname[group]
    return ""
@}
@c endfile
@end example

The @code{getgrgid} function is similar, it takes a numeric group-id and
looks up the information associated with that group-id:

@cindex @code{getgrgid} user-defined function
@example
@c file eg/lib/groupawk.in
function getgrgid(gid)
@{
    _gr_init()
    if (gid in _gr_bygid)
        return _gr_bygid[gid]
    return ""
@}
@c endfile
@end example

The @code{getgruser} function does not have a C counterpart. It takes a
username and returns the list of groups that have the user as a member:

@cindex @code{getgruser} user-defined function
@example
@c file eg/lib/groupawk.in
function getgruser(user)
@{
    _gr_init()
    if (user in _gr_groupsbyuser)
        return _gr_groupsbyuser[user]
    return ""
@}
@c endfile
@end example

The @code{getgrent} function steps through the database one entry at a time.
It uses @code{_gr_count} to track its position in the list:

@cindex @code{getgrent} user-defined function
@example
@c file eg/lib/groupawk.in
function getgrent()
@{
    _gr_init()
    if (++_gr_count in _gr_bycount)
        return _gr_bycount[_gr_count]
    return ""
@}
@c endfile
@end example

The @code{endgrent} function resets @code{_gr_count} to zero so that @code{getgrent} can
start over again:

@cindex @code{endgrent} user-defined function
@example
@c file eg/lib/groupawk.in
function endgrent()
@{
    _gr_count = 0
@}
@c endfile
@end example

As with the user database routines, each function calls @code{_gr_init} to
initialize the arrays.  Doing so only incurs the extra overhead of running
@command{grcat} if these functions are used (as opposed to moving the body of
@code{_gr_init} into a @code{BEGIN} rule).

Most of the work is in scanning the database and building the various
associative arrays.  The functions that the user calls are themselves very
simple, relying on @command{awk}'s associative arrays to do work.

The @command{id} program in @ref{Id Program, ,Printing out User Information},
uses these functions.

@node Sample Programs, Language History, Library Functions, Top
@chapter Practical @command{awk} Programs

@ref{Library Functions, ,A Library of @command{awk} Functions},
presents the idea that reading programs in a language contributes to
learning that language.  This @value{CHAPTER} continues that theme,
presenting a potpourri of @command{awk} programs for your reading
enjoyment.
@ifnotinfo
There are three sections.
The first describes how to run the programs presented
in this @value{CHAPTER}.

The second presents @command{awk}
versions of several common POSIX utilities.
These are programs that you are hopefully already familiar with,
and therefore, whose problems are understood.
By reimplementing these programs in @command{awk},
you can focus on the @command{awk}-related aspects of solving
the programming problem.

The third is a grab bag of interesting programs.
These solve a number of different data-manipulation and management
problems.  Many of the programs are short, which emphasizes @command{awk}'s
ability to do a lot in just a few lines of code.
@end ifnotinfo

Many of these programs use the library functions presented in
@ref{Library Functions, ,A Library of @command{awk} Functions}.

@menu
* Running Examples::            How to run these examples.
* Clones::                      Clones of common utilities.
* Miscellaneous Programs::      Some interesting @command{awk} programs.
@end menu

@node Running Examples, Clones, Sample Programs, Sample Programs
@section Running the Example Programs

To run a given program, you would typically do something like this:

@example
awk -f @var{program} -- @var{options} @var{files}
@end example

@noindent
Here, @var{program} is the name of the @command{awk} program (such as
@file{cut.awk}), @var{options} are any command-line options for the
program that start with a @samp{-}, and @var{files} are the actual @value{DF}s.

If your system supports the @samp{#!} executable interpreter mechanism
(@pxref{Executable Scripts, , Executable @command{awk} Programs}),
you can instead run your program directly:

@example
cut.awk -c1-8 myfiles > results
@end example

If your @command{awk} is not @command{gawk}, you may instead need to use this:

@example
cut.awk -- -c1-8 myfiles > results
@end example

@node Clones, Miscellaneous Programs, Running Examples, Sample Programs
@section Reinventing Wheels for Fun and Profit

This @value{SECTION} presents a number of POSIX utilities that are implemented in
@command{awk}.  Reinventing these programs in @command{awk} is often enjoyable,
because the algorithms can be very clearly expressed, and the code is usually
very concise and simple.  This is true because @command{awk} does so much for you.

It should be noted that these programs are not necessarily intended to
replace the installed versions on your system.  Instead, their
purpose is to illustrate @command{awk} language programming for ``real world''
tasks.

The programs are presented in alphabetical order.

@menu
* Cut Program::                 The @command{cut} utility.
* Egrep Program::               The @command{egrep} utility.
* Id Program::                  The @command{id} utility.
* Split Program::               The @command{split} utility.
* Tee Program::                 The @command{tee} utility.
* Uniq Program::                The @command{uniq} utility.
* Wc Program::                  The @command{wc} utility.
@end menu

@node Cut Program, Egrep Program, Clones, Clones
@subsection Cutting out Fields and Columns

@cindex @command{cut} utility
The @command{cut} utility selects, or ``cuts,'' characters or fields
from its standard input and sends them to its standard output.
Fields are separated by tabs by default,
but you may supply a command-line option to change the field
@dfn{delimiter} (i.e., the field separator character). @command{cut}'s
definition of fields is less general than @command{awk}'s.

A common use of @command{cut} might be to pull out just the login name of
logged-on users from the output of @command{who}.  For example, the following
pipeline generates a sorted, unique list of the logged-on users:

@example
who | cut -c1-8 | sort | uniq
@end example

The options for @command{cut} are:

@table @code
@item -c @var{list}
Use @var{list} as the list of characters to cut out.  Items within the list
may be separated by commas, and ranges of characters can be separated with
dashes.  The list @samp{1-8,15,22-35} specifies characters 1 through
8, 15, and 22 through 35.

@item -f @var{list}
Use @var{list} as the list of fields to cut out.

@item -d @var{delim}
Use @var{delim} as the field separator character instead of the tab
character.

@item -s
Suppress printing of lines that do not contain the field delimiter.
@end table

The @command{awk} implementation of @command{cut} uses the @code{getopt} library
function (@pxref{Getopt Function, ,Processing Command-Line Options})
and the @code{join} library function
(@pxref{Join Function, ,Merging an Array into a String}).

The program begins with a comment describing the options, the library
functions needed, and a @code{usage} function that prints out a usage
message and exits.  @code{usage} is called if invalid arguments are
supplied:

@cindex @code{cut.awk} program
@example
@c file eg/prog/cut.awk
# cut.awk --- implement cut in awk
@c endfile
@ignore
@c file eg/prog/cut.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/prog/cut.awk
# Options:
#    -f list     Cut fields
#    -d c        Field delimiter character
#    -c list     Cut characters
#
#    -s          Suppress lines without the delimiter
#
# Requires getopt and join library functions

@group
function usage(    e1, e2)
@{
    e1 = "usage: cut [-f list] [-d c] [-s] [files...]"
    e2 = "usage: cut [-c list] [files...]"
    print e1 > "/dev/stderr"
    print e2 > "/dev/stderr"
    exit 1
@}
@end group
@c endfile
@end example

@noindent
The variables @code{e1} and @code{e2} are used so that the function
fits nicely on the
@ifnotinfo
page.
@end ifnotinfo
@ifnottex
screen.
@end ifnottex

Next comes a @code{BEGIN} rule that parses the command-line options.
It sets @code{FS} to a single tab character, because that is @command{cut}'s
default field separator.  The output field separator is also set to be the
same as the input field separator.  Then @code{getopt} is used to step
through the command-line options.  One or the other of the variables
@code{by_fields} or @code{by_chars} is set to true, to indicate that
processing should be done by fields or by characters, respectively.
When cutting by characters, the output field separator is set to the null
string.

@example
@c file eg/prog/cut.awk
BEGIN    \
@{
    FS = "\t"    # default
    OFS = FS
    while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) @{
        if (c == "f") @{
            by_fields = 1
            fieldlist = Optarg
        @} else if (c == "c") @{
            by_chars = 1
            fieldlist = Optarg
            OFS = ""
        @} else if (c == "d") @{
            if (length(Optarg) > 1) @{
                printf("Using first character of %s" \
                " for delimiter\n", Optarg) > "/dev/stderr"
                Optarg = substr(Optarg, 1, 1)
            @}
            FS = Optarg
            OFS = FS
            if (FS == " ")    # defeat awk semantics
                FS = "[ ]"
        @} else if (c == "s")
            suppress++
        else
            usage()
    @}

    for (i = 1; i < Optind; i++)
        ARGV[i] = ""
@c endfile
@end example

Special care is taken when the field delimiter is a space.  Using
a single space (@code{@w{" "}}) for the value of @code{FS} is
incorrect---@command{awk} would separate fields with runs of spaces,
tabs, and/or newlines, and we want them to be separated with individual
spaces.  Also, note that after @code{getopt} is through, we have to
clear out all the elements of @code{ARGV} from 1 to @code{Optind},
so that @command{awk} does not try to process the command-line options
as @value{FN}s.

After dealing with the command-line options, the program verifies that the
options make sense.  Only one or the other of @option{-c} and @option{-f}
should be used, and both require a field list.  Then the program calls
either @code{set_fieldlist} or @code{set_charlist} to pull apart the
list of fields or characters:

@example
@c file eg/prog/cut.awk
    if (by_fields && by_chars)
        usage()

    if (by_fields == 0 && by_chars == 0)
        by_fields = 1    # default

    if (fieldlist == "") @{
        print "cut: needs list for -c or -f" > "/dev/stderr"
        exit 1
    @}

    if (by_fields)
        set_fieldlist()
    else
        set_charlist()
@}
@c endfile
@end example

@code{set_fieldlist}  is used to split the field list apart at the commas,
and into an array.  Then, for each element of the array, it looks to
see if it is actually a range, and if so, splits it apart. The range
is verified to make sure the first number is smaller than the second.
Each number in the list is added to the @code{flist} array, which
simply lists the fields that will be printed.  Normal field splitting
is used.  The program lets @command{awk} handle the job of doing the
field splitting:

@example
@c file eg/prog/cut.awk
function set_fieldlist(        n, m, i, j, k, f, g)
@{
    n = split(fieldlist, f, ",")
    j = 1    # index in flist
    for (i = 1; i <= n; i++) @{
        if (index(f[i], "-") != 0) @{ # a range
            m = split(f[i], g, "-")
@group
            if (m != 2 || g[1] >= g[2]) @{
                printf("bad field list: %s\n",
                                  f[i]) > "/dev/stderr"
                exit 1
            @}
@end group
            for (k = g[1]; k <= g[2]; k++)
                flist[j++] = k
        @} else
            flist[j++] = f[i]
    @}
    nfields = j - 1
@}
@c endfile
@end example

The @code{set_charlist} function is more complicated than @code{set_fieldlist}.
The idea here is to use @command{gawk}'s @code{FIELDWIDTHS} variable
(@pxref{Constant Size, ,Reading Fixed-Width Data}),
which describes constant width input.  When using a character list, that is
exactly what we have.

Setting up @code{FIELDWIDTHS} is more complicated than simply listing the
fields that need to be printed.  We have to keep track of the fields to
print and also the intervening characters that have to be skipped.
For example, suppose you wanted characters 1 through 8, 15, and
22 through 35.  You would use @samp{-c 1-8,15,22-35}.  The necessary value
for @code{FIELDWIDTHS} is @code{@w{"8 6 1 6 14"}}.  This yields five
fields, and the fields to print
are @code{$1}, @code{$3}, and @code{$5}.
The intermediate fields are @dfn{filler},
which is stuff in between the desired data.
@code{flist} lists the fields to print, and @code{t} tracks the
complete field list, including filler fields:

@example
@c file eg/prog/cut.awk
function set_charlist(    field, i, j, f, g, t,
                          filler, last, len)
@{
    field = 1   # count total fields
    n = split(fieldlist, f, ",")
    j = 1       # index in flist
    for (i = 1; i <= n; i++) @{
        if (index(f[i], "-") != 0) @{ # range
            m = split(f[i], g, "-")
            if (m != 2 || g[1] >= g[2]) @{
                printf("bad character list: %s\n",
                               f[i]) > "/dev/stderr"
                exit 1
            @}
            len = g[2] - g[1] + 1
            if (g[1] > 1)  # compute length of filler
                filler = g[1] - last - 1
            else
                filler = 0
@group
            if (filler)
                t[field++] = filler
@end group
            t[field++] = len  # length of field
            last = g[2]
            flist[j++] = field - 1
        @} else @{
            if (f[i] > 1)
                filler = f[i] - last - 1
            else
                filler = 0
            if (filler)
                t[field++] = filler
            t[field++] = 1
            last = f[i]
            flist[j++] = field - 1
        @}
    @}
    FIELDWIDTHS = join(t, 1, field - 1)
    nfields = j - 1
@}
@c endfile
@end example

Next is the rule that actually processes the data.  If the @option{-s} option
is given, then @code{suppress} is true.  The first @code{if} statement
makes sure that the input record does have the field separator.  If
@command{cut} is processing fields, @code{suppress} is true, and the field
separator character is not in the record, then the record is skipped.

If the record is valid, then @command{gawk} has split the data
into fields, either using the character in @code{FS} or using fixed-length
fields and @code{FIELDWIDTHS}.  The loop goes through the list of fields
that should be printed.  The corresponding field is printed if it contains data.
If the next field also has data, then the separator character is
written out between the fields:

@example
@c file eg/prog/cut.awk
@{
    if (by_fields && suppress && index($0, FS) != 0)
        next

    for (i = 1; i <= nfields; i++) @{
        if ($flist[i] != "") @{
            printf "%s", $flist[i]
            if (i < nfields && $flist[i+1] != "")
                printf "%s", OFS
        @}
    @}
    print ""
@}
@c endfile
@end example

This version of @command{cut} relies on @command{gawk}'s @code{FIELDWIDTHS}
variable to do the character-based cutting.  While it is possible in
other @command{awk} implementations to use @code{substr}
(@pxref{String Functions, ,String Manipulation Functions}),
it is also extremely painful.
The @code{FIELDWIDTHS} variable supplies an elegant solution to the problem
of picking the input line apart by characters.

@c Exercise: Rewrite using split with "".

@node Egrep Program, Id Program, Cut Program, Clones
@subsection Searching for Regular Expressions in Files

@cindex @command{egrep} utility
The @command{egrep} utility searches files for patterns.  It uses regular
expressions that are almost identical to those available in @command{awk}
(@pxref{Regexp, ,Regular Expressions}).
It is used in the following manner:

@example
egrep @r{[} @var{options} @r{]} '@var{pattern}' @var{files} @dots{}
@end example

The @var{pattern} is a regular expression.  In typical usage, the regular
expression is quoted to prevent the shell from expanding any of the
special characters as @value{FN} wildcards.  Normally, @command{egrep}
prints the lines that matched.  If multiple @value{FN}s are provided on
the command line, each output line is preceded by the name of the file
and a colon.

The options to @command{egrep} are as follows:

@table @code
@item -c
Print out a count of the lines that matched the pattern, instead of the
lines themselves.

@item -s
Be silent.  No output is produced and the exit value indicates whether
the pattern was matched.

@item -v
Invert the sense of the test. @command{egrep} prints the lines that do
@emph{not} match the pattern and exits successfully if the pattern is not
matched.

@item -i
Ignore case distinctions in both the pattern and the input data.

@item -l
Only print (list) the names of the files that matched, not the lines that matched.

@item -e @var{pattern}
Use @var{pattern} as the regexp to match.  The purpose of the @option{-e}
option is to allow patterns that start with a @samp{-}.
@end table

This version uses the @code{getopt} library function
(@pxref{Getopt Function, ,Processing Command-Line Options})
and the file transition library program
(@pxref{Filetrans Function, ,Noting @value{DDF} Boundaries}).

The program begins with a descriptive comment and then a @code{BEGIN} rule
that processes the command-line arguments with @code{getopt}.  The @option{-i}
(ignore case) option is particularly easy with @command{gawk}; we just use the
@code{IGNORECASE} built-in variable
(@pxref{Built-in Variables}):

@cindex @code{egrep.awk} program
@example
@c file eg/prog/egrep.awk
# egrep.awk --- simulate egrep in awk
@c endfile
@ignore
@c file eg/prog/egrep.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/prog/egrep.awk
# Options:
#    -c    count of lines
#    -s    silent - use exit value
#    -v    invert test, success if no match
#    -i    ignore case
#    -l    print filenames only
#    -e    argument is pattern
#
# Requires getopt and file transition library functions

BEGIN @{
    while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) @{
        if (c == "c")
            count_only++
        else if (c == "s")
            no_print++
        else if (c == "v")
            invert++
        else if (c == "i")
            IGNORECASE = 1
        else if (c == "l")
            filenames_only++
        else if (c == "e")
            pattern = Optarg
        else
            usage()
    @}
@c endfile
@end example

Next comes the code that handles the @command{egrep}-specific behavior. If no
pattern is supplied with @option{-e}, the first non-option on the
command line is used.  The @command{awk} command-line arguments up to @code{ARGV[Optind]}
are cleared, so that @command{awk} won't try to process them as files.  If no
files are specified, the standard input is used, and if multiple files are
specified, we make sure to note this so that the @value{FN}s can precede the
matched lines in the output:

@example
@c file eg/prog/egrep.awk
    if (pattern == "")
        pattern = ARGV[Optind++]

    for (i = 1; i < Optind; i++)
        ARGV[i] = ""
    if (Optind >= ARGC) @{
        ARGV[1] = "-"
        ARGC = 2
    @} else if (ARGC - Optind > 1)
        do_filenames++

#    if (IGNORECASE)
#        pattern = tolower(pattern)
@}
@c endfile
@end example

The last two lines are commented out, since they are not needed in
@command{gawk}.  They should be uncommented if you have to use another version
of @command{awk}.

The next set of lines should be uncommented if you are not using
@command{gawk}.  This rule translates all the characters in the input line
into lowercase if the @option{-i} option is specified.@footnote{It
also introduces a subtle bug;
if a match happens, we output the translated line, not the original.}
The rule is
commented out since it is not necessary with @command{gawk}:

@c Exercise: Fix this, w/array and new line as key to original line

@example
@c file eg/prog/egrep.awk
#@{
#    if (IGNORECASE)
#        $0 = tolower($0)
#@}
@c endfile
@end example

The @code{beginfile} function is called by the rule in @file{ftrans.awk}
when each new file is processed.  In this case, it is very simple; all it
does is initialize a variable @code{fcount} to zero. @code{fcount} tracks
how many lines in the current file matched the pattern.
(Naming the parameter @code{junk} shows we know that @code{beginfile}
is called with a parameter, but that we're not interested in its value.):

@example
@c file eg/prog/egrep.awk
function beginfile(junk)
@{
    fcount = 0
@}
@c endfile
@end example

The @code{endfile} function is called after each file has been processed.
It affects the output only when the user wants a count of the number of lines that
matched.  @code{no_print} is true only if the exit status is desired.
@code{count_only} is true if line counts are desired.  @command{egrep}
therefore only prints line counts if printing and counting are enabled.
The output format must be adjusted depending upon the number of files to
process.  Finally, @code{fcount} is added to @code{total}, so that we
know how many lines altogether matched the pattern:

@example
@c file eg/prog/egrep.awk
function endfile(file)
@{
    if (! no_print && count_only)
        if (do_filenames)
            print file ":" fcount
        else
            print fcount

    total += fcount
@}
@c endfile
@end example

The following rule does most of the work of matching lines. The variable
@code{matches} is true if the line matched the pattern. If the user
wants lines that did not match, the sense of @code{matches} is inverted
using the @samp{!} operator. @code{fcount} is incremented with the value of
@code{matches}, which is either one or zero, depending upon a
successful or unsuccessful match.  If the line does not match, the
@code{next} statement just moves on to the next record.

A number of additional tests are made, but they are only done if we
are not counting lines.  First, if the user only wants exit status
(@code{no_print} is true), then it is enough to know that @emph{one}
line in this file matched, and we can skip on to the next file with
@code{nextfile}.  Similarly, if we are only printing @value{FN}s, we can
print the @value{FN}, and then skip to the next file with @code{nextfile}.
Finally, each line is printed, with a leading @value{FN} and colon
if necessary:

@cindex @code{!} operator
@example
@c file eg/prog/egrep.awk
@{
    matches = ($0 ~ pattern)
    if (invert)
        matches = ! matches

    fcount += matches    # 1 or 0

    if (! matches)
        next

    if (! count_only) @{
        if (no_print)
            nextfile

        if (filenames_only) @{
            print FILENAME
            nextfile
        @}

        if (do_filenames)
            print FILENAME ":" $0
        else
            print
    @}
@}
@c endfile
@end example

The @code{END} rule takes care of producing the correct exit status. If
there are no matches, the exit status is one, otherwise it is zero:

@example
@c file eg/prog/egrep.awk
END    \
@{
    if (total == 0)
        exit 1
    exit 0
@}
@c endfile
@end example

The @code{usage} function prints a usage message in case of invalid options,
and then exits:

@example
@c file eg/prog/egrep.awk
function usage(    e)
@{
    e = "Usage: egrep [-csvil] [-e pat] [files ...]"
    e = e "\n\tegrep [-csvil] pat [files ...]"
    print e > "/dev/stderr"
    exit 1
@}
@c endfile
@end example

The variable @code{e} is used so that the function fits nicely
on the printed page.

@cindex backslash continuation
Just a note on programming style: you may have noticed that the @code{END}
rule uses backslash continuation, with the open brace on a line by
itself.  This is so that it more closely resembles the way functions
are written.  Many of the examples
in this @value{CHAPTER}
use this style. You can decide for yourself if you like writing
your @code{BEGIN} and @code{END} rules this way
or not.

@node Id Program, Split Program, Egrep Program, Clones
@subsection Printing out User Information

@cindex @command{id} utility
The @command{id} utility lists a user's real and effective user-id numbers,
real and effective group-id numbers, and the user's group set, if any.
@command{id} only prints the effective user-id and group-id if they are
different from the real ones.  If possible, @command{id} also supplies the
corresponding user and group names.  The output might look like this:

@example
$ id
@print{} uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty)
@end example

This information is part of what is provided by @command{gawk}'s
@code{PROCINFO} array (@pxref{Built-in Variables}).
However, the @command{id} utility provides a more palatable output than just
individual numbers.

Here is a simple version of @command{id} written in @command{awk}.
It uses the user database library functions
(@pxref{Passwd Functions, ,Reading the User Database})
and the group database library functions
(@pxref{Group Functions, ,Reading the Group Database}):

The program is fairly straightforward.  All the work is done in the
@code{BEGIN} rule.  The user and group ID numbers are obtained from
@code{PROCINFO}.
The code is repetitive.  The entry in the user database for the real user-id
number is split into parts at the @samp{:}. The name is the first field.
Similar code is used for the effective user-id number and the group
numbers.

@cindex @code{id.awk} program
@example
@c file eg/prog/id.awk
# id.awk --- implement id in awk
#
# Requires user and group library functions
@c endfile
@ignore
@c file eg/prog/id.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993
# Revised February 1996

@c endfile
@end ignore
@c file eg/prog/id.awk
# output is:
# uid=12(foo) euid=34(bar) gid=3(baz) \
#             egid=5(blat) groups=9(nine),2(two),1(one)

@group
BEGIN    \
@{
    uid = PROCINFO["uid"]
    euid = PROCINFO["euid"]
    gid = PROCINFO["gid"]
    egid = PROCINFO["egid"]
@end group

    printf("uid=%d", uid)
    pw = getpwuid(uid)
    if (pw != "") @{
        split(pw, a, ":")
        printf("(%s)", a[1])
    @}

    if (euid != uid) @{
        printf(" euid=%d", euid)
        pw = getpwuid(euid)
        if (pw != "") @{
            split(pw, a, ":")
            printf("(%s)", a[1])
        @}
    @}

    printf(" gid=%d", gid)
    pw = getgrgid(gid)
    if (pw != "") @{
        split(pw, a, ":")
        printf("(%s)", a[1])
    @}

    if (egid != gid) @{
        printf(" egid=%d", egid)
        pw = getgrgid(egid)
        if (pw != "") @{
            split(pw, a, ":")
            printf("(%s)", a[1])
        @}
    @}

    for (i = 1; ("group" i) in PROCINFO; i++) @{
        if (i == 1)
            printf(" groups=")
        group = PROCINFO["group" i]
        printf("%d", group)
        pw = getgrgid(group)
        if (pw != "") @{
            split(pw, a, ":")
            printf("(%s)", a[1])
        @}
        if (("group" (i+1)) in PROCINFO)
            printf(",")
    @}

    print ""
@}
@c endfile
@end example

@cindex @code{in} operator
The test in the @code{for} loop is worth noting.
Any supplementary groups in the @code{PROCINFO} array have the
indices @code{"group1"} through @code{"group@var{N}"} for some
@var{N}; i.e., the total number of supplementary groups.
The problem is, we don't know in advance how many of these groups
there are.

This loop works by starting at one, concatenating the value with
@code{"group"}, and then using @code{in} to see if that value is
in the array.  Eventually, @code{i} is incremented past
the last group in the array and the loop exits.

The loop is also correct if there are @emph{no} supplementary
groups; then the condition is false the first time it's
tested, and the loop body never executes.

@c exercise!!!
@ignore
The POSIX version of @command{id} takes arguments that control which
information is printed.  Modify this version to accept the same
arguments and perform in the same way.
@end ignore

@node Split Program, Tee Program, Id Program, Clones
@subsection Splitting a Large File into Pieces

@cindex @code{split} utility
The @code{split} program splits large text files into smaller pieces.
The usage is as follows:

@example
split @r{[}-@var{count}@r{]} file @r{[} @var{prefix} @r{]}
@end example

By default,
the output files are named @file{xaa}, @file{xab}, and so on. Each file has
1000 lines in it, with the likely exception of the last file. To change the
number of lines in each file, supply a number on the command line
preceded with a minus; e.g., @samp{-500} for files with 500 lines in them
instead of 1000.  To change the name of the output files to something like
@file{myfileaa}, @file{myfileab}, and so on, supply an additional
argument that specifies the @value{FN} prefix.

Here is a version of @code{split} in @command{awk}. It uses the @code{ord} and
@code{chr} functions presented in
@ref{Ordinal Functions, ,Translating Between Characters and Numbers}.

The program first sets its defaults, and then tests to make sure there are
not too many arguments.  It then looks at each argument in turn.  The
first argument could be a minus followed by a number. If it is, this happens
to look like a negative number, so it is made positive, and that is the
count of lines.  The data @value{FN} is skipped over and the final argument
is used as the prefix for the output @value{FN}s:

@cindex @code{split.awk} program
@example
@c file eg/prog/split.awk
# split.awk --- do split in awk
#
# Requires ord and chr library functions
@c endfile
@ignore
@c file eg/prog/split.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/prog/split.awk
# usage: split [-num] [file] [outname]

BEGIN @{
    outfile = "x"    # default
    count = 1000
    if (ARGC > 4)
        usage()

    i = 1
    if (ARGV[i] ~ /^-[0-9]+$/) @{
        count = -ARGV[i]
        ARGV[i] = ""
        i++
    @}
    # test argv in case reading from stdin instead of file
    if (i in ARGV)
        i++    # skip data file name
    if (i in ARGV) @{
        outfile = ARGV[i]
        ARGV[i] = ""
    @}

    s1 = s2 = "a"
    out = (outfile s1 s2)
@}
@c endfile
@end example

The next rule does most of the work. @code{tcount} (temporary count) tracks
how many lines have been printed to the output file so far. If it is greater
than @code{count}, it is time to close the current file and start a new one.
@code{s1} and @code{s2} track the current suffixes for the @value{FN}. If
they are both @samp{z}, the file is just too big.  Otherwise, @code{s1}
moves to the next letter in the alphabet and @code{s2} starts over again at
@samp{a}:

@c else on separate line here for page breaking
@example
@c file eg/prog/split.awk
@{
    if (++tcount > count) @{
        close(out)
        if (s2 == "z") @{
            if (s1 == "z") @{
                printf("split: %s is too large to split\n",
                       FILENAME) > "/dev/stderr"
                exit 1
            @}
            s1 = chr(ord(s1) + 1)
            s2 = "a"
        @}
@group
        else
            s2 = chr(ord(s2) + 1)
@end group
        out = (outfile s1 s2)
        tcount = 1
    @}
    print > out
@}
@c endfile
@end example

@c Exercise: do this with just awk builtin functions, index("abc..."), substr, etc.

@noindent
The @code{usage} function simply prints an error message and exits:

@example
@c file eg/prog/split.awk
function usage(   e)
@{
    e = "usage: split [-num] [file] [outname]"
    print e > "/dev/stderr"
    exit 1
@}
@c endfile
@end example

@noindent
The variable @code{e} is used so that the function
fits nicely on the
@ifinfo
screen.
@end ifinfo
@ifnotinfo
page.
@end ifnotinfo

This program is a bit sloppy; it relies on @command{awk} to close the last file
for it automatically, instead of doing it in an @code{END} rule.
It also assumes that letters are contiguous in the character set,
which isn't true for EBCDIC systems.
@c BFD...

@node Tee Program, Uniq Program, Split Program, Clones
@subsection Duplicating Output into Multiple Files

@cindex @code{tee} utility
The @code{tee} program is known as a ``pipe fitting.''  @code{tee} copies
its standard input to its standard output and also duplicates it to the
files named on the command line.  Its usage is as follows:

@example
tee @r{[}-a@r{]} file @dots{}
@end example

The @option{-a} option tells @code{tee} to append to the named files, instead of
truncating them and starting over.

The @code{BEGIN} rule first makes a copy of all the command-line arguments
into an array named @code{copy}.
@code{ARGV[0]} is not copied, since it is not needed.
@code{tee} cannot use @code{ARGV} directly, since @command{awk} attempts to
process each @value{FN} in @code{ARGV} as input data.

@cindex flag variables
If the first argument is @option{-a}, then the flag variable
@code{append} is set to true, and both @code{ARGV[1]} and
@code{copy[1]} are deleted. If @code{ARGC} is less than two, then no
@value{FN}s were supplied and @code{tee} prints a usage message and exits.
Finally, @command{awk} is forced to read the standard input by setting
@code{ARGV[1]} to @code{"-"} and @code{ARGC} to two: 

@c NEXT ED: Add more leading commentary in this program
@cindex @code{tee.awk} program
@example
@c file eg/prog/tee.awk
# tee.awk --- tee in awk
@c endfile
@ignore
@c file eg/prog/tee.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993
# Revised December 1995

@c endfile
@end ignore
@c file eg/prog/tee.awk
BEGIN    \
@{
    for (i = 1; i < ARGC; i++)
        copy[i] = ARGV[i]

    if (ARGV[1] == "-a") @{
        append = 1
        delete ARGV[1]
        delete copy[1]
        ARGC--
    @}
    if (ARGC < 2) @{
        print "usage: tee [-a] file ..." > "/dev/stderr"
        exit 1
    @}
    ARGV[1] = "-"
    ARGC = 2
@}
@c endfile
@end example

The single rule does all the work.  Since there is no pattern, it is
executed for each line of input.  The body of the rule simply prints the
line into each file on the command line, and then to the standard output:

@example
@c file eg/prog/tee.awk
@{
    # moving the if outside the loop makes it run faster
    if (append)
        for (i in copy)
            print >> copy[i]
    else
        for (i in copy)
            print > copy[i]
    print
@}
@c endfile
@end example

@noindent
It is also possible to write the loop this way:

@example
for (i in copy)
    if (append)
        print >> copy[i]
    else
        print > copy[i]
@end example

@noindent
This is more concise but it is also less efficient.  The @samp{if} is
tested for each record and for each output file.  By duplicating the loop
body, the @samp{if} is only tested once for each input record.  If there are
@var{N} input records and @var{M} output files, the first method only
executes @var{N} @samp{if} statements, while the second executes
@var{N}@code{*}@var{M} @samp{if} statements.

Finally, the @code{END} rule cleans up by closing all the output files:

@example
@c file eg/prog/tee.awk
END    \
@{
    for (i in copy)
        close(copy[i])
@}
@c endfile
@end example

@node Uniq Program, Wc Program, Tee Program, Clones
@subsection Printing Non-Duplicated Lines of Text

@cindex @command{uniq} utility
The @command{uniq} utility reads sorted lines of data on its standard
input, and by default removes duplicate lines.  In other words, it only
prints unique lines---hence the name.  @command{uniq} has a number of
options. The usage is as follows:

@example
uniq @r{[}-udc @r{[}-@var{n}@r{]]} @r{[}+@var{n}@r{]} @r{[} @var{input file} @r{[} @var{output file} @r{]]}
@end example

The option meanings are:

@table @code
@item -d
Only print repeated lines.

@item -u
Only print non-repeated lines.

@item -c
Count lines. This option overrides @option{-d} and @option{-u}.  Both repeated
and non-repeated lines are counted.

@item -@var{n}
Skip @var{n} fields before comparing lines.  The definition of fields
is similar to @command{awk}'s default: non-whitespace characters separated
by runs of spaces and/or tabs.

@item +@var{n}
Skip @var{n} characters before comparing lines.  Any fields specified with
@samp{-@var{n}} are skipped first.

@item @var{input file}
Data is read from the input file named on the command line, instead of from
the standard input.

@item @var{output file}
The generated output is sent to the named output file, instead of to the
standard output.
@end table

Normally @command{uniq} behaves as if both the @option{-d} and
@option{-u} options are provided.

@command{uniq} uses the
@code{getopt} library function
(@pxref{Getopt Function, ,Processing Command-Line Options})
and the @code{join} library function
(@pxref{Join Function, ,Merging an Array into a String}).

The program begins with a @code{usage} function and then a brief outline of
the options and their meanings in a comment.
The @code{BEGIN} rule deals with the command-line arguments and options. It
uses a trick to get @code{getopt} to handle options of the form @samp{-25},
treating such an option as the option letter @samp{2} with an argument of
@samp{5}. If indeed two or more digits are supplied (@code{Optarg} looks
like a number), @code{Optarg} is
concatenated with the option digit and then the result is added to zero to make
it into a number.  If there is only one digit in the option, then
@code{Optarg} is not needed. @code{Optind} must be decremented so that
@code{getopt} processes it next time.  This code is admittedly a bit
tricky.

If no options are supplied, then the default is taken, to print both
repeated and non-repeated lines.  The output file, if provided, is assigned
to @code{outputfile}.  Early on, @code{outputfile} is initialized to the
standard output, @file{/dev/stdout}:

@cindex @code{uniq.awk} program
@example
@c file eg/prog/uniq.awk
@group
# uniq.awk --- do uniq in awk
#
# Requires getopt and join library functions
@end group
@c endfile
@ignore
@c file eg/prog/uniq.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/prog/uniq.awk
function usage(    e)
@{
    e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]"
    print e > "/dev/stderr"
    exit 1
@}

# -c    count lines. overrides -d and -u
# -d    only repeated lines
# -u    only non-repeated lines
# -n    skip n fields
# +n    skip n characters, skip fields first

BEGIN   \
@{
    count = 1
    outputfile = "/dev/stdout"
    opts = "udc0:1:2:3:4:5:6:7:8:9:"
    while ((c = getopt(ARGC, ARGV, opts)) != -1) @{
        if (c == "u")
            non_repeated_only++
        else if (c == "d")
            repeated_only++
        else if (c == "c")
            do_count++
        else if (index("0123456789", c) != 0) @{
            # getopt requires args to options
            # this messes us up for things like -5
            if (Optarg ~ /^[0-9]+$/)
                fcount = (c Optarg) + 0
            else @{
                fcount = c + 0
                Optind--
            @}
        @} else
            usage()
    @}

    if (ARGV[Optind] ~ /^\+[0-9]+$/) @{
        charcount = substr(ARGV[Optind], 2) + 0
        Optind++
    @}

    for (i = 1; i < Optind; i++)
        ARGV[i] = ""

    if (repeated_only == 0 && non_repeated_only == 0)
        repeated_only = non_repeated_only = 1

    if (ARGC - Optind == 2) @{
        outputfile = ARGV[ARGC - 1]
        ARGV[ARGC - 1] = ""
    @}
@}
@c endfile
@end example

The following function, @code{are_equal}, compares the current line,
@code{$0}, to the
previous line, @code{last}.  It handles skipping fields and characters.
If no field count and no character count are specified, @code{are_equal}
simply returns one or zero depending upon the result of a simple string
comparison of @code{last} and @code{$0}.  Otherwise, things get more
complicated.
If fields have to be skipped, each line is broken into an array using
@code{split}
(@pxref{String Functions, ,String Manipulation Functions});
the desired fields are then joined back into a line using @code{join}.
The joined lines are stored in @code{clast} and @code{cline}.
If no fields are skipped, @code{clast} and @code{cline} are set to
@code{last} and @code{$0}, respectively.
Finally, if characters are skipped, @code{substr} is used to strip off the
leading @code{charcount} characters in @code{clast} and @code{cline}.  The
two strings are then compared and @code{are_equal} returns the result:

@example
@c file eg/prog/uniq.awk
function are_equal(    n, m, clast, cline, alast, aline)
@{
    if (fcount == 0 && charcount == 0)
        return (last == $0)

    if (fcount > 0) @{
        n = split(last, alast)
        m = split($0, aline)
        clast = join(alast, fcount+1, n)
        cline = join(aline, fcount+1, m)
    @} else @{
        clast = last
        cline = $0
    @}
    if (charcount) @{
        clast = substr(clast, charcount + 1)
        cline = substr(cline, charcount + 1)
    @}

    return (clast == cline)
@}
@c endfile
@end example

The following two rules are the body of the program.  The first one is
executed only for the very first line of data.  It sets @code{last} equal to
@code{$0}, so that subsequent lines of text have something to be compared to.

The second rule does the work. The variable @code{equal} is one or zero,
depending upon the results of @code{are_equal}'s comparison. If @command{uniq}
is counting repeated lines, and the lines are equal, then it increments the @code{count} variable.
Otherwise it prints the line and resets @code{count},
since the two lines are not equal.

If @command{uniq} is not counting, and if the lines are equal, @code{count} is incremented.
Nothing is printed, since the point is to remove duplicates.
Otherwise, if @command{uniq} is counting repeated lines and more than
one line is seen, or if @command{uniq} is counting non-repeated lines
and only one line is seen, then the line is printed, and @code{count}
is reset.

Finally, similar logic is used in the @code{END} rule to print the final
line of input data:

@example
@c file eg/prog/uniq.awk
NR == 1 @{
    last = $0
    next
@}

@{
    equal = are_equal()

    if (do_count) @{    # overrides -d and -u
        if (equal)
            count++
        else @{
            printf("%4d %s\n", count, last) > outputfile
            last = $0
            count = 1    # reset
        @}
        next
    @}

    if (equal)
        count++
    else @{
        if ((repeated_only && count > 1) ||
            (non_repeated_only && count == 1))
                print last > outputfile
        last = $0
        count = 1
    @}
@}

END @{
    if (do_count)
        printf("%4d %s\n", count, last) > outputfile
    else if ((repeated_only && count > 1) ||
            (non_repeated_only && count == 1))
        print last > outputfile
@}
@c endfile
@end example

@node Wc Program, , Uniq Program, Clones
@subsection Counting Things

@cindex @command{wc} utility
The @command{wc} (word count) utility counts lines, words, and characters in
one or more input files. Its usage is as follows:

@example
wc @r{[}-lwc@r{]} @r{[} @var{files} @dots{} @r{]}
@end example

If no files are specified on the command line, @command{wc} reads its standard
input. If there are multiple files, it also prints total counts for all
the files.  The options and their meanings are shown in the following list:

@table @code
@item -l
Only count lines.

@item -w
Only count words.
A ``word'' is a contiguous sequence of non-whitespace characters, separated
by spaces and/or tabs.  Happily, this is the normal way @command{awk} separates
fields in its input data.

@item -c
Only count characters.
@end table

Implementing @command{wc} in @command{awk} is particularly elegant,
since @command{awk} does a lot of the work for us; it splits lines into
words (i.e., fields) and counts them, it counts lines (i.e., records),
and it can easily tell us how long a line is.

This uses the @code{getopt} library function
(@pxref{Getopt Function, ,Processing Command-Line Options})
and the file transition functions
(@pxref{Filetrans Function, ,Noting @value{DDF} Boundaries}).

This version has one notable difference from traditional versions of
@command{wc}: it always prints the counts in the order lines, words,
and characters.  Traditional versions note the order of the @option{-l},
@option{-w}, and @option{-c} options on the command line, and print the
counts in that order.

The @code{BEGIN} rule does the argument processing.  The variable
@code{print_total} is true if more than one file is named on the
command line:

@cindex @code{wc.awk} program
@example
@c file eg/prog/wc.awk
# wc.awk --- count lines, words, characters
@c endfile
@ignore
@c file eg/prog/wc.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993
@c endfile
@end ignore
@c file eg/prog/wc.awk

# Options:
#    -l    only count lines
#    -w    only count words
#    -c    only count characters
#
# Default is to count lines, words, characters
#
# Requires getopt and file transition library functions

BEGIN @{
    # let getopt print a message about
    # invalid options. we ignore them
    while ((c = getopt(ARGC, ARGV, "lwc")) != -1) @{
        if (c == "l")
            do_lines = 1
        else if (c == "w")
            do_words = 1
        else if (c == "c")
            do_chars = 1
    @}
    for (i = 1; i < Optind; i++)
        ARGV[i] = ""

    # if no options, do all
    if (! do_lines && ! do_words && ! do_chars)
        do_lines = do_words = do_chars = 1

    print_total = (ARGC - i > 2)
@}
@c endfile
@end example

The @code{beginfile} function is simple; it just resets the counts of lines,
words, and characters to zero, and saves the current @value{FN} in
@code{fname}:

@c NEXT ED: make it lines = words = chars = 0
@example
@c file eg/prog/wc.awk
function beginfile(file)
@{
    chars = lines = words = 0
    fname = FILENAME
@}
@c endfile
@end example

The @code{endfile} function adds the current file's numbers to the running
totals of lines, words, and characters.  It then prints out those numbers
for the file that was just read. It relies on @code{beginfile} to reset the
numbers for the following @value{DF}:

@c NEXT ED: make order for += be lines, words, chars
@example
@c file eg/prog/wc.awk
function endfile(file)
@{
    tchars += chars
    tlines += lines
    twords += words
    if (do_lines)
        printf "\t%d", lines
@group
    if (do_words)
        printf "\t%d", words
@end group
    if (do_chars)
        printf "\t%d", chars
    printf "\t%s\n", fname
@}
@c endfile
@end example

There is one rule that is executed for each line. It adds the length of
the record, plus one, to @code{chars}.  Adding one plus the record length
is needed because the newline character separating records (the value
of @code{RS}) is not part of the record itself, and thus not included
in its length.  Next, @code{lines} is incremented for each line read,
and @code{words} is incremented by the value of @code{NF}, which is the
number of ``words'' on this line:@footnote{@command{wc} can't just use
the value of @code{FNR} in @code{endfile}.  If you examine the code in
@ref{Filetrans Function, ,Noting @value{DDF} Boundaries},
you will see that @code{FNR} has already been reset by the time
@code{endfile} is called.}
@c ONE DAY: make the above an exercise, instead of giving away the answer.

@example
@c file eg/prog/wc.awk
# do per line
@{
    chars += length($0) + 1    # get newline
    lines++
    words += NF
@}
@c endfile
@end example

Finally, the @code{END} rule simply prints the totals for all the files.

@example
@c file eg/prog/wc.awk
END @{
    if (print_total) @{
        if (do_lines)
            printf "\t%d", tlines
        if (do_words)
            printf "\t%d", twords
        if (do_chars)
            printf "\t%d", tchars
        print "\ttotal"
    @}
@}
@c endfile
@end example

@node Miscellaneous Programs, , Clones, Sample Programs
@section A Grab Bag of @command{awk} Programs

This @value{SECTION} is a large ``grab bag'' of miscellaneous programs.
We hope you find them both interesting and enjoyable.

@menu
* Dupword Program::             Finding duplicated words in a document.
* Alarm Program::               An alarm clock.
* Translate Program::           A program similar to the @command{tr} utility.
* Labels Program::              Printing mailing labels.
* Word Sorting::                A program to produce a word usage count.
* History Sorting::             Eliminating duplicate entries from a history
                                file.
* Extract Program::             Pulling out programs from Texinfo source
                                files.
* Simple Sed::                  A Simple Stream Editor.
* Igawk Program::               A wrapper for @command{awk} that includes
                                files.
@end menu

@node Dupword Program, Alarm Program, Miscellaneous Programs, Miscellaneous Programs
@subsection Finding Duplicated Words in a Document

A common error when writing large amounts of prose is to accidentally
duplicate words.  Typically you will see this in text as something like ``the
the program does the following @dots{}.''  When the text is online, often
the duplicated words occur at the end of one line and the beginning of
another, making them very difficult to spot.
@c as here!

This program, @file{dupword.awk}, scans through a file one line at a time
and looks for adjacent occurrences of the same word.  It also saves the last
word on a line (in the variable @code{prev}) for comparison with the first
word on the next line.

@cindex Texinfo
The first two statements make sure that the line is all lowercase,
so that, for example, ``The'' and ``the'' compare equal to each other.
The next statement replaces non-alphanumeric and non-whitespace characters
with spaces, so that punctuation does not affect the comparison either.
The characters are replaced with spaces so that formatting controls
don't create nonsense words (e.g., the Texinfo @samp{@@code@{NF@}}
becomes @samp{codeNF} if punctuation is simply deleted).  The record is
then re-split into fields, yielding just the actual words on the line,
and insuring that there are no empty fields.

If there are no fields left after removing all the punctuation, the
current record is skipped.  Otherwise, the program loops through each
word, comparing it to the previous one:

@cindex @code{dupword.awk} program
@example
@c file eg/prog/dupword.awk
# dupword.awk --- find duplicate words in text
@c endfile
@ignore
@c file eg/prog/dupword.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# December 1991
# Revised October 2000

@c endfile
@end ignore
@c file eg/prog/dupword.awk
@{
    $0 = tolower($0)
    gsub(/[^[:alnum:][:blank:]]/, " ");
    $0 = $0         # re-split
    if (NF == 0)
        next
    if ($1 == prev)
        printf("%s:%d: duplicate %s\n",
            FILENAME, FNR, $1)
    for (i = 2; i <= NF; i++)
        if ($i == $(i-1))
            printf("%s:%d: duplicate %s\n",
                FILENAME, FNR, $i)
    prev = $NF
@}
@c endfile
@end example

@node Alarm Program, Translate Program, Dupword Program, Miscellaneous Programs
@subsection An Alarm Clock Program
@cindex insomnia, cure for
@cindex Robbins, Arnold
@quotation
@i{Nothing cures insomnia like a ringing alarm clock.}@*
Arnold Robbins
@end quotation

The following program is a simple ``alarm clock'' program.
You give it a time of day and an optional message.  At the specified time,
it prints the message on the standard output. In addition, you can give it
the number of times to repeat the message as well as a delay between
repetitions.

This program uses the @code{gettimeofday} function from
@ref{Gettimeofday Function, ,Managing the Time of Day}.

All the work is done in the @code{BEGIN} rule.  The first part is argument
checking and setting of defaults: the delay, the count, and the message to
print.  If the user supplied a message without the ASCII BEL
character (known as the ``alert'' character, @code{"\a"}), then it is added to
the message.  (On many systems, printing the ASCII BEL generates some sort
of audible alert. Thus when the alarm goes off, the system calls attention
to itself in case the user is not looking at their computer or terminal.):

@cindex @code{alarm.awk} program
@example
@c file eg/prog/alarm.awk
# alarm.awk --- set an alarm
#
# Requires gettimeofday library function
@c endfile
@ignore
@c file eg/prog/alarm.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/prog/alarm.awk
# usage: alarm time [ "message" [ count [ delay ] ] ]

BEGIN    \
@{
    # Initial argument sanity checking
    usage1 = "usage: alarm time ['message' [count [delay]]]"
    usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])

    if (ARGC < 2) @{
        print usage1 > "/dev/stderr"
        print usage2 > "/dev/stderr"
        exit 1
    @} else if (ARGC == 5) @{
        delay = ARGV[4] + 0
        count = ARGV[3] + 0
        message = ARGV[2]
    @} else if (ARGC == 4) @{
        count = ARGV[3] + 0
        message = ARGV[2]
    @} else if (ARGC == 3) @{
        message = ARGV[2]
    @} else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) @{
        print usage1 > "/dev/stderr"
        print usage2 > "/dev/stderr"
        exit 1
    @}

    # set defaults for once we reach the desired time
    if (delay == 0)
        delay = 180    # 3 minutes
@group
    if (count == 0)
        count = 5
@end group
    if (message == "")
        message = sprintf("\aIt is now %s!\a", ARGV[1])
    else if (index(message, "\a") == 0)
        message = "\a" message "\a"
@c endfile
@end example

The next @value{SECTION} of code turns the alarm time into hours and minutes,
converts it (if necessary) to a 24-hour clock, and then turns that
time into a count of the seconds since midnight.  Next it turns the current
time into a count of seconds since midnight.  The difference between the two
is how long to wait before setting off the alarm:

@example
@c file eg/prog/alarm.awk
    # split up alarm time
    split(ARGV[1], atime, ":")
    hour = atime[1] + 0    # force numeric
    minute = atime[2] + 0  # force numeric

    # get current broken down time
    gettimeofday(now)

    # if time given is 12-hour hours and it's after that
    # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
    # then add 12 to real hour
    if (hour < 12 && now["hour"] > hour)
        hour += 12

    # set target time in seconds since midnight
    target = (hour * 60 * 60) + (minute * 60)

    # get current time in seconds since midnight
    current = (now["hour"] * 60 * 60) + \
               (now["minute"] * 60) + now["second"]

    # how long to sleep for
    naptime = target - current
    if (naptime <= 0) @{
        print "time is in the past!" > "/dev/stderr"
        exit 1
    @}
@c endfile
@end example

@cindex @command{sleep} utility
Finally, the program uses the @code{system} function
(@pxref{I/O Functions, ,Input/Output Functions})
to call the @command{sleep} utility.  The @command{sleep} utility simply pauses
for the given number of seconds.  If the exit status is not zero,
the program assumes that @command{sleep} was interrupted and exits. If
@command{sleep} exited with an OK status (zero), then the program prints the
message in a loop, again using @command{sleep} to delay for however many
seconds are necessary:

@example
@c file eg/prog/alarm.awk
    # zzzzzz..... go away if interrupted
    if (system(sprintf("sleep %d", naptime)) != 0)
        exit 1

    # time to notify!
    command = sprintf("sleep %d", delay)
    for (i = 1; i <= count; i++) @{
        print message
        # if sleep command interrupted, go away
        if (system(command) != 0)
            break
    @}

    exit 0
@}
@c endfile
@end example

@node Translate Program, Labels Program, Alarm Program, Miscellaneous Programs
@subsection Transliterating Characters

@cindex @command{tr} utility
The system @command{tr} utility transliterates characters.  For example, it is
often used to map uppercase letters into lowercase for further processing:

@example
@var{generate data} | tr 'A-Z' 'a-z' | @var{process data} @dots{}
@end example

@command{tr} requires two lists of characters.@footnote{On some older
System V systems,
@command{tr} may require that the lists be written as
range expressions enclosed in square brackets (@samp{[a-z]}) and quoted,
to prevent the shell from attempting a @value{FN} expansion.  This is
not a feature.}  When processing the input, the first character in the
first list is replaced with the first character in the second list,
the second character in the first list is replaced with the second
character in the second list, and so on.  If there are more characters
in the ``from'' list than in the ``to'' list, the last character of the
``to'' list is used for the remaining characters in the ``from'' list.

Some time ago,
@c early or mid-1989!
a user proposed that a transliteration function should
be added to @command{gawk}.
@c Wishing to avoid gratuitous new features,
@c at least theoretically
The following program was written to
prove that character transliteration could be done with a user-level
function.  This program is not as complete as the system @command{tr} utility
but it does most of the job.

The @command{translate} program demonstrates one of the few weaknesses
of standard @command{awk}: dealing with individual characters is very
painful, requiring repeated use of the @code{substr}, @code{index},
and @code{gsub} built-in functions
(@pxref{String Functions, ,String Manipulation Functions}).@footnote{This
program was written before @command{gawk} acquired the ability to
split each character in a string into separate array elements.}
@c Exercise: How might you use this new feature to simplify the program?

There are two functions.  The first, @code{stranslate}, takes three
arguments:

@table @code
@item from
A list of characters to translate from.

@item to
A list of characters to translate to.

@item target
The string to do the translation on.
@end table

Associative arrays make the translation part fairly easy. @code{t_ar} holds
the ``to'' characters, indexed by the ``from'' characters.  Then a simple
loop goes through @code{from}, one character at a time.  For each character
in @code{from}, if the character appears in @code{target}, @code{gsub}
is used to change it to the corresponding @code{to} character.

The @code{translate} function simply calls @code{stranslate} using @code{$0}
as the target.  The main program sets two global variables, @code{FROM} and
@code{TO}, from the command line, and then changes @code{ARGV} so that
@command{awk} reads from the standard input.

Finally, the processing rule simply calls @code{translate} for each record:

@cindex @code{translate.awk} program
@example
@c file eg/prog/translate.awk
# translate.awk --- do tr-like stuff
@c endfile
@ignore
@c file eg/prog/translate.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# August 1989

@c endfile
@end ignore
@c file eg/prog/translate.awk
# Bugs: does not handle things like: tr A-Z a-z, it has
# to be spelled out. However, if `to' is shorter than `from',
# the last character in `to' is used for the rest of `from'.

function stranslate(from, to, target,     lf, lt, t_ar, i, c)
@{
    lf = length(from)
    lt = length(to)
    for (i = 1; i <= lt; i++)
        t_ar[substr(from, i, 1)] = substr(to, i, 1)
    if (lt < lf)
        for (; i <= lf; i++)
            t_ar[substr(from, i, 1)] = substr(to, lt, 1)
    for (i = 1; i <= lf; i++) @{
        c = substr(from, i, 1)
        if (index(target, c) > 0)
            gsub(c, t_ar[c], target)
    @}
    return target
@}

function translate(from, to)
@{
    return $0 = stranslate(from, to, $0)
@}

# main program
BEGIN @{
@group
    if (ARGC < 3) @{
        print "usage: translate from to" > "/dev/stderr"
        exit
    @}
@end group
    FROM = ARGV[1]
    TO = ARGV[2]
    ARGC = 2
    ARGV[1] = "-"
@}

@{
    translate(FROM, TO)
    print
@}
@c endfile
@end example

While it is possible to do character transliteration in a user-level
function, it is not necessarily efficient, and we (the @command{gawk}
authors) started to consider adding a built-in function.  However,
shortly after writing this program, we learned that the System V Release 4
@command{awk} had added the @code{toupper} and @code{tolower} functions
(@pxref{String Functions, ,String Manipulation Functions}).
These functions handle the vast majority of the
cases where character transliteration is necessary, and so we chose to
simply add those functions to @command{gawk} as well and then leave well
enough alone.

An obvious improvement to this program would be to set up the
@code{t_ar} array only once, in a @code{BEGIN} rule. However, this
assumes that the ``from'' and ``to'' lists
will never change throughout the lifetime of the program.

@node Labels Program, Word Sorting, Translate Program, Miscellaneous Programs
@subsection Printing Mailing Labels

Here is a ``real world''@footnote{``Real world'' is defined as
``a program actually used to get something done.''}
program.  This
script reads lists of names and
addresses and generates mailing labels.  Each page of labels has 20 labels
on it, two across and ten down.  The addresses are guaranteed to be no more
than five lines of data.  Each address is separated from the next by a blank
line.

The basic idea is to read 20 labels worth of data.  Each line of each label
is stored in the @code{line} array.  The single rule takes care of filling
the @code{line} array and printing the page when 20 labels have been read.

The @code{BEGIN} rule simply sets @code{RS} to the empty string, so that
@command{awk} splits records at blank lines
(@pxref{Records, ,How Input Is Split into Records}).
It sets @code{MAXLINES} to 100, since 100 is the maximum number
of lines on the page (20 * 5 = 100).

Most of the work is done in the @code{printpage} function.
The label lines are stored sequentially in the @code{line} array.  But they
have to print horizontally; @code{line[1]} next to @code{line[6]},
@code{line[2]} next to @code{line[7]}, and so on.  Two loops are used to
accomplish this.  The outer loop, controlled by @code{i}, steps through
every 10 lines of data; this is each row of labels.  The inner loop,
controlled by @code{j}, goes through the lines within the row.
As @code{j} goes from 0 to 4, @samp{i+j} is the @code{j}'th line in
the row, and @samp{i+j+5} is the entry next to it.  The output ends up
looking something like this:

@example
line 1          line 6
line 2          line 7
line 3          line 8
line 4          line 9
line 5          line 10
@dots{}
@end example

As a final note, an extra blank line is printed at lines 21 and 61, to keep
the output lined up on the labels.  This is dependent on the particular
brand of labels in use when the program was written.  You will also note
that there are two blank lines at the top and two blank lines at the bottom.

The @code{END} rule arranges to flush the final page of labels; there may
not have been an even multiple of 20 labels in the data:

@cindex @code{labels.awk} program
@example
@c file eg/prog/labels.awk
# labels.awk --- print mailing labels
@c endfile
@ignore
@c file eg/prog/labels.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# June 1992
@c endfile
@end ignore
@c file eg/prog/labels.awk

# Each label is 5 lines of data that may have blank lines.
# The label sheets have 2 blank lines at the top and 2 at
# the bottom.

BEGIN    @{ RS = "" ; MAXLINES = 100 @}

function printpage(    i, j)
@{
    if (Nlines <= 0)
        return

    printf "\n\n"        # header

    for (i = 1; i <= Nlines; i += 10) @{
        if (i == 21 || i == 61)
            print ""
        for (j = 0; j < 5; j++) @{
            if (i + j > MAXLINES)
                break
            printf "   %-41s %s\n", line[i+j], line[i+j+5]
        @}
        print ""
    @}

    printf "\n\n"        # footer

    for (i in line)
        line[i] = ""
@}

# main rule
@{
    if (Count >= 20) @{
        printpage()
        Count = 0
        Nlines = 0
    @}
    n = split($0, a, "\n")
    for (i = 1; i <= n; i++)
        line[++Nlines] = a[i]
    for (; i <= 5; i++)
        line[++Nlines] = ""
    Count++
@}

END    \
@{
    printpage()
@}
@c endfile
@end example

@node Word Sorting, History Sorting, Labels Program, Miscellaneous Programs
@subsection Generating Word Usage Counts

@c NEXT ED: Rewrite this whole section and example
The following @command{awk} program prints
the number of occurrences of each word in its input.  It illustrates the
associative nature of @command{awk} arrays by using strings as subscripts.  It
also demonstrates the @samp{for @var{index} in @var{array}} mechanism.
Finally, it shows how @command{awk} is used in conjunction with other
utility programs to do a useful task of some complexity with a minimum of
effort.  Some explanations follow the program listing:

@example
# Print list of word frequencies
@{
    for (i = 1; i <= NF; i++)
        freq[$i]++
@}

END @{
    for (word in freq)
        printf "%s\t%d\n", word, freq[word]
@}
@end example

@c Exercise: Use asort() here

This program has two rules.  The
first rule, because it has an empty pattern, is executed for every input line.
It uses @command{awk}'s field-accessing mechanism
(@pxref{Fields, ,Examining Fields}) to pick out the individual words from
the line, and the built-in variable @code{NF} (@pxref{Built-in Variables})
to know how many fields are available.
For each input word, it increments an element of the array @code{freq} to
reflect that the word has been seen an additional time.

The second rule, because it has the pattern @code{END}, is not executed
until the input has been exhausted.  It prints out the contents of the
@code{freq} table that has been built up inside the first action.
This program has several problems that would prevent it from being
useful by itself on real text files:

@itemize @bullet
@item
Words are detected using the @command{awk} convention that fields are
separated just by whitespace.  Other characters in the input (except
newlines) don't have any special meaning to @command{awk}.  This means that
punctuation characters count as part of words.

@item
The @command{awk} language considers upper- and lowercase characters to be
distinct.  Therefore, ``bartender'' and ``Bartender'' are not treated
as the same word.  This is undesirable, since in normal text, words
are capitalized if they begin sentences, and a frequency analyzer should not
be sensitive to capitalization.

@item
The output does not come out in any useful order.  You're more likely to be
interested in which words occur most frequently or in having an alphabetized
table of how frequently each word occurs.
@end itemize

@cindex @command{sort} utility
The way to solve these problems is to use some of @command{awk}'s more advanced
features.  First, we use @code{tolower} to remove
case distinctions.  Next, we use @code{gsub} to remove punctuation
characters.  Finally, we use the system @command{sort} utility to process the
output of the @command{awk} script.  Here is the new version of
the program:

@cindex @code{wordfreq.awk} program
@example
@c file eg/prog/wordfreq.awk
# wordfreq.awk --- print list of word frequencies

@{
    $0 = tolower($0)    # remove case distinctions
    # remove punctuation
    gsub(/[^[:alnum:]_[:blank:]]/, "", $0)
    for (i = 1; i <= NF; i++)
        freq[$i]++
@}

END @{
    for (word in freq)
        printf "%s\t%d\n", word, freq[word]
@}
@c endfile
@end example

Assuming we have saved this program in a file named @file{wordfreq.awk},
and that the data is in @file{file1}, the following pipeline:

@example
awk -f wordfreq.awk file1 | sort +1 -nr
@end example

@noindent
produces a table of the words appearing in @file{file1} in order of
decreasing frequency.  The @command{awk} program suitably massages the
data and produces a word frequency table, which is not ordered.

The @command{awk} script's output is then sorted by the @command{sort}
utility and printed on the terminal.  The options given to @command{sort}
specify a sort that uses the second field of each input line (skipping
one field), that the sort keys should be treated as numeric quantities
(otherwise @samp{15} would come before @samp{5}), and that the sorting
should be done in descending (reverse) order.

The @command{sort} could even be done from within the program, by changing
the @code{END} action to:

@example
@c file eg/prog/wordfreq.awk
END @{
    sort = "sort +1 -nr"
    for (word in freq)
        printf "%s\t%d\n", word, freq[word] | sort
    close(sort)
@}
@c endfile
@end example

This way of sorting must be used on systems that do not
have true pipes at the command-line (or batch-file) level.
See the general operating system documentation for more information on how
to use the @command{sort} program.

@node History Sorting, Extract Program, Word Sorting, Miscellaneous Programs
@subsection Removing Duplicates from Unsorted Text

The @command{uniq} program
(@pxref{Uniq Program, ,Printing Non-Duplicated Lines of Text}),
removes duplicate lines from @emph{sorted} data.

Suppose, however, you need to remove duplicate lines from a @value{DF} but
that you want to preserve the order the lines are in.  A good example of
this might be a shell history file.  The history file keeps a copy of all
the commands you have entered, and it is not unusual to repeat a command
several times in a row.  Occasionally you might want to compact the history
by removing duplicate entries.  Yet it is desirable to maintain the order
of the original commands.

This simple program does the job.  It uses two arrays.  The @code{data}
array is indexed by the text of each line.
For each line, @code{data[$0]} is incremented.
If a particular line has not
been seen before, then @code{data[$0]} is zero.
In this case, the text of the line is stored in @code{lines[count]}.
Each element of @code{lines} is a unique command, and the indices of
@code{lines} indicate the order in which those lines are encountered.
The @code{END} rule simply prints out the lines, in order:

@cindex Rakitzis, Byron
@cindex @code{histsort.awk} program
@example
@c file eg/prog/histsort.awk
# histsort.awk --- compact a shell history file
# Thanks to Byron Rakitzis for the general idea
@c endfile
@ignore
@c file eg/prog/histsort.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993

@c endfile
@end ignore
@c file eg/prog/histsort.awk
@group
@{
    if (data[$0]++ == 0)
        lines[++count] = $0
@}
@end group

END @{
    for (i = 1; i <= count; i++)
        print lines[i]
@}
@c endfile
@end example

This program also provides a foundation for generating other useful
information.  For example, using the following @code{print} statement in the
@code{END} rule indicates how often a particular command is used:

@example
print data[lines[i]], lines[i]
@end example

This works because @code{data[$0]} is incremented each time a line is
seen.

@node Extract Program, Simple Sed, History Sorting, Miscellaneous Programs
@subsection Extracting Programs from Texinfo Source Files

@ifnotinfo
Both this chapter and the previous chapter
(@ref{Library Functions, ,A Library of @command{awk} Functions})
present a large number of @command{awk} programs.
@end ifnotinfo
@ifinfo
The nodes
@ref{Library Functions, ,A Library of @command{awk} Functions},
and @ref{Sample Programs, ,Practical @command{awk} Programs},
are the top level nodes for a large number of @command{awk} programs.
@end ifinfo
If you want to experiment with these programs, it is tedious to have to type
them in by hand.  Here we present a program that can extract parts of a
Texinfo input file into separate files.

@cindex Texinfo
This @value{DOCUMENT} is written in Texinfo, the GNU project's document
formatting
language.
A single Texinfo source file can be used to produce both
printed and online documentation.
@ifnotinfo
Texinfo is fully documented in the book
@cite{Texinfo---The GNU Documentation Format},
available from the Free Software Foundation.
@end ifnotinfo
@ifinfo
The Texinfo language is described fully, starting with
@ref{Top}.
@end ifinfo

For our purposes, it is enough to know three things about Texinfo input
files:

@itemize @bullet
@item
The ``at'' symbol (@samp{@@}) is special in Texinfo, much as
the backslash (@samp{\}) is in C
or @command{awk}.  Literal @samp{@@} symbols are represented in Texinfo source
files as @samp{@@@@}.

@item
Comments start with either @samp{@@c} or @samp{@@comment}.
The file extraction program works by using special comments that start
at the beginning of a line.

@item
Lines containing @samp{@@group} and @samp{@@end group} commands bracket
example text that should not be split across a page boundary.
(Unfortunately, @TeX{} isn't always smart enough to do things exactly right
and we have to give it some help.)
@end itemize

The following program, @file{extract.awk}, reads through a Texinfo source
file and does two things, based on the special comments.
Upon seeing @samp{@w{@@c system @dots{}}},
it runs a command, by extracting the command text from the
control line and passing it on to the @code{system} function
(@pxref{I/O Functions, ,Input/Output Functions}).
Upon seeing @samp{@@c file @var{filename}}, each subsequent line is sent to
the file @var{filename}, until @samp{@@c endfile} is encountered.
The rules in @file{extract.awk} match either @samp{@@c} or
@samp{@@comment} by letting the @samp{omment} part be optional.
Lines containing @samp{@@group} and @samp{@@end group} are simply removed.
@file{extract.awk} uses the @code{join} library function
(@pxref{Join Function, ,Merging an Array into a String}).

The example programs in the online Texinfo source for @cite{@value{TITLE}}
(@file{gawk.texi}) have all been bracketed inside @samp{file} and
@samp{endfile} lines.  The @command{gawk} distribution uses a copy of
@file{extract.awk} to extract the sample programs and install many
of them in a standard directory where @command{gawk} can find them.
The Texinfo file looks something like this:

@example
@dots{}
This program has a @@code@{BEGIN@} rule,
that prints a nice message:

@@example
@@c file examples/messages.awk
BEGIN @@@{ print "Don't panic!" @@@}
@@c end file
@@end example

It also prints some final advice:

@@example
@@c file examples/messages.awk
END @@@{ print "Always avoid bored archeologists!" @@@}
@@c end file
@@end example
@dots{}
@end example

@file{extract.awk} begins by setting @code{IGNORECASE} to one, so that
mixed upper- and lowercase letters in the directives won't matter.

The first rule handles calling @code{system}, checking that a command is
given (@code{NF} is at least three) and also checking that the command
exits with a zero exit status, signifying OK:

@cindex @code{extract.awk} program
@example
@c file eg/prog/extract.awk
# extract.awk --- extract files and run programs
#                 from texinfo files
@c endfile
@ignore
@c file eg/prog/extract.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# May 1993
# Revised September 2000

@c endfile
@end ignore
@c file eg/prog/extract.awk
BEGIN    @{ IGNORECASE = 1 @}

/^@@c(omment)?[ \t]+system/    \
@{
    if (NF < 3) @{
        e = (FILENAME ":" FNR)
        e = (e  ": badly formed `system' line")
        print e > "/dev/stderr"
        next
    @}
    $1 = ""
    $2 = ""
    stat = system($0)
    if (stat != 0) @{
        e = (FILENAME ":" FNR)
        e = (e ": warning: system returned " stat)
        print e > "/dev/stderr"
    @}
@}
@c endfile
@end example

@noindent
The variable @code{e} is used so that the function
fits nicely on the
@ifnotinfo
page.
@end ifnotinfo
@ifnottex
screen.
@end ifnottex

The second rule handles moving data into files.  It verifies that a
@value{FN} is given in the directive.  If the file named is not the
current file, then the current file is closed.  Keeping the current file
open until a new file is encountered allows the use of the @samp{>}
redirection for printing the contents, keeping open file management
simple.

The @samp{for} loop does the work.  It reads lines using @code{getline}
(@pxref{Getline, ,Explicit Input with @code{getline}}).
For an unexpected end of file, it calls the @code{@w{unexpected_eof}}
function.  If the line is an ``endfile'' line, then it breaks out of
the loop.
If the line is an @samp{@@group} or @samp{@@end group} line, then it
ignores it and goes on to the next line.
Similarly, comments within examples are also ignored.

Most of the work is in the following few lines.  If the line has no @samp{@@}
symbols, the program can print it directly.
Otherwise, each leading @samp{@@} must be stripped off.
To remove the @samp{@@} symbols, the line is split into separate elements of
the array @code{a}, using the @code{split} function
(@pxref{String Functions, ,String Manipulation Functions}).
The @samp{@@} symbol is used as the separator character.
Each element of @code{a} that is empty indicates two successive @samp{@@}
symbols in the original line.  For each two empty elements (@samp{@@@@} in
the original file), we have to add a single @samp{@@} symbol back in.

When the processing of the array is finished, @code{join} is called with the
value of @code{SUBSEP}, to rejoin the pieces back into a single
line.  That line is then printed to the output file:

@example
@c file eg/prog/extract.awk
/^@@c(omment)?[ \t]+file/    \
@{
    if (NF != 3) @{
        e = (FILENAME ":" FNR ": badly formed `file' line")
        print e > "/dev/stderr"
        next
    @}
    if ($3 != curfile) @{
        if (curfile != "")
            close(curfile)
        curfile = $3
    @}

    for (;;) @{
        if ((getline line) <= 0)
            unexpected_eof()
        if (line ~ /^@@c(omment)?[ \t]+endfile/)
            break
        else if (line ~ /^@@(end[ \t]+)?group/)
            continue
        else if (line ~ /^@@c(omment+)?[ \t]+/)
            continue
        if (index(line, "@@") == 0) @{
            print line > curfile
            continue
        @}
        n = split(line, a, "@@")
        # if a[1] == "", means leading @@,
        # don't add one back in.
        for (i = 2; i <= n; i++) @{
            if (a[i] == "") @{ # was an @@@@
                a[i] = "@@"
                if (a[i+1] == "")
                    i++
            @}
        @}
        print join(a, 1, n, SUBSEP) > curfile
    @}
@}
@c endfile
@end example

An important thing to note is the use of the @samp{>} redirection.
Output done with @samp{>} only opens the file once; it stays open and
subsequent output is appended to the file
(@pxref{Redirection, , Redirecting Output of @code{print} and @code{printf}}).
This makes it easy to mix program text and explanatory prose for the same
sample source file (as has been done here!) without any hassle.  The file is
only closed when a new data @value{FN} is encountered or at the end of the
input file.

Finally, the function @code{@w{unexpected_eof}} prints an appropriate
error message and then exits.
The @code{END} rule handles the final cleanup, closing the open file:

@c function lb put on same line for page breaking. sigh
@example
@c file eg/prog/extract.awk
@group
function unexpected_eof() @{
    printf("%s:%d: unexpected EOF or error\n",
        FILENAME, FNR) > "/dev/stderr"
    exit 1
@}
@end group

END @{
    if (curfile)
        close(curfile)
@}
@c endfile
@end example

@node Simple Sed, Igawk Program, Extract Program, Miscellaneous Programs
@subsection A Simple Stream Editor

@cindex @command{sed} utility
@cindex stream editor
The @command{sed} utility is a ``stream editor,'' a program that reads a
stream of data, makes changes to it, and passes it on.
It is often used to make global changes to a large file or to a stream
of data generated by a pipeline of commands.
While @command{sed} is a complicated program in its own right, its most common
use is to perform global substitutions in the middle of a pipeline:

@example
command1 < orig.data | sed 's/old/new/g' | command2 > result
@end example

Here, @samp{s/old/new/g} tells @command{sed} to look for the regexp
@samp{old} on each input line and globally replace it with the text
@samp{new}, (i.e., all the occurrences on a line).  This is similar to
@command{awk}'s @code{gsub} function
(@pxref{String Functions, ,String Manipulation Functions}).

The following program, @file{awksed.awk}, accepts at least two command-line
arguments: the pattern to look for and the text to replace it with. Any
additional arguments are treated as data @value{FN}s to process. If none
are provided, the standard input is used:

@cindex Brennan, Michael
@cindex @command{awksed.awk} program
@cindex simple stream editor
@cindex stream editor, simple
@example
@c file eg/prog/awksed.awk
# awksed.awk --- do s/foo/bar/g using just print
#    Thanks to Michael Brennan for the idea
@c endfile
@ignore
@c file eg/prog/awksed.awk
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# August 1995

@c endfile
@end ignore
@c file eg/prog/awksed.awk
function usage()
@{
    print "usage: awksed pat repl [files...]" > "/dev/stderr"
    exit 1
@}

BEGIN @{
    # validate arguments
    if (ARGC < 3)
        usage()

    RS = ARGV[1]
    ORS = ARGV[2]

    # don't use arguments as files
    ARGV[1] = ARGV[2] = ""
@}

@group
# look ma, no hands!
@{
    if (RT == "")
        printf "%s", $0
    else
        print
@}
@end group
@c endfile
@end example

The program relies on @command{gawk}'s ability to have @code{RS} be a regexp,
as well as on the setting of @code{RT} to the actual text that terminates the
record (@pxref{Records, ,How Input Is Split into Records}).

The idea is to have @code{RS} be the pattern to look for. @command{gawk}
automatically sets @code{$0} to the text between matches of the pattern.
This is text that we want to keep, unmodified.  Then, by setting @code{ORS}
to the replacement text, a simple @code{print} statement outputs the
text we want to keep, followed by the replacement text.

There is one wrinkle to this scheme, which is what to do if the last record
doesn't end with text that matches @code{RS}.  Using a @code{print}
statement unconditionally prints the replacement text, which is not correct.
However, if the file did not end in text that matches @code{RS}, @code{RT}
is set to the null string.  In this case, we can print @code{$0} using
@code{printf}
(@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}).

The @code{BEGIN} rule handles the setup, checking for the right number
of arguments and calling @code{usage} if there is a problem. Then it sets
@code{RS} and @code{ORS} from the command-line arguments and sets
@code{ARGV[1]} and @code{ARGV[2]} to the null string, so that they are
not treated as @value{FN}s
(@pxref{ARGC and ARGV, , Using @code{ARGC} and @code{ARGV}}).

The @code{usage} function prints an error message and exits.
Finally, the single rule handles the printing scheme outlined above,
using @code{print} or @code{printf} as appropriate, depending upon the
value of @code{RT}.

@ignore
Exercise, compare the performance of this version with the more
straightforward:

BEGIN {
    pat = ARGV[1]
    repl = ARGV[2]
    ARGV[1] = ARGV[2] = ""
}

{ gsub(pat, repl); print }

Exercise: what are the advantages and disadvantages of this version vs. sed?
  Advantage: egrep regexps
             speed (?)
  Disadvantage: no & in replacement text

Others?
@end ignore

@node Igawk Program, , Simple Sed, Miscellaneous Programs
@subsection An Easy Way to Use Library Functions

Using library functions in @command{awk} can be very beneficial. It
encourages code reuse and the writing of general functions. Programs are
smaller and therefore clearer.
However, using library functions is only easy when writing @command{awk}
programs; it is painful when running them, requiring multiple @option{-f}
options.  If @command{gawk} is unavailable, then so too is the @env{AWKPATH}
environment variable and the ability to put @command{awk} functions into a
library directory (@pxref{Options, ,Command-Line Options}).
It would be nice to be able to write programs in the following manner:

@example
# library functions
@@include getopt.awk
@@include join.awk
@dots{}

# main program
BEGIN @{
    while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
        @dots{}
    @dots{}
@}
@end example

The following program, @file{igawk.sh}, provides this service.
It simulates @command{gawk}'s searching of the @env{AWKPATH} variable
and also allows @dfn{nested} includes; i.e., a file that is included
with @samp{@@include} can contain further @samp{@@include} statements.
@command{igawk} makes an effort to only include files once, so that nested
includes don't accidentally include a library function twice.

@command{igawk} should behave just like @command{gawk} externally.  This
means it should accept all of @command{gawk}'s command-line arguments,
including the ability to have multiple source files specified via
@option{-f}, and the ability to mix command-line and library source files.

The program is written using the POSIX Shell (@command{sh}) command language.
The way the program works is as follows:

@enumerate
@item
Loop through the arguments, saving anything that doesn't represent
@command{awk} source code for later, when the expanded program is run.

@item
For any arguments that do represent @command{awk} text, put the arguments into
a temporary file that will be expanded.  There are two cases:

@enumerate a
@item
Literal text, provided with @option{--source} or @option{--source=}.  This
text is just echoed directly.  The @command{echo} program automatically
supplies a trailing newline.

@item
Source @value{FN}s provided with @option{-f}.  We use a neat trick and echo
@samp{@@include @var{filename}} into the temporary file.  Since the file
inclusion program works the way @command{gawk} does, this gets the text
of the file included into the program at the correct point.
@end enumerate

@item
Run an @command{awk} program (naturally) over the temporary file to expand
@samp{@@include} statements.  The expanded program is placed in a second
temporary file.

@item
Run the expanded program with @command{gawk} and any other original command-line
arguments that the user supplied (such as the data @value{FN}s).
@end enumerate

The initial part of the program turns on shell tracing if the first
argument is @samp{debug}.  Otherwise, a shell @code{trap} statement
arranges to clean up any temporary files on program exit or upon an
interrupt.

@c 2e: For the temp file handling, go with Darrel's ig=${TMP:-/tmp}/igs.$$
@c 2e: or something as similar as possible.

The next part loops through all the command-line arguments.
There are several cases of interest:

@table @code
@item --
This ends the arguments to @command{igawk}.  Anything else should be passed on
to the user's @command{awk} program without being evaluated.

@item -W
This indicates that the next option is specific to @command{gawk}.  To make
argument processing easier, the @option{-W} is appended to the front of the
remaining arguments and the loop continues.  (This is an @command{sh}
programming trick.  Don't worry about it if you are not familiar with
@command{sh}.)

@item -v@r{,} -F
These are saved and passed on to @command{gawk}.

@item -f@r{,} --file@r{,} --file=@r{,} -Wfile=
The @value{FN} is saved to the temporary file @file{/tmp/ig.s.$$} with an
@samp{@@include} statement.
The @command{sed} utility is used to remove the leading option part of the
argument (e.g., @samp{--file=}).

@item --source@r{,} --source=@r{,} -Wsource=
The source text is echoed into @file{/tmp/ig.s.$$}.

@item --version@r{,} -Wversion
@command{igawk} prints its version number, runs @samp{gawk --version}
to get the @command{gawk} version information, and then exits.
@end table

If none of the @option{-f}, @option{--file}, @option{-Wfile}, @option{--source},
or @option{-Wsource} arguments are supplied, then the first non-option argument
should be the @command{awk} program.  If there are no command-line
arguments left, @command{igawk} prints an error message and exits.
Otherwise, the first argument is echoed into @file{/tmp/ig.s.$$}.
In any case, after the arguments have been processed,
@file{/tmp/ig.s.$$} contains the complete text of the original @command{awk}
program.

@cindex @command{sed} utility
@cindex stream editor
The @samp{$$} in @command{sh} represents the current process ID number.
It is often used in shell programs to generate unique temporary @value{FN}s.
This allows multiple users to run @command{igawk} without worrying
that the temporary @value{FN}s will clash.
The program is as follows:

@cindex @code{igawk.sh} program
@example
@c file eg/prog/igawk.sh
#! /bin/sh
# igawk --- like gawk but do @@include processing
@c endfile
@ignore
@c file eg/prog/igawk.sh
#
# Arnold Robbins, arnold@@gnu.org, Public Domain
# July 1993

@c endfile
@end ignore
@c file eg/prog/igawk.sh
if [ "$1" = debug ]
then
    set -x
    shift
else
    # cleanup on exit, hangup, interrupt, quit, termination
    trap 'rm -f /tmp/ig.[se].$$' 0 1 2 3 15
fi

while [ $# -ne 0 ] # loop over arguments
do
    case $1 in
    --)     shift; break;;

    -W)     shift
            set -- -W"$@@"
            continue;;

    -[vF])  opts="$opts $1 '$2'"
            shift;;

    -[vF]*) opts="$opts '$1'" ;;

    -f)     echo @@include "$2" >> /tmp/ig.s.$$
            shift;;

    -f*)    f=`echo "$1" | sed 's/-f//'`
            echo @@include "$f" >> /tmp/ig.s.$$ ;;

    -?file=*)    # -Wfile or --file
            f=`echo "$1" | sed 's/-.file=//'`
            echo @@include "$f" >> /tmp/ig.s.$$ ;;

    -?file)      # get arg, $2
            echo @@include "$2" >> /tmp/ig.s.$$
            shift;;

    -?source=*)  # -Wsource or --source
            t=`echo "$1" | sed 's/-.source=//'`
            echo "$t" >> /tmp/ig.s.$$ ;;

    -?source)    # get arg, $2
            echo "$2" >> /tmp/ig.s.$$
            shift;;

    -?version)
            echo igawk: version 1.0 1>&2
            gawk --version
            exit 0 ;;

    -[W-]*) opts="$opts '$1'" ;;

    *)      break;;
    esac
    shift
done

if [ ! -s /tmp/ig.s.$$ ]
then
@group
    if [ -z "$1" ]
    then
         echo igawk: no program! 1>&2
         exit 1
@end group
    else
        echo "$1" > /tmp/ig.s.$$
        shift
    fi
fi

# at this point, /tmp/ig.s.$$ has the program
@c endfile
@end example

The @command{awk} program to process @samp{@@include} directives
reads through the program, one line at a time, using @code{getline}
(@pxref{Getline, ,Explicit Input with @code{getline}}).  The input
@value{FN}s and @samp{@@include} statements are managed using a stack.
As each @samp{@@include} is encountered, the current @value{FN} is
``pushed'' onto the stack and the file named in the @samp{@@include}
directive becomes the current @value{FN}.  As each file is finished,
the stack is ``popped,'' and the previous input file becomes the current
input file again.  The process is started by making the original file
the first one on the stack.

The @code{pathto} function does the work of finding the full path to
a file.  It simulates @command{gawk}'s behavior when searching the
@env{AWKPATH} environment variable
(@pxref{AWKPATH Variable, ,The @env{AWKPATH} Environment Variable}).
If a @value{FN} has a @samp{/} in it, no path search is done. Otherwise,
the @value{FN} is concatenated with the name of each directory in
the path, and an attempt is made to open the generated @value{FN}.
The only way to test if a file can be read in @command{awk} is to go
ahead and try to read it with @code{getline}; this is what @code{pathto}
does.@footnote{On some very old versions of @command{awk}, the test
@samp{getline junk < t} can loop forever if the file exists but is empty.
Caveat emptor.} If the file can be read, it is closed and the @value{FN}
is returned:

@ignore
An alternative way to test for the file's existence would be to call
@samp{system("test -r " t)}, which uses the @command{test} utility to
see if the file exists and is readable.  The disadvantage to this method
is that it requires creating an extra process and can thus be slightly
slower.
@end ignore

@example
@c file eg/prog/igawk.sh
gawk -- '
# process @@include directives

function pathto(file,    i, t, junk)
@{
    if (index(file, "/") != 0)
        return file

    for (i = 1; i <= ndirs; i++) @{
        t = (pathlist[i] "/" file)
@group
        if ((getline junk < t) > 0) @{
            # found it
            close(t)
            return t
        @}
@end group
    @}
    return ""
@}
@c endfile
@end example

The main program is contained inside one @code{BEGIN} rule.  The first thing it
does is set up the @code{pathlist} array that @code{pathto} uses.  After
splitting the path on @samp{:}, null elements are replaced with @code{"."},
which represents the current directory:

@example
@c file eg/prog/igawk.sh
BEGIN @{
    path = ENVIRON["AWKPATH"]
    ndirs = split(path, pathlist, ":")
    for (i = 1; i <= ndirs; i++) @{
        if (pathlist[i] == "")
            pathlist[i] = "."
    @}
@c endfile
@end example

The stack is initialized with @code{ARGV[1]}, which will be @file{/tmp/ig.s.$$}.
The main loop comes next.  Input lines are read in succession. Lines that
do not start with @samp{@@include} are printed verbatim.
If the line does start with @samp{@@include}, the @value{FN} is in @code{$2}.
@code{pathto} is called to generate the full path.  If it cannot, then we
print an error message and continue.

The next thing to check is if the file is included already.  The
@code{processed} array is indexed by the full @value{FN} of each included
file and it tracks this information for us.  If the file is
seen again, a warning message is printed. Otherwise, the new @value{FN} is
pushed onto the stack and processing continues.

Finally, when @code{getline} encounters the end of the input file, the file
is closed and the stack is popped.  When @code{stackptr} is less than zero,
the program is done:

@example
@c file eg/prog/igawk.sh
    stackptr = 0
    input[stackptr] = ARGV[1] # ARGV[1] is first file

    for (; stackptr >= 0; stackptr--) @{
        while ((getline < input[stackptr]) > 0) @{
            if (tolower($1) != "@@include") @{
                print
                continue
            @}
            fpath = pathto($2)
@group
            if (fpath == "") @{
                printf("igawk:%s:%d: cannot find %s\n",
                    input[stackptr], FNR, $2) > "/dev/stderr"
                continue
            @}
@end group
            if (! (fpath in processed)) @{
                processed[fpath] = input[stackptr]
                input[++stackptr] = fpath  # push onto stack
            @} else
                print $2, "included in", input[stackptr],
                    "already included in",
                    processed[fpath] > "/dev/stderr"
        @}
        close(input[stackptr])
    @}
@}' /tmp/ig.s.$$ > /tmp/ig.e.$$
@c endfile
@end example

The last step is to call @command{gawk} with the expanded program,
along with the original
options and command-line arguments that the user supplied.  @command{gawk}'s
exit status is passed back on to @command{igawk}'s calling program:

@c this causes more problems than it solves, so leave it out.
@ignore
The special file @file{/dev/null} is passed as a @value{DF} to @command{gawk}
to handle an interesting case. Suppose that the user's program only has
a @code{BEGIN} rule and there are no @value{DF}s to read.
The program should exit without reading any @value{DF}s.
However, suppose that an included library file defines an @code{END}
rule of its own. In this case, @command{gawk} will hang, reading standard
input. In order to avoid this, @file{/dev/null} is explicitly added to the
command-line. Reading from @file{/dev/null} always returns an immediate
end of file indication.

@c Hmm. Add /dev/null if $# is 0?  Still messes up ARGV. Sigh.
@end ignore

@example
@c file eg/prog/igawk.sh
eval gawk -f /tmp/ig.e.$$ $opts -- "$@@"

exit $?
@c endfile
@end example

This version of @command{igawk} represents my third attempt at this program.
There are three key simplifications that make the program work better:

@itemize @bullet
@item
Using @samp{@@include} even for the files named with @option{-f} makes building
the initial collected @command{awk} program much simpler; all the
@samp{@@include} processing can be done once.

@item
The @code{pathto} function doesn't try to save the line read with
@code{getline} when testing for the file's accessibility.  Trying to save
this line for use with the main program complicates things considerably.
@c what problem does this engender though - exercise
@c answer, reading from "-" or /dev/stdin

@item
Using a @code{getline} loop in the @code{BEGIN} rule does it all in one
place.  It is not necessary to call out to a separate loop for processing
nested @samp{@@include} statements.
@end itemize

Also, this program illustrates that it is often worthwhile to combine
@command{sh} and @command{awk} programming together.  You can usually
accomplish quite a lot, without having to resort to low-level programming
in C or C++, and it is frequently easier to do certain kinds of string
and argument manipulation using the shell than it is in @command{awk}.

Finally, @command{igawk} shows that it is not always necessary to add new
features to a program; they can often be layered on top.  With @command{igawk},
there is no real reason to build @samp{@@include} processing into
@command{gawk} itself.

@cindex search path
@cindex directory search
@cindex path, search
@cindex search path, for source files
As an additional example of this, consider the idea of having two
files in a directory in the search path:

@table @file
@item default.awk
This file contains a set of default library functions, such
as @code{getopt} and @code{assert}.

@item site.awk
This file contains library functions that are specific to a site or
installation; i.e., locally developed functions.
Having a separate file allows @file{default.awk} to change with
new @command{gawk} releases, without requiring the system administrator to
update it each time by adding the local functions.
@end table

One user
@c Karl Berry, karl@ileaf.com, 10/95
suggested that @command{gawk} be modified to automatically read these files
upon startup.  Instead, it would be very simple to modify @command{igawk}
to do this. Since @command{igawk} can process nested @samp{@@include}
directives, @file{default.awk} could simply contain @samp{@@include}
statements for the desired library functions.

@c Exercise: make this change

@ignore
@c Try this
@iftex
@page
@headings off
@majorheading III@ @ @ Appendixes
Part III provides the appendixes, the Glossary, and two licenses that cover
the @command{gawk} source code and this @value{DOCUMENT}, respectively.
It contains the following appendixes:

@itemize @bullet
@item
@ref{Language History, ,The Evolution of the @command{awk} Language}.

@item
@ref{Installation, ,Installing @command{gawk}}.

@item
@ref{Notes, ,Implementation Notes}.

@item
@ref{Basic Concepts, ,Basic Programming Concepts}.

@item
@ref{Glossary}.

@item
@ref{Copying, ,GNU General Public License}.

@item
@ref{GNU Free Documentation License}.
@end itemize

@page
@evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @|
@oddheading  @| @| @strong{@thischapter}@ @ @ @thispage
@end iftex
@end ignore

@node Language History, Installation, Sample Programs, Top
@appendix The Evolution of the @command{awk} Language

This @value{DOCUMENT} describes the GNU implementation of @command{awk}, which follows
the POSIX specification.
Many long-time @command{awk} users learned @command{awk} programming
with the original @command{awk} implementation in Version 7 Unix.
(This implementation was the basis for @command{awk} in Berkeley Unix,
through 4.3--Reno.  Subsequent versions of Berkeley Unix, and systems
derived from 4.4BSD--Lite, use various versions of @command{gawk}
for their @command{awk}.)
This @value{CHAPTER} briefly describes the
evolution of the @command{awk} language, with cross references to other parts
of the @value{DOCUMENT} where you can find more information.

@menu
* V7/SVR3.1::                   The major changes between V7 and System V
                                Release 3.1.
* SVR4::                        Minor changes between System V Releases 3.1
                                and 4.
* POSIX::                       New features from the POSIX standard.
* BTL::                         New features from the Bell Laboratories
                                version of @command{awk}.
* POSIX/GNU::                   The extensions in @command{gawk} not in POSIX
                                @command{awk}.
* Contributors::                The major contributors to @command{gawk}.
@end menu

@node V7/SVR3.1, SVR4, Language History, Language History
@appendixsec Major Changes Between V7 and SVR3.1

The @command{awk} language evolved considerably between the release of
Version 7 Unix (1978) and the new version that was first made generally available in
System V Release 3.1 (1987).  This @value{SECTION} summarizes the changes, with
cross-references to further details:

@itemize @bullet
@item
The requirement for @samp{;} to separate rules on a line
(@pxref{Statements/Lines, ,@command{awk} Statements Versus Lines}).

@item
User-defined functions and the @code{return} statement
(@pxref{User-defined, ,User-Defined Functions}).

@item
The @code{delete} statement (@pxref{Delete, ,The @code{delete} Statement}).

@item
The @code{do}-@code{while} statement
(@pxref{Do Statement, ,The @code{do}-@code{while} Statement}).

@item
The built-in functions @code{atan2}, @code{cos}, @code{sin}, @code{rand}, and
@code{srand} (@pxref{Numeric Functions}).

@item
The built-in functions @code{gsub}, @code{sub}, and @code{match}
(@pxref{String Functions, ,String Manipulation Functions}).

@item
The built-in functions @code{close} and @code{system}
(@pxref{I/O Functions, ,Input/Output Functions}).

@item
The @code{ARGC}, @code{ARGV}, @code{FNR}, @code{RLENGTH}, @code{RSTART},
and @code{SUBSEP} built-in variables (@pxref{Built-in Variables}).

@item
The conditional expression using the ternary operator @samp{?:}
(@pxref{Conditional Exp, ,Conditional Expressions}).

@item
The exponentiation operator @samp{^}
(@pxref{Arithmetic Ops, ,Arithmetic Operators}) and its assignment operator
form @samp{^=} (@pxref{Assignment Ops, ,Assignment Expressions}).

@item
C-compatible operator precedence, which breaks some old @command{awk}
programs (@pxref{Precedence, ,Operator Precedence (How Operators Nest)}).

@item
Regexps as the value of @code{FS}
(@pxref{Field Separators, ,Specifying How Fields Are Separated}) and as the
third argument to the @code{split} function
(@pxref{String Functions, ,String Manipulation Functions}).

@item
Dynamic regexps as operands of the @samp{~} and @samp{!~} operators
(@pxref{Regexp Usage, ,How to Use Regular Expressions}).

@item
The escape sequences @samp{\b}, @samp{\f}, and @samp{\r}
(@pxref{Escape Sequences}).
(Some vendors have updated their old versions of @command{awk} to
recognize @samp{\b}, @samp{\f}, and @samp{\r}, but this is not
something you can rely on.)

@item
Redirection of input for the @code{getline} function
(@pxref{Getline, ,Explicit Input with @code{getline}}).

@item
Multiple @code{BEGIN} and @code{END} rules
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).

@item
Multidimensional arrays
(@pxref{Multi-dimensional, ,Multidimensional Arrays}).
@end itemize

@node SVR4, POSIX, V7/SVR3.1, Language History
@appendixsec Changes Between SVR3.1 and SVR4

@cindex @command{awk} language, V.4 version
The System V Release 4 (1989) version of Unix @command{awk} added these features
(some of which originated in @command{gawk}):

@itemize @bullet
@item
The @code{ENVIRON} variable (@pxref{Built-in Variables}).
@c gawk and MKS awk

@item
Multiple @option{-f} options on the command line
(@pxref{Options, ,Command-Line Options}).
@c MKS awk

@item
The @option{-v} option for assigning variables before program execution begins
(@pxref{Options, ,Command-Line Options}).
@c GNU, Bell Laboratories & MKS together

@item
The @option{--} option for terminating command-line options.

@item
The @samp{\a}, @samp{\v}, and @samp{\x} escape sequences
(@pxref{Escape Sequences}).
@c GNU, for ANSI C compat

@item
A defined return value for the @code{srand} built-in function
(@pxref{Numeric Functions}).

@item
The @code{toupper} and @code{tolower} built-in string functions
for case translation
(@pxref{String Functions, ,String Manipulation Functions}).

@item
A cleaner specification for the @samp{%c} format-control letter in the
@code{printf} function
(@pxref{Control Letters, ,Format-Control Letters}).

@item
The ability to dynamically pass the field width and precision (@code{"%*.*d"})
in the argument list of the @code{printf} function
(@pxref{Control Letters, ,Format-Control Letters}).

@item
The use of regexp constants, such as @code{/foo/}, as expressions, where
they are equivalent to using the matching operator, as in @samp{$0 ~ /foo/}
(@pxref{Using Constant Regexps, ,Using Regular Expression Constants}).

@item
Processing of escape sequences inside command-line variable assignments
(@pxref{Assignment Options, ,Assigning Variables on the Command Line}).
@end itemize

@node POSIX, BTL, SVR4, Language History
@appendixsec Changes Between SVR4 and POSIX @command{awk}

The POSIX Command Language and Utilities standard for @command{awk} (1992)
introduced the following changes into the language:

@itemize @bullet
@item
The use of @option{-W} for implementation-specific options
(@pxref{Options, ,Command-Line Options}).

@item
The use of @code{CONVFMT} for controlling the conversion of numbers
to strings (@pxref{Conversion, ,Conversion of Strings and Numbers}).

@item
The concept of a numeric string and tighter comparison rules to go
with it (@pxref{Typing and Comparison, ,Variable Typing and Comparison Expressions}).

@item
More complete documentation of many of the previously undocumented
features of the language.
@end itemize

The following common extensions are not permitted by the POSIX
standard:

@c IMPORTANT! Keep this list in sync with the one in node Options

@itemize @bullet
@item
@code{\x} escape sequences are not recognized
(@pxref{Escape Sequences}).

@item
Newlines do not act as whitespace to separate fields when @code{FS} is
equal to a single space
(@pxref{Fields, ,Examining Fields}).

@item
Newlines are not allowed after @samp{?} or @samp{:}
(@pxref{Conditional Exp, ,Conditional Expressions}).

@item
The synonym @code{func} for the keyword @code{function} is not
recognized (@pxref{Definition Syntax, ,Function Definition Syntax}).

@item
The operators @samp{**} and @samp{**=} cannot be used in
place of @samp{^} and @samp{^=} (@pxref{Arithmetic Ops, ,Arithmetic Operators},
and @ref{Assignment Ops, ,Assignment Expressions}).

@item
Specifying @samp{-Ft} on the command line does not set the value
of @code{FS} to be a single tab character
(@pxref{Field Separators, ,Specifying How Fields Are Separated}).

@item
The @code{fflush} built-in function is not supported
(@pxref{I/O Functions, ,Input/Output Functions}).
@end itemize

@node BTL, POSIX/GNU, POSIX, Language History
@appendixsec Extensions in the Bell Laboratories @command{awk}

@cindex extensions, Bell Laboratories @command{awk}
@cindex Kernighan, Brian
Brian Kernighan, one of the original designers of Unix @command{awk},
has made his version available via his home page
(@pxref{Other Versions, ,Other Freely Available @command{awk} Implementations}).
This @value{SECTION} describes extensions in his version of @command{awk} that are
not in POSIX @command{awk}.

@itemize @bullet
@item
The @samp{-mf @var{N}} and @samp{-mr @var{N}} command-line options
to set the maximum number of fields and the maximum
record size, respectively
(@pxref{Options, ,Command-Line Options}).
As a side note, his @command{awk} no longer needs these options;
it continues to accept them to avoid breaking old programs.

@item
The @code{fflush} built-in function for flushing buffered output
(@pxref{I/O Functions, ,Input/Output Functions}).

@item
The @samp{**} and @samp{**=} operators
(@pxref{Arithmetic Ops, ,Arithmetic Operators}
and
@ref{Assignment Ops, ,Assignment Expressions}).

@item
The use of @code{func} as an abbreviation for @code{function}
(@pxref{Definition Syntax, ,Function Definition Syntax}).

@ignore
@item
The @code{SYMTAB} array, that allows access to @command{awk}'s internal symbol
table. This feature is not documented, largely because
it is somewhat shakily implemented. For instance, you cannot access arrays
or array elements through it.
@end ignore
@end itemize

The Bell Laboratories @command{awk} also incorporates the following extensions,
originally developed for @command{gawk}:

@itemize @bullet
@item
The @samp{\x} escape sequence
(@pxref{Escape Sequences}).

@item
The @file{/dev/stdin}, @file{/dev/stdout}, and @file{/dev/stderr}
special files
(@pxref{Special Files, ,Special @value{FFN}s in @command{gawk}}).

@item
The ability for @code{FS} and for the third
argument to @code{split} to be null strings
(@pxref{Single Character Fields, , Making Each Character a Separate Field}).

@item
The @code{nextfile} statement
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).

@item
The ability to delete all of an array at once with @samp{delete @var{array}}
(@pxref{Delete, ,The @code{delete} Statement}).
@end itemize

@node POSIX/GNU, Contributors, BTL, Language History
@appendixsec Extensions in @command{gawk} Not in POSIX @command{awk}

@ignore
I've tried to follow this general order, esp. for the 3.0 and 3.1 sections:
       variables
       special files
       language changes (e.g., hex constants)
       differences in standard awk functions
       new gawk functions
       new keywords
       new command-line options
       new ports
Within each category, be alphabetical.
@end ignore

@cindex compatibility mode
The GNU implementation, @command{gawk}, adds a large number of features.
This @value{SECTION} lists them in the order they were added to @command{gawk}.
They can all be disabled with either the @option{--traditional} or
@option{--posix} options
(@pxref{Options, ,Command-Line Options}).

Version 2.10 of @command{gawk} introduced the following features:

@itemize @bullet
@item
The @env{AWKPATH} environment variable for specifying a path search for
the @option{-f} command-line option
(@pxref{Options, ,Command-Line Options}).

@item
The @code{IGNORECASE} variable and its effects
(@pxref{Case-sensitivity, ,Case Sensitivity in Matching}).

@item
The @file{/dev/stdin}, @file{/dev/stdout}, @file{/dev/stderr} and
@file{/dev/fd/@var{N}} special @value{FN}s
(@pxref{Special Files, ,Special @value{FFN}s in @command{gawk}}).
@end itemize

Version 2.13 of @command{gawk} introduced the following features:

@itemize @bullet
@item
The @code{FIELDWIDTHS} variable and its effects
(@pxref{Constant Size, ,Reading Fixed-Width Data}).

@item
The @code{systime} and @code{strftime} built-in functions for obtaining
and printing timestamps
(@pxref{Time Functions, ,Using @command{gawk}'s Timestamp Functions}).

@item
The @option{-W lint} option to provide error and portability checking
for both the source code and at runtime
(@pxref{Options, ,Command-Line Options}).

@item
The @option{-W compat} option to turn off the GNU extensions
(@pxref{Options, ,Command-Line Options}).

@item
The @option{-W posix} option for full POSIX compliance
(@pxref{Options, ,Command-Line Options}).
@end itemize

Version 2.14 of @command{gawk} introduced the following feature:

@itemize @bullet
@item
The @code{next file} statement for skipping to the next @value{DF}
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).
@end itemize

Version 2.15 of @command{gawk} introduced the following features:

@itemize @bullet
@item
The @code{ARGIND} variable, which tracks the movement of @code{FILENAME}
through @code{ARGV}  (@pxref{Built-in Variables}).

@item
The @code{ERRNO} variable, which contains the system error message when
@code{getline} returns @minus{}1 or when @code{close} fails
(@pxref{Built-in Variables}).

@item
The @file{/dev/pid}, @file{/dev/ppid}, @file{/dev/pgrpid}, and
@file{/dev/user} @value{FN} interpretation
(@pxref{Special Files, ,Special @value{FFN}s in @command{gawk}}).

@item
The ability to delete all of an array at once with @samp{delete @var{array}}
(@pxref{Delete, ,The @code{delete} Statement}).

@item
The ability to use GNU-style long-named options that start with @option{--}
(@pxref{Options, ,Command-Line Options}).

@item
The @option{--source} option for mixing command-line and library
file source code
(@pxref{Options, ,Command-Line Options}).
@end itemize

Version 3.0 of @command{gawk} introduced the following features:

@itemize @bullet
@item
@code{IGNORECASE} changed, now applying to string comparison as well
as regexp operations
(@pxref{Case-sensitivity, ,Case Sensitivity in Matching}).

@item
The @code{RT} variable that contains the input text that
matched @code{RS}
(@pxref{Records, ,How Input Is Split into Records}).

@item
Full support for both POSIX and GNU regexps
(@pxref{Regexp, , Regular Expressions}).

@item
The @code{gensub} function for more powerful text manipulation
(@pxref{String Functions, ,String Manipulation Functions}).

@item
The @code{strftime} function acquired a default time format,
allowing it to be called with no arguments
(@pxref{Time Functions, ,Using @command{gawk}'s Timestamp Functions}).

@item
The ability for @code{FS} and for the third
argument to @code{split} to be null strings
(@pxref{Single Character Fields, , Making Each Character a Separate Field}).

@item
The ability for @code{RS} to be a regexp
(@pxref{Records, ,How Input Is Split into Records}).

@item
The @code{next file} statement became @code{nextfile}
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).

@item
The @option{--lint-old} option to
warn about constructs that are not available in
the original Version 7 Unix version of @command{awk}
(@pxref{V7/SVR3.1, ,Major Changes Between V7 and SVR3.1}).

@item
The @option{-m} option and the @code{fflush} function from the
Bell Laboratories research version of @command{awk}
(@pxref{Options, ,Command-Line Options}; also
@pxref{I/O Functions, ,Input/Output Functions}).

@item
The @option{--re-interval} option to provide interval expressions in regexps
(@pxref{Regexp Operators, , Regular Expression Operators}).

@item
The @option{--traditional} option was added as a better name for
@option{--compat} (@pxref{Options, ,Command-Line Options}).

@item
The use of GNU Autoconf to control the configuration process
(@pxref{Quick Installation, , Compiling @command{gawk} for Unix}).

@item
Amiga support
(@pxref{Amiga Installation, ,Installing @command{gawk} on an Amiga}).

@end itemize

Version 3.1 of @command{gawk} introduced the following features:

@itemize @bullet
@item
The @code{BINMODE} special variable for non-POSIX systems,
which allows binary I/O for input and/or output files
(@pxref{PC Using, ,Using @command{gawk} on PC Operating Systems}).

@item
The @code{LINT} special variable, which dynamically controls lint warnings
(@pxref{Built-in Variables}).

@item
The @code{PROCINFO} array for providing process-related information
(@pxref{Built-in Variables}).

@item
The @code{TEXTDOMAIN} special variable for setting an application's
internationalization text domain
(@pxref{Built-in Variables},
and
@ref{Internationalization, ,Internationalization with @command{gawk}}).

@item
The ability to use octal and hexadecimal constants in @command{awk}
program source code
(@pxref{Non-decimal-numbers, ,Octal and Hexadecimal Numbers}).

@item
The @samp{|&} operator for two-way I/O to a coprocess
(@pxref{Two-way I/O, ,Two-Way Communications with Another Process}).

@item
The @file{/inet} special files for TCP/IP networking using @samp{|&}
(@pxref{TCP/IP Networking, , Using @command{gawk} for Network Programming}).

@item
The optional second argument to @code{close} that allows closing one end
of a two-way pipe to a coprocess
(@pxref{Two-way I/O, ,Two-Way Communications with Another Process}).

@item
The optional third argument to the @code{match} function
for capturing text-matching subexpressions within a regexp
(@pxref{String Functions, , String Manipulation Functions}).

@item
Positional specifiers in @code{printf} formats for
making translations easier
(@pxref{Printf Ordering, , Rearranging @code{printf} Arguments}).

@item
The @code{asort} function for sorting arrays
(@pxref{Array Sorting, ,Sorting Array Values and Indices with @command{gawk}}).

@item
The @code{bindtextdomain} and @code{dcgettext} functions
for internationalization
(@pxref{Programmer i18n, ,Internationalizing @command{awk} Programs}).

@item
The @code{extension} built-in function and the ability to add
new built-in functions dynamically
(@pxref{Dynamic Extensions, , Adding New Built-in Functions to @command{gawk}}).

@item
The @code{mktime} built-in function for creating timestamps
(@pxref{Time Functions, ,Using @command{gawk}'s Timestamp Functions}).

@item
The
@code{and},
@code{or},
@code{xor},
@code{compl},
@code{lshift},
@code{rshift},
and
@code{strtonum} built-in
functions
(@pxref{Bitwise Functions, ,Using @command{gawk}'s Bit Manipulation Functions}).

@item
@cindex @code{next file} statement
The support for @samp{next file} as two words was removed completely
(@pxref{Nextfile Statement, ,Using @command{gawk}'s @code{nextfile} Statement}).

@item
The @option{--dump-variables} option to print a list of all global variables
(@pxref{Options, ,Command-Line Options}).

@item
The @option{--gen-po} command-line option and the use of a leading
underscore to mark strings that should be translated
(@pxref{String Extraction, ,Extracting Marked Strings}).

@item
The @option{--non-decimal-data} option to allow non-decimal
input data
(@pxref{Non-decimal Data, ,Allowing Non-Decimal Input Data}).

@item
The @option{--profile} option and @command{pgawk}, the
profiling version of @command{gawk}, for producing execution
profiles of @command{awk} programs
(@pxref{Profiling, ,Profiling Your @command{awk} Programs}).

@item
The @option{--enable-portals} configuration option to enable special treatment of
pathnames that begin with @file{/p} as BSD portals
(@pxref{Portal Files, , Using @command{gawk} with BSD Portals}).

@item
The use of GNU Automake to help in standardizing the configuration process
(@pxref{Quick Installation, , Compiling @command{gawk} for Unix}).

@item
The use of GNU @code{gettext} for @command{gawk}'s own message output
(@pxref{Gawk I18N, ,@command{gawk} Can Speak Your Language}).

@item
BeOS support
(@pxref{BeOS Installation, , Installing @command{gawk} on BeOS}).

@item
Tandem support
(@pxref{Tandem Installation, ,Installing @command{gawk} on a Tandem}).

@item
The Atari port became officially unsupported
(@pxref{Atari Installation, ,Installing @command{gawk} on the Atari ST}).

@item
The source code now uses new-style function definitions, with
@command{ansi2knr} to convert the code on systems with old compilers.

@end itemize

@c XXX ADD MORE STUFF HERE

@node Contributors, , POSIX/GNU, Language History
@appendixsec Major Contributors to @command{gawk}
@cindex contributors to @command{gawk}
@quotation
@i{Always give credit where credit is due.}@*
Anonymous
@end quotation

This @value{SECTION} names the major contributors to @command{gawk}
and/or this @value{DOCUMENT}, in approximate chronological order:

@itemize @bullet
@item
@cindex Aho, Alfred
@cindex Weinberger, Peter
@cindex Kernighan, Brian
Dr.@: Alfred V.@: Aho,
Dr.@: Peter J.@: Weinberger, and
Dr.@: Brian W.@: Kernighan, all of Bell Laboratories,
designed and implemented Unix @command{awk},
from which @command{gawk} gets the majority of its feature set.

@item
@cindex Rubin, Paul
Paul Rubin
did the initial design and implementation in 1986, and wrote
the first draft (around 40 pages) of this @value{DOCUMENT}.

@item
@cindex Fenlason, Jay
Jay Fenlason
finished the initial implementation.

@item
@cindex Close, Diane
Diane Close
revised the first draft of this @value{DOCUMENT}, bringing it
to around 90 pages.

@item
@cindex Stallman, Richard
Richard Stallman
helped finish the implementation and the initial draft of this
@value{DOCUMENT}.
He is also the founder of the FSF and the GNU project.

@item
@cindex Woods, John
John Woods
contributed parts of the code (mostly fixes) in
the initial version of @command{gawk}.

@item
@cindex Trueman, David
In 1988,
David Trueman
took over primary maintenance of @command{gawk},
making it compatible with ``new'' @command{awk}, and
greatly improving its performance.

@item
@cindex Rankin, Pat
Pat Rankin
provided the VMS port and its documentation.

@item
@cindex Kwok, Conrad
@cindex Garfinkle, Scott
@cindex Williams, Kent
Conrad Kwok,
Scott Garfinkle,
and
Kent Williams
did the initial ports to MS-DOS with various versions of MSC.

@item
@cindex Peterson, Hal
Hal Peterson
provided help in porting @command{gawk} to Cray systems.

@item
@cindex Rommel, Kai Uwe
Kai Uwe Rommel
provided the port to OS/2 and its documentation.

@item
@cindex Jaegermann, Michal
Michal Jaegermann
provided the port to Atari systems and its documentation.
He continues to provide portability checking with DEC Alpha
systems, and has done a lot of work to make sure @command{gawk}
works on non-32-bit systems.

@item
@cindex Fish, Fred
Fred Fish
provided the port to Amiga systems and its documentation.

@item
@cindex Deifik, Scott
Scott Deifik
currently maintains the MS-DOS port.

@item
@cindex Grigera, Juan
Juan Grigera
maintains the port to Win32 systems.

@item
@cindex Hankerson, Darrel
Dr.@: Darrel Hankerson
acts as coordinator for the various ports to different PC platforms
and creates binary distributions for various PC operating systems.
He is also instrumental in keeping the documentation up to date for
the various PC platforms.

@item
@cindex Zoulas, Christos
Christos Zoulas
provided the @code{extension}
built-in function for dynamically adding new modules.

@item
@cindex Kahrs, J@"urgen
J@"urgen Kahrs
contributed the initial version of the TCP/IP networking
code and documentation, and motivated the inclusion of the @samp{|&} operator.

@item
@cindex Davies, Stephen
Stephen Davies
provided the port to Tandem systems and its documentation.

@item
@cindex Brown, Martin
Martin Brown
provided the port to BeOS and its documentation.

@item
@cindex Peters, Arno
Arno Peters
did the initial work to convert @command{gawk} to use
GNU Automake and @code{gettext}.

@item
@cindex Broder, Alan J.@:
Alan J.@: Broder
provided the initial version of the @code{asort} function
as well as the code for the new optional third argument to the @code{match} function.

@item
@cindex Robbins, Arnold
Arnold Robbins
has been working on @command{gawk} since 1988, at first
helping David Trueman, and as the primary maintainer since around 1994.
@end itemize

@node Installation, Notes, Language History, Top
@appendix Installing @command{gawk}

@cindex Linux
@cindex GNU/Linux
This appendix provides instructions for installing @command{gawk} on the
various platforms that are supported by the developers.  The primary
developer supports GNU/Linux (and Unix), whereas the other ports are
contributed.
@xref{Bugs, , Reporting Problems and Bugs},
for the electronic mail addresses of the people who did
the respective ports.

@menu
* Gawk Distribution::           What is in the @command{gawk} distribution.
* Unix Installation::           Installing @command{gawk} under various
                                versions of Unix.
* Non-Unix Installation::       Installation on Other Operating Systems.
* Unsupported::                 Systems whose ports are no longer supported.
* Bugs::                        Reporting Problems and Bugs.
* Other Versions::              Other freely available @command{awk}
                                implementations.
@end menu

@node Gawk Distribution, Unix Installation, Installation, Installation
@appendixsec The @command{gawk} Distribution

This @value{SECTION} describes how to get the @command{gawk}
distribution, how to extract it, and then what is in the various files and
subdirectories.

@menu
* Getting::                     How to get the distribution.
* Extracting::                  How to extract the distribution.
* Distribution contents::       What is in the distribution.
@end menu

@node Getting, Extracting, Gawk Distribution, Gawk Distribution
@appendixsubsec Getting the @command{gawk} Distribution
@cindex getting @command{gawk}
@cindex anonymous @command{ftp}
@cindex @command{ftp}, anonymous
@cindex source code, @command{gawk}
@cindex @command{gawk}, source code
There are three ways to get GNU software:

@itemize @bullet
@item
Copy it from someone else who already has it.

@cindex FSF
@cindex Free Software Foundation
@item
Order @command{gawk} directly from the Free Software Foundation.
Software distributions are available for Unix, MS-DOS, and VMS, on
tape and CD-ROM.  Their address is:

@display
Free Software Foundation
59 Temple Place, Suite 330
Boston, MA  02111-1307 USA
Phone: +1-617-542-5942
Fax (including Japan): +1-617-542-2652
Email: @email{gnu@@gnu.org}
URL: @uref{http://www.gnu.org/}
@end display

@noindent
Ordering from the FSF directly contributes to the support of the foundation
and to the production of more free software.

@item
Retrieve @command{gawk} by using anonymous @command{ftp} to the Internet host
@code{gnudist.gnu.org}, in the directory @file{/gnu/gawk}.
@end itemize

The GNU software archive is mirrored around the world.
The up-to-date list of mirror sites is available from
@uref{http://www.gnu.org/order/ftp.html, the main FSF web site}.
Try to use one of the mirrors; they
will be less busy, and you can usually find one closer to your site.

@node Extracting, Distribution contents, Getting, Gawk Distribution
@appendixsubsec Extracting the Distribution
@command{gawk} is distributed as a @code{tar} file compressed with the
GNU Zip program, @code{gzip}.

Once you have the distribution (for example,
@file{gawk-@value{VERSION}.@value{PATCHLEVEL}.tar.gz}),
use @code{gzip} to expand the
file and then use @code{tar} to extract it.  You can use the following
pipeline to produce the @command{gawk} distribution:

@example
# Under System V, add 'o' to the tar options
gzip -d -c gawk-@value{VERSION}.@value{PATCHLEVEL}.tar.gz | tar -xvpf -
@end example

@noindent
This creates a directory named @file{gawk-@value{VERSION}.@value{PATCHLEVEL}}
in the current directory.

The distribution @value{FN} is of the form
@file{gawk-@var{V}.@var{R}.@var{P}.tar.gz}.
The @var{V} represents the major version of @command{gawk},
the @var{R} represents the current release of version @var{V}, and
the @var{P} represents a @dfn{patch level}, meaning that minor bugs have
been fixed in the release.  The current patch level is @value{PATCHLEVEL},
but when retrieving distributions, you should get the version with the highest
version, release, and patch level.  (Note, however, that patch levels greater than
or equal to 80 denote ``beta'' or non-production software; you might not want
to retrieve such a version unless you don't mind experimenting.)
If you are not on a Unix system, you need to make other arrangements
for getting and extracting the @command{gawk} distribution.  You should consult
a local expert.

@node Distribution contents, , Extracting, Gawk Distribution
@appendixsubsec Contents of the @command{gawk} Distribution

The @command{gawk} distribution has a number of C source files,
documentation files,
subdirectories, and files related to the configuration process
(@pxref{Unix Installation, ,Compiling and Installing @command{gawk} on Unix}),
as well as several subdirectories related to different non-Unix
operating systems:

@table @asis
@item Various @samp{.c}, @samp{.y}, and @samp{.h} files:
These files are the actual @command{gawk} source code.
@end table

@table @file
@item README
@itemx README_d/README.*
Descriptive files: @file{README} for @command{gawk} under Unix and the
rest for the various hardware and software combinations.

@item INSTALL
A file providing an overview of the configuration and installation process.

@item ChangeLog
A detailed list of source code changes as bugs are fixed or improvements made.

@item NEWS
A list of changes to @command{gawk} since the last release or patch.

@item COPYING
The GNU General Public License.

@item FUTURES
A brief list of features and changes being contemplated for future
releases, with some indication of the time frame for the feature, based
on its difficulty.

@item LIMITATIONS
A list of those factors that limit @command{gawk}'s performance.
Most of these depend on the hardware or operating system software, and
are not limits in @command{gawk} itself.

@item POSIX.STD
A description of one area where the POSIX standard for @command{awk} is
incorrect as well as how @command{gawk} handles the problem.

@cindex artificial intelligence, using @command{gawk}
@cindex AI programming, using @command{gawk}
@item doc/awkforai.txt
A short article describing why @command{gawk} is a good language for
AI (Artificial Intelligence) programming.

@item doc/README.card
@itemx doc/ad.block
@itemx doc/awkcard.in
@itemx doc/cardfonts
@itemx doc/colors
@itemx doc/macros
@itemx doc/no.colors
@itemx doc/setter.outline
The @command{troff} source for a five-color @command{awk} reference card.
A modern version of @command{troff} such as GNU @command{troff} (@command{groff}) is
needed to produce the color version. See the file @file{README.card}
for instructions if you have an older @command{troff}.

@item doc/gawk.1
The @command{troff} source for a manual page describing @command{gawk}.
This is distributed for the convenience of Unix users.

@cindex Texinfo
@item doc/gawk.texi
The Texinfo source file for this @value{DOCUMENT}.
It should be processed with @TeX{} to produce a printed document, and
with @command{makeinfo} to produce an Info or HTML file.

@item doc/gawk.info
The generated Info file for this @value{DOCUMENT}.

@item doc/gawkinet.texi
The Texinfo source file for
@ifinfo
@xref{Top}.
@end ifinfo
@ifnotinfo
@cite{TCP/IP Internetworking with @command{gawk}}.
@end ifnotinfo
It should be processed with @TeX{} to produce a printed document and
with @command{makeinfo} to produce an Info or HTML file.

@item doc/gawkinet.info
The generated Info file for
@cite{TCP/IP Internetworking with @command{gawk}}.

@item doc/igawk.1
The @command{troff} source for a manual page describing the @command{igawk}
program presented in
@ref{Igawk Program, ,An Easy Way to Use Library Functions}.

@item doc/Makefile.in
The input file used during the configuration process to generate the
actual @file{Makefile} for creating the documentation.

@item Makefile.am
@itemx */Makefile.am
Files used by the GNU @command{automake} software for generating
the @file{Makefile.in} files used by @command{autoconf} and
@command{configure}.

@item Makefile.in
@itemx acconfig.h
@itemx acinclude.m4
@itemx aclocal.m4
@itemx configh.in
@itemx configure.in
@itemx configure
@itemx custom.h
@itemx missing_d/*
@itemx m4/*
These files and subdirectories are used when configuring @command{gawk}
for various Unix systems.  They are explained in
@ref{Unix Installation, ,Compiling and Installing @command{gawk} on Unix}.

@item intl/*
@itemx po/*
The @file{intl} directory provides the GNU @code{gettext} library, which implements
@command{gawk}'s internationalization features, while the @file{po} library
contains message translations.

@item awklib/extract.awk
@itemx awklib/Makefile.am
@itemx awklib/Makefile.in
@itemx awklib/eg/*
The @file{awklib} directory contains a copy of @file{extract.awk}
(@pxref{Extract Program, ,Extracting Programs from Texinfo Source Files}),
which can be used to extract the sample programs from the Texinfo
source file for this @value{DOCUMENT}. It also contains a @file{Makefile.in} file, which
@command{configure} uses to generate a @file{Makefile}.
@file{Makefile.am} is used by GNU Automake to create @file{Makefile.in}.
The library functions from
@ref{Library Functions, , A Library of @command{awk} Functions},
and the @command{igawk} program from
@ref{Igawk Program, , An Easy Way to Use Library Functions},
are included as ready-to-use files in the @command{gawk} distribution.
They are installed as part of the installation process.
The rest of the programs in this @value{DOCUMENT} are available in appropriate
subdirectories of @file{awklib/eg}.

@item unsupported/atari/*
Files needed for building @command{gawk} on an Atari ST
(@pxref{Atari Installation, ,Installing @command{gawk} on the Atari ST}, for details).

@item unsupported/tandem/*
Files needed for building @command{gawk} on a Tandem
(@pxref{Tandem Installation, ,Installing @command{gawk} on a Tandem}, for details).

@item posix/*
Files needed for building @command{gawk} on POSIX-compliant systems.

@item pc/*
Files needed for building @command{gawk} under MS-DOS, MS Windows and OS/2
(@pxref{PC Installation, ,Installation on PC Operating Systems}, for details).

@item vms/*
Files needed for building @command{gawk} under VMS
(@pxref{VMS Installation, ,How to Compile and Install @command{gawk} on VMS}, for details).

@item test/*
A test suite for
@command{gawk}.  You can use @samp{make check} from the top-level @command{gawk}
directory to run your version of @command{gawk} against the test suite.
If @command{gawk} successfully passes @samp{make check}, then you can
be confident of a successful port.
@end table

@node Unix Installation, Non-Unix Installation, Gawk Distribution, Installation
@appendixsec Compiling and Installing @command{gawk} on Unix

Usually, you can compile and install @command{gawk} by typing only two
commands.  However, if you use an unusual system, you may need
to configure @command{gawk} for your system yourself.

@menu
* Quick Installation::               Compiling @command{gawk} under Unix.
* Additional Configuration Options:: Other compile-time options.
* Configuration Philosophy::         How it's all supposed to work.
@end menu

@node Quick Installation, Additional Configuration Options, Unix Installation, Unix Installation
@appendixsubsec Compiling @command{gawk} for Unix

@cindex installation, unix
After you have extracted the @command{gawk} distribution, @command{cd}
to @file{gawk-@value{VERSION}.@value{PATCHLEVEL}}.  Like most GNU software,
@command{gawk} is configured
automatically for your Unix system by running the @command{configure} program.
This program is a Bourne shell script that is generated automatically using
GNU @command{autoconf}.
@ifnotinfo
(The @command{autoconf} software is
described fully in
@cite{Autoconf---Generating Automatic Configuration Scripts},
which is available from the Free Software Foundation.)
@end ifnotinfo
@ifinfo
(The @command{autoconf} software is described fully starting with
@ref{Top}.)
@end ifinfo

To configure @command{gawk}, simply run @command{configure}:

@example
sh ./configure
@end example

This produces a @file{Makefile} and @file{config.h} tailored to your system.
The @file{config.h} file describes various facts about your system.
You might want to edit the @file{Makefile} to
change the @code{CFLAGS} variable, which controls
the command-line options that are passed to the C compiler (such as
optimization levels or compiling for debugging).

Alternatively, you can add your own values for most @command{make}
variables on the command line, such as @code{CC} and @code{CFLAGS}, when
running @command{configure}:

@example
CC=cc CFLAGS=-g sh ./configure
@end example

@noindent
See the file @file{INSTALL} in the @command{gawk} distribution for
all the details.

After you have run @command{configure} and possibly edited the @file{Makefile},
type:

@example
make
@end example

@noindent
Shortly thereafter, you should have an executable version of @command{gawk}.
That's all there is to it!
To verify that @command{gawk} is working properly,
run @samp{make check}.  All of the tests should succeed.
If these steps do not work, or if any of the tests fail,
check the files in the @file{README_d} directory to see if you've
found a known problem.  If the failure is not described there,
please send in a bug report
(@pxref{Bugs, ,Reporting Problems and Bugs}.)

@node Additional Configuration Options, Configuration Philosophy, Quick Installation, Unix Installation
@appendixsubsec Additional Configuration Options

There are several additional options you may use on the @command{configure}
command line when compiling @command{gawk} from scratch.

@table @code
@cindex @code{--enable-portals} configuration option
@cindex configuration option, @code{--enable-portals}
@item --enable-portals
This option causes @command{gawk} to treat pathnames that begin
with @file{/p} as BSD portal files when doing two-way I/O with
the @samp{|&} operator
(@pxref{Portal Files, , Using @command{gawk} with BSD Portals}).

@cindex Linux
@cindex GNU/Linux
@cindex @code{--with-included-gettext} configuration option
@cindex configuration option, @code{--with-included-gettext}
@item --with-included-gettext
Use the version of the @code{gettext} library that comes with @command{gawk}.
This option should be used on systems that do @emph{not} use @value{PVERSION} 2 (or later)
of the GNU C library.
All known modern GNU/Linux systems use Glibc 2.  Use this option on any other system.

@cindex @code{--disable-nls} configuration option
@cindex configuration option, @code{--disable-nls}
@item --disable-nls
Disable all message translation facilities.
This is usually not desirable, but it may bring you some slight performance
improvement.
You should also use this option if @option{--with-included-gettext}
doesn't work on your system.
@end table

@node Configuration Philosophy, , Additional Configuration Options, Unix Installation
@appendixsubsec The Configuration Process

@cindex configuring @command{gawk}
This @value{SECTION} is of interest only if you know something about using the
C language and the Unix operating system.

The source code for @command{gawk} generally attempts to adhere to formal
standards wherever possible.  This means that @command{gawk} uses library
routines that are specified by the ISO C standard and by the POSIX
operating system interface standard.  When using an ISO C compiler,
function prototypes are used to help improve the compile-time checking.

Many Unix systems do not support all of either the ISO or the
POSIX standards.  The @file{missing_d} subdirectory in the @command{gawk}
distribution contains replacement versions of those functions that are
most likely to be missing.

The @file{config.h} file that @command{configure} creates contains
definitions that describe features of the particular operating system
where you are attempting to compile @command{gawk}.  The three things
described by this file are: what header files are available, so that
they can be correctly included, what (supposedly) standard functions
are actually available in your C libraries, and various miscellaneous
facts about your variant of Unix.  For example, there may not be an
@code{st_blksize} element in the @code{stat} structure.  In this case,
@samp{HAVE_ST_BLKSIZE} is undefined.

@cindex @code{custom.h} configuration file
It is possible for your C compiler to lie to @command{configure}. It may
do so by not exiting with an error when a library function is not
available.  To get around this, edit the file @file{custom.h}.
Use an @samp{#ifdef} that is appropriate for your system, and either
@code{#define} any constants that @command{configure} should have defined but
didn't, or @code{#undef} any constants that @command{configure} defined and
should not have.  @file{custom.h} is automatically included by
@file{config.h}.

It is also possible that the @command{configure} program generated by
@command{autoconf} will not work on your system in some other fashion.
If you do have a problem, the file @file{configure.in} is the input for
@command{autoconf}.  You may be able to change this file and generate a
new version of @command{configure} that works on your system
(@pxref{Bugs, ,Reporting Problems and Bugs},
for information on how to report problems in configuring @command{gawk}).
The same mechanism may be used to send in updates to @file{configure.in}
and/or @file{custom.h}.

@node Non-Unix Installation, Unsupported, Unix Installation, Installation
@appendixsec Installation on Other Operating Systems

This @value{SECTION} describes how to install @command{gawk} on
various non-Unix systems.

@menu
* Amiga Installation::          Installing @command{gawk} on an Amiga.
* BeOS Installation::           Installing @command{gawk} on BeOS.
* PC Installation::             Installing and Compiling @command{gawk} on
                                MS-DOS and OS/2.
* VMS Installation::            Installing @command{gawk} on VMS.
@end menu

@node Amiga Installation, BeOS Installation, Non-Unix Installation, Non-Unix Installation
@appendixsubsec Installing @command{gawk} on an Amiga

@cindex amiga
@cindex installation, amiga
You can install @command{gawk} on an Amiga system using a Unix emulation
environment, available via anonymous @command{ftp} from
@code{ftp.ninemoons.com} in the directory @file{pub/ade/current}.
This includes a shell based on @command{pdksh}.  The primary component of
this environment is a Unix emulation library, @file{ixemul.lib}.
@c could really use more background here, who wrote this, etc.

A more complete distribution for the Amiga is available on
the Geek Gadgets CD-ROM, available from:

@display
CRONUS
1840 E. Warner Road #105-265
Tempe, AZ 85284  USA
US Toll Free: (800) 804-0833
Phone: +1-602-491-0442
FAX: +1-602-491-0048
Email: @email{info@@ninemoons.com}
WWW: @uref{http://www.ninemoons.com}
Anonymous @command{ftp} site: @code{ftp.ninemoons.com}
@end display

Once you have the distribution, you can configure @command{gawk} simply by
running @command{configure}:

@example
configure -v m68k-amigaos
@end example

Then run @command{make} and you should be all set!
If these steps do not work, please send in a bug report
(@pxref{Bugs, ,Reporting Problems and Bugs}).

@node BeOS Installation, PC Installation, Amiga Installation, Non-Unix Installation
@appendixsubsec Installing @command{gawk} on BeOS
@cindex BeOS
@cindex installation, beos

@c From email contributed by Martin Brown, mc@whoever.com
Since BeOS DR9, all the tools that you should need to build @code{gawk} are
included with BeOS. The process is basically identical to the Unix process
of running @command{configure} and then @command{make}. Full instructions are given below.

You can compile @command{gawk} under BeOS by extracting the standard sources
and running @command{configure}. You @emph{must} specify the location
prefix for the installation directory. For BeOS DR9 and beyond, the best directory to
use is @file{/boot/home/config}, so the @command{configure} command is:

@example
configure --prefix=/boot/home/config
@end example

This installs the compiled application into @file{/boot/home/config/bin},
which is already specified in the standard @env{PATH}.

Once the configuration process is completed, you can run @command{make},
and then @samp{make install}:

@example
$ make
@dots{}
$ make install
@end example

BeOS uses @command{bash} as its shell; thus, you use @command{gawk} the same way you would
under Unix.
If these steps do not work, please send in a bug report
(@pxref{Bugs, ,Reporting Problems and Bugs}).

@c Rewritten by Scott Deifik <scottd@amgen.com>
@c and Darrel Hankerson <hankedr@mail.auburn.edu>

@node PC Installation, VMS Installation, BeOS Installation, Non-Unix Installation
@appendixsubsec Installation on PC Operating Systems

@cindex installation, pc operating systems
This @value{SECTION} covers installation and usage of @command{gawk} on x86 machines
running DOS, any version of Windows, or OS/2.
In this @value{SECTION}, the term ``Win32''
refers to any of Windows-95/98/ME/NT/2000.

The limitations of DOS (and DOS shells under Windows or OS/2) has meant
that various ``DOS extenders'' are often used with programs such as
@command{gawk}.  The varying capabilities of Microsoft Windows 3.1
and Win32 can add to the confusion.  For an overview of the
considerations, please refer to @file{README_d/README.pc} in the
distribution.

@menu
* PC Binary Installation::      Installing a prepared distribution.
* PC Compiling::                Compiling @command{gawk} for MS-DOS, Win32,
                                and OS/2.
* PC Using::                    Running @command{gawk} on MS-DOS, Win32 and
                                OS/2.
@end menu

@node PC Binary Installation, PC Compiling, PC Installation, PC Installation
@appendixsubsubsec Installing a Prepared Distribution for PC Systems

If you have received a binary distribution prepared by the DOS
maintainers, then @command{gawk} and the necessary support files appear
under the @file{gnu} directory, with executables in @file{gnu/bin},
libraries in @file{gnu/lib/awk}, and manual pages under @file{gnu/man}.
This is designed for easy installation to a @file{/gnu} directory on your
drive---however, the files can be installed anywhere provided @env{AWKPATH} is
set properly.  Regardless of the installation directory, the first line of
@file{igawk.cmd} and @file{igawk.bat} (in @file{gnu/bin}) may need to be
edited.

The binary distribution contains a separate file describing the
contents. In particular, it may include more than one version of the
@command{gawk} executable. OS/2 binary distributions may have a
different arrangement, but installation is similar.

@node PC Compiling, PC Using, PC Binary Installation, PC Installation
@appendixsubsubsec Compiling @command{gawk} for PC Operating Systems

@command{gawk} can be compiled for MS-DOS, Win32, and OS/2 using the GNU
development tools from DJ Delorie (DJGPP; MS-DOS only) or Eberhard
Mattes (EMX; MS-DOS, Win32 and OS/2).  Microsoft Visual C/C++ can be used
to build a Win32 version, and Microsoft C/C++ can be
used to build 16-bit versions for MS-DOS and OS/2.  The file
@file{README_d/README.pc} in the @command{gawk} distribution contains
additional notes, and @file{pc/Makefile} contains important information on
compilation options.

To build @command{gawk}, copy the files in the @file{pc} directory
(@emph{except} for @file{ChangeLog}) to the directory with the rest of
the @command{gawk} sources. The @file{Makefile} contains a configuration
section with comments and may need to be edited in order to work with
your @command{make} utility.

The @file{Makefile} contains a number of targets for building various MS-DOS,
Win32, and OS/2 versions. A list of targets is printed if the @command{make}
command is given without a target. As an example, to build @command{gawk}
using the DJGPP tools, enter @samp{make djgpp}.

Using @command{make} to run the standard tests and to install @command{gawk}
requires additional Unix-like tools, including @command{sh}, @command{sed}, and
@command{cp}. In order to run the tests, the @file{test/*.ok} files may need to
be converted so that they have the usual DOS-style end-of-line markers. Most
of the tests work properly with Stewartson's shell along with the
companion utilities or appropriate GNU utilities.  However, some editing of
@file{test/Makefile} is required. It is recommended that you copy the file
@file{pc/Makefile.tst} over the file @file{test/Makefile} as a
replacement. Details can be found in @file{README_d/README.pc}
and in the file @file{pc/Makefile.tst}.

@node PC Using, , PC Compiling, PC Installation
@appendixsubsubsec Using @command{gawk} on PC Operating Systems

@cindex search path
@cindex directory search
@cindex path, search
@cindex search path, for source files
The OS/2 and MS-DOS versions of @command{gawk} search for program files as
described in @ref{AWKPATH Variable, ,The @env{AWKPATH} Environment Variable}.
However, semicolons (rather than colons) separate elements
in the @env{AWKPATH} variable. If @env{AWKPATH} is not set or is empty,
then the default search path is @code{@w{".;c:/lib/awk;c:/gnu/lib/awk"}}.

An @command{sh}-like shell (as opposed to @command{command.com} under MS-DOS
or @command{cmd.exe} under OS/2) may be useful for @command{awk} programming.
Ian Stewartson has written an excellent shell for MS-DOS and OS/2,
Daisuke Aoyama has ported GNU @command{bash} to MS-DOS using the DJGPP tools,
and several shells are available for OS/2, including @command{ksh}.  The file
@file{README_d/README.pc} in the @command{gawk} distribution contains
information on these shells.  Users of Stewartson's shell on DOS should
examine its documentation for handling command lines; in particular,
the setting for @command{gawk} in the shell configuration may need to be
changed and the @code{ignoretype} option may also be of interest.

@cindex @code{BINMODE} variable
Under OS/2 and DOS, @command{gawk} (and many other text programs) silently
translate end-of-line @code{"\r\n"} to @code{"\n"} on input and @code{"\n"}
to @code{"\r\n"} on output.  A special @code{BINMODE} variable allows
control over these translations and is interpreted as follows.

@itemize @bullet
@item
If @code{BINMODE} is @samp{"r"}, or
@code{(BINMODE & 1)} is nonzero, then
binary mode is set on read (i.e., no translations on reads).

@item
If @code{BINMODE} is @code{"w"}, or
@code{(BINMODE & 2)} is nonzero, then
binary mode is set on write (i.e., no translations on writes).

@item
If @code{BINMODE} is @code{"rw"} or @code{"wr"},
binary mode is set for both read and write
(same as @code{(BINMODE & 3)}).

@item
@code{BINMODE=@var{non-null-string}} is
the same as @samp{BINMODE=3} (i.e., no translations on
reads or writes).  However, @command{gawk} issues a warning
message if the string is not one of @code{"rw"} or @code{"wr"}.
@end itemize

@noindent
The modes for standard input and standard output are set one time
only (after the
command line is read, but before processing any of the @command{awk} program).
Setting @code{BINMODE} for standard input or
standard output is accomplished by using an
appropriate @samp{-v BINMODE=@var{N}} option on the command line.
@code{BINMODE} is set at the time a file or pipe is opened and cannot be
changed mid-stream.

The name @code{BINMODE} was chosen to match @command{mawk}
(@pxref{Other Versions, , Other Freely Available @command{awk} Implementations}).
Both @command{mawk} and @command{gawk} handle @code{BINMODE} similarly; however,
@command{mawk} adds a @samp{-W BINMODE=@var{N}} option and an environment
variable that can set @code{BINMODE}, @code{RS}, and @code{ORS}.  The
files @file{binmode[1-3].awk} (under @file{gnu/lib/awk} in some of the
prepared distributions) have been chosen to match @command{mawk}'s @samp{-W
BINMODE=@var{N}} option.  These can be changed or discarded; in particular,
the setting of @code{RS} giving the fewest ``surprises'' is open to debate.
@command{mawk} uses @samp{RS = "\r\n"} if binary mode is set on read, which is
appropriate for files with the DOS-style end-of-line.

To Illustrate, the following examples set binary mode on writes for standard
output and other files, and set @code{ORS} as the ``usual'' DOS-style
end-of-line:

@example
gawk -v BINMODE=2 -v ORS="\r\n" @dots{}
@end example

@noindent
or:

@example
gawk -v BINMODE=w -f binmode2.awk @dots{}
@end example

@noindent
These give the same result as the @samp{-W BINMODE=2} option in
@command{mawk}.
The following changes the record separator to @code{"\r\n"} and sets binary
mode on reads, but does not affect the mode on standard input:

@example
gawk -v RS="\r\n" --source "BEGIN @{ BINMODE = 1 @}" @dots{}
@end example

@noindent
or:

@example
gawk -f binmode1.awk @dots{}
@end example

@noindent
With proper quoting, in the first example the setting of @code{RS} can be
moved into the @code{BEGIN} rule.

@node VMS Installation, , PC Installation, Non-Unix Installation
@appendixsubsec How to Compile and Install @command{gawk} on VMS

@c based on material from Pat Rankin <rankin@eql.caltech.edu>

@cindex installation, vms
This @value{SUBSECTION} describes how to compile and install @command{gawk} under VMS.

@menu
* VMS Compilation::             How to compile @command{gawk} under VMS.
* VMS Installation Details::    How to install @command{gawk} under VMS.
* VMS Running::                 How to run @command{gawk} under VMS.
* VMS POSIX::                   Alternate instructions for VMS POSIX.
@end menu

@node VMS Compilation, VMS Installation Details, VMS Installation, VMS Installation
@appendixsubsubsec Compiling @command{gawk} on VMS

To compile @command{gawk} under VMS, there is a @code{DCL} command procedure that
issues all the necessary @code{CC} and @code{LINK} commands. There is
also a @file{Makefile} for use with the @code{MMS} utility.  From the source
directory, use either:

@example
$ @@[.VMS]VMSBUILD.COM
@end example

@noindent
or:

@example
$ MMS/DESCRIPTION=[.VMS]DESCRIP.MMS GAWK
@end example

Depending upon which C compiler you are using, follow one of the sets
of instructions in this table:

@table @asis
@item VAX C V3.x
Use either @file{vmsbuild.com} or @file{descrip.mms} as is.  These use
@code{CC/OPTIMIZE=NOLINE}, which is essential for Version 3.0.

@item VAX C V2.x
You must have Version 2.3 or 2.4; older ones won't work.  Edit either
@file{vmsbuild.com} or @file{descrip.mms} according to the comments in them.
For @file{vmsbuild.com}, this just entails removing two @samp{!} delimiters.
Also edit @file{config.h} (which is a copy of file @file{[.config]vms-conf.h})
and comment out or delete the two lines @samp{#define __STDC__ 0} and
@samp{#define VAXC_BUILTINS} near the end.

@item GNU C
Edit @file{vmsbuild.com} or @file{descrip.mms}; the changes are different
from those for VAX C V2.x but equally straightforward.  No changes to
@file{config.h} are needed.

@item DEC C
Edit @file{vmsbuild.com} or @file{descrip.mms} according to their comments.
No changes to @file{config.h} are needed.
@end table

@command{gawk} has been tested under VAX/VMS 5.5-1 using VAX C V3.2, and
GNU C 1.40 and 2.3.  It should work without modifications for VMS V4.6 and up.

@node VMS Installation Details, VMS Running, VMS Compilation, VMS Installation
@appendixsubsubsec Installing @command{gawk} on VMS

To install @command{gawk}, all you need is a ``foreign'' command, which is
a @code{DCL} symbol whose value begins with a dollar sign. For example:

@example
$ GAWK :== $disk1:[gnubin]GAWK
@end example

@noindent
Substitute the actual location of @command{gawk.exe} for
@samp{$disk1:[gnubin]}. The symbol should be placed in the
@file{login.com} of any user who wants to run @command{gawk},
so that it is defined every time the user logs on.
Alternatively, the symbol may be placed in the system-wide
@file{sylogin.com} procedure, which allows all users
to run @command{gawk}.

Optionally, the help entry can be loaded into a VMS help library:

@example
$ LIBRARY/HELP SYS$HELP:HELPLIB [.VMS]GAWK.HLP
@end example

@noindent
(You may want to substitute a site-specific help library rather than
the standard VMS library @samp{HELPLIB}.)  After loading the help text,
the command:

@example
$ HELP GAWK
@end example

@noindent
provides information about both the @command{gawk} implementation and the
@command{awk} programming language.

The logical name @samp{AWK_LIBRARY} can designate a default location
for @command{awk} program files.  For the @option{-f} option, if the specified
@value{FN} has no device or directory path information in it, @command{gawk}
looks in the current directory first, then in the directory specified
by the translation of @samp{AWK_LIBRARY} if the file is not found.
If, after searching in both directories, the file still is not found,
@command{gawk} appends the suffix @samp{.awk} to the filename and retries
the file search.  If @samp{AWK_LIBRARY} is not defined, that
portion of the file search fails benignly.

@node VMS Running, VMS POSIX, VMS Installation Details, VMS Installation
@appendixsubsubsec Running @command{gawk} on VMS

Command-line parsing and quoting conventions are significantly different
on VMS, so examples in this @value{DOCUMENT} or from other sources often need minor
changes.  They @emph{are} minor though, and all @command{awk} programs
should run correctly.

Here are a couple of trivial tests:

@example
$ gawk -- "BEGIN @{print ""Hello, World!""@}"
$ gawk -"W" version
! could also be -"W version" or "-W version"
@end example

@noindent
Note that uppercase and mixed-case text must be quoted.

The VMS port of @command{gawk} includes a @code{DCL}-style interface in addition
to the original shell-style interface (see the help entry for details).
One side effect of dual command-line parsing is that if there is only a
single parameter (as in the quoted string program above), the command
becomes ambiguous.  To work around this, the normally optional @option{--}
flag is required to force Unix style rather than @code{DCL} parsing.  If any
other dash-type options (or multiple parameters such as @value{DF}s to
process) are present, there is no ambiguity and @option{--} can be omitted.

@cindex search path
@cindex directory search
@cindex path, search
@cindex search path, for source files
The default search path, when looking for @command{awk} program files specified
by the @option{-f} option, is @code{"SYS$DISK:[],AWK_LIBRARY:"}.  The logical
name @samp{AWKPATH} can be used to override this default.  The format
of @samp{AWKPATH} is a comma-separated list of directory specifications.
When defining it, the value should be quoted so that it retains a single
translation and not a multitranslation @code{RMS} searchlist.

@node VMS POSIX, , VMS Running, VMS Installation
@appendixsubsubsec Building and Using @command{gawk} on VMS POSIX

Ignore the instructions above, although @file{vms/gawk.hlp} should still
be made available in a help library.  The source tree should be unpacked
into a container file subsystem rather than into the ordinary VMS filesystem.
Make sure that the two scripts, @file{configure} and
@file{vms/posix-cc.sh}, are executable; use @samp{chmod +x} on them if
necessary.  Then execute the following two commands:

@example
psx> CC=vms/posix-cc.sh configure
psx> make CC=c89 gawk
@end example

@noindent
The first command constructs files @file{config.h} and @file{Makefile} out
of templates, using a script to make the C compiler fit @command{configure}'s
expectations.  The second command compiles and links @command{gawk} using
the C compiler directly; ignore any warnings from @command{make} about being
unable to redefine @code{CC}.  @command{configure} takes a very long
time to execute, but at least it provides incremental feedback as it runs.

This has been tested with VAX/VMS V6.2, VMS POSIX V2.0, and DEC C V5.2.

Once built, @command{gawk} works like any other shell utility.  Unlike
the normal VMS port of @command{gawk}, no special command-line manipulation is
needed in the VMS POSIX environment.

@node Unsupported, Bugs, Non-Unix Installation, Installation
@appendixsec Unsupported Operating System Ports

This sections describes systems for which
the @command{gawk} port is no longer supported.

@menu
* Atari Installation::          Installing @command{gawk} on the Atari ST.
* Tandem Installation::         Installing @command{gawk} on a Tandem.
@end menu

@node Atari Installation, Tandem Installation, Unsupported, Unsupported
@appendixsubsec Installing @command{gawk} on the Atari ST

The Atari port is no longer supported.  It is
included for those who might want to use it but it is no longer being
actively maintained.

@c based on material from Michal Jaegermann <michal@gortel.phys.ualberta.ca>
@cindex atari
@cindex installation, atari
There are no substantial differences when installing @command{gawk} on
various Atari models.  Compiled @command{gawk} executables do not require
a large amount of memory with most @command{awk} programs, and should run on all
Motorola processor-based models (called further ST, even if that is not
exactly right).

In order to use @command{gawk}, you need to have a shell, either text or
graphics, that does not map all the characters of a command line to
uppercase.  Maintaining case distinction in option flags is very
important (@pxref{Options, ,Command-Line Options}).
These days this is the default and it may only be a problem for some
very old machines.  If your system does not preserve the case of option
flags, you need to upgrade your tools.  Support for I/O
redirection is necessary to make it easy to import @command{awk} programs
from other environments.  Pipes are nice to have but not vital.

@menu
* Atari Compiling::             Compiling @command{gawk} on Atari.
* Atari Using::                 Running @command{gawk} on Atari.
@end menu

@node Atari Compiling, Atari Using, Atari Installation, Atari Installation
@appendixsubsubsec Compiling @command{gawk} on the Atari ST

A proper compilation of @command{gawk} sources when @code{sizeof(int)}
differs from @code{sizeof(void *)} requires an ISO C compiler. An initial
port was done with @command{gcc}.  You may actually prefer executables
where @code{int}s are four bytes wide but the other variant works as well.

You may need quite a bit of memory when trying to recompile the @command{gawk}
sources, as some source files (@file{regex.c} in particular) are quite
big.  If you run out of memory compiling such a file, try reducing the
optimization level for this particular file, which may help.

@cindex Linux
@cindex GNU/Linux
With a reasonable shell (@command{bash} will do), you have a pretty good chance
that the @command{configure} utility will succeed, and in particular if
you run GNU/Linux, MiNT or a similar operating system.  Otherwise
sample versions of @file{config.h} and @file{Makefile.st} are given in the
@file{atari} subdirectory and can be edited and copied to the
corresponding files in the main source directory.  Even if
@command{configure} produces something, it might be advisable to compare
its results with the sample versions and possibly make adjustments.

Some @command{gawk} source code fragments depend on a preprocessor define
@samp{atarist}.  This basically assumes the TOS environment with @command{gcc}.
Modify these sections as appropriate if they are not right for your
environment.  Also see the remarks about @env{AWKPATH} and @code{envsep} in
@ref{Atari Using, ,Running @command{gawk} on the Atari ST}.

As shipped, the sample @file{config.h} claims that the @code{system}
function is missing from the libraries, which is not true, and an
alternative implementation of this function is provided in
@file{unsupported/atari/system.c}.
Depending upon your particular combination of
shell and operating system, you might want to change the file to indicate
that @code{system} is available.

@node Atari Using, , Atari Compiling, Atari Installation
@appendixsubsubsec Running @command{gawk} on the Atari ST

An executable version of @command{gawk} should be placed, as usual,
anywhere in your @env{PATH} where your shell can find it.

While executing, the Atari version of @command{gawk} creates a number of temporary files.  When
using @command{gcc} libraries for TOS, @command{gawk} looks for either of
the environment variables, @env{TEMP} or @env{TMPDIR}, in that order.
If either one is found, its value is assumed to be a directory for
temporary files.  This directory must exist, and if you can spare the
memory, it is a good idea to put it on a RAM drive.  If neither
@env{TEMP} nor @env{TMPDIR} are found, then @command{gawk} uses the
current directory for its temporary files.

The ST version of @command{gawk} searches for its program files, as described in
@ref{AWKPATH Variable, ,The @env{AWKPATH} Environment Variable}.
The default value for the @env{AWKPATH} variable is taken from
@code{DEFPATH} defined in @file{Makefile}. The sample @command{gcc}/TOS
@file{Makefile} for the ST in the distribution sets @code{DEFPATH} to
@code{@w{".,c:\lib\awk,c:\gnu\lib\awk"}}.  The search path can be
modified by explicitly setting @env{AWKPATH} to whatever you want.
Note that colons cannot be used on the ST to separate elements in the
@env{AWKPATH} variable, since they have another reserved meaning.
Instead, you must use a comma to separate elements in the path.  When
recompiling, the separating character can be modified by initializing
the @code{envsep} variable in @file{unsupported/atari/gawkmisc.atr} to another
value.

Although @command{awk} allows great flexibility in doing I/O redirections
from within a program, this facility should be used with care on the ST
running under TOS.  In some circumstances, the OS routines for file-handle
pool processing lose track of certain events, causing the
computer to crash and requiring a reboot.  Often a warm reboot is
sufficient.  Fortunately, this happens infrequently and in rather
esoteric situations.  In particular, avoid having one part of an
@command{awk} program using @code{print} statements explicitly redirected
to @file{/dev/stdout}, while other @code{print} statements use the
default standard output, and a calling shell has redirected standard
output to a file.
@c 10/2000: Is this still true, now that gawk does /dev/stdout internally?

When @command{gawk} is compiled with the ST version of @command{gcc} and its
usual libraries, it accepts both @samp{/} and @samp{\} as path separators.
While this is convenient, it should be remembered that this removes one
technically valid character (@samp{/}) from your @value{FN}.
It may also create problems for external programs called via the @code{system}
function, which may not support this convention.  Whenever it is possible
that a file created by @command{gawk} will be used by some other program,
use only backslashes.  Also remember that in @command{awk}, backslashes in
strings have to be doubled in order to get literal backslashes
(@pxref{Escape Sequences}).

@node Tandem Installation, , Atari Installation, Unsupported
@appendixsubsec Installing @command{gawk} on a Tandem
@cindex tandem
@cindex installation, tandem

The Tandem port is only minimally supported.
The port's contributor no longer has access to a Tandem system.

@c This section based on README.Tandem by Stephen Davies (scldad@sdc.com.au)
The Tandem port was done on a Cyclone machine running D20.
The port is pretty clean and all facilities seem to work except for
the I/O piping facilities
(@pxref{Getline/Pipe, , Using @code{getline} from a Pipe},
@ref{Getline/Variable/Pipe, ,Using @code{getline} into a Variable from a Pipe},
and
@ref{Redirection, ,Redirecting Output of @code{print} and @code{printf}}),
which is just too foreign a concept for Tandem.

To build a Tandem executable from source, download all of the files so
that the @value{FN}s on the Tandem box conform to the restrictions of D20.
For example, @file{array.c} becomes @file{ARRAYC}, and @file{awk.h}
becomes @file{AWKH}.  The totally Tandem-specific files are in the
@file{tandem} ``subvolume'' (@file{unsupported/tandem} in the @command{gawk}
distribution) and should be copied to the main source directory before
building @command{gawk}.

The file @file{compit} can then be used to compile and bind an executable.
Alas, there is no @command{configure} or @command{make}.

Usage is the same as for Unix, except that D20 requires all @samp{@{} and
@samp{@}} characters to be escaped with @samp{~} on the command line
(but @emph{not} in script files). Also, the standard Tandem syntax for
@samp{/in filename,out filename/} must be used instead of the usual
Unix @samp{<} and @samp{>} for file redirection.  (Redirection options
on @code{getline}, @code{print} etc., are supported.)

The @samp{-mr @var{val}} option
(@pxref{Options, ,Command-Line Options})
has been ``stolen'' to enable Tandem users to process fixed-length
records with no ``end-of-line'' character. That is, @samp{-mr 74} tells
@command{gawk} to read the input file as fixed 74-byte records.

@node Bugs, Other Versions, Unsupported, Installation
@appendixsec Reporting Problems and Bugs
@cindex archeologists
@quotation
@i{There is nothing more dangerous than a bored archeologist.}@*
The Hitchhiker's Guide to the Galaxy
@end quotation
@c the radio show, not the book. :-)

@cindex bug reports
@cindex problem reports
@cindex reporting bugs
@cindex reporting problems
If you have problems with @command{gawk} or think that you have found a bug,
please report it to the developers; we cannot promise to do anything
but we might well want to fix it.

Before reporting a bug, make sure you have actually found a real bug.
Carefully reread the documentation and see if it really says you can do
what you're trying to do.  If it's not clear whether you should be able
to do something or not, report that too; it's a bug in the documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible @command{awk} program and input @value{DF} that
reproduces the problem.  Then send us the program and @value{DF},
some idea of what kind of Unix system you're using,
the compiler you used to compile @command{gawk}, and the exact results
@command{gawk} gave you.  Also say what you expected to occur; this helps
us decide whether the problem is really in the documentation.

@cindex @code{bug-gawk@@gnu.org} bug reporting address
@cindex emaill address for bug reports, @code{bug-gawk@@gnu.org}
@cindex bug reports, email address, @code{bug-gawk@@gnu.org}
Once you have a precise problem, send email to @email{bug-gawk@@gnu.org}.

@cindex Robbins, Arnold
Please include the version number of @command{gawk} you are using.
You can get this information with the command @samp{gawk --version}.
Using this address automatically sends a carbon copy of your
mail to me.  If necessary, I can be reached directly at
@email{arnold@@gnu.org}.  The bug reporting address is preferred since the
email list is archived at the GNU Project.
@emph{All email should be in English, since that is my native language.}

@cindex @code{comp.lang.awk} Usenet news group
@strong{Caution:} Do @emph{not} try to report bugs in @command{gawk} by
posting to the Usenet/Internet newsgroup @code{comp.lang.awk}.
While the @command{gawk} developers do occasionally read this newsgroup,
there is no guarantee that we will see your posting.  The steps described
above are the official recognized ways for reporting bugs.

Non-bug suggestions are always welcome as well.  If you have questions
about things that are unclear in the documentation or are just obscure
features, ask me; I will try to help you out, although I
may not have the time to fix the problem.  You can send me electronic
mail at the Internet address noted previously.

If you find bugs in one of the non-Unix ports of @command{gawk}, please send
an electronic mail message to the person who maintains that port.  They
are named in the following list, as well as in the @file{README} file in the @command{gawk}
distribution.  Information in the @file{README} file should be considered
authoritative if it conflicts with this @value{DOCUMENT}.

The people maintaining the non-Unix ports of @command{gawk} are
as follows:

@ignore
@table @asis
@cindex Fish, Fred
@item Amiga
Fred Fish, @email{fnf@@ninemoons.com}.

@cindex Brown, Martin
@item BeOS
Martin Brown, @email{mc@@whoever.com}.

@cindex Deifik, Scott
@cindex Hankerson, Darrel
@item MS-DOS
Scott Deifik, @email{scottd@@amgen.com} and
Darrel Hankerson, @email{hankedr@@mail.auburn.edu}.

@cindex Grigera, Juan
@item MS-Windows
Juan Grigera, @email{juan@@biophnet.unlp.edu.ar}.

@cindex Rommel, Kai Uwe
@item OS/2
Kai Uwe Rommel, @email{rommel@@ars.de}.

@cindex Davies, Stephen
@item Tandem
Stephen Davies, @email{scldad@@sdc.com.au}.

@cindex Rankin, Pat
@item VMS
Pat Rankin, @email{rankin@@eql.caltech.edu}.
@end table
@end ignore

@multitable {MS-Windows} {123456789012345678901234567890123456789001234567890}
@cindex Fish, Fred
@item Amiga @tab Fred Fish, @email{fnf@@ninemoons.com}.

@cindex Brown, Martin
@item BeOS @tab Martin Brown, @email{mc@@whoever.com}.

@cindex Deifik, Scott
@cindex Hankerson, Darrel
@item MS-DOS @tab Scott Deifik, @email{scottd@@amgen.com} and
Darrel Hankerson, @email{hankedr@@mail.auburn.edu}.

@cindex Grigera, Juan
@item MS-Windows @tab Juan Grigera, @email{juan@@biophnet.unlp.edu.ar}.

@cindex Rommel, Kai Uwe
@item OS/2 @tab Kai Uwe Rommel, @email{rommel@@ars.de}.

@cindex Davies, Stephen
@item Tandem @tab Stephen Davies, @email{scldad@@sdc.com.au}.

@cindex Rankin, Pat
@item VMS @tab Pat Rankin, @email{rankin@@eql.caltech.edu}.
@end multitable

If your bug is also reproducible under Unix, please send a copy of your
report to the @email{bug-gawk@@gnu.org} email list as well.

@node Other Versions, , Bugs, Installation
@appendixsec Other Freely Available @command{awk} Implementations
@cindex other @command{awk} implementations
@ignore
From: emory!amc.com!brennan (Michael Brennan)
Subject: C++ comments in awk programs
To: arnold@gnu.ai.mit.edu (Arnold Robbins)
Date: Wed, 4 Sep 1996 08:11:48 -0700 (PDT)

@end ignore
@cindex Brennan, Michael
@quotation
@i{It's kind of fun to put comments like this in your awk code.}@*
@ @ @ @ @ @ @code{// Do C++ comments work? answer: yes! of course}@*
Michael Brennan
@end quotation

There are three other freely available @command{awk} implementations.
This @value{SECTION} briefly describes where to get them:

@table @asis
@cindex Kernighan, Brian
@cindex Unix @command{awk}, source code
@cindex source code, Unix @command{awk}
@item Unix @command{awk}
Brian Kernighan has made his implementation of
@command{awk} freely available.
You can retrieve this version via the World Wide Web from
his home page.@footnote{@uref{http://cm.bell-labs.com/who/bwk}}
It is available in several archive formats:

@table @asis
@item Shell archive
@uref{http://cm.bell-labs.com/who/bwk/awk.shar}

@item Compressed @command{tar} file
@uref{http://cm.bell-labs.com/who/bwk/awk.tar.gz}

@item Zip file
@uref{http://cm.bell-labs.com/who/bwk/awk.zip}
@end table

This version requires an ISO C (1990 standard) compiler;
the C compiler from
GCC (the GNU Compiler Collection)
works quite nicely.

@xref{BTL, ,Extensions in the Bell Laboratories @command{awk}},
for a list of extensions in this @command{awk} that are not in POSIX @command{awk}.

@cindex GPL
@cindex General Public License
@cindex GNU General Public License
@cindex Brennan, Michael
@cindex @command{mawk}, source code
@cindex source code, @command{mawk}
@item @command{mawk}
Michael Brennan has written an independent implementation of @command{awk},
called @command{mawk}.  It is available under the GPL
(@pxref{Copying, ,GNU General Public License}),
just as @command{gawk} is.

You can get it via anonymous @command{ftp} to the host
@code{@w{ftp.whidbey.net}}.  Change directory to @file{/pub/brennan}.
Use ``binary'' or ``image'' mode, and retrieve @file{mawk1.3.3.tar.gz}
(or the latest version that is there).

@command{gunzip} may be used to decompress this file. Installation
is similar to @command{gawk}'s
(@pxref{Unix Installation, , Compiling and Installing @command{gawk} on Unix}).

@cindex extensions, @command{mawk}
@command{mawk} has the following extensions that are not in POSIX @command{awk}:

@itemize @bullet
@item
The @code{fflush} built-in function for flushing buffered output
(@pxref{I/O Functions, ,Input/Output Functions}).

@item
The @samp{**} and @samp{**=} operators
(@pxref{Arithmetic Ops, ,Arithmetic Operators}
and also see
@ref{Assignment Ops, ,Assignment Expressions}).

@item
The use of @code{func} as an abbreviation for @code{function}
(@pxref{Definition Syntax, ,Function Definition Syntax}).

@item
The @samp{\x} escape sequence
(@pxref{Escape Sequences}).

@item
The @file{/dev/stdout}, and @file{/dev/stderr}
special files
(@pxref{Special Files, ,Special @value{FFN}s in @command{gawk}}).
Use @code{"-"} instead of @code{"/dev/stdin"} with @command{mawk}.

@item
The ability for @code{FS} and for the third
argument to @code{split} to be null strings
(@pxref{Single Character Fields, , Making Each Character a Separate Field}).

@item
The ability to delete all of an array at once with @samp{delete @var{array}}
(@pxref{Delete, ,The @code{delete} Statement}).

@item
The ability for @code{RS} to be a regexp
(@pxref{Records, ,How Input Is Split into Records}).

@item
The @code{BINMODE} special variable for non-Unix operating systems
(@pxref{PC Using, ,Using @command{gawk} on PC Operating Systems}).
@end itemize

The next version of @command{mawk} will support @code{nextfile}.

@cindex Sumner, Andrew
@cindex @command{awka} compiler for @command{awk} programs
@cindex @command{awka}, source code
@cindex source code, @command{awka}
@item @command{awka}
Written by Andrew Sumner,
@command{awka} translates @command{awk} programs into C, compiles them,
and links them with a library of functions that provides the core
@command{awk} functionality.
It also has a number of extensions.

@cindex GPL
@cindex General Public License
@cindex GNU General Public License
@cindex LGPL
@cindex Lesser General Public License
@cindex GNU Lesser General Public License
The @command{awk} translator is released under the GPL, and the library
is under the LGPL.

@ignore
To get @command{awka}, go to its home page at
Go to @uref{http://awka.sourceforge.net}.
@end ignore
To get @command{awka}, go to @uref{http://awka.sourceforge.net}.
You can reach Andrew Sumner at @email{andrew_sumner@@bigfoot.com}.
@end table

@node Notes, Basic Concepts, Installation, Top
@appendix Implementation Notes

This appendix contains information mainly of interest to implementors and
maintainers of @command{gawk}.  Everything in it applies specifically to
@command{gawk} and not to other implementations.

@menu
* Compatibility Mode::          How to disable certain @command{gawk}
                                extensions.
* Additions::                   Making Additions To @command{gawk}.
* Dynamic Extensions::          Adding new built-in functions to
                                @command{gawk}.
* Future Extensions::           New features that may be implemented one day.
@end menu

@node Compatibility Mode, Additions, Notes, Notes
@appendixsec Downward Compatibility and Debugging

@xref{POSIX/GNU, ,Extensions in @command{gawk} Not in POSIX @command{awk}},
for a summary of the GNU extensions to the @command{awk} language and program.
All of these features can be turned off by invoking @command{gawk} with the
@option{--traditional} option or with the @option{--posix} option.

If @command{gawk} is compiled for debugging with @samp{-DDEBUG}, then there
is one more option available on the command line:

@table @code
@item -W parsedebug
@itemx --parsedebug
Print out the parse stack information as the program is being parsed.
@end table

This option is intended only for serious @command{gawk} developers
and not for the casual user.  It probably has not even been compiled into
your version of @command{gawk}, since it slows down execution.

@node Additions, Dynamic Extensions, Compatibility Mode, Notes
@appendixsec Making Additions to @command{gawk}

If you find that you want to enhance @command{gawk} in a significant
fashion, you are perfectly free to do so.  That is the point of having
free software; the source code is available and you are free to change
it as you want (@pxref{Copying, ,GNU General Public License}).

This @value{SECTION} discusses the ways you might want to change @command{gawk}
as well as any considerations you should bear in mind.

@menu
* Adding Code::                 Adding code to the main body of
                                @command{gawk}.
* New Ports::                   Porting @command{gawk} to a new operating
                                system.
@end menu

@node Adding Code, New Ports, Additions, Additions
@appendixsubsec Adding New Features

@cindex adding new features
@cindex features, adding to @command{gawk}
You are free to add any new features you like to @command{gawk}.
However, if you want your changes to be incorporated into the @command{gawk}
distribution, there are several steps that you need to take in order to
make it possible for me to include your changes:

@enumerate 1
@item
Before building the new feature into @command{gawk} itself,
consider writing it as an extension module
(@pxref{Dynamic Extensions, ,Adding New Built-in Functions to @command{gawk}}).
If that's not possible, continue with the rest of the steps in this list.

@item
Get the latest version.
It is much easier for me to integrate changes if they are relative to
the most recent distributed version of @command{gawk}.  If your version of
@command{gawk} is very old, I may not be able to integrate them at all.
(@xref{Getting, ,Getting the @command{gawk} Distribution},
for information on getting the latest version of @command{gawk}.)

@item
@ifnotinfo
Follow the @cite{GNU Coding Standards}.
@end ifnotinfo
@ifinfo
See @inforef{Top, , Version, standards, GNU Coding Standards}.
@end ifinfo
This document describes how GNU software should be written. If you haven't
read it, please do so, preferably @emph{before} starting to modify @command{gawk}.
(The @cite{GNU Coding Standards} are available from
the GNU Project's
@command{ftp}
site, at
@uref{ftp://gnudist.gnu.org/gnu/GNUInfo/standards.text}.
Texinfo, Info, and DVI versions are also available.)

@cindex @command{gawk}, coding style
@cindex coding style used in @command{gawk}
@item
Use the @command{gawk} coding style.
The C code for @command{gawk} follows the instructions in the
@cite{GNU Coding Standards}, with minor exceptions.  The code is formatted
using the traditional ``K&R'' style, particularly as regards to the placement
of braces and the use of tabs.  In brief, the coding rules for @command{gawk}
are as follows:

@itemize @bullet
@item
Use ANSI/ISO style (prototype) function headers when defining functions.

@item
Put the name of the function at the beginning of its own line.

@item
Put the return type of the function, even if it is @code{int}, on the
line above the line with the name and arguments of the function.

@item
Put spaces around parentheses used in control structures
(@code{if}, @code{while}, @code{for}, @code{do}, @code{switch},
and @code{return}).

@item
Do not put spaces in front of parentheses used in function calls.

@item
Put spaces around all C operators and after commas in function calls.

@item
Do not use the comma operator to produce multiple side effects, except
in @code{for} loop initialization and increment parts, and in macro bodies.

@item
Use real tabs for indenting, not spaces.

@item
Use the ``K&R'' brace layout style.

@item
Use comparisons against @code{NULL} and @code{'\0'} in the conditions of
@code{if}, @code{while}, and @code{for} statements, as well as in the @code{case}s
of @code{switch} statements, instead of just the
plain pointer or character value.

@item
Use the @code{TRUE}, @code{FALSE} and @code{NULL} symbolic constants
and the character constant @code{'\0'} where appropriate, instead of @code{1}
and @code{0}.

@item
Use the @code{ISALPHA}, @code{ISDIGIT}, etc.@: macros, instead of the
traditional lowercase versions; these macros are better behaved for
non-ASCII character sets.

@item
Provide one-line descriptive comments for each function.

@item
Do not use @samp{#elif}. Many older Unix C compilers cannot handle it.

@item
Do not use the @code{alloca} function for allocating memory off the stack.
Its use causes more portability trouble than is worth the minor benefit of not having
to free the storage. Instead, use @code{malloc} and @code{free}.
@end itemize

@strong{Note:}
If I have to reformat your code to follow the coding style used in
@command{gawk}, I may not bother to integrate your changes at all.

@item
Be prepared to sign the appropriate paperwork.
In order for the FSF to distribute your changes, you must either place
those changes in the public domain and submit a signed statement to that
effect, or assign the copyright in your changes to the FSF.
Both of these actions are easy to do and @emph{many} people have done so
already. If you have questions, please contact me
(@pxref{Bugs, , Reporting Problems and Bugs}),
or @email{gnu@@gnu.org}.

@cindex Texinfo
@item
Update the documentation.
Along with your new code, please supply new sections and/or chapters
for this @value{DOCUMENT}.  If at all possible, please use real
Texinfo, instead of just supplying unformatted ASCII text (although
even that is better than no documentation at all).
Conventions to be followed in @cite{@value{TITLE}} are provided
after the @samp{@@bye} at the end of the Texinfo source file.
If possible, please update the @command{man} page as well.

You will also have to sign paperwork for your documentation changes.

@item
Submit changes as context diffs or unified diffs.
Use @samp{diff -c -r -N} or @samp{diff -u -r -N} to compare
the original @command{gawk} source tree with your version.
(I find context diffs to be more readable but unified diffs are
more compact.)
I recommend using the GNU version of @command{diff}.
Send the output produced by either run of @command{diff} to me when you
submit your changes.
(@xref{Bugs, , Reporting Problems and Bugs}, for the electronic mail
information.)

Using this format makes it easy for me to apply your changes to the
master version of the @command{gawk} source code (using @code{patch}).
If I have to apply the changes manually, using a text editor, I may
not do so, particularly if there are lots of changes.

@item
Include an entry for the @file{ChangeLog} file with your submission.
This helps further minimize the amount of work I have to do,
making it easier for me to accept patches.
@end enumerate

Although this sounds like a lot of work, please remember that while you
may write the new code, I have to maintain it and support it. If it
isn't possible for me to do that with a minimum of extra work, then I
probably will not.

@node New Ports, , Adding Code, Additions
@appendixsubsec Porting @command{gawk} to a New Operating System

@cindex porting @command{gawk}
If you want to port @command{gawk} to a new operating system, there are
several steps to follow:

@enumerate 1
@item
Follow the guidelines in
@ifinfo
@ref{Adding Code, ,Adding New Features},
@end ifinfo
@ifnotinfo
the previous @value{SECTION}
@end ifnotinfo
concerning coding style, submission of diffs, and so on.

@item
When doing a port, bear in mind that your code must co-exist peacefully
with the rest of @command{gawk} and the other ports. Avoid gratuitous
changes to the system-independent parts of the code. If at all possible,
avoid sprinkling @samp{#ifdef}s just for your port throughout the
code.

@cindex GPL
@cindex General Public License
@cindex GNU General Public License
If the changes needed for a particular system affect too much of the
code, I probably will not accept them.  In such a case, you can, of course,
distribute your changes on your own, as long as you comply
with the GPL
(@pxref{Copying, ,GNU General Public License}).

@item
A number of the files that come with @command{gawk} are maintained by other
people at the Free Software Foundation.  Thus, you should not change them
unless it is for a very good reason; i.e., changes are not out of the
question, but changes to these files are scrutinized extra carefully.
The files are @file{getopt.h}, @file{getopt.c},
@file{getopt1.c}, @file{regex.h}, @file{regex.c}, @file{dfa.h},
@file{dfa.c}, @file{install-sh}, and @file{mkinstalldirs}.

@item
Be willing to continue to maintain the port.
Non-Unix operating systems are supported by volunteers who maintain
the code needed to compile and run @command{gawk} on their systems. If noone
volunteers to maintain a port, it becomes unsupported and it may
be necessary to remove it from the distribution.

@item
Supply an appropriate @file{gawkmisc.???} file.
Each port has its own @file{gawkmisc.???} that implements certain
operating system specific functions. This is cleaner than a plethora of
@samp{#ifdef}s scattered throughout the code.  The @file{gawkmisc.c} in
the main source directory includes the appropriate
@file{gawkmisc.???} file from each subdirectory.
Be sure to update it as well.

Each port's @file{gawkmisc.???} file has a suffix reminiscent of the machine
or operating system for the port---for example, @file{pc/gawkmisc.pc} and
@file{vms/gawkmisc.vms}. The use of separate suffixes, instead of plain
@file{gawkmisc.c}, makes it possible to move files from a port's subdirectory
into the main subdirectory, without accidentally destroying the real
@file{gawkmisc.c} file.  (Currently, this is only an issue for the
PC operating system ports.)

@item
Supply a @file{Makefile} as well as any other C source and header files that are
necessary for your operating system.  All your code should be in a
separate subdirectory, with a name that is the same as, or reminiscent
of, either your operating system or the computer system.  If possible,
try to structure things so that it is not necessary to move files out
of the subdirectory into the main source directory.  If that is not
possible, then be sure to avoid using names for your files that
duplicate the names of files in the main source directory.

@item
Update the documentation.
Please write a section (or sections) for this @value{DOCUMENT} describing the
installation and compilation steps needed to compile and/or install
@command{gawk} for your system.

@item
Be prepared to sign the appropriate paperwork.
In order for the FSF to distribute your code, you must either place
your code in the public domain and submit a signed statement to that
effect, or assign the copyright in your code to the FSF.
@ifinfo
Both of these actions are easy to do and @emph{many} people have done so
already. If you have questions, please contact me, or
@email{gnu@@gnu.org}.
@end ifinfo
@end enumerate

Following these steps makes it much easier to integrate your changes
into @command{gawk} and have them co-exist happily with other
operating systems' code that is already there.

In the code that you supply and maintain, feel free to use a
coding style and brace layout that suits your taste.

@node Dynamic Extensions, Future Extensions, Additions, Notes
@appendixsec Adding New Built-in Functions to @command{gawk}
@cindex Robinson, Will
@cindex robot, the
@cindex Lost In Space
@quotation
@i{Danger Will Robinson!  Danger!!@*
Warning! Warning!}@*
The Robot
@end quotation

@cindex Linux
@cindex GNU/Linux
Beginning with @command{gawk} 3.1, it is possible to add new built-in
functions to @command{gawk} using dynamically loaded libraries. This
facility is available on systems (such as GNU/Linux) that support
the @code{dlopen} and @code{dlsym} functions.
This @value{SECTION} describes how to write and use dynamically
loaded extentions for @command{gawk}.
Experience with programming in
C or C++ is necessary when reading this @value{SECTION}.

@strong{Caution:} The facilities described in this @value{SECTION}
are very much subject to change in the next @command{gawk} release.
Be aware that you may have to re-do everything, perhaps from scratch,
upon the next release.

@menu
* Internals::                   A brief look at some @command{gawk} internals.
* Sample Library::              A example of new functions.
@end menu

@node Internals, Sample Library, Dynamic Extensions, Dynamic Extensions
@appendixsubsec A Minimal Introduction to @command{gawk} Internals

The truth is that @command{gawk} was not designed for simple extensibility.
The facilities for adding functions using shared libraries work, but
are something of a ``bag on the side.''  Thus, this tour is
brief and simplistic; would-be @command{gawk} hackers are encouraged to
spend some time reading the source code before trying to write
extensions based on the material presented here.  Of particular note
are the files @file{awk.h}, @file{builtin.c}, and @file{eval.c}.
Reading @file{awk.y} in order to see how the parse tree is built
would also be of use.

With the disclaimers out of the way, the following types, structure
members, functions, and macros are declared in @file{awk.h} and are of
use when writing extensions.  The next @value{SECTION}
shows how they are used:

@table @code
@cindex @code{AWKNUM} internal type
@cindex internal type, @code{AWKNUM}
@item AWKNUM
An @code{AWKNUM} is the internal type of @command{awk}
floating-point numbers.  Typically, it is a C @code{double}.

@cindex @code{NODE} internal type
@cindex internal type, @code{NODE}
@item NODE
Just about everything is done using objects of type @code{NODE}.
These contain both strings and numbers, as well as variables and arrays.

@cindex @code{force_number} internal function
@cindex internal function, @code{force_number}
@item AWKNUM force_number(NODE *n)
This macro forces a value to be numeric. It returns the actual
numeric value contained in the node.
It may end up calling an internal @command{gawk} function.

@cindex @code{force_string} internal function
@cindex internal function, @code{force_string}
@item void force_string(NODE *n)
This macro guarantees that a @code{NODE}'s string value is current.
It may end up calling an internal @command{gawk} function.
It also guarantees that the string is zero-terminated.

@cindex @code{param_cnt} internal variable
@cindex internal variable, @code{param_cnt}
@item n->param_cnt
The number of parameters actually passed in a function call at runtime.

@cindex @code{stptr} internal variable
@cindex @code{stlen} internal variable
@cindex internal variable, @code{stptr}
@cindex internal variable, @code{stlen}
@item n->stptr
@itemx n->stlen
The data and length of a @code{NODE}'s string value, respectively.
The string is @emph{not} guaranteed to be zero-terminated.
If you need to pass the string value to a C library function, save
the value in @code{n->stptr[n->stlen]}, assign @code{'\0'} to it,
call the routine, and then restore the value.

@cindex @code{type} internal variable
@cindex internal variable, @code{type}
@item n->type
The type of the @code{NODE}. This is a C @code{enum}. Values should
be either @code{Node_var} or @code{Node_var_array} for function
parameters.

@cindex @code{vname} internal variable
@cindex internal variable, @code{vname}
@item n->vname
The ``variable name'' of a node.  This is not of much use inside
externally written extensions.

@cindex @code{assoc_clear} internal function
@cindex internal function, @code{assoc_clear}
@item void assoc_clear(NODE *n)
Clears the associative array pointed to by @code{n}.
Make sure that @samp{n->type == Node_var_array} first.

@cindex @code{assoc_lookup} internal function
@cindex internal function, @code{assoc_lookup}
@item NODE **assoc_lookup(NODE *symbol, NODE *subs, int reference)
Finds, and installs if necessary, array elements.
@code{symbol} is the array, @code{subs} is the subscript.
This is usually a value created with @code{tmp_string} (see below).
@code{reference} should be @code{TRUE} if it is an error to use the
value before it is created. Typically, @code{FALSE} is the
correct value to use from extension functions.

@cindex @code{make_string} internal function
@cindex internal function, @code{make_string}
@item NODE *make_string(char *s, size_t len)
Take a C string and turn it into a pointer to a @code{NODE} that
can be stored appropriately.  This is permanent storage; understanding
of @command{gawk} memory management is helpful.

@cindex @code{make_number} internal function
@cindex internal function, @code{make_number}
@item NODE *make_number(AWKNUM val)
Take an @code{AWKNUM} and turn it into a pointer to a @code{NODE} that
can be stored appropriately.  This is permanent storage; understanding
of @command{gawk} memory management is helpful.

@cindex @code{tmp_string} internal function
@item NODE *tmp_string(char *s, size_t len);
@cindex internal function, @code{tmp_string}
Take a C string and turn it into a pointer to a @code{NODE} that
can be stored appropriately.  This is temporary storage; understanding
of @command{gawk} memory management is helpful.

@cindex @code{tmp_number} internal function
@item NODE *tmp_number(AWKNUM val)
@cindex internal function, @code{tmp_number}
Take an @code{AWKNUM} and turn it into a pointer to a @code{NODE} that
can be stored appropriately.  This is temporary storage;
understanding of @command{gawk} memory management is helpful.

@cindex @code{dupnode} internal function
@cindex internal function, @code{dupnode}
@item NODE *dupnode(NODE *n)
Duplicate a node.  In most cases, this increments an internal
reference count instead of actually duplicating the entire @code{NODE};
understanding of @command{gawk} memory management is helpful.

@cindex @code{free_temp} internal macro
@cindex internal macro, @code{free_temp}
@item void free_temp(NODE *n)
This macro releases the memory associated with a @code{NODE}
allocated with @code{tmp_string} or @code{tmp_number}.
Understanding of @command{gawk} memory management is helpful.

@cindex @code{make_builtin} internal function
@cindex internal function, @code{make_builtin}
@item void make_builtin(char *name, NODE *(*func)(NODE *), int count)
Register a C function pointed to by @code{func} as new built-in
function @code{name}. @code{name} is a regular C string. @code{count}
is the maximum number of arguments that the function takes.
The function should be written in the following manner:

@example
/* do_xxx --- do xxx function for gawk */

NODE *
do_xxx(NODE *tree)
@{
    @dots{}
@}
@end example

@cindex @code{get_argument} internal function
@cindex internal function, @code{get_argument}
@item NODE *get_argument(NODE *tree, int i)
This function is called from within a C extension function to get
the @code{i}'th argument from the function call.
The first argument is argument zero.

@cindex @code{set_value} internal function
@item void set_value(NODE *tree)
@cindex internal function, @code{set_value}
This function is called from within a C extension function to set
the return value from the extension function.  This value is
what the @command{awk} program sees as the return value from the
new @command{awk} function.

@cindex @code{update_ERRNO} internal function
@item void update_ERRNO(void)
@cindex internal function, @code{update_ERRNO}
This function is called from within a C extension function to set
the value of @command{gawk}'s @code{ERRNO} variable, based on the current
value of the C @code{errno} variable.
It is provided as a convenience.
@end table

An argument that is supposed to be an array needs to be handled with
some extra code, in case the array being passed in is actually
from a function parameter.
The following ``boiler plate'' code shows how to do this:

@smallexample
NODE *the_arg;

the_arg = get_argument(tree, 2); /* assume need 3rd arg, 0-based */

/* if a parameter, get it off the stack */
if (the_arg->type == Node_param_list)
    the_arg = stack_ptr[the_arg->param_cnt];

/* parameter referenced an array, get it */
if (the_arg->type == Node_array_ref)
    the_arg = the_arg->orig_array;

/* check type */
if (the_arg->type != Node_var && the_arg->type != Node_var_array)
    fatal("newfunc: third argument is not an array");

/* force it to be an array, if necessary, clear it */
the_arg->type = Node_var_array;
assoc_clear(the_arg);
@end smallexample

Again, you should spend time studying the @command{gawk} internals;
don't just blindly copy this code.

@node Sample Library, , Internals, Dynamic Extensions
@appendixsubsec Directory and File Operation Built-ins

Two useful functions that are not in @command{awk} are @code{chdir}
(so that an @command{awk} program can change its directory) and
@code{stat} (so that an @command{awk} program can gather information about
a file).
This @value{SECTION} implements these functions for @command{gawk} in an
external extension library.

@menu
* Internal File Description::   What the new functions will do.
* Internal File Ops::           The code for internal file operations.
* Using Internal File Ops::     How to use an external extension.
@end menu

@node Internal File Description, Internal File Ops, Sample Library, Sample Library
@appendixsubsubsec Using @code{chdir} and @code{stat}

This @value{SECTION} shows how to use the new functions at the @command{awk}
level once they've been integrated into the running @command{gawk}
interpreter.
Using @code{chdir} is very straightforward. It takes one argument,
the new directory to change to:

@example
@dots{}
newdir = "/home/arnold/funstuff"
ret = chdir(newdir)
if (ret < 0) @{
    printf("could not change to %s: %s\n",
                   newdir, ERRNO) > "/dev/stderr"
    exit 1
@}
@dots{}
@end example

The return value is negative if the @code{chdir} failed,
and @code{ERRNO}
(@pxref{Built-in Variables})
is set to a string indicating the error.

Using @code{stat} is a bit more complicated.
The C @code{stat} function fills in a structure that has a fair
amount of information.
The right way to model this in @command{awk} is to fill in an associative
array with the appropriate information:

@c broke printf for page breaking
@example
file = "/home/arnold/.profile"
fdata[1] = "x"    # force `fdata' to be an array
ret = stat(file, fdata)
if (ret < 0) @{
    printf("could not stat %s: %s\n",
             file, ERRNO) > "/dev/stderr"
    exit 1
@}
printf("size of %s is %d bytes\n", file, fdata["size"])
@end example

The @code{stat} function always clears the data array, even if
the @code{stat} fails.  It fills in the following elements:

@table @code
@item "name"
The name of the file that was @code{stat}'ed.

@item "dev"
@itemx "ino"
The file's device and inode numbers, respectively.

@item "mode"
The file's mode, as a numeric value. This includes both the file's
type and its permissions.

@item "nlink"
The number of hard links (directory entries) the file has.

@item "uid"
@itemx "gid"
The numeric user and group ID numbers of the file's owner.

@item "size"
The size in bytes of the file.

@item "blocks"
The number of disk blocks the file actually occupies. This may not
be a function of the file's size if the file has holes.

@item "atime"
@itemx "mtime"
@itemx "ctime"
The file's last access, modification, and inode update times,
respectively.  These are numeric timestamps, suitable for formatting
with @code{strftime}
(@pxref{Built-in, ,Built-in Functions}).

@item "pmode"
The file's ``printable mode.''  This is a string representation of
the file's type and permissions, such as what is produced by
@samp{ls -l}---for example, @code{"drwxr-xr-x"}.

@item "type"
A printable string representation of the file's type.  The value
is one of the following:

@table @code
@item "blockdev"
@itemx "chardev"
The file is a block or character device (``special file'').

@ignore
@item "door"
The file is a Solaris ``door'' (special file used for
interprocess communications).
@end ignore

@item "directory"
The file is a directory.

@item "fifo"
The file is a named-pipe (also known as a FIFO).

@item "file"
The file is just a regular file.

@item "socket"
The file is an @code{AF_UNIX} (``Unix domain'') socket in the
filesystem.

@item "symlink"
The file is a symbolic link.
@end table
@end table

Several additional elements may be present depending upon the operating
system and the type of the file.  You can test for them in your @command{awk}
program by using the @code{in} operator
(@pxref{Reference to Elements, ,Referring to an Array Element}):

@table @code
@item "blksize"
The preferred block size for I/O to the file. This field is not
present on all POSIX-like systems in the C @code{stat} structure.

@item "linkval"
If the file is a symbolic link, this element is the name of the
file the link points to (i.e., the value of the link).

@item "rdev"
@itemx "major"
@itemx "minor"
If the file is a block or character device file, then these values
represent the numeric device number and the major and minor components
of that number, respectively.
@end table

@node Internal File Ops, Using Internal File Ops, Internal File Description, Sample Library
@appendixsubsubsec C Code for @code{chdir} and @code{stat}

@cindex Linux
@cindex GNU/Linux
Here is the C code for these extensions.  They were written for
GNU/Linux.  The code needs some more work for complete portability
to other POSIX-compliant systems:@footnote{This version is edited
slightly for presentation.  The complete version can be found in
@file{extension/filefuncs.c} in the @command{gawk} distribution.}

@c break line for page breaking
@example
#include "awk.h"

#include <sys/sysmacros.h>

/*  do_chdir --- provide dynamically loaded
                 chdir() builtin for gawk */

static NODE *
do_chdir(tree)
NODE *tree;
@{
    NODE *newdir;
    int ret = -1;

    newdir = get_argument(tree, 0);
@end example

The file includes the @code{"awk.h"} header file for definitions
for the @command{gawk} internals.  It includes @code{<sys/sysmacros.h>}
for access to the @code{major} and @code{minor} macros.

@cindex conventions, programming
@cindex programming conventions
By convention, for an @command{awk} function @code{foo}, the function that
implements it is called @samp{do_foo}.  The function should take
a @samp{NODE *} argument, usually called @code{tree}, that
represents the argument list to the function.  The @code{newdir}
variable represents the new directory to change to, retrieved
with @code{get_argument}.  Note that the first argument is
numbered zero.

This code actually accomplishes the @code{chdir}. It first forces
the argument to be a string and passes the string value to the
@code{chdir} system call. If the @code{chdir} fails, @code{ERRNO}
is updated.
The result of @code{force_string} has to be freed with @code{free_temp}:

@example
    if (newdir != NULL) @{
        (void) force_string(newdir);
        ret = chdir(newdir->stptr);
        if (ret < 0)
            update_ERRNO();

        free_temp(newdir);
    @}
@end example

Finally, the function returns the return value to the @command{awk} level,
using @code{set_value}. Then it must return a value from the call to
the new built-in (this value ignored by the interpreter):

@example
    /* Set the return value */
    set_value(tmp_number((AWKNUM) ret));

    /* Just to make the interpreter happy */
    return tmp_number((AWKNUM) 0);
@}
@end example

The @code{stat} built-in is more involved.  First comes a function
that turns a numeric mode into a printable representation
(e.g., 644 becomes @samp{-rw-r--r--}). This is omitted here for brevity:

@c break line for page breaking
@example
/* format_mode --- turn a stat mode field
                   into something readable */

static char *
format_mode(fmode)
unsigned long fmode;
@{
    @dots{}
@}
@end example

Next comes the actual @code{do_stat} function itself.  First come the
variable declarations and argument checking:

@ignore
Changed message for page breaking. Used to be:
    "stat: called with incorrect number of arguments (%d), should be 2",
@end ignore
@example
/* do_stat --- provide a stat() function for gawk */

static NODE *
do_stat(tree)
NODE *tree;
@{
    NODE *file, *array;
    struct stat sbuf;
    int ret;
    char *msg;
    NODE **aptr;
    char *pmode;    /* printable mode */
    char *type = "unknown";

    /* check arg count */
    if (tree->param_cnt != 2)
        fatal(
    "stat: called with %d arguments, should be 2",
            tree->param_cnt);
@end example

Then comes the actual work. First, we get the arguments.
Then, we always clear the array.  To get the file information,
we use @code{lstat}, in case the file is a symbolic link.
If there's an error, we set @code{ERRNO} and return:

@c comment made multiline for page breaking
@example
    /*
     * directory is first arg,
     * array to hold results is second
     */
    file = get_argument(tree, 0);
    array = get_argument(tree, 1);

    /* empty out the array */
    assoc_clear(array);

    /* lstat the file, if error, set ERRNO and return */
    (void) force_string(file);
    ret = lstat(file->stptr, & sbuf);
    if (ret < 0) @{
        update_ERRNO();

        set_value(tmp_number((AWKNUM) ret));

        free_temp(file);
        return tmp_number((AWKNUM) 0);
    @}
@end example

Now comes the tedious part: filling in the array.  Only a few of the
calls are shown here, since they all follow the same pattern:

@example
    /* fill in the array */
    aptr = assoc_lookup(array, tmp_string("name", 4), FALSE);
    *aptr = dupnode(file);

    aptr = assoc_lookup(array, tmp_string("mode", 4), FALSE);
    *aptr = make_number((AWKNUM) sbuf.st_mode);

    aptr = assoc_lookup(array, tmp_string("pmode", 5), FALSE);
    pmode = format_mode(sbuf.st_mode);
    *aptr = make_string(pmode, strlen(pmode));
@end example

When done, we free the temporary value containing the @value{FN},
set the return value, and return:

@example
    free_temp(file);

    /* Set the return value */
    set_value(tmp_number((AWKNUM) ret));

    /* Just to make the interpreter happy */
    return tmp_number((AWKNUM) 0);
@}
@end example

@cindex conventions, programming
@cindex programming conventions
Finally, it's necessary to provide the ``glue'' that loads the
new function(s) into @command{gawk}.  By convention, each library has
a routine named @code{dlload} that does the job:

@example
/* dlload --- load new builtins in this library */

NODE *
dlload(tree, dl)
NODE *tree;
void *dl;
@{
    make_builtin("chdir", do_chdir, 1);
    make_builtin("stat", do_stat, 2);
    return tmp_number((AWKNUM) 0);
@}
@end example

And that's it!  As an exercise, consider adding functions to
implement system calls such as @code{chown}, @code{chmod}, and @code{umask}.

@node Using Internal File Ops, , Internal File Ops, Sample Library
@appendixsubsubsec Integrating the Extensions

@cindex Linux
@cindex GNU/Linux
Now that the code is written, it must be possible to add it at
runtime to the running @command{gawk} interpreter.  First, the
code must be compiled.  Assuming that the functions are in
a file named @file{filefuncs.c}, and @var{idir} is the location
of the @command{gawk} include files,
the following steps create
a GNU/Linux shared library:

@example
$ gcc -shared -DHAVE_CONFIG_H -c -O -g -I@var{idir} filefuncs.c
$ ld -o filefuncs.so -shared filefuncs.o
@end example

@cindex @code{extension} built-in function
Once the library exists, it is loaded by calling the @code{extension}
built-in function.
This function takes two arguments: the name of the
library to load and the name of a function to call when the library
is first loaded. This function adds the new functions to @command{gawk}.
It returns the value returned by the initialization function
within the shared library:

@example
# file testff.awk
BEGIN @{
    extension("./filefuncs.so", "dlload")

    chdir(".")  # no-op

    data[1] = 1 # force `data' to be an array
    print "Info for testff.awk"
    ret = stat("testff.awk", data)
    print "ret =", ret
    for (i in data)
        printf "data[\"%s\"] = %s\n", i, data[i]
    print "testff.awk modified:",
        strftime("%m %d %y %H:%M:%S", data["mtime"])
@}
@end example

Here are the results of running the program:

@example
$ gawk -f testff.awk 
@print{} Info for testff.awk
@print{} ret = 0
@print{} data["blksize"] = 4096
@print{} data["mtime"] = 932361936
@print{} data["mode"] = 33188
@print{} data["type"] = file
@print{} data["dev"] = 2065
@print{} data["gid"] = 10
@print{} data["ino"] = 878597
@print{} data["ctime"] = 971431797
@print{} data["blocks"] = 2
@print{} data["nlink"] = 1
@print{} data["name"] = testff.awk
@print{} data["atime"] = 971608519
@print{} data["pmode"] = -rw-r--r--
@print{} data["size"] = 607
@print{} data["uid"] = 2076
@print{} testff.awk modified: 07 19 99 08:25:36
@end example

@node Future Extensions, , Dynamic Extensions, Notes
@appendixsec Probable Future Extensions
@ignore
From emory!scalpel.netlabs.com!lwall Tue Oct 31 12:43:17 1995
Return-Path: <emory!scalpel.netlabs.com!lwall>
Message-Id: <9510311732.AA28472@scalpel.netlabs.com>
To: arnold@skeeve.atl.ga.us (Arnold D. Robbins)
Subject: Re: May I quote you?
In-Reply-To: Your message of "Tue, 31 Oct 95 09:11:00 EST."
             <m0tAHPQ-00014MC@skeeve.atl.ga.us>
Date: Tue, 31 Oct 95 09:32:46 -0800
From: Larry Wall <emory!scalpel.netlabs.com!lwall>

: Greetings. I am working on the release of gawk 3.0. Part of it will be a
: thoroughly updated manual. One of the sections deals with planned future
: extensions and enhancements.  I have the following at the beginning
: of it:
:
: @cindex PERL
: @cindex Wall, Larry
: @display
: @i{AWK is a language similar to PERL, only considerably more elegant.} @*
: Arnold Robbins
: @sp 1
: @i{Hey!} @*
: Larry Wall
: @end display
:
: Before I actually release this for publication, I wanted to get your
: permission to quote you.  (Hopefully, in the spirit of much of GNU, the
: implied humor is visible... :-)

I think that would be fine.

Larry
@end ignore
@cindex PERL
@cindex Wall, Larry
@cindex Robbins, Arnold
@quotation
@i{AWK is a language similar to PERL, only considerably more elegant.}@*
Arnold Robbins

@i{Hey!}@*
Larry Wall
@end quotation

This @value{SECTION} briefly lists extensions and possible improvements
that indicate the directions we are
currently considering for @command{gawk}.  The file @file{FUTURES} in the
@command{gawk} distribution lists these extensions as well.

Following is a list of probable future changes visible at the
@command{awk} language level:

@c these are ordered by likelihood
@table @asis
@item Loadable Module Interface
It is not clear that the @command{awk}-level interface to the
modules facility is as good as it should be.  The interface needs to be
redesigned, particularly taking namespace issues into account, as
well as possibly including issues such as library search path order
and versioning.

@item @code{RECLEN} variable for fixed length records
Along with @code{FIELDWIDTHS}, this would speed up the processing of
fixed-length records.
@code{PROCINFO["RS"]} would be @code{"RS"} or @code{"RECLEN"},
depending upon which kind of record processing is in effect.

@item Additional @code{printf} specifiers
The 1999 ISO C standard added a number of additional @code{printf}
format specifiers.  These should be evaluated for possible inclusion
in @command{gawk}.

@ignore
@item A @samp{%'d} flag
Add @samp{%'d} for putting in commas in formatting numeric values.
@end ignore

@item Databases
It may be possible to map a GDBM/NDBM/SDBM file into an @command{awk} array.

@item Large Character Sets
It would be nice if @command{gawk} could handle UTF-8 and other
character sets that are larger than eight bits.

@item More @code{lint} warnings
There are more things that could be checked for portability.
@end table

Following is a list of probable improvements that will make @command{gawk}'s
source code easier to work with:

@table @asis
@item Loadable Module Mechanics
The current extension mechanism works
(@pxref{Dynamic Extensions, ,Adding New Built-in Functions to @command{gawk}}),
but is rather primitive. It requires a fair amount of manual work
to create and integrate a loadable module.
Nor is the current mechanism as portable as might be desired.
The GNU @command{libtool} package provides a number of features that
would make using loadable modules much easier.
@command{gawk} should be changed to use @command{libtool}.

@item Loadable Module Internals
The API to its internals that @command{gawk} ``exports'' should be revised.
Too many things are needlessly exposed.  A new API should be designed
and implemented to make module writing easier.

@item Better Array Subscript Management
@command{gawk}'s management of array subscript storage could use revamping,
so that using the same value to index multiple arrays only
stores one copy of the index value.

@item Integrating the DBUG Library
Integrating Fred Fish's DBUG library would be helpful during development,
but it's a lot of work to do.
@end table

Following is a list of probable improvements that will make @command{gawk}
perform better:

@table @asis
@item An Improved Version of @code{dfa}
The @code{dfa} pattern matcher from GNU @command{grep} has some
problems. Either a new version or a fixed one will deal with some
important regexp matching issues.

@c NEXT ED: remove this item. awka and mawk do these respectively
@item Compilation of @command{awk} programs
@command{gawk} uses a Bison (YACC-like)
parser to convert the script given it into a syntax tree; the syntax
tree is then executed by a simple recursive evaluator.  This method incurs
a lot of overhead, since the recursive evaluator performs many procedure
calls to do even the simplest things.

It should be possible for @command{gawk} to convert the script's parse tree
into a C program which the user would then compile, using the normal
C compiler and a special @command{gawk} library to provide all the needed
functions (regexps, fields, associative arrays, type coercion, and so on).

An easier possibility might be for an intermediate phase of @command{gawk} to
convert the parse tree into a linear byte code form like the one used
in GNU Emacs Lisp.  The recursive evaluator would then be replaced by
a straight line byte code interpreter that would be intermediate in speed
between running a compiled program and doing what @command{gawk} does
now.
@end table

Finally,
the programs in the test suite could use documenting in this @value{DOCUMENT}.

@xref{Additions, ,Making Additions to @command{gawk}},
if you are interested in tackling any of these projects.

@node Basic Concepts, Glossary, Notes, Top
@appendix Basic Programming Concepts
@cindex basic programming concepts
@cindex programming concepts, basic

This @value{APPENDIX} attempts to define some of the basic concepts
and terms that are used throughout the rest of this @value{DOCUMENT}.
As this @value{DOCUMENT} is specifically about @command{awk},
and not about computer programming in general, the coverage here
is by necessity fairly cursory and simplistic.
(If you need more background, there are many
other introductory texts that you should refer to instead.)

@menu
* Basic High Level::            The high level view.
* Basic Data Typing::           A very quick intro to data types.
* Floating Point Issues::       Stuff to know about floating-point numbers.
@end menu

@node Basic High Level, Basic Data Typing, Basic Concepts, Basic Concepts
@appendixsec What a Program Does

@cindex processing data
At the most basic level, the job of a program is to process
some input data and produce results.

@c NEXT ED: Use real images here
@iftex
@tex
\expandafter\ifx\csname graph\endcsname\relax \csname newbox\endcsname\graph\fi
\expandafter\ifx\csname graphtemp\endcsname\relax \csname newdimen\endcsname\graphtemp\fi
\setbox\graph=\vtop{\vskip 0pt\hbox{%
    \special{pn 20}%
    \special{pa 2425 200}%
    \special{pa 2850 200}%
    \special{fp}%
    \special{sh 1.000}%
    \special{pn 20}%
    \special{pa 2750 175}%
    \special{pa 2850 200}%
    \special{pa 2750 225}%
    \special{pa 2750 175}%
    \special{fp}%
    \special{pn 20}%
    \special{pa 850 200}%
    \special{pa 1250 200}%
    \special{fp}%
    \special{sh 1.000}%
    \special{pn 20}%
    \special{pa 1150 175}%
    \special{pa 1250 200}%
    \special{pa 1150 225}%
    \special{pa 1150 175}%
    \special{fp}%
    \special{pn 20}%
    \special{pa 2950 400}%
    \special{pa 3650 400}%
    \special{pa 3650 0}%
    \special{pa 2950 0}%
    \special{pa 2950 400}%
    \special{fp}%
    \special{pn 10}%
    \special{ar 1800 200 450 200 0 6.28319}%
    \graphtemp=.5ex\advance\graphtemp by 0.200in
    \rlap{\kern 3.300in\lower\graphtemp\hbox to 0pt{\hss Results\hss}}%
    \graphtemp=.5ex\advance\graphtemp by 0.200in
    \rlap{\kern 1.800in\lower\graphtemp\hbox to 0pt{\hss Program\hss}}%
    \special{pn 10}%
    \special{pa 0 400}%
    \special{pa 700 400}%
    \special{pa 700 0}%
    \special{pa 0 0}%
    \special{pa 0 400}%
    \special{fp}%
    \graphtemp=.5ex\advance\graphtemp by 0.200in
    \rlap{\kern 0.350in\lower\graphtemp\hbox to 0pt{\hss Data\hss}}%
    \hbox{\vrule depth0.400in width0pt height 0pt}%
    \kern 3.650in
  }%
}%
\centerline{\box\graph}
@end tex
@end iftex
@ifnottex
@example
                  _______
+------+         /       \         +---------+
| Data | -----> < Program > -----> | Results |
+------+         \_______/         +---------+
@end example
@end ifnottex

@cindex compiled programs
@cindex programs, compiled
@cindex interpreted programs
@cindex programs, interpreted
The ``program'' in the figure can be either a compiled
program@footnote{Compiled programs are typically written
in lower-level languages such as C, C++, Fortran, or Ada,
and then translated, or @dfn{compiled}, into a form that
the computer can execute directly.}
(such as @command{ls}),
or it may be @dfn{interpreted}.  In the latter case, a machine-executable
program such as @command{awk} reads your program, and then uses the
instructions in your program to process the data.

@cindex programming, basic steps
When you write a program, it usually consists
of the following, very basic set of steps:

@c NEXT ED: Use real images here
@iftex
@tex
\expandafter\ifx\csname graph\endcsname\relax \csname newbox\endcsname\graph\fi
\expandafter\ifx\csname graphtemp\endcsname\relax \csname newdimen\endcsname\graphtemp\fi
\setbox\graph=\vtop{\vskip 0pt\hbox{%
    \graphtemp=.5ex\advance\graphtemp by 0.600in
    \rlap{\kern 2.800in\lower\graphtemp\hbox to 0pt{\hss Yes\hss}}%
    \graphtemp=.5ex\advance\graphtemp by 0.100in
    \rlap{\kern 3.300in\lower\graphtemp\hbox to 0pt{\hss No\hss}}%
    \special{pn 8}%
    \special{pa 2100 1000}%
    \special{pa 1600 1000}%
    \special{pa 1600 1000}%
    \special{pa 1600 300}%
    \special{fp}%
    \special{sh 1.000}%
    \special{pn 8}%
    \special{pa 1575 400}%
    \special{pa 1600 300}%
    \special{pa 1625 400}%
    \special{pa 1575 400}%
    \special{fp}%
    \special{pn 8}%
    \special{pa 2600 500}%
    \special{pa 2600 900}%
    \special{fp}%
    \special{sh 1.000}%
    \special{pn 8}%
    \special{pa 2625 800}%
    \special{pa 2600 900}%
    \special{pa 2575 800}%
    \special{pa 2625 800}%
    \special{fp}%
    \special{pn 8}%
    \special{pa 3200 200}%
    \special{pa 4000 200}%
    \special{fp}%
    \special{sh 1.000}%
    \special{pn 8}%
    \special{pa 3900 175}%
    \special{pa 4000 200}%
    \special{pa 3900 225}%
    \special{pa 3900 175}%
    \special{fp}%
    \special{pn 8}%
    \special{pa 1400 200}%
    \special{pa 2100 200}%
    \special{fp}%
    \special{sh 1.000}%
    \special{pn 8}%
    \special{pa 2000 175}%
    \special{pa 2100 200}%
    \special{pa 2000 225}%
    \special{pa 2000 175}%
    \special{fp}%
    \special{pn 8}%
    \special{ar 2600 1000 400 100 0 6.28319}%
    \graphtemp=.5ex\advance\graphtemp by 1.000in
    \rlap{\kern 2.600in\lower\graphtemp\hbox to 0pt{\hss Process\hss}}%
    \special{pn 8}%
    \special{pa 2200 400}%
    \special{pa 3100 400}%
    \special{pa 3100 0}%
    \special{pa 2200 0}%
    \special{pa 2200 400}%
    \special{fp}%
    \graphtemp=.5ex\advance\graphtemp by 0.200in
    \rlap{\kern 2.688in\lower\graphtemp\hbox to 0pt{\hss More Data?\hss}}%
    \special{pn 8}%
    \special{ar 650 200 650 200 0 6.28319}%
    \graphtemp=.5ex\advance\graphtemp by 0.200in
    \rlap{\kern 0.613in\lower\graphtemp\hbox to 0pt{\hss Initialization\hss}}%
    \special{pn 8}%
    \special{ar 0 200 0 0 0 6.28319}%
    \special{pn 8}%
    \special{ar 4550 200 450 100 0 6.28319}%
    \graphtemp=.5ex\advance\graphtemp by 0.200in
    \rlap{\kern 4.600in\lower\graphtemp\hbox to 0pt{\hss Clean Up\hss}}%
    \hbox{\vrule depth1.100in width0pt height 0pt}%
    \kern 5.000in
  }%
}%
\centerline{\box\graph}
@end tex
@end iftex
@ifnottex
@example
                              ______
+----------------+           / More \  No       +----------+
| Initialization | -------> <  Data  > -------> | Clean Up |
+----------------+    ^      \   ?  /           +----------+
                      |       +--+-+
                      |          | Yes
                      |          |
                      |          V
                      |     +---------+
                      +-----+ Process |
                            +---------+
@end example
@end ifnottex

@table @asis
@item Initialization
These are the things you do before actually starting to process
data, such as checking arguments, initializing any data you need
to work with, and so on.
This step corresponds to @command{awk}'s @code{BEGIN} rule
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).

If you were baking a cake, this might consist of laying out all the
mixing bowls and the baking pan, and making sure you have all the
ingredients that you need.

@item Processing
This is where the actual work is done.  Your program reads data,
one logical chunk at a time, and processes it as appropriate.

In most programming languages, you have to manually manage the reading
of data, checking to see if there is more each time you read a chunk.
@command{awk}'s pattern-action paradigm
(@pxref{Getting Started, ,Getting Started with @command{awk}})
handles the mechanics of this for you.

In baking a cake, the processing corresponds to the actual labor:
breaking eggs, mixing the flour, water, and other ingredients, and then putting the cake
into the oven.

@item Clean Up
Once you've processed all the data, you may have things you need to
do before exiting.
This step corresponds to @command{awk}'s @code{END} rule
(@pxref{BEGIN/END, ,The @code{BEGIN} and @code{END} Special Patterns}).

After the cake comes out of the oven, you still have to wrap it in
plastic wrap to keep anyone from tasting it, as well as wash
the mixing bowls and other utensils.
@end table

@cindex algorithm, definition of
An @dfn{algorithm} is a detailed set of instructions necessary to accomplish
a task, or process data.  It is much the same as a recipe for baking
a cake.  Programs implement algorithms.  Often, it is up to you to design
the algorithm and implement it, simultaneously.

@cindex record, definition of
@cindex fields, definition of
The ``logical chunks'' we talked about previously are called @dfn{records},
similar to the records a company keeps on employees, a school keeps for
students, or a doctor keeps for patients.
Each record has many component parts, such as first and last names,
date of birth, address, and so on.  The component parts are referred
to as the @dfn{fields} of the record.

The act of reading data is termed @dfn{input}, and that of
generating results, not too surprisingly, is termed @dfn{output}.
They are often referred to together as ``Input/Output,''
and even more often, as ``I/O'' for short.
(You will also see ``input'' and ``output'' used as verbs.)

@cindex data-driven languages
@cindex language, data-driven
@command{awk} manages the reading of data for you, as well as the
breaking it up into records and fields.  Your program's job is to
tell @command{awk} what to with the data.  You do this by describing
@dfn{patterns} in the data to look for, and @dfn{actions} to execute
when those patterns are seen.  This @dfn{data-driven} nature of
@command{awk} programs usually makes them both easier to write
and easier to read.

@node Basic Data Typing, Floating Point Issues, Basic High Level, Basic Concepts
@appendixsec Data Values in a Computer

@cindex variable, definition of
In a program,
you keep track of information and values in things called @dfn{variables}.
A variable is just a name for a given value, such as @code{first_name},
@code{last_name}, @code{address}, and so on.
@command{awk} has several pre-defined variables, and it has
special names to refer to the current input record
and the fields of the record.
You may also group multiple
associated values under one name, as an array.

@cindex values, numeric
@cindex values, string
@cindex scalar, definition of
Data, particularly in @command{awk}, consists of either numeric
values, such as 42 or 3.1415927, or string values.
String values are essentially anything that's not a number, such as a name.
Strings are sometimes referred to as @dfn{character data}, since they
store the individual characters that comprise them.
Individual variables, as well as numeric and string variables, are
referred to as @dfn{scalar} values.  
Groups of values, such as arrays, are not scalars.

@cindex integer, definition of
@cindex floating-point, definition of
Within computers, there are two kinds of numeric values: @dfn{integers},
and @dfn{floating-point}.
In school, integer values were referred to as ``whole'' numbers---that is,
numbers without any fractional part, such as 1, 42, or @minus{}17.
The advantage to integer numbers is that they represent values exactly.
The disadvantage is that their range is limited.  On most modern systems,
this range is @minus{}2,147,483,648 to 2,147,483,647.

@cindex unsigned integers
@cindex integer, unsigned
Integer values come in two flavors: @dfn{signed} and @dfn{unsigned}.
Signed values may be negative or positive, with the range of values just
described.
Unsigned values are always positive.  On most modern systems,
the range is from 0 to 4,294,967,295.

@cindex double-precision floating-point, definition of
@cindex single-precision floating-point, definition of
Floating-point numbers represent what are called ``real'' numbers; i.e.,
those that do have a fractional part, such as 3.1415927.
The advantage to floating-point numbers is that they
can represent a much larger range of values.
The disadvantage is that there are numbers that they cannot represent
exactly.
@command{awk} uses @dfn{double-precision} floating-point numbers, which
can hold more digits than @dfn{single-precision}
floating-point numbers.
Floating-point issues are discussed more fully in
@ref{Floating Point Issues, ,Floating-Point Number Caveats}.

At the very lowest level, computers store values as groups of binary digits,
or @dfn{bits}.  Modern computers group bits into groups of eight, called @dfn{bytes}.
Advanced applications sometimes have to manipulate bits directly,
and @command{gawk} provides functions for doing so.

@cindex null string, definition of
@cindex empty string, definition of
While you are probably used to the idea of a number without a value (i.e., zero),
it takes a bit more getting used to the idea of zero-length character data.
Nevertheless, such a thing exists.
It is called the @dfn{null string}.
The null string is character data that has no value.
In other words, it is empty.  It is written in @command{awk} programs
like this: @code{""}.

Humans are used to working in decimal; i.e., base 10.  In base 10,
numbers go from 0 to 9, and then ``roll over'' into the next
column.  (Remember grade school? 42 is 4 times 10 plus 2.)

There are other number bases though.  Computers commonly use base 2
or @dfn{binary}, base 8 or @dfn{octal}, and base 16 or @dfn{hexadecimal}.
In binary, each column represents two times the value in the column to
its right. Each column may contain either a 0 or a 1.
Thus, binary 1010 represents 1 times 8, plus 0 times 4, plus 1 times 2,
plus 0 times 1, or decimal 10.
Octal and hexadecimal are discussed more in
@ref{Non-decimal-numbers, ,Octal and Hexadecimal Numbers}.

Programs are written in programming languages.
Hundreds, if not thousands, of programming languages exist.
One of the most popular is the C programming language.
The C language had a very strong influence on the design of
the @command{awk} language.

@cindex Kernighan, Brian
@cindex Ritchie, Dennis
There have been several versions of C.  The first is often referred to
as ``K&R'' C, after the initials of Brian Kernighan and Dennis Ritchie,
the authors of the first book on C.  (Dennis Ritchie created the language,
and Brian Kernighan was one of the creators of @command{awk}.)

In the mid-1980's, an effort began to produce an international standard
for C.  This work culminated in 1989, with the production of the ANSI
standard for C.  This standard became an ISO standard in 1990.
Where it makes sense, POSIX @command{awk} is compatible with 1990 ISO C.

In 1999, a revised ISO C standard was approved and released.
Future versions of @command{gawk} will be as compatible as possible
with this standard.

@node Floating Point Issues, , Basic Data Typing, Basic Concepts
@appendixsec Floating-Point Number Caveats

As mentioned earlier, floating-point numbers represent what are called
``real'' numbers; i.e., those that have a fractional part.  @command{awk}
uses double-precision floating-point numbers to represent all
numeric values.  This @value{SECTION} describes some of the issues
involved in using floating-point numbers.

There is a very nice paper on floating-point arithmetic by
David Goldberg, @cite{What Every
Computer Scientist Should Know About Floating-point Arithmetic},
@cite{ACM Computing Surveys} @strong{23}, 1 (1991-03),
5-48.@footnote{@uref{http://www.validgh.com/goldberg/paper.ps}}
This is worth reading if you are interested in the details,
but it does require a background in Computer Science.

Internally, @command{awk} keeps both the numeric value
(double-precision floating-point) and the string value for a variable.
Separately, @command{awk} keeps
track of what type the variable has
(@pxref{Typing and Comparison, ,Variable Typing and Comparison Expressions}),
which plays a role in how variables are used in comparisons.

It is important to note that the string value for a number may not
reflect the full value (all the digits) that the numeric value
actually contains.
The following program (@file{values.awk}) illustrates this:

@example
@{
   $1 = $2 + $3
   # see it for what it is
   printf("$1 = %.12g\n", $1)
   # use CONVFMT
   a = "<" $1 ">"
   print "a =", a
@group
   # use OFMT
   print "$1 =", $1
@end group
@}
@end example

@noindent
This program shows the full value of the sum of @code{$2} and @code{$3}
using @code{printf}, and then prints the string values obtained
from both automatic conversion (via @code{CONVFMT}) and
from printing (via @code{OFMT}).

Here is what happens when the program is run:

@example
$ echo 2 3.654321 1.2345678 | awk -f values.awk
@print{} $1 = 4.8888888
@print{} a = <4.88889>
@print{} $1 = 4.88889
@end example

This makes it clear that the full numeric value is different from
what the default string representations show.

@code{CONVFMT}'s default value is @code{"%.6g"}, which yields a value with
at least six significant digits.  For some applications, you might want to
change it to specify more precision.
On most modern machines, most of the time,
17 digits is enough to capture a floating-point number's
value exactly.@footnote{Pathological cases can require up to
752 digits (!), but we doubt that you need to worry about this.}

@cindex floating-point, precision issues
Unlike numbers in the abstract sense (such as what you studied in high school
or college math), numbers stored in computers are limited in certain ways.
They cannot represent an infinite number of digits, nor can they always
represent things exactly.
In particular,
floating-point numbers cannot
always represent values exactly.  Here is an example:

@example
$ awk '@{ printf("%010d\n", $1 * 100) @}'
515.79
@print{} 0000051579
515.80
@print{} 0000051579
515.81
@print{} 0000051580
515.82
@print{} 0000051582
@kbd{Ctrl-d}
@end example

@noindent
This shows that some values can be represented exactly,
whereas others are only approximated.  This is not a ``bug''
in @command{awk}, but simply an artifact of how computers
represent numbers.

@cindex negative zero
@cindex positive zero
@cindex zero, negative vs.@: positive
@cindex floating-point, positive and negative values for zero
Another peculiarity of floating-point numbers on modern systems
is that they often have more than one representation for the number zero!
In particular, it is possible to represent ``minus zero'' as well as
regular, or ``positive'' zero.

This example shows that negative and positive zero are distinct values
when stored internally, but that they are in fact equal to each other,
as well as to ``regular'' zero:

@smallexample
$ gawk 'BEGIN @{ mz = -0 ; pz = 0 
> printf "-0 = %g, +0 = %g, (-0 == +0) -> %d\n", mz, pz, mz == pz
> printf "mz == 0 -> %d, pz == 0 -> %d\n", mz == 0, pz == 0
> @}'
@print{} -0 = -0, +0 = 0, (-0 == +0) -> 1
@print{} mz == 0 -> 1, pz == 0 -> 1
@end smallexample

It helps to keep this in mind should you process numeric data
that contains negative zero values; the fact that the zero is negative
is noted and can affect comparisons.

@node Glossary, Copying, Basic Concepts, Top
@unnumbered Glossary

@table @asis
@item Action
A series of @command{awk} statements attached to a rule.  If the rule's
pattern matches an input record, @command{awk} executes the
rule's action.  Actions are always enclosed in curly braces.
(@xref{Action Overview, ,Actions}.)

@cindex Spencer, Henry
@cindex @command{sed} utility
@cindex amazing @command{awk} assembler (@command{aaa})
@item Amazing @command{awk} Assembler
Henry Spencer at the University of Toronto wrote a retargetable assembler
completely as @command{sed} and @command{awk} scripts.  It is thousands
of lines long, including machine descriptions for several eight-bit
microcomputers.  It is a good example of a program that would have been
better written in another language.
You can get it from @uref{ftp://ftp.freefriends.org/arnold/Awkstuff/aaa.tgz}.

@cindex amazingly workable formatter (@command{awf})
@cindex @command{awf} (amazingly workable formatter) program
@item Amazingly Workable Formatter (@command{awf})
Henry Spencer at the University of Toronto wrote a formatter that accepts
a large subset of the @samp{nroff -ms} and @samp{nroff -man} formatting
commands, using @command{awk} and @command{sh}.
It is available over the Internet
from @uref{ftp://ftp.freefriends.org/arnold/Awkstuff/awf.tgz}.

@item Anchor
The regexp metacharacters @samp{^} and @samp{$}, which force the match
to the beginning or end of the string, respectively.

@cindex ANSI
@item ANSI
The American National Standards Institute.  This organization produces
many standards, among them the standards for the C and C++ programming
languages.
These standards often become international standards as well. See also
``ISO.''

@item Array
A grouping of multiple values under the same name.
Most languages just provide sequential arrays.
@command{awk} provides associative arrays.

@item Assertion
A statement in a program that a condition is true at this point in the program.
Useful for reasoning about how a program is supposed to behave.

@item Assignment
An @command{awk} expression that changes the value of some @command{awk}
variable or data object.  An object that you can assign to is called an
@dfn{lvalue}.  The assigned values are called @dfn{rvalues}.
@xref{Assignment Ops, ,Assignment Expressions}.

@item Associative Array
Arrays in which the indices may be numbers or strings, not just
sequential integers in a fixed range.

@item @command{awk} Language
The language in which @command{awk} programs are written.

@item @command{awk} Program
An @command{awk} program consists of a series of @dfn{patterns} and
@dfn{actions}, collectively known as @dfn{rules}.  For each input record
given to the program, the program's rules are all processed in turn.
@command{awk} programs may also contain function definitions.

@item @command{awk} Script
Another name for an @command{awk} program.

@item Bash
The GNU version of the standard shell
@iftex
(the @b{B}ourne-@b{A}gain @b{SH}ell).
@end iftex
@ifnottex
(the Bourne-Again SHell).
@end ifnottex
See also ``Bourne Shell.''

@item BBS
See ``Bulletin Board System.''

@item Bit
Short for ``Binary Digit.''
All values in computer memory ultimately reduce to binary digits: values
that are either zero or one.
Groups of bits may be interpreted differently---as integers,
floating-point numbers, character data, addresses of other
memory objects, or other data.
@command{awk} lets you work with floating-point numbers and strings.
@command{gawk} lets you manipulate bit values with the built-in
functions described in
@ref{Bitwise Functions, ,Using @command{gawk}'s Bit Manipulation Functions}.

Computers are often defined by how many bits they use to represent integer
values.  Typical systems are 32-bit systems, but 64-bit systems are
becoming increasingly popular, and 16-bit systems are waning in
popularity.

@item Boolean Expression
Named after the English mathematician Boole. See also ``Logical Expression.''

@item Bourne Shell
The standard shell (@file{/bin/sh}) on Unix and Unix-like systems,
originally written by Steven R.@: Bourne.
Many shells (@command{bash}, @command{ksh}, @command{pdksh}, @command{zsh}) are
generally upwardly compatible with the Bourne shell.

@item Built-in Function
The @command{awk} language provides built-in functions that perform various
numerical, I/O-related, and string computations.  Examples are
@code{sqrt} (for the square root of a number) and @code{substr} (for a
substring of a string).
@command{gawk} provides functions for timestamp management, bit manipulation,
and runtime string translation.
(@xref{Built-in, ,Built-in Functions}.)

@item Built-in Variable
@code{ARGC},
@code{ARGV},
@code{CONVFMT},
@code{ENVIRON},
@code{FILENAME},
@code{FNR},
@code{FS},
@code{NF},
@code{NR},
@code{OFMT},
@code{OFS},
@code{ORS},
@code{RLENGTH},
@code{RSTART},
@code{RS},
and
@code{SUBSEP}
are the variables that have special meaning to @command{awk}.
In addition,
@code{ARGIND},
@code{BINMODE},
@code{ERRNO},
@code{FIELDWIDTHS},
@code{IGNORECASE},
@code{LINT},
@code{PROCINFO},
@code{RT},
and
@code{TEXTDOMAIN}
are the variables that have special meaning to @command{gawk}.
Changing some of them affects @command{awk}'s running environment.
(@xref{Built-in Variables}.)

@item Braces
See ``Curly Braces.''

@item Bulletin Board System
A computer system allowing users to log in and read and/or leave messages
for other users of the system, much like leaving paper notes on a bulletin
board.

@item C
The system programming language that most GNU software is written in.  The
@command{awk} programming language has C-like syntax, and this @value{DOCUMENT}
points out similarities between @command{awk} and C when appropriate.

In general, @command{gawk} attempts to be as similar to the 1990 version
of ISO C as makes sense.  Future versions of @command{gawk} may adopt features
from the newer 1999 standard, as appropriate.

@item C++
A popular object-oriented programming language derived from C.

@cindex ISO 8859-1
@cindex ISO Latin-1
@cindex character sets (machine character encodings)
@item Character Set
The set of numeric codes used by a computer system to represent the
characters (letters, numbers, punctuation, etc.) of a particular country
or place. The most common character set in use today is ASCII (American
Standard Code for Information Interchange).  Many European
countries use an extension of ASCII known as ISO-8859-1 (ISO Latin-1).

@cindex @command{chem} utility
@item CHEM
A preprocessor for @command{pic} that reads descriptions of molecules
and produces @command{pic} input for drawing them.
It was written in @command{awk}
by Brian Kernighan and Jon Bentley, and is available from
@uref{http://cm.bell-labs.com/netlib/typesetting/chem.gz}.

@item Coprocess
A subordinate program with which two-way communications is possible.

@cindex compiled programs
@item Compiler
A program that translates human-readable source code into
machine-executable object code.  The object code is then executed
directly by the computer.
See also ``Interpreter.''

@item Compound Statement
A series of @command{awk} statements, enclosed in curly braces.  Compound
statements may be nested.
(@xref{Statements, ,Control Statements in Actions}.)

@item Concatenation
Concatenating two strings means sticking them together, one after another,
producing a new string.  For example, the string @samp{foo} concatenated with
the string @samp{bar} gives the string @samp{foobar}.
(@xref{Concatenation, ,String Concatenation}.)

@item Conditional Expression
An expression using the @samp{?:} ternary operator, such as
@samp{@var{expr1} ? @var{expr2} : @var{expr3}}.  The expression
@var{expr1} is evaluated; if the result is true, the value of the whole
expression is the value of @var{expr2}; otherwise the value is
@var{expr3}.  In either case, only one of @var{expr2} and @var{expr3}
is evaluated. (@xref{Conditional Exp, ,Conditional Expressions}.)

@item Comparison Expression
A relation that is either true or false, such as @samp{(a < b)}.
Comparison expressions are used in @code{if}, @code{while}, @code{do},
and @code{for}
statements, and in patterns to select which input records to process.
(@xref{Typing and Comparison, ,Variable Typing and Comparison Expressions}.)

@item Curly Braces
The characters @samp{@{} and @samp{@}}.  Curly braces are used in
@command{awk} for delimiting actions, compound statements, and function
bodies.

@cindex dark corner
@item Dark Corner
An area in the language where specifications often were (or still
are) not clear, leading to unexpected or undesirable behavior.
Such areas are marked in this @value{DOCUMENT} with
@iftex
the picture of a flashlight in the margin
@end iftex
@ifnottex
``(d.c.)'' in the text
@end ifnottex
and are indexed under the heading ``dark corner.''

@item Data Driven
A description of @command{awk} programs, where you specify the data you
are interested in processing, and what to do when that data is seen.

@item Data Objects
These are numbers and strings of characters.  Numbers are converted into
strings and vice versa, as needed.
(@xref{Conversion, ,Conversion of Strings and Numbers}.)

@item Deadlock
The situation in which two communicating processes are each waiting
for the other to perform an action.

@item Double-Precision
An internal representation of numbers that can have fractional parts.
Double-precision numbers keep track of more digits than do single-precision
numbers, but operations on them are sometimes more expensive.  This is the way
@command{awk} stores numeric values.  It is the C type @code{double}.

@item Dynamic Regular Expression
A dynamic regular expression is a regular expression written as an
ordinary expression.  It could be a string constant, such as
@code{"foo"}, but it may also be an expression whose value can vary.
(@xref{Computed Regexps, , Using Dynamic Regexps}.)

@item Environment
A collection of strings, of the form @var{name@code{=}val}, that each
program has available to it. Users generally place values into the
environment in order to provide information to various programs. Typical
examples are the environment variables @env{HOME} and @env{PATH}.

@item Empty String
See ``Null String.''

@cindex epoch, definition of
@item Epoch
The date used as the ``beginning of time'' for timestamps.
Time values in Unix systems are represented as seconds since the epoch,
with library functions available for converting these values into
standard date and time formats.

The epoch on Unix and POSIX systems is 1970-01-01 00:00:00 UTC.
See also ``GMT'' and ``UTC.''

@item Escape Sequences
A special sequence of characters used for describing non-printing
characters, such as @samp{\n} for newline or @samp{\033} for the ASCII
ESC (Escape) character. (@xref{Escape Sequences}.)

@item FDL
See ``Free Documentation License.''

@item Field
When @command{awk} reads an input record, it splits the record into pieces
separated by whitespace (or by a separator regexp that you can
change by setting the built-in variable @code{FS}).  Such pieces are
called fields.  If the pieces are of fixed length, you can use the built-in
variable @code{FIELDWIDTHS} to describe their lengths.
(@xref{Field Separators, ,Specifying How Fields Are Separated},
and
@ref{Constant Size, ,Reading Fixed-Width Data}.)

@item Flag
A variable whose truth value indicates the existence or non-existence
of some condition.

@item Floating-Point Number
Often referred to in mathematical terms as a ``rational'' or real number,
this is just a number that can have a fractional part.
See also ``Double-Precision'' and ``Single-Precision.''

@item Format
Format strings are used to control the appearance of output in the
@code{strftime} and @code{sprintf} functions, and are used in the
@code{printf} statement as well.  Also, data conversions from numbers to strings
are controlled by the format string contained in the built-in variable
@code{CONVFMT}. (@xref{Control Letters, ,Format-Control Letters}.)

@item Free Documentation License
This document describes the terms under which this @value{DOCUMENT}
is published and may be copied. (@xref{GNU Free Documentation License}.)

@item Function
A specialized group of statements used to encapsulate general
or program-specific tasks.  @command{awk} has a number of built-in
functions, and also allows you to define your own.
(@xref{Functions}.)

@item FSF
See ``Free Software Foundation.''

@cindex FSF
@cindex Free Software Foundation
@cindex Stallman, Richard
@item Free Software Foundation
A non-profit organization dedicated
to the production and distribution of freely distributable software.
It was founded by Richard M.@: Stallman, the author of the original
Emacs editor.  GNU Emacs is the most widely used version of Emacs today.

@item @command{gawk}
The GNU implementation of @command{awk}.

@cindex GPL
@cindex General Public License
@cindex GNU General Public License
@item General Public License
This document describes the terms under which @command{gawk} and its source
code may be distributed. (@xref{Copying, ,GNU General Public License}.)

@item GMT
``Greenwich Mean Time.''
This is the old term for UTC.
It is the time of day used as the epoch for Unix and POSIX systems.
See also ``Epoch'' and ``UTC.''

@cindex FSF
@cindex Free Software Foundation
@cindex GNU Project
@item GNU
``GNU's not Unix''.  An on-going project of the Free Software Foundation
to create a complete, freely distributable, POSIX-compliant computing
environment.

@item GNU/Linux
A variant of the GNU system using the Linux kernel, instead of the
Free Software Foundation's Hurd kernel.
Linux is a stable, efficient, full-featured clone of Unix that has
been ported to a variety of architectures.
It is most popular on PC-class systems, but runs well on a variety of
other systems too.
The Linux kernel source code is available under the terms of the GNU General
Public License, which is perhaps its most important aspect.

@item GPL
See ``General Public License.''

@item Hexadecimal
Base 16 notation, where the digits are @code{0}--@code{9} and
@code{A}--@code{F}, with @samp{A}
representing 10, @samp{B} representing 11, and so on, up to @samp{F} for 15.
Hexadecimal numbers are written in C using a leading @samp{0x},
to indicate their base.  Thus, @code{0x12} is 18 (1 times 16 plus 2).

@item I/O
Abbreviation for ``Input/Output,'' the act of moving data into and/or
out of a running program.

@item Input Record
A single chunk of data that is read in by @command{awk}.  Usually, an @command{awk} input
record consists of one line of text.
(@xref{Records, ,How Input Is Split into Records}.)

@item Integer
A whole number, i.e., a number that does not have a fractional part.

@item Internationalization
The process of writing or modifying a program so
that it can use multiple languages without requiring
further source code changes.

@cindex interpreted programs
@item Interpreter
A program that reads human-readable source code directly, and uses
the instructions in it to process data and produce results.
@command{awk} is typically (but not always) implemented as an interpreter.
See also ``Compiler.''

@item Interval Expression
A component of a regular expression that lets you specify repeated matches of
some part of the regexp.  Interval expressions were not traditionally available
in @command{awk} programs.

@cindex ISO
@item ISO
The International Standards Organization.
This organization produces international standards for many things, including
programming languages, such as C and C++.
In the computer arena, important standards like those for C, C++, and POSIX
become both American national and ISO international standards simultaneously.
This @value{DOCUMENT} refers to Standard C as ``ISO C'' throughout.

@item Keyword
In the @command{awk} language, a keyword is a word that has special
meaning.  Keywords are reserved and may not be used as variable names.

@command{gawk}'s keywords are:
@code{BEGIN},
@code{END},
@code{if},
@code{else},
@code{while},
@code{do@dots{}while},
@code{for},
@code{for@dots{}in},
@code{break},
@code{continue},
@code{delete},
@code{next},
@code{nextfile},
@code{function},
@code{func},
and
@code{exit}.

@cindex LGPL
@cindex Lesser General Public License
@cindex GNU Lesser General Public License
@item Lesser General Public License
This document describes the terms under which binary library archives
or shared objects,
and their source code may be distributed.

@item Linux
See ``GNU/Linux.''

@item LGPL
See ``Lesser General Public License.''

@item Localization
The process of providing the data necessary for an
internationalized program to work in a particular language.

@item Logical Expression
An expression using the operators for logic, AND, OR, and NOT, written
@samp{&&}, @samp{||}, and @samp{!} in @command{awk}. Often called Boolean
expressions, after the mathematician who pioneered this kind of
mathematical logic.

@item Lvalue
An expression that can appear on the left side of an assignment
operator.  In most languages, lvalues can be variables or array
elements.  In @command{awk}, a field designator can also be used as an
lvalue.

@item Matching
The act of testing a string against a regular expression.  If the
regexp describes the contents of the string, it is said to @dfn{match} it.

@item Metacharacters
Characters used within a regexp that do not stand for themselves.
Instead, they denote regular expression operations, such as repetition,
grouping, or alternation.

@item Null String
A string with no characters in it.  It is represented explicitly in
@command{awk} programs by placing two double quote characters next to
each other (@code{""}).  It can appear in input data by having two successive
occurrences of the field separator appear next to each other.

@item Number
A numeric-valued data object.  Modern @command{awk} implementations use
double-precision floating-point to represent numbers.
Very old @command{awk} implementations use single-precision floating-point.

@item Octal
Base-eight notation, where the digits are @code{0}--@code{7}.
Octal numbers are written in C using a leading @samp{0},
to indicate their base.  Thus, @code{013} is 11 (one times 8 plus 3).

@cindex P1003.2 POSIX standard
@item P1003.2
See ``POSIX.''

@item Pattern
Patterns tell @command{awk} which input records are interesting to which
rules.

A pattern is an arbitrary conditional expression against which input is
tested.  If the condition is satisfied, the pattern is said to @dfn{match}
the input record.  A typical pattern might compare the input record against
a regular expression. (@xref{Pattern Overview, ,Pattern Elements}.)

@item POSIX
The name for a series of standards
@c being developed by the IEEE
that specify a Portable Operating System interface.  The ``IX'' denotes
the Unix heritage of these standards.  The main standard of interest for
@command{awk} users is
@cite{IEEE Standard for Information Technology, Standard 1003.2-1992,
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities}.
Informally, this standard is often referred to as simply ``P1003.2.''

@item Precedence
The order in which operations are performed when operators are used
without explicit parentheses.

@item Private
Variables and/or functions that are meant for use exclusively by library
functions and not for the main @command{awk} program. Special care must be
taken when naming such variables and functions.
(@xref{Library Names,  ,  Naming Library Function Global Variables}.)

@item Range (of input lines)
A sequence of consecutive lines from the input file(s).  A pattern
can specify ranges of input lines for @command{awk} to process or it can
specify single lines. (@xref{Pattern Overview, ,Pattern Elements}.)

@item Recursion
When a function calls itself, either directly or indirectly.
If this isn't clear, refer to the entry for ``recursion.''

@item Redirection
Redirection means performing input from something other than the standard input
stream, or performing output to something other than the standard output stream.

You can redirect the output of the @code{print} and @code{printf} statements
to a file or a system command, using the @samp{>}, @samp{>>}, @samp{|}, and @samp{|&}
operators.  You can redirect input to the @code{getline} statement using
the @samp{<}, @samp{|}, and @samp{|&} operators.
(@xref{Redirection, ,Redirecting Output of @code{print} and @code{printf}},
and @ref{Getline, ,Explicit Input with @code{getline}}.)

@item Regexp
Short for @dfn{regular expression}.  A regexp is a pattern that denotes a
set of strings, possibly an infinite set.  For example, the regexp
@samp{R.*xp} matches any string starting with the letter @samp{R}
and ending with the letters @samp{xp}.  In @command{awk}, regexps are
used in patterns and in conditional expressions.  Regexps may contain
escape sequences. (@xref{Regexp, ,Regular Expressions}.)

@item Regular Expression
See ``regexp.''

@item Regular Expression Constant
A regular expression constant is a regular expression written within
slashes, such as @code{/foo/}.  This regular expression is chosen
when you write the @command{awk} program and cannot be changed during
its execution. (@xref{Regexp Usage, ,How to Use Regular Expressions}.)

@item Rule
A segment of an @command{awk} program that specifies how to process single
input records.  A rule consists of a @dfn{pattern} and an @dfn{action}.
@command{awk} reads an input record; then, for each rule, if the input record
satisfies the rule's pattern, @command{awk} executes the rule's action.
Otherwise, the rule does nothing for that input record.

@item Rvalue
A value that can appear on the right side of an assignment operator.
In @command{awk}, essentially every expression has a value. These values
are rvalues.

@item Scalar
A single value, be it a number or a string.
Regular variables are scalars; arrays and functions are not.

@item Search Path
In @command{gawk}, a list of directories to search for @command{awk} program source files.
In the shell, a list of directories to search for executable programs.

@item Seed
The initial value, or starting point, for a sequence of random numbers.

@item @command{sed}
See ``Stream Editor.''

@item Shell
The command interpreter for Unix and POSIX-compliant systems.
The shell works both interactively, and as a programming language
for batch files, or shell scripts.

@item Short-Circuit
The nature of the @command{awk} logical operators @samp{&&} and @samp{||}.
If the value of the entire expression is determinable from evaluating just
the lefthand side of these operators, the righthand side is not
evaluated.
(@xref{Boolean Ops, ,Boolean Expressions}.)

@item Side Effect
A side effect occurs when an expression has an effect aside from merely
producing a value.  Assignment expressions, increment and decrement
expressions, and function calls have side effects.
(@xref{Assignment Ops, ,Assignment Expressions}.)

@item Single-Precision
An internal representation of numbers that can have fractional parts.
Single-precision numbers keep track of fewer digits than do double-precision
numbers, but operations on them are sometimes less expensive in terms of CPU time.
This is the type used by some very old versions of @command{awk} to store
numeric values.  It is the C type @code{float}.

@item Space
The character generated by hitting the space bar on the keyboard.

@item Special File
A @value{FN} interpreted internally by @command{gawk}, instead of being handed
directly to the underlying operating system---for example, @file{/dev/stderr}.
(@xref{Special Files, ,Special @value{FFN}s in @command{gawk}}.)

@item Stream Editor
A program that reads records from an input stream and processes them one
or more at a time.  This is in contrast with batch programs, which may
expect to read their input files in entirety before starting to do
anything, as well as with interactive programs which require input from the
user.

@item String
A datum consisting of a sequence of characters, such as @samp{I am a
string}.  Constant strings are written with double quotes in the
@command{awk} language and may contain escape sequences.
(@xref{Escape Sequences}.)

@item Tab
The character generated by hitting the @kbd{TAB} key on the keyboard.
It usually expands to up to eight spaces upon output.

@item Text Domain
A unique name that identifies an application.
Used for grouping messages that are translated at runtime
into the local language.

@item Timestamp
A value in the ``seconds since the epoch'' format used by Unix
and POSIX systems.  Used for the @command{gawk} functions
@code{mktime}, @code{strftime}, and @code{systime}.
See also ``Epoch'' and ``UTC.''

@cindex Linux
@cindex GNU/Linux
@cindex Unix
@cindex BSD-based operating systems
@cindex NetBSD
@cindex FreeBSD
@cindex OpenBSD
@item Unix
A computer operating system originally developed in the early 1970's at
AT&T Bell Laboratories.  It initially became popular in universities around
the world and later moved into commercial environments as a software
development system and network server system. There are many commercial
versions of Unix, as well as several work-alike systems whose source code
is freely available (such as GNU/Linux, NetBSD, FreeBSD, and OpenBSD).

@item UTC
The accepted abbreviation for ``Universal Coordinated Time.''
This is standard time in Greenwich, England, which is used as a
reference time for day and date calculations.
See also ``Epoch'' and ``GMT.''

@item Whitespace
A sequence of space, tab, or newline characters occurring inside an input
record or a string.
@end table

@node Copying, GNU Free Documentation License, Glossary, Top
@unnumbered GNU General Public License
@center Version 2, June 1991

@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@c fakenode --- for prepinfo
@unnumberedsec Preamble

  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

  Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

  The precise terms and conditions for copying, distribution and
modification follow.

@ifnotinfo
@c fakenode --- for prepinfo
@unnumberedsec Terms and Conditions for Copying, Distribution and Modification
@end ifnotinfo
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate 0
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.

@item
You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

@enumerate a
@item
Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,

@item
Accompany it with the information you received as to the offer
to distribute corresponding source code.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
@end enumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable.  However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.

If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.

@item
You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices.  Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

@item
The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and ``any
later version'', you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.

@item
If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@ifnotinfo
@c fakenode --- for prepinfo
@heading NO WARRANTY
@end ifnotinfo
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW@.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE@.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU@.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@ifnotinfo
@c fakenode --- for prepinfo
@heading END OF TERMS AND CONDITIONS
@end ifnotinfo
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@c fakenode --- for prepinfo
@unnumberedsec How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.

  To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the ``copyright'' line and a pointer to where the full notice is found.

@smallexample
@var{one line to give the program's name and an idea of what it does.}
Copyright (C) @var{year}  @var{name of author}

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE@.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
@end smallexample

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

@smallexample
Gnomovision version 69, Copyright (C) @var{year} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'.  This is free software, and you are welcome
to redistribute it under certain conditions; type `show c'
for details.
@end smallexample

The hypothetical commands @samp{show w} and @samp{show c} should show
the appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than @samp{show w} and
@samp{show c}; they could even be mouse-clicks or menu items---whatever
suits your program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the program, if
necessary.  Here is a sample; alter the names:

@smallexample
@group
Yoyodyne, Inc., hereby disclaims all copyright
interest in the program `Gnomovision'
(which makes passes at compilers) written
by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end group
@end smallexample

This General Public License does not permit incorporating your program into
proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Lesser General
Public License instead of this License.

@node GNU Free Documentation License, Index, Copying, Top
@unnumbered GNU Free Documentation License
@center Version 1.1, March 2000
@cindex FDL
@cindex Free Documentation License
@cindex GNU Free Documentation License

@display
Copyright (C) 2000  Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display
@sp 1
@enumerate 0
@item
PREAMBLE

The purpose of this License is to make a manual, textbook, or other
written document ``free'' in the sense of freedom: to assure everyone
the effective freedom to copy and redistribute it, with or without
modifying it, either commercially or noncommercially.  Secondarily,
this License preserves for the author and publisher a way to get
credit for their work, while not being considered responsible for
modifications made by others.

This License is a kind of ``copyleft'', which means that derivative
works of the document must themselves be free in the same sense.  It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does.  But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book.  We recommend this License
principally for works whose purpose is instruction or reference.

@sp 1
@item
APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work that contains a
notice placed by the copyright holder saying it can be distributed
under the terms of this License.  The ``Document'', below, refers to any
such manual or work.  Any member of the public is a licensee, and is
addressed as ``you''.

A ``Modified Version'' of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A ``Secondary Section'' is a named appendix or a front-matter section of
the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document's overall subject
(or to related matters) and contains nothing that could fall directly
within that overall subject.  (For example, if the Document is in part a
textbook of mathematics, a Secondary Section may not explain any
mathematics.)  The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The ``Invariant Sections'' are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License.

The ``Cover Texts'' are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License.

A ``Transparent'' copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, whose contents can be viewed and edited directly and
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters.  A copy made in an otherwise Transparent file
format whose markup has been designed to thwart or discourage
subsequent modification by readers is not Transparent.  A copy that is
not ``Transparent'' is called ``Opaque''.

Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input format, SGML
or XML using a publicly available DTD, and standard-conforming simple
HTML designed for human modification.  Opaque formats include
PostScript, PDF, proprietary formats that can be read and edited only
by proprietary word processors, SGML or XML for which the DTD and/or
processing tools are not generally available, and the
machine-generated HTML produced by some word processors for output
purposes only.

The ``Title Page'' means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page.  For works in
formats which do not have any title page as such, ``Title Page'' means
the text near the most prominent appearance of the work's title,
preceding the beginning of the body of the text.
@sp 1
@item
VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License.  You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute.  However, you may accept
compensation in exchange for copies.  If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.
@sp 1
@item
COPYING IN QUANTITY

If you publish printed copies of the Document numbering more than 100,
and the Document's license notice requires Cover Texts, you must enclose
the copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover.  Both covers must also clearly and legibly identify
you as the publisher of these copies.  The front cover must present
the full title with all words of the title equally prominent and
visible.  You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a publicly-accessible computer-network location containing a complete
Transparent copy of the Document, free of added material, which the
general network-using public has access to download anonymously at no
charge using public-standard network protocols.  If you use the latter
option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this
Transparent copy will remain thus accessible at the stated location
until at least one year after the last time you distribute an Opaque
copy (directly or through your agents or retailers) of that edition to
the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.
@sp 1
@item
MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it.  In addition, you must do these things in the Modified Version:

@enumerate A
@item
Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document).  You may use the same title as a previous version
if the original publisher of that version gives permission.

@item
List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has less than five).

@item
State on the Title page the name of the publisher of the
Modified Version, as the publisher.

@item
Preserve all the copyright notices of the Document.

@item
Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

@item
Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

@item
Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document's license notice.

@item
Include an unaltered copy of this License.

@item
Preserve the section entitled ``History'', and its title, and add to
it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page.  If
there is no section entitled ``History'' in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

@item
Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on.  These may be placed in the ``History'' section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

@item
In any section entitled ``Acknowledgements'' or ``Dedications'',
preserve the section's title, and preserve in the section all the
substance and tone of each of the contributor acknowledgements
and/or dedications given therein.

@item
Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

@item
Delete any section entitled ``Endorsements''.  Such a section
may not be included in the Modified Version.

@item
Do not retitle any existing section as ``Endorsements''
or to conflict in title with any Invariant Section.
@end enumerate

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section entitled ``Endorsements'', provided it contains
nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity.  If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.
@sp 1
@item
COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy.  If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections entitled ``History''
in the various original documents, forming one section entitled
``History''; likewise combine any sections entitled ``Acknowledgements'',
and any sections entitled ``Dedications''.  You must delete all sections
entitled ``Endorsements.''
@sp 1
@item
COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.
@sp 1
@item
AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, does not as a whole count as a Modified Version
of the Document, provided no compilation copyright is claimed for the
compilation.  Such a compilation is called an ``aggregate'', and this
License does not apply to the other self-contained works thus compiled
with the Document, on account of their being thus compiled, if they
are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one quarter
of the entire aggregate, the Document's Cover Texts may be placed on
covers that surround only the Document within the aggregate.
Otherwise they must appear on covers around the whole aggregate.
@sp 1
@item
TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License provided that you also include the
original English version of this License.  In case of a disagreement
between the translation and the original English version of this
License, the original English version will prevail.
@sp 1
@item
TERMINATION

You may not copy, modify, sublicense, or distribute the Document except
as expressly provided for under this License.  Any other attempt to
copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License.  However,
parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such
parties remain in full compliance.
@sp 1
@item
FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
@uref{http://www.gnu.org/copyleft/}.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License ``or any later version'' applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.

@end enumerate

@c fakenode --- for prepinfo
@unnumberedsec ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

@smallexample
@group

  Copyright (C)  @var{year}  @var{your name}.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.1
  or any later version published by the Free Software Foundation;
  with the Invariant Sections being @var{list their titles}, with the
  Front-Cover Texts being @var{list}, and with the Back-Cover Texts being @var{list}.
  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.
@end group
@end smallexample
If you have no Invariant Sections, write ``with no Invariant Sections''
instead of saying which ones are invariant.  If you have no
Front-Cover Texts, write ``no Front-Cover Texts'' instead of
``Front-Cover Texts being @var{list}''; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.

@node Index, , GNU Free Documentation License, Top
@unnumbered Index
@printindex cp

@bye

Unresolved Issues:
------------------
1. From ADR.

   Robert J. Chassell points out that awk programs should have some indication
   of how to use them.  It would be useful to perhaps have a "programming
   style" section of the manual that would include this and other tips.

2. The default AWKPATH search path should be configurable via `configure'
   The default and how this changes needs to be documented.

Consistency issues:
	/.../ regexps are in @code, not @samp
	".." strings are in @code, not @samp
	no @print before @dots
	values of expressions in the text (@code{x} has the value 15),
		should be in roman, not @code
	Use   tab   and not   TAB
	Use   ESC   and not   ESCAPE
	Use   space and not   blank	to describe the space bar's character
	The term "blank" is thus basically reserved for "blank lines" etc.
	To make dark corners work, the @value{DARKCORNER} has to be outside
		closing `.' of a sentence and after (pxref{...}).  This is
		a change from earlier versions.
	" " should have an @w{} around it
	Use "non-" everywhere
	Use @command{ftp} when talking about anonymous ftp
	Use uppercase and lowercase, not "upper-case" and "lower-case"
		or "upper case" and "lower case"
	Use "single precision" and "double precision", not "single-precision" or "double-precision"
	Use alphanumeric, not alpha-numeric
	Use POSIX-compliant, not POSIX compliant
	Use --foo, not -Wfoo when describing long options
	Use "Bell Laboratories", but not "Bell Labs".
	Use "behavior" instead of "behaviour".
	Use "zeros" instead of "zeroes".
	Use "nonzero" not "non-zero".
	Use "runtime" not "run time" or "run-time".
	Use "command-line" not "command line".
	Use "online" not "on-line".
	Use "whitespace" not "white space".
	Use "Input/Output", not "input/output". Also "I/O", not "i/o".
	Use "lefthand"/"righthand", not "left-hand"/"right-hand".
	Use "workaround", not "work-around".
	Use "startup"/"cleanup", not "start-up"/"clean-up"
	Use @code{do}, and not @code{do}-@code{while}, except where
		actually discussing the do-while.
	The words "a", "and", "as", "between", "for", "from", "in", "of",
		"on", "that", "the", "to", "with", and "without",
		should not be capitalized in @chapter, @section etc.
		"Into" and "How" should.
	Search for @dfn; make sure important items are also indexed.
	"e.g." should always be followed by a comma.
	"i.e." should always be followed by a comma.
	The numbers zero through ten should be spelled out, except when
		talking about file descriptor numbers. > 10 and < 0, it's
		ok to use numbers.
	In tables, put command-line options in @code, while in the text,
		put them in @option.
	When using @strong, use "Note:" or "Caution:" with colons and
		not exclamation points.  Do not surround the paragraphs
		with @quotation ... @end quotation.
	For most cases, do NOT put a comma before "and", "or" or "but".
		But exercise taste with this rule.
	Don't show the awk command with a program in quotes when it's
		just the program.  I.e.

			{
				....
			}

		not
			awk '{
				...
			}'
		
	Do show it when showing command-line arguments, data files, etc, even
		if there is no output shown.

	Use numbered lists only to show a sequential series of steps.

	Use @code{xxx} for the xxx operator in indexing statements, not @samp.

Date: Wed, 13 Apr 94 15:20:52 -0400
From: rms@gnu.org (Richard Stallman)
To: gnu-prog@gnu.org
Subject: A reminder: no pathnames in GNU

It's a GNU convention to use the term "file name" for the name of a
file, never "pathname".  We use the term "path" for search paths,
which are lists of file names.  Using it for a single file name as
well is potentially confusing to users.

So please check any documentation you maintain, if you think you might
have used "pathname".

Note that "file name" should be two words when it appears as ordinary
text.  It's ok as one word when it's a metasyntactic variable, though.

------------------------
ORA uses filename, thus the macro.

Suggestions:
------------
Enhance FIELDWIDTHS with some way to indicate "the rest of the record".
E.g., a length of 0 or -1 or something.  May be "n"?

Make FIELDWIDTHS be an array?

% Next edition:
%	1. Talk about common extensions, those in nawk, gawk, mawk
%	2. Use @code{foo} for variables and @code{foo()} for functions
%	3. Standardize the error messages from the functions and programs
%	   in Chapters 12 and 13.
%	4. Nuke the BBS stuff and use something that won't be obsolete
%	5. Reorg chapters 5 & 7 like so:
%Chapter 5:
% - Constants, Variables, and Conversions
%   + Constant Expressions
%   + Using Regular Expression Constants
%   + Variables
%   + Conversion of Strings and Numbers
% - Operators
%   + Arithmetic Operators
%   + String Concatenation
%   + Assignment Expressions
%   + Increment and Decrement Operators
% - Truth Values and Conditions
%   + True and False in Awk
%   + Boolean Expressions
%   + Conditional Expressions
% - Function Calls
% - Operator Precedence
%
%Chapter 7:
%  - Array Basics
%    + Introduction to Arrays
%    + Referring to an Array Element
%    + Assigning Array Elements
%    + Basic Array Example
%    + Scanning All Elements of an Array
%  - The delete Statement
%  - Using Numbers to Subscript Arrays
%  - Using Uninitialized Variables as Subscripts
%  - Multidimensional Arrays
%    + Scanning Multidimensional Arrays
%  - Sorting Array Values and Indices with gawk