176 lines
5.4 KiB
Groff
176 lines
5.4 KiB
Groff
.\" Copyright (c) 1980, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)csh.4 8.1 (Berkeley) 6/8/93
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.\" $FreeBSD$
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.\"
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.nr H1 3
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.NH
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Other, less commonly used, shell features
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.NH 2
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Loops at the terminal; variables as vectors
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.PP
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It is occasionally useful to use the
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.I foreach
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control structure at the terminal to aid in performing a number
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of similar commands.
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For instance, there were at one point three shells in use on the Cory \s-2UNIX\s0
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system at Cory Hall,
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`/bin/sh',
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`/bin/nsh',
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and
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`/bin/csh'.
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To count the number of persons using each shell one could have issued
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the commands
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.DS
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% grep \-c csh$ /etc/passwd
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27
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% grep \-c nsh$ /etc/passwd
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128
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% grep \-c \-v sh$ /etc/passwd
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430
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%
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.DE
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Since these commands are very similar we can use
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.I foreach
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to do this more easily.
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.DS
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% foreach i (\'sh$\' \'csh$\' \'\-v sh$\')
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? grep \-c $i /etc/passwd
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? end
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27
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128
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430
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%
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.DE
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Note here that the shell prompts for
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input with `? ' when reading the body of the loop.
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.PP
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Very useful with loops are variables which contain lists of filenames
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or other words.
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You can, for example, do
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.DS
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% set a=(\`ls\`)
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% echo $a
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csh.n csh.rm
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% ls
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csh.n
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csh.rm
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% echo $#a
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2
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%
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.DE
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The
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.I set
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command here gave the variable
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.I a
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a list of all the filenames in the current directory as value.
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We can then iterate over these names to perform any chosen function.
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.PP
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The output of a command within `\`' characters is converted by
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the shell to a list of words.
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You can also place the `\`' quoted string within `"' characters
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to take each (non-empty) line as a component of the variable;
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preventing the lines from being split into words at blanks and tabs.
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A modifier `:x' exists which can be used later to expand each component
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of the variable into another variable splitting it into separate words
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at embedded blanks and tabs.
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.NH 2
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Braces { ... } in argument expansion
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.PP
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Another form of filename expansion, alluded
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to before involves the characters `{' and `}'.
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These characters specify that the contained strings, separated by `,'
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are to be consecutively substituted into the containing characters
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and the results expanded left to right.
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Thus
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.DS
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A{str1,str2,...strn}B
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.DE
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expands to
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.DS
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Astr1B Astr2B ... AstrnB
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.DE
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This expansion occurs before the other filename expansions, and may
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be applied recursively (i.e. nested).
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The results of each expanded string are sorted separately, left
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to right order being preserved.
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The resulting filenames are not required to exist if no other expansion
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mechanisms are used.
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This means that this mechanism can be used to generate arguments which are
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not filenames, but which have common parts.
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.PP
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A typical use of this would be
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.DS
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mkdir ~/{hdrs,retrofit,csh}
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.DE
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to make subdirectories `hdrs', `retrofit' and `csh'
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in your home directory.
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This mechanism is most useful when the common prefix is longer
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than in this example, i.e.
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.DS
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chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
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.DE
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.NH 2
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Command substitution
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.PP
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A command enclosed in `\`' characters is replaced, just before
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filenames are expanded, by the output from that command.
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Thus it is possible to do
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.DS
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set pwd=\`pwd\`
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.DE
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to save the current directory in the variable
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.I pwd
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or to do
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.DS
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ex \`grep \-l TRACE *.c\`
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.DE
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to run the editor
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.I ex
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supplying as arguments those files whose names end in `.c'
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which have the string `TRACE' in them.*
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.FS
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*Command expansion also occurs in input redirected with `<<'
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and within `"' quotations.
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Refer to the shell manual section for full details.
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.FE
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.NH 2
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Other details not covered here
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.PP
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In particular circumstances it may be necessary to know the exact
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nature and order of different substitutions performed by the shell.
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The exact meaning of certain combinations of quotations is also
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occasionally important.
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These are detailed fully in its manual section.
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.PP
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The shell has a number of command line option flags mostly of use
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in writing \s-2UNIX\s0 programs,
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and debugging shell scripts.
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See the csh(1) manual section for a list of these options.
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.bp
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