csh.3

早期freebsd实现

.\" Copyright (c) 1980, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"	@(#)csh.3	8.1 (Berkeley) 6/8/93
.\"
.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