path: root/bin/sh/sh.1

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.\"	@(#)sh.1	8.6 (Berkeley) 5/4/95
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.na
.TH SH 1
.SH NAME
sh \- command interpreter (shell)
.SH SYNOPSIS
sh [-/+aCefnpuvxIimsVEb] [-/+o longname] [arg ...]
.SH DESCRIPTION
.LP
Sh is the standard command interpreter for the system.
The current version of sh is in the process of being changed to
conform with the POSIX 1003.2 and 1003.2a specifications for
the shell.  This version has many features which make it appear
similar in some respects to the Korn shell, but it is not a Korn
shell clone (run GNU's bash if you want that).  Only features
designated by POSIX, plus a few Berkeley extensions, are being
incorporated into this shell.  We expect POSIX conformance by the
time 4.4 BSD is released.
This man page is not intended to be a tutorial or a complete
specification of the shell.
.sp 2
.B Overview
.sp
.LP
The shell is a command that reads lines from
either a file or the terminal, interprets them, and
generally executes other commands. It is the program that is running
when a user logs into the system (although a user can select
a different shell with the chsh(1) command).
The shell
implements a language that has flow control constructs,
a macro facility that provides a variety of features in
addition to data storage, along with built in history and line
editing capabilities.  It incorporates many features to
aid interactive use and has the advantage that the interpretative
language is common to both interactive and non-interactive
use (shell scripts).  That is, commands can be typed directly
to the running shell or can be put into a file and the file
can be executed directly by the shell.
.sp 2
.B Invocation
.sp
.LP
If no args are present and if the standard input of the shell
is connected to a terminal (or if the -i flag is set), the shell
is considered an interactive shell.  An interactive shell
generally prompts before each command and handles programming
and command errors differently (as described below).
When first starting, the shell inspects argument 0, and
if it begins with a dash '-', the shell is also considered
a login shell.  This is normally done automatically by the system
when the user first logs in. A login shell first reads commands
from the files /etc/profile and .profile if they exist.
If the environment variable ENV is set on entry to a shell,
or is set in the .profile of a login shell, the shell next reads
commands from the file named in ENV.  Therefore, a user should
place commands that are to be executed only at login time in
the .profile file, and commands that are executed for every
shell inside the ENV file.  To set the ENV variable to some
file, place the following line in your .profile of your home
directory
.nf

		ENV=$HOME/.shinit; export ENV

.fi
substituting for ``.shinit'' any filename you wish.
Since the ENV file is read for
every invocation of the shell, including shell scripts and
non-interactive shells, the following paradigm is useful
for restricting commands in the ENV file to interactive invocations.
Place commands within the ``case'' and ``esac'' below (these
commands are described later):
.nf

	case $- in *i*)
		# commands for interactive use only
		...
	esac

.fi
If command line arguments besides the options have been
specified, then the shell treats the first argument as the
name of a file from which to read commands (a shell script), and
the remaining arguments are set as the positional parameters
of the shell ($1, $2, etc).  Otherwise, the shell reads commands
from its standard input.
.sp 2
.B Argument List Processing
.sp
.LP
All of the single letter options have a corresponding name
that can be used as an argument to the '-o' option. The
set -o name is provided next to the single letter option in
the description below.
Specifying a dash ``-'' turns the option on, while using a plus ``+''
disables the option.
The following options can be set from the command line or
with the set(1) builtin (described later).
.TP
-a    allexport
Export all variables assigned to.
(UNIMPLEMENTED for 4.4alpha)
.TP
-C    noclobber
Don't overwrite existing files with ``>''.
(UNIMPLEMENTED for 4.4alpha)
.TP
-e    errexit
If not interactive, exit immediately if any
untested command fails.
The exit status of a command is considered to be
explicitly tested if the command is used to control
an if, elif, while, or until; or if the command is the left
hand operand of an ``&&'' or ``||'' operator.

.TP
-f    noglob
Disable pathname expansion.
.TP
-n    noexec
If not interactive, read commands but do not
execute them.  This is useful for checking the
syntax of shell scripts.
.TP
-p    privileged
Turn on privileged mode.  This mode is enabled on startup
if either the effective user or group id is not equal to the
real user or group id.  Turning this mode off sets the
effective user and group ids to the real user and group ids.
Also on interactive shells and when enabled, this mode sources
/etc/suid_profile (instead of ~/.profile) after /etc/profile
and ignores the contents of the \fBENV\fP variable.
.TP
-u    nounset
Write a message to standard error when attempting
to expand a variable that is not set, and if the
shell is not interactive, exit immediately.
(UNIMPLEMENTED for 4.4alpha)
.TP
-v    verbose
The shell writes its input to standard error
as it is read.  Useful for debugging.
.TP
-x    xtrace
Write each command to standard error (preceded
by a '+ ') before it is executed.  Useful for
debugging.
.TP
-I    ignoreeof
Ignore EOF's from input when interactive.
.TP
-i    interactive
Force the shell to behave interactively.
.TP
-m    monitor
Turn on job control (set automatically when
interactive).
.TP
-s    stdin
Read commands from standard input (set automatically
if no file arguments are present).  This option has
no effect when set after the shell has already started
running (i.e. with set(1)).
.TP
-V    vi
Enable the built-in vi(1) command line editor (disables
-E if it has been set).
.TP
-E    emacs
Enable the built-in emacs(1) command line editor (disables
-V if it has been set).
.TP
-b    notify
Enable asynchronous notification of background job
completion.
(UNIMPLEMENTED for 4.4alpha)
.LP
.sp 2
.B Lexical Structure
.sp
.LP
The shell reads input in terms of lines from a file and breaks
it up into words at whitespace (blanks and tabs), and at
certain sequences of
characters that are special to the shell called ``operators''.
There are two types of operators: control operators and
redirection operators (their meaning is discussed later).
Following is a list of operators:
.nf
.sp
Control operators: &  &&  (  )  ;  ;; | || <newline>
.sp
Redirection operator:  <  >  >|  <<  >>  <&  >&  <<-  <>
.sp
.fi
.sp 2
.B Quoting
.sp
.LP
Quoting is used to remove the special meaning of certain characters
or words to the shell, such as operators, whitespace, or
keywords.  There are three types of quoting: matched single quotes,
matched double quotes, and backslash.
.sp 2
.B Backslash
.sp
.LP
A backslash preserves the literal meaning of the following
character, with the exception of <newline>.  A backslash preceding
a <newline> is treated as a line continuation.
.sp 2
.B Single Quotes
.sp
.LP
Enclosing characters in single quotes preserves the literal
meaning of all the characters (except single quotes, making
it impossible to put single-quotes in a single-quoted string).
.sp 2
.B Double Quotes
.sp
.LP
Enclosing characters within double quotes preserves the literal
meaning of all characters except dollarsign ($), backquote (`),
and backslash (\\).  The backslash inside double quotes is
historically weird, and serves to quote only the following
characters: $  `  "  \\  <newline>.
Otherwise it remains literal.
.sp 2
.B Reserved Words
.sp
.LP
Reserved words are words that have special meaning to the
shell and are recognized at the beginning of a line and
after a control operator.  The following are reserved words:
.nf

   !	elif	fi	while	case
   else	for	then	{	}
   do	done	until	if	esac

.fi
Their meaning is discussed later.
.sp 2
.B Aliases
.sp
.LP
An alias is a name and corresponding value set using the alias(1)
builtin command.  Whenever a reserved word may occur (see above),
and after checking for reserved words, the shell
checks the word to see if it matches an alias. If it does,
it replaces it in the input stream with its value.  For example,
if there is an alias called ``lf'' with the value ``ls -F'',
then the input
.nf

   lf foobar <return>

	would become

   ls -F foobar <return>

.fi
.LP
Aliases provide a convenient way for naive users to
create shorthands for commands without having to learn how
to create functions with arguments.  They can also be
used to create lexically obscure code.  This use is discouraged.
.sp 2
.B Commands
.sp
.LP
The shell interprets the words it reads according to a
language, the specification of which is outside the scope
of this man page (refer to the BNF in the POSIX 1003.2
document).  Essentially though, a line is read and if
the first word of the line (or after a control operator)
is not a reserved word, then the shell has recognized a
simple command.  Otherwise, a complex command or some
other special construct may have been recognized.
.sp 2
.B Simple Commands
.sp
.LP
If a simple command has been recognized, the shell performs
the following actions:
.sp
1) Leading words of the form ``name=value'' are
stripped off and assigned to the environment of
the simple command.  Redirection operators and
their arguments (as described below) are stripped
off and saved for processing.
.sp
2) The remaining words are expanded as described in
the section called ``Expansions'', and the
first remaining word is considered the command
name and the command is located.  The remaining
words are considered the arguments of the command.
If no command name resulted, then the ``name=value''
variable assignments recognized in 1) affect the
current shell.
.sp
3) Redirections are performed as described in
the next section.
.sp 2
.B Redirections
.sp
.LP
Redirections are used to change where a command reads its input
or sends its output.  In general, redirections open, close, or
duplicate an existing reference to a file.  The overall format
used for redirection is:
.nf

		[n] redir-op file

.fi
where redir-op is one of the redirection operators mentioned
previously.  Following is a list of the possible redirections.
The [n] is an optional number, as in '3' (not '[3]'), that
refers to a file descriptor.
.TP
[n]> file
Redirect standard output (or n) to file.
.TP
[n]>| file
Same, but override the -C option.
.TP
[n]>> file
Append standard output (or n) to file.
.TP
[n]< file
Redirect standard input (or n) from file.
.TP
[n1]<&n2
Duplicate standard input (or n1) from
file descriptor n2.
.TP
[n]<&-
Close standard input (or n).
.TP
[n1]>&n2
Duplicate standard output (or n) from
n2.
.TP
[n]>&-
Close standard output (or n).
.TP
[n]<> file
Open file for reading and writing on
standard input (or n).
.LP
The following redirection is often called a ``here-document''.
.nf

    [n]<< delimiter
        here-doc-text...
    delimiter

.fi
All the text on successive lines up to the delimiter is
saved away and made available to the command on standard
input, or file descriptor n if it is specified.  If the delimiter
as specified on the initial line is quoted, then the here-doc-text
is treated literally, otherwise the text is subjected to
parameter expansion, command substitution, and arithmetic
expansion (as described in the section on ``Expansions''). If
the operator is ``<<-'' instead of ``<<'', then leading tabs
in the here-doc-text are stripped.
.sp 2
.B Search and Execution
.sp
.LP
There are three types of commands: shell functions,
builtin commands, and normal programs -- and the
command is searched for (by name) in that order.  They
each are executed in a different way.
.LP
When a shell function is executed, all of the shell positional
parameters (except $0, which remains unchanged) are
set to the arguments of the shell function.
The variables which are explicitly placed in the environment of
the command (by placing assignments to them before the
function name) are made local to the function and are set
to the values given. Then the command given in the function
definition is executed.   The positional parameters are
restored to their original values when the command completes.
This all occurs within the current shell.
.LP
Shell builtins are executed internally to the shell, without
spawning a new process.
.LP
Otherwise, if the command name doesn't match a function
or builtin, the command is searched for as a normal
program in the filesystem (as described in the next section).
When a normal program is executed, the shell runs the program,
passing the arguments and the environment to the
program. If the program is not a normal executable file
(i.e., if it does not begin with the "magic number"
whose ASCII representation is "#!", so execve(2) returns
ENOEXEC then) the shell
will interpret the program in a subshell.  The child shell will
reinitialize itself in this case, so that the effect will
be as if a new shell had been invoked to handle the ad-hoc shell
script, except that the location of hashed commands located in
the parent shell will be remembered by the child.
.LP
Note that previous versions of this document
and the source code itself misleadingly and sporadically
refer to a shell script without a magic number
as a "shell procedure".
.sp 2
.B Path Search
.sp
.LP
When locating a command, the shell first looks to see if
it has a shell function by that name.  Then it looks for a
builtin command by that name.  If a builtin command is not found,
one of two things happen:
.sp
1) Command names containing a slash are simply executed without
performing any searches.
.sp
2) The shell searches each entry in PATH in turn for the command.
The value of the PATH variable should be a series of
entries separated by colons.  Each entry consists of a
directory name.
The current directory
may be indicated implicitly by an empty directory name,
or explicitly by a single period.
.sp 2
.B Command Exit Status
.sp
.LP
Each command has an exit status that can influence the behavior
of other shell commands.  The paradigm is that a command exits
with zero for normal or success, and non-zero for failure,
error, or a false indication.  The man page for each command
should indicate the various exit codes and what they mean.
Additionally, the builtin commands return exit codes, as does
an executed shell function.
.sp 2
.B Complex Commands
.sp
.LP
Complex commands are combinations of simple commands
with control operators or reserved words, together creating a larger complex
command.  More generally, a command is one of the following:
.nf

  - simple command

  - pipeline

  - list or compound-list

  - compound command

  - function definition

.fi
.LP
Unless otherwise stated, the exit status of a command is
that of the last simple command executed by the command.
.sp 2
.B Pipelines
.sp
.LP
A pipeline is a sequence of one or more commands separated
by the control operator |.  The standard output of all but
the last command is connected to the standard input
of the next command.  The standard output of the last
command is inherited from the shell, as usual.
.LP
The format for a pipeline is:
.nf

[!] command1 [ | command2 ...]

.fi
.LP
The standard output of command1 is connected to the standard
input of command2. The standard input, standard output, or
both of a command is considered to be assigned by the
pipeline before any redirection specified by redirection
operators that are part of the command.
.LP
If the pipeline is not in the background (discussed later),
the shell waits for all commands to complete.
.LP
If the reserved word ! does not precede the pipeline, the
exit status is the exit status of the last command specified
in the pipeline.  Otherwise, the exit status is the logical
NOT of the exit status of the last command.  That is, if
the last command returns zero, the exit status is 1; if
the last command returns greater than zero, the exit status
is zero.
.LP
Because pipeline assignment of standard input or standard
output or both takes place before redirection, it can be
modified by redirection.  For example:
.nf

$ command1 2>&1 | command2

.fi
sends both the standard output and standard error of command1
to the standard input of command2.
.LP
A ; or <newline> terminator causes the preceding
AND-OR-list (described next) to be executed sequentially; a & causes
asynchronous execution of the preceding AND-OR-list.
.LP
Note that unlike some other shells, each process in the
pipeline is a child of the invoking shell (unless it
is a shell builtin, in which case it executes in the
current shell -- but any effect it has on the
environment is wiped).
.sp 2
.B Background Commands -- &
.sp
.LP
If a command is terminated by the control operator ampersand
(&), the shell executes the command asynchronously -- that is,
the shell does not wait for
the command to finish before executing the next command.
.LP
The format for running a command in background is:
.nf

command1 & [command2 & ...]

.fi
If the shell is not interactive, the standard input of an
asynchronous command is set to /dev/null.
.sp 2
.B Lists -- Generally Speaking
.sp
.LP
A list is a sequence of zero or more commands separated by
newlines, semicolons, or ampersands,
and optionally terminated by one of these three characters.
The commands in a
list are executed in the order they are written.
If command is followed by an ampersand, the shell starts the
command and immediately proceed onto the next command;
otherwise it waits for the command to terminate before
proceeding to the next one.
.sp 2
.B Short-Circuit List Operators
.sp
.LP
``&&'' and ``||'' are AND-OR list operators.  ``&&'' executes
the first command, and then executes the second command
iff the exit status of the first command is zero.  ``||''
is similar, but executes the second command iff the exit
status of the first command is nonzero.  ``&&'' and ``||''
both have the same priority.
.sp 2
.B Flow-Control Constructs -- if, while, for, case
.sp
.LP
The syntax of the if command is
.nf

    if list
    then list
    [ elif list
    then    list ] ...
    [ else list ]
    fi

.fi
The syntax of the while command is
.nf

    while list
    do   list
    done

.fi
The two lists are executed repeatedly while the exit status of the
first list is zero.  The until command is similar, but has the word
until in place of while, which causes it to
repeat until the exit status of the first list is zero.
.LP
The syntax of the for command is
.nf

    for variable in word...
    do   list
    done

.fi
The words are expanded, and then the list is executed
repeatedly with the variable set to each word in turn.  do
and done may be replaced with ``{'' and ``}''.
.LP
The syntax of the break and continue command is
.nf

    break [ num ]
    continue [ num ]

.fi
Break terminates the num innermost for or while loops.
Continue continues with the next iteration of the innermost loop.
These are implemented as builtin commands.
.LP
The syntax of the case command is
.nf

    case word in
    pattern) list ;;
    ...
    esac

.fi
.LP
The pattern can actually be one or more patterns (see Shell
Patterns described later), separated by ``|'' characters.
.sp 2
.B Grouping Commands Together
.sp
.LP
Commands may be grouped by writing either
.nf

    (list)

.fi
or
.nf

    { list; }

.fi
The first of these executes the commands in a subshell.
Builtin commands grouped into a (list) will not affect
the current shell.
The second form does not fork another shell so is
slightly more efficient.
Grouping commands together this way allows you to
redirect their output as though they were one program:
.nf

    { echo -n "hello"; echo " world" } > greeting

.fi
.sp 2
.B Functions
.sp
.LP
The syntax of a function definition is
.nf

    name ( ) command

.fi
.LP
A function definition is an executable statement; when
executed it installs a function named name and returns an
exit status of zero.   The command is normally a list
enclosed between ``{'' and ``}''.
.LP
Variables may be declared to be local to a function by
using a local command.  This should appear as the first
statement of a function, and the syntax is
.nf

    local [ variable | - ] ...

.fi
Local is implemented as a builtin command.
.LP
When a variable is made local, it inherits the initial
value and exported and readonly flags from the variable
with the same name in the surrounding scope, if there is
one.  Otherwise, the variable is initially unset.   The shell
uses dynamic scoping, so that if you make the variable x
local to function f, which then calls function g, references
to the variable x made inside g will refer to the
variable x declared inside f, not to the global variable
named x.
.LP
The only special parameter than can be made local is
``-''.  Making ``-'' local any shell options that are
changed via the set command inside the function to be
restored to their original values when the function
returns.
.LP
The syntax of the return command is
.nf

    return [ exitstatus ]

.fi
It terminates the currently executing function.  Return is
implemented as a builtin command.
.sp 2
.B Variables and Parameters
.sp
.LP
The shell maintains a set of parameters.  A parameter
denoted by a name is called a variable. When starting up,
the shell turns all the environment variables into shell
variables.  New variables can be set using the form
.nf

    name=value

.fi
.LP
Variables set by the user must have a name consisting solely
of alphabetics, numerics, and underscores - the first of which
must not be numeric.  A parameter can also be denoted by a number
or a special character as explained below.
.sp 2
.B Positional Parameters
.sp
.LP
A positional parameter is a parameter denoted by a number (n > 0).
The shell sets these initially to the values of its command
line arguments that follow the name of the shell script.
The set(1) builtin can also be used to set or reset them.
.sp 2
.B Special Parameters
.sp
.LP
A special parameter is a parameter denoted by one of the following
special characters.  The value of the parameter is listed
next to its character.
.TP
*
Expands to the positional parameters, starting from one.  When
the expansion occurs within a double-quoted string
it expands to a single field with the value of each parameter
separated by the first character of the IFS variable, or by a
<space> if IFS is unset.
.TP
@
Expands to the positional parameters, starting from one.  When
the expansion occurs within double-quotes, each positional
parameter expands as a separate argument.
If there are no positional parameters, the
expansion of @ generates zero arguments, even when @ is
double-quoted.  What this basically means, for example, is
if $1 is ``abc'' and $2 is ``def ghi'', then "$@" expands to
the two arguments:

"abc"   "def ghi"
.TP
#
Expands to the number of positional parameters.
.TP
?
Expands to the exit status of the most recent pipeline.
.TP
- (Hyphen)
Expands to the current option flags (the single-letter
option names concatenated into a string) as specified on
invocation, by the set builtin command, or implicitly
by the shell.
.TP
$
Expands to the process ID of the invoked shell.  A subshell
retains the same value of $ as its parent.
.TP
!
Expands to the process ID of the most recent background
command executed from the current shell.  For a
pipeline, the process ID is that of the last command in the
pipeline.
.TP
0 (Zero.)
Expands to the name of the shell or shell script.
.LP
.sp 2
.B Word Expansions
.sp
.LP
This clause describes the various expansions that are
performed on words.  Not all expansions are performed on
every word, as explained later.
.LP
Tilde expansions, parameter expansions, command substitutions,
arithmetic expansions, and quote removals that occur within
a single word expand to a single field.  It is only field
splitting or pathname expansion that can create multiple
fields from a single word. The single exception to this
rule is the expansion of the special parameter @ within
double-quotes, as was described above.
.LP
The order of word expansion is:
.LP
(1)  Tilde Expansion, Parameter Expansion, Command Substitution,
Arithmetic Expansion (these all occur at the same time).
.LP
(2)  Field Splitting is performed on fields
generated by step (1) unless the IFS variable is null.
.LP
(3)  Pathname Expansion (unless set -f is in effect).
.LP
(4)  Quote Removal.
.LP
The $ character is used to introduce parameter expansion, command
substitution, or arithmetic evaluation.
.sp 2
.B Tilde Expansion (substituting a user's home directory)
.sp
.LP
A word beginning with an unquoted tilde character (~) is
subjected to tilde expansion.  All the characters up to
a slash (/) or the end of the word are treated as a username
and are replaced with the user's home directory.  If the
username is missing (as in ~/foobar), the tilde is replaced
with the value of the HOME variable (the current user's
home directory).

.sp 2
.B Parameter Expansion
.sp
.LP
The format for parameter expansion is as follows:
.nf

    ${expression}

.fi
where expression consists of all characters until the matching }.  Any }
escaped by a backslash or within a quoted string, and characters in
embedded arithmetic expansions, command substitutions, and variable
expansions, are not examined in determining the matching }.
.LP
The simplest form for parameter expansion is:
.nf

    ${parameter}

.fi
The value, if any, of parameter is substituted.
.LP
The parameter name or symbol can be enclosed in braces, which are
optional except for positional parameters with more than one digit or
when parameter is followed by a character that could be interpreted as
part of the name.
If a parameter expansion occurs inside
double-quotes:
.LP
1) Pathname expansion is not performed on the results of the
expansion.
.LP
2) Field splitting is not performed on the results of the
expansion, with the exception of @.
.LP
In addition, a parameter expansion can be modified by using one of the
following formats.
.sp
.TP
${parameter:-word}
Use Default Values.  If parameter is unset or
null, the expansion of word is
substituted; otherwise, the value of
parameter is substituted.
.TP
${parameter:=word}
Assign Default Values.  If parameter is unset
or null, the expansion of word is
assigned to parameter.  In all cases, the
final value of parameter is
substituted.  Only variables, not positional
parameters or special parameters, can be
assigned in this way.
.TP
${parameter:?[word]}
Indicate Error if Null or Unset.  If
parameter is unset or null, the expansion of
word (or a message indicating it is unset if
word is omitted) is written to standard
error and the shell exits with a nonzero
exit status.  Otherwise, the value of
parameter is substituted.  An
interactive shell need not exit.
.TP
${parameter:+word}
Use Alternate Value.  If parameter is unset
or null, null is substituted;
otherwise, the expansion of word is
substituted.
.LP
In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omission
of the colon results in a test for a parameter that is only unset.
.TP
${#parameter}
String Length.  The length in characters of
the value of parameter.
.LP
The following four varieties of parameter expansion provide for substring
processing.  In each case, pattern matching notation (see Shell Patterns),
rather
than regular expression notation, is used to evaluate the patterns.
If parameter is * or @, the result of the expansion is unspecified.
Enclosing the full parameter expansion string in double-quotes does not
cause the following four varieties of pattern characters to be quoted,
whereas quoting characters within the braces has this effect.
.TP
${parameter%word}
Remove Smallest Suffix Pattern.  The word
is expanded to produce a pattern.  The
parameter expansion then results in
parameter, with the smallest portion of the
suffix matched by the pattern deleted.
.TP
${parameter%%word}
Remove Largest Suffix Pattern.  The word
is expanded to produce a pattern.  The
parameter expansion then results in
parameter, with the largest portion of the
suffix matched by the pattern deleted.
.TP
${parameter#word}
Remove Smallest Prefix Pattern.  The word
is expanded to produce a pattern.  The
parameter expansion then results in
parameter, with the smallest portion of the
prefix matched by the pattern deleted.
.TP
${parameter##word}
Remove Largest Prefix Pattern.  The word
is expanded to produce a pattern.  The
parameter expansion then results in
parameter, with the largest portion of the
prefix matched by the pattern deleted.
.LP
.sp 2
.B Command Substitution
.sp
.LP
Command substitution allows the output of a command to be substituted in
place of the command name itself.  Command substitution occurs when
the command is enclosed as follows:
.nf

       $(command)

.fi
or (``backquoted'' version):
.nf

       `command`

.fi
.LP
The shell expands the command substitution by executing command in a
subshell environment and replacing the command substitution
with the
standard output of the command, removing sequences of one or more
<newline>s at the end of the substitution.  (Embedded <newline>s before
the end of the output are not removed; however, during field
splitting, they may be translated into <space>s, depending on the value
of IFS and quoting that is in effect.)

.sp 2
.B Arithmetic Expansion
.sp
.LP
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic
expansion is as follows:
.nf

       $((expression))

.fi
The expression is treated as if it were in double-quotes, except
that a double-quote inside the expression is not treated specially.  The
shell expands all tokens in the expression for parameter expansion,
command substitution, and quote removal.
.LP
Next, the shell treats this as an arithmetic expression and
substitutes the value of the expression.

.sp 2
.B White Space Splitting (Field Splitting)
.sp
.LP
After parameter expansion, command substitution, and
arithmetic expansion the shell scans the results of
expansions and substitutions that did not occur in double-quotes for
field splitting and multiple fields can result.
.LP
The shell treats each character of the IFS as a delimiter and use
the delimiters to split the results of parameter expansion and command
substitution into fields.

.sp 2
.B Pathname Expansion (File Name Generation)
.sp
.LP
Unless the -f flag is set, file name generation is performed
after word splitting is complete.  Each word is
viewed as a series of patterns, separated by slashes.  The
process of expansion replaces the word with the names of
all existing files whose names can be formed by replacing
each pattern with a string that matches the specified pattern.
There are two restrictions on this: first, a pattern cannot match
a string containing a slash, and second,
a pattern cannot match a string starting with a period
unless the first character of the pattern is a period.
The next section describes the patterns used for both
Pathname Expansion and the case(1) command.

.sp 2
.B Shell Patterns
.sp
.LP
A pattern consists of normal characters, which match themselves,
and meta-characters.   The meta-characters are
``!'', ``*'', ``?'', and ``[''.  These  characters lose
their special meanings if they are quoted.  When command
or variable substitution is performed and the dollar sign
or back quotes are not double quoted, the value of the
variable or the output of the command is scanned for these
characters and they are turned into meta-characters.
.LP
An asterisk (``*'') matches any string of characters.   A
question mark matches any single character. A left
bracket (``['') introduces a character class.  The end of
the character class is indicated by a ``]''; if the ``]''
is missing then the ``['' matches a ``['' rather than
introducing a character class.  A character class matches
any of the characters between the square brackets.   A
range of characters may be specified using a minus sign.
The character class may be complemented by making an
exclamation point the first character of the character
class.
.LP
To include a ``]'' in a character class, make it the first
character listed (after the ``!'', if any).  To include a
minus sign, make it the first or last character listed

.sp 2
.B Builtins
.sp
.LP
This section lists the builtin commands which
are builtin because they need to perform some  operation
that can't be performed by a separate process. In addition to
these, there are several other commands that may
be builtin for efficiency (e.g. printf(1), echo(1), test(1),
etc).
.TP
:
A null command that returns a 0 (true) exit value.
.TP
\&. file
The commands in the specified file are read and executed by the shell.
.TP
alias  [ name[=string] ...  ]
If name=string is specified, the shell defines the
alias ``name'' with value ``string''.  If just ``name''
is specified, the value of the alias ``name'' is printed.
With no arguments, the alias builtin prints the
names and values of all defined aliases (see unalias).
.TP
bg [ job ] ...
Continue the specified jobs (or the current job if no
jobs are given) in the background.
.TP
command command arg...
Execute the specified builtin command.  (This is useful when you
have a shell function with the same name
as a builtin command.)
.TP
cd [ directory ]
Switch to the specified directory (default $HOME).
If the an entry for CDPATH appears in the environment
of the cd command or the shell variable CDPATH is set
and the directory name does not begin with a slash,
then the directories listed in CDPATH will be
searched for the specified directory.  The format of
CDPATH is the same as that of PATH. In an interactive shell,
the cd command will print out the name of
the directory that it actually switched to if this is
different from the name that the user gave.  These
may be different either because the CDPATH mechanism
was used or because a symbolic link was crossed.
.TP
eval string...
Concatenate all the arguments with spaces.  Then
re-parse and execute the command.
.TP
exec [ command arg...  ]
Unless command is omitted, the shell process is
replaced with the specified program (which must be a
real program, not a shell builtin or function).   Any
redirections on the exec command are marked as permanent,
so that they are not undone when the exec command finishes.
.TP
exit [ exitstatus ]
Terminate the shell process.  If exitstatus is given
it is used as the exit status of the shell; otherwise
the exit status of the preceding command is used.
.TP
export name...
The specified names are exported so that they will
appear in the environment of subsequent commands.
The only way to un-export a variable is to unset it.
The shell allows the value of a variable to be set at the
same time it is exported by writing
.nf

    export name=value

.fi
With no arguments the export command lists the names
of all exported variables.
.TP
fc  [-e editor] [first [last]]
.TP
fc  -l [-nr] [first [last]]
.TP
fc  -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands
previously entered to an interactive shell.
.RS +.5i
.TP 2
-e editor
Use the editor named by editor to edit the commands.  The
editor string is a command name, subject to search via the
PATH variable.  The value in the FCEDIT variable
is used as a default when -e is not specified.  If
FCEDIT is null or unset, the value of the EDITOR
variable is used.  If EDITOR is null or unset,
ed(1) is used as the editor.
.TP 2
-l (ell)
List the commands rather than invoking
an editor on them.  The commands are written in the
sequence indicated by the first and last operands, as
affected by -r, with each command preceded by the command
number.
.TP 2
-n
Suppress command numbers when listing with -l.
.TP 2
-r
Reverse the order of the commands listed (with -l) or
edited (with neither -l nor -s).
.TP 2
-s
Re-execute the command without invoking an editor.
.TP 2
first
.TP 2
last
Select the commands to list or edit.  The number of
previous commands that can be accessed are determined
by the value of the HISTSIZE variable.  The value of first
or last or both are one of the following:
.TP 2
[+]number
A positive number representing a command
number; command numbers can be displayed
with the -l option.
.TP 2
-number
A negative decimal number representing the
command that was executed number of
commands previously.  For example, -1 is
the immediately previous command.
.TP 2
string
A string indicating the most recently
entered command that begins with that
string.  If the old=new operand is not also
specified with -s, the string form of the
first operand cannot contain an embedded
equal sign.
.TP
The following environment variables affect the execution of fc:
.TP 2
FCEDIT
Name of the editor to use.
.TP 2
HISTSIZE
The number of previous commands that are accessable.
.RE
.TP
fg [ job ]
Move the specified job or the current job to the
foreground.
.TP
getopts optstring var
The POSIX getopts command.
The getopts command deprecates the older getopt command.
The first argument should be a series of letters, each possibly
followed by a colon which indicates that the option takes an argument.
The specified variable is set to the parsed option.  The index of
the next argument is placed into the shell variable OPTIND.
If an option takes an argument, it is placed into the shell variable
OPTARG.  If an invalid option is encountered, var is set to '?'.
It returns a false value (1) when it encounters the end of the options.
.TP
hash -rv command...
The shell maintains a hash table which remembers the
locations of commands.  With no arguments whatsoever,
the hash command  prints out the contents of this
table.  Entries which have not been looked at since
the last cd command are marked with an asterisk; it
is possible for these entries to be invalid.
.sp
With arguments, the hash command removes the specified commands
from the hash table (unless they are
functions) and then locates them.   With the -v
option, hash prints the locations of the commands as
it finds them.  The -r option causes the hash command
to delete all the entries in the hash table except
for functions.
.TP
jobid [ job ]
Print the process id's of the processes in the job.
If the job argument is omitted, use the current job.
.TP
jobs
This command lists out all the background processes
which are children of the current shell process.
.TP
pwd
Print the current directory.  The builtin command may
differ from the program of the same name because the
builtin command remembers what the current directory
is rather than recomputing it each time.  This makes
it faster.  However, if the current directory is
renamed, the builtin version of pwd will continue to
print the old name for the directory.
.TP
read [ -p prompt ] [ -e ] variable...
The prompt is printed if the -p option is specified
and the standard input is a terminal.  Then a line is
read from the standard input.  The trailing newline
is deleted from the line and the line is split as
described in the section on word splitting above, and
the pieces are assigned to the variables in order.
If there are more pieces than variables, the remaining
pieces (along with the characters in IFS that
separated them) are assigned to the last variable.
If there are more variables than pieces, the remaining
variables are assigned the null string.
.sp
The -e option causes any backslashes in the input to
be treated specially.  If a backslash is followed by
a newline, the backslash and the newline will be
deleted.   If a backslash is followed by any other
character, the backslash will be deleted and the following
character will be treated as though it were
not in IFS, even if it is.
.TP
readonly name...
The specified names are marked as read only, so that
they cannot be subsequently modified or unset.  The shell
allows the value of a variable to be set at the same
time it is marked read only by writing
.TP
readonly name=value
With no arguments the readonly command lists the
names of all read only variables.
.TP
set [ { -options | +options | -- } ] arg...
The set command performs three different functions.
.sp
With no arguments, it lists the values of all shell
variables.
.sp
If options are given, it sets the specified option
flags, or clears them as described in the section
called ``Argument List Processing''.
.sp
The third use of the set command is to set the values
of the shell's positional parameters to the specified
args.   To change the positional parameters without
changing any options, use ``--'' as the first argument
to set.  If no args are present, the set command
will clear all the positional parameters (equivalent
to executing ``shift $#''.
.TP
setvar variable value
Assigns value to variable. (In general it is better
to write variable=value rather than using setvar.
Setvar is intended to be used in functions that
assign values to variables whose names are passed as
parameters.)
.TP
shift [ n ]
Shift the positional parameters n times.  A shift
sets the value of $1 to the value of $2, the value of
$2 to the value of $3, and so on, decreasing the
value of $# by one. If there are zero positional
parameters, shifting doesn't do anything.
.TP
trap [ action ] signal...
Cause the shell to parse and execute action when any
of the specified signals are received. The signals
are specified by signal number. Action may be null
or omitted; the former causes the specified signal to
be ignored and the latter causes the default action
to be taken. When the shell forks off a subshell, it
resets trapped (but not ignored) signals to the
default action. The trap command has no effect on
signals that were ignored on entry to the shell.
.TP
ulimit [ -HSacdflmnust ] [ limit ]
Set or display resource limits (see getrlimit(2)).
If ``limit'' is specified, the named resource will be set;
otherwise the current resource value will be displayed.
.br
If ``-H'' is specified, the hard limits will be
set or displayed.  While everybody is allowed to reduce a
hard limit, only the superuser can increase it.  Option ``-S''
specifies the soft limits instead.  When displaying limits,
only one of ``-S'' or ``-H'' can be given.  The default is
to display the soft limits, and to set both, the hard and
the soft limits.
.br
Option ``-a'' requests to display all resources.  The parameter
``limit'' is not acceptable in this mode.
.br
The remaining options specify which resource value is to be
displayed or modified.  They are mutually exclusive.
.RS +.7i
.TP 2
-c coredumpsize
The maximal size of core dump files, in 512-byte blocks.
.TP 2
-d datasize
The maximal size of the data segment of a process, in kilobytes.
.TP 2
-f filesize
The maximal size of a file, in 512-byte blocks.  This is the
default.
.TP 2
-l lockedmem
The maximal size of memory that can be locked by a process, in
kilobytes.
.TP 2
-m memoryuse
The maximal resident set size of a process, in kilobytes.
.TP 2
-n nofiles
The maximal number of descriptors that could be opened by a process.
.TP 2
-u userproc
The maximal number of simultaneous processes for this user ID.
.TP 2
-s stacksize
The maximal size of the stack segment, in kilobytes.
.TP 2
-t time
The maximal amount of CPU time to be used by each process, in seconds.
.RE
.TP
umask [ mask ]
Set the value of umask (see umask(2)) to the specified
octal value. If the argument is omitted, the
umask value is printed.
.TP
unalias [-a] [name]
If ``name'' is specified, the shell removes that alias.
If ``-a'' is specified, all aliases are removed.
.TP
unset name...
The specified variables and functions are unset and
unexported. If a given name corresponds to both a
variable and a function, both the variable and the
function are unset.
.TP
wait [ job ]
Wait for the specified job to complete and return the
exit status of the last process in the job. If the
argument is omitted, wait for all jobs to complete
and the return an exit status of zero.
.LP
.sp 2
.B Command Line Editing
.sp
.LP
When sh is being used interactively from a terminal, the current command
and the command history (see fc in Builtins) can be edited using vi-mode
command-line editing. This mode uses commands, described below, similar
to a subset of those described in the vi man page.
The command set -o vi enables vi-mode editing and place sh into vi
insert mode.
With vi-mode enabled, sh can be switched between insert mode and command
mode. The editor is not described in full here, but will be in a later
document. It's similar to vi: typing <ESC> will throw you into
command VI command mode. Hitting <return> while in command mode
will pass the line to the shell.
.SH HISTORY
A
.I sh
command appeared in
Version 1 AT&T UNIX.