path: root/bin/csh/USD.doc/csh.3

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.nr H1 2
.NH
Shell control structures and command scripts
.NH 2
Introduction
.PP
It is possible to place commands in files and to cause shells to be
invoked to read and execute commands from these files,
which are called
.I "shell scripts."
We here detail those features of the shell useful to the writers of such
scripts.
.NH 2
Make
.PP
It is important to first note what shell scripts are
.I not
useful for.
There is a program called
.I make
which is very useful for maintaining a group of related files
or performing sets of operations on related files.
For instance a large program consisting of one or more files
can have its dependencies described in a
.I makefile
which contains definitions of the commands used to create these
different files when changes occur.
Definitions of the means for printing listings, cleaning up the directory
in which the files reside, and installing the resultant programs
are easily, and most appropriately placed in this
.I makefile.
This format is superior and preferable to maintaining a group of shell
procedures to maintain these files.
.PP
Similarly when working on a document a
.I makefile
may be created which defines how different versions of the document
are to be created and which options of
.I nroff
or
.I troff
are appropriate.
.NH 2
Invocation and the argv variable
.PP
A
.I csh
command script may be interpreted by saying
.DS
% csh script ...
.DE
where
.I script
is the name of the file containing a group of
.I csh
commands and
`\&...' is replaced by a sequence of arguments.
The shell places these arguments in the variable
.I argv
and then begins to read commands from the script.
These parameters are then available through the same mechanisms
which are used to reference any other shell variables.
.PP
If you make the file
`script'
executable by doing
.DS
chmod 755 script
.DE
and place a shell comment at the beginning of the shell script
(i.e. begin the file with a `#' character)
then a `/bin/csh' will automatically be invoked to execute `script' when
you type
.DS
script
.DE
If the file does not begin with a `#' then the standard shell
`/bin/sh' will be used to execute it.
This allows you to convert your older shell scripts to use
.I csh
at your convenience.
.NH 2
Variable substitution
.PP
After each input line is broken into words and history substitutions
are done on it, the input line is parsed into distinct commands.
Before each command is executed a mechanism know as
.I "variable substitution"
is done on these words.
Keyed by the character `$' this substitution replaces the names
of variables by their values.
Thus
.DS
echo $argv
.DE
when placed in a command script would cause the current value of the
variable
.I argv
to be echoed to the output of the shell script.
It is an error for
.I argv
to be unset at this point.
.PP
A number of notations are provided for accessing components and attributes
of variables.
The notation
.DS
$?name
.DE
expands to `1' if name is
.I set
or to `0'
if name is not
.I set.
It is the fundamental mechanism used for checking whether particular
variables have been assigned values.
All other forms of reference to undefined variables cause errors.
.PP
The notation
.DS
$#name
.DE
expands to the number of elements in the variable
.I name.
Thus
.DS
% set argv=(a b c)
% echo $?argv
1
% echo $#argv
3
% unset argv
% echo $?argv
0
% echo $argv
Undefined variable: argv.
%
.DE
.PP
It is also possible to access the components of a variable
which has several values.
Thus
.DS
$argv[1]
.DE
gives the first component of
.I argv
or in the example above `a'.
Similarly
.DS
$argv[$#argv]
.DE
would give `c',
and
.DS
$argv[1\-2]
.DE
would give `a b'. Other notations useful in shell scripts are
.DS
$\fIn\fR
.DE
where
.I n
is an integer as a shorthand for
.DS
$argv[\fIn\fR\|]
.DE
the
.I n\|th
parameter and
.DS
$*
.DE
which is a shorthand for
.DS
$argv
.DE
The form
.DS
$$
.DE
expands to the process number of the current shell.
Since this process number is unique in the system it can
be used in generation of unique temporary file names.
The form
.DS
$<
.DE
is quite special and is replaced by the next line of input read from
the shell's standard input (not the script it is reading).  This is
useful for writing shell scripts that are interactive, reading
commands from the terminal, or even writing a shell script that
acts as a filter, reading lines from its input file. Thus the sequence
.DS
echo 'yes or no?\ec'
set a=($<)
.DE
would write out the prompt `yes or no?' without a newline and then
read the answer into the variable `a'.  In this case `$#a' would be
`0' if either a blank line or end-of-file (^D) was typed.
.PP
One minor difference between `$\fIn\fR\|' and `$argv[\fIn\fR\|]'
should be noted here.
The form
`$argv[\fIn\fR\|]'
will yield an error if
.I n
is not in the range
`1\-$#argv'
while `$n'
will never yield an out of range subscript error.
This is for compatibility with the way older shells handled parameters.
.PP
Another important point is that it is never an error to give a subrange
of the form `n\-'; if there are less than
.I n
components of the given variable then no words are substituted.
A range of the form `m\-n' likewise returns an empty vector without giving
an error when \fIm\fR exceeds the number of elements of the given variable,
provided the subscript \fIn\fR is in range.
.NH 2
Expressions
.PP
In order for interesting shell scripts to be constructed it
must be possible to evaluate expressions in the shell based on the
values of variables.
In fact, all the arithmetic operations of the language C are available
in the shell
with the same precedence that they have in C.
In particular, the operations `==' and `!=' compare strings
and the operators `&&' and `|\|\||' implement the boolean and/or operations.
The special operators `=~' and `!~' are similar to `==' and `!=' except
that the string on the right side can have pattern matching characters
(like *, ? or []) and the test is whether the string on the left matches
the pattern on the right.
.PP
The shell also allows file enquiries of the form
.DS
\-? filename
.DE
where `?' is replace by a number of single characters.
For instance the expression primitive
.DS
\-e filename
.DE
tell whether the file
`filename'
exists.
Other primitives test for read, write and execute access to the file,
whether it is a directory, or has non-zero length.
.PP
It is possible to test whether a command terminates normally,
by a primitive of the
form `{ command }' which returns true, i.e. `1' if the command
succeeds exiting normally with exit status 0, or `0' if the command
terminates abnormally or with exit status non-zero.
If more detailed information about the execution status of a command
is required, it can be executed and the variable `$status' examined
in the next command.
Since `$status' is set by every command, it is very transient.
It can be saved if it is inconvenient to use it only in the single
immediately following command.
.PP
For a full list of expression components available see the manual
section for the shell.
.NH 2
Sample shell script
.PP
A sample shell script which makes use of the expression mechanism
of the shell and some of its control structure follows:
.DS
% cat copyc
#
# Copyc copies those C programs in the specified list
# to the directory ~/backup if they differ from the files
# already in ~/backup
#
set noglob
foreach i ($argv) 

        if ($i !~ *.c) continue  # not a .c file so do nothing

        if (! \-r ~/backup/$i:t) then
                echo $i:t not in backup... not cp\e\'ed
                continue
        endif

        cmp \-s $i ~/backup/$i:t # to set $status

        if ($status != 0) then
                echo new backup of $i
                cp $i ~/backup/$i:t
        endif
end
.DE
.PP
This script makes use of the
.I foreach
command, which causes the shell to execute the commands between the
.I foreach
and the matching
.I end
for each of the values given between `(' and `)' with the named
variable, in this case `i' set to successive values in the list.
Within this loop we may use the command
.I break
to stop executing the loop 
and
.I continue
to prematurely terminate one iteration
and begin the next.
After the
.I foreach
loop the iteration variable
(\fIi\fR in this case)
has the value at the last iteration.
.PP
We set the variable
.I noglob
here to prevent filename expansion of the members of
.I argv.
This is a good idea, in general, if the arguments to a shell script
are filenames which have already been expanded or if the arguments
may contain filename expansion metacharacters.
It is also possible to quote each use of a `$' variable expansion,
but this is harder and less reliable.
.PP
The other control construct used here is a statement of the form
.DS
\fBif\fR ( expression ) \fBthen\fR
	command
	...
\fBendif\fR
.DE
The placement of the keywords here is
.B not
flexible due to the current implementation of the shell.\(dg
.FS
\(dgThe following two formats are not currently acceptable to the shell:
.sp
.in +5
.nf
\fBif\fR ( expression )		# \fBWon't work!\fR
\fBthen\fR
	command
	...
\fBendif\fR
.fi
.in -5
.sp
and
.sp
.in +5
.nf
\fBif\fR ( expression ) \fBthen\fR command \fBendif\fR		# \fBWon't work\fR
.in -5
.fi
.FE
.PP
The shell does have another form of the if statement of the form
.DS
\fBif\fR ( expression ) \fBcommand\fR
.DE
which can be written
.DS
\fBif\fR ( expression ) \e
	command
.DE
Here we have escaped the newline for the sake of appearance.
The command must not involve `\||\|', `&' or `;'
and must not be another control command.
The second form requires the final `\e' to
.B immediately
precede the end-of-line.
.PP
The more general
.I if
statements above also admit a sequence of
.I else\-if
pairs followed by a single
.I else
and an
.I endif,
e.g.:
.DS
\fBif\fR ( expression ) \fBthen\fR
	commands
\fBelse\fR \fBif\fR (expression ) \fBthen\fR
	commands
\&...

\fBelse\fR
	commands
\fBendif\fR
.DE
.PP
Another important mechanism used in shell scripts is the `:' modifier.
We can use the modifier `:r' here to extract a root of a filename or
`:e' to extract the
.I extension.
Thus if the variable
.I i
has the value
`/mnt/foo.bar'
then
.sp
.in +5
.nf
% echo $i $i:r $i:e
/mnt/foo.bar /mnt/foo bar
%
.sp
.in -5
.fi
shows how the `:r' modifier strips off the trailing `.bar' and the
the `:e' modifier leaves only the `bar'.
Other modifiers will take off the last component of a pathname leaving
the head `:h' or all but the last component of a pathname leaving the
tail `:t'.
These modifiers are fully described in the
.I csh
manual pages in the User's Reference Manual.
It is also possible to use the
.I "command substitution"
mechanism described in the next major section to perform modifications
on strings to then reenter the shell's environment.
Since each usage of this mechanism involves the creation of a new process,
it is much more expensive to use than the `:' modification mechanism.\(dd
.FS
\(dd It is also important to note that
the current implementation of the shell limits the number of `:' modifiers
on a `$' substitution to 1.
Thus
.sp
.nf
.in +5
% echo $i $i:h:t
/a/b/c /a/b:t
%
.in -5
.fi
.sp
does not do what one would expect.
.FE
Finally, we note that the character `#' lexically introduces a shell
comment in shell scripts (but not from the terminal).
All subsequent characters on the input line after a `#' are discarded
by the shell.
This character can be quoted using `\'' or `\e' to place it in
an argument word.
.NH 2
Other control structures
.PP
The shell also has control structures
.I while
and
.I switch
similar to those of C.
These take the forms
.DS
\fBwhile\fR ( expression )
	commands
\fBend\fR
.DE
and
.DS
\fBswitch\fR ( word )

\fBcase\fR str1:
	commands
	\fBbreaksw\fR

\& ...

\fBcase\fR strn:
	commands
	\fBbreaksw\fR

\fBdefault:\fR
	commands
	\fBbreaksw\fR

\fBendsw\fR
.DE
For details see the manual section for
.I csh.
C programmers should note that we use
.I breaksw
to exit from a
.I switch
while
.I break
exits a
.I while
or
.I foreach
loop.
A common mistake to make in
.I csh
scripts is to use
.I break
rather than
.I breaksw
in switches.
.PP
Finally,
.I csh
allows a
.I goto
statement, with labels looking like they do in C, i.e.:
.DS
loop:
	commands
	\fBgoto\fR loop
.DE
.NH 2
Supplying input to commands
.PP
Commands run from shell scripts receive by default the standard
input of the shell which is running the script.
This is different from previous shells running
under \s-2UNIX\s0.  It allows shell scripts to fully participate
in pipelines, but mandates extra notation for commands which are to take
inline data.
.PP
Thus we need a metanotation for supplying inline data to commands in
shell scripts.
As an example, consider this script which runs the editor to
delete leading blanks from the lines in each argument file:
.DS
% cat deblank
# deblank \-\- remove leading blanks
foreach i ($argv)
ed \- $i << \'EOF\'
1,$s/^[ ]*//
w
q
\&\'EOF\'
end
%
.DE
The notation `<< \'EOF\''
means that the standard input for the
.I ed
command is to come from the text in the shell script file
up to the next line consisting of exactly `\'EOF\''.
The fact that the `EOF' is enclosed in `\'' characters, i.e. quoted,
causes the shell to not perform variable substitution on the
intervening lines.
In general, if any part of the word following the `<<' which the
shell uses to terminate the text to be given to the command is quoted
then these substitutions will not be performed.
In this case since we used the form `1,$' in our editor script
we needed to insure that this `$' was not variable substituted.
We could also have insured this by preceding the `$' here with a `\e',
i.e.:
.DS
1,\e$s/^[ ]*//
.DE
but quoting the `EOF' terminator is a more reliable way of achieving the
same thing.
.NH 2
Catching interrupts
.PP
If our shell script creates temporary files, we may wish to catch
interruptions of the shell script so that we can clean up
these files.
We can then do
.DS
onintr label
.DE
where
.I label
is a label in our program.
If an interrupt is received the shell will do a
`goto label'
and we can remove the temporary files and then do an
.I exit
command (which is built in to the shell)
to exit from the shell script.
If we wish to exit with a non-zero status we can do
.DS
exit(1)
.DE
e.g. to exit with status `1'.
.NH 2
What else?
.PP
There are other features of the shell useful to writers of shell
procedures.
The
.I verbose
and
.I echo
options and the related
.I \-v
and
.I \-x
command line options can be used to help trace the actions of the shell.
The
.I \-n
option causes the shell only to read commands and not to execute
them and may sometimes be of use.
.PP
One other thing to note is that
.I csh
will not execute shell scripts which do not begin with the
character `#', that is shell scripts that do not begin with a comment.
Similarly, the `/bin/sh' on your system may well defer to `csh'
to interpret shell scripts which begin with `#'.
This allows shell scripts for both shells to live in harmony.
.PP
There is also another quotation mechanism using `"' which allows
only some of the expansion mechanisms we have so far discussed to occur
on the quoted string and serves to make this string into a single word
as `\'' does.
.bp