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.nr H1 3 Other, less commonly used, shell features Loops at the terminal; variables as vectors

It is occasionally useful to use the foreach control structure at the terminal to aid in performing a number of similar commands. For instance, there were at one point three shells in use on the Cory \s-2UNIX\s0 system at Cory Hall, `/bin/sh', `/bin/nsh', and `/bin/csh'. To count the number of persons using each shell one could have issued the commands % grep -c csh$ /etc/passwd 27 % grep -c nsh$ /etc/passwd 128 % grep -c -v sh$ /etc/passwd 430 % Since these commands are very similar we can use foreach to do this more easily. % foreach i (\'sh$\' \'csh$\' \'-v sh$\') ? grep -c $i /etc/passwd ? end 27 128 430 % Note here that the shell prompts for input with `? ' when reading the body of the loop.

Very useful with loops are variables which contain lists of filenames or other words. You can, for example, do % set a=(\`ls\`) % echo $a csh.n csh.rm % ls csh.n csh.rm % echo $#a 2 % The set command here gave the variable a a list of all the filenames in the current directory as value. We can then iterate over these names to perform any chosen function.

The output of a command within `\`' characters is converted by the shell to a list of words. You can also place the `\`' quoted string within `"' characters to take each (non-empty) line as a component of the variable; preventing the lines from being split into words at blanks and tabs. A modifier `:x' exists which can be used later to expand each component of the variable into another variable splitting it into separate words at embedded blanks and tabs. Braces { ... } in argument expansion

Another form of filename expansion, alluded to before involves the characters `{' and `}'. These characters specify that the contained strings, separated by `,' are to be consecutively substituted into the containing characters and the results expanded left to right. Thus A{str1,str2,...strn}B expands to Astr1B Astr2B ... AstrnB This expansion occurs before the other filename expansions, and may be applied recursively (i.e. nested). The results of each expanded string are sorted separately, left to right order being preserved. The resulting filenames are not required to exist if no other expansion mechanisms are used. This means that this mechanism can be used to generate arguments which are not filenames, but which have common parts.

A typical use of this would be mkdir ~/{hdrs,retrofit,csh} to make subdirectories `hdrs', `retrofit' and `csh' in your home directory. This mechanism is most useful when the common prefix is longer than in this example, i.e. chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}} Command substitution

A command enclosed in `\`' characters is replaced, just before filenames are expanded, by the output from that command. Thus it is possible to do set pwd=\`pwd\` to save the current directory in the variable pwd or to do ex \`grep -l TRACE *.c\` to run the editor ex supplying as arguments those files whose names end in `.c' which have the string `TRACE' in them.* .FS *Command expansion also occurs in input redirected with `<<' and within `"' quotations. Refer to the shell manual section for full details. .FE Other details not covered here

In particular circumstances it may be necessary to know the exact nature and order of different substitutions performed by the shell. The exact meaning of certain combinations of quotations is also occasionally important. These are detailed fully in its manual section.

The shell has a number of command line option flags mostly of use in writing \s-2UNIX\s0 programs, and debugging shell scripts. See the csh(1) manual section for a list of these options. .bp