Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming csh: C Shell
Starting a Loop at a Terminal
It is occasionally useful to use the foreach control structure at the terminal to aid in
performing a number of similar commands. For instance, if we want to know how many
people have the Bourne shell (/bin/sh) as their login shell, we can give this command:
% grep -c /bin/sh$ /etc/passwd
if we want to know how many people have the C shell {/bin/csh) as their login shell, we
can give this command:
% grep -c /bin/csh$ /etc/passwd
Since these commands are very similar, we can invoke this foreach command at the
terminal to find how many have the Bourne shell and how many have the C shell:
% foreach i ('/bin/sh$' '/bin/csh$')
? grep -c $i /etc/passwd
? end
Note: When the foreach command is entered at the terminal, the C shell prompts for
input with "?" until the end com mand is given.
Also useful with loops are variables that contain lists of file names or other words. For
example, examine the following terminal session:
% set a= (ls)
% echo $a
csh.n csh.rm
% Is
csh.n
csh.rm
% echo $#a
2
The set command here gave the variable a a list of all the file names in the current
directory as its value. We can then iterate over these names to perform any chosen
function.
When the C shell encounters a command enclosed in accent marks (), it executes the
command and takes the standard output of the command as the value of the expression
formed by the delimited command.
The output of a command within accent marks () is converted by the C shell to a list of
words. You can also place the quoted string within double quotation marks (") to take
each (nonempty) line as a component of the variable. This prevents 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 by splitting the original
variable into separate words at embedded blanks and tabs.
8-19