9.11 Repeating a Command with a foreach Loop

When some people need to repeat a command on several files, the first thing they think of is C shell history (11.5 ) :

-v
 

% cat -t -v /usr/fran/report | pg


   ...
% ^fran/report^rob/file3


cat -t -v /usr/rob/file3 | pg
   ...
% ^3^21


cat -t -v /usr/rob/file21 | pg
   ...
%

That kind of thing can be easier with the C shell's foreach loop. (In the Bourne and Korn shells, use a for (9.12 ) loop.) You give the loop a list of the words that will change each time the command line is run. In this example, it's a list of filenames. The loop will step through the words, one by one, storing a word into a shell variable (6.8 ) , then running the command(s). The loop goes on until it has read all the words. For example:

% foreach file (/usr/fran/report /usr/rob/file3 /usr/rob/file21)


? cat -t -v $file | pg


? end


   ...Shell runs cat -t -v /usr/fran/report | pg
...
   ...Shell runs cat -t -v /usr/rob/file3 | pg
...
   ...Shell runs cat -t -v /usr/rob/file21 | pg
...
%

The question marks (? ) are secondary prompts (9.13 ) ; the C shell will keep printing them until you type the command end . Then the loop runs.

The list between the parentheses doesn't have to be filenames. Among other things, you can use wildcards (1.16 ) , backquotes (9.16 ) (command substitution), variables (6.8 , 6.1 ) , and the C shell's handy curly brace ({} ) operators (9.5 ) . For example, you could have typed the above loop this way:

% foreach file (/usr/fran/report /usr/rob/file{3,21})


? cat -t -v $file | pg


? end

If you want the loop to stop before or after running each command, add the C shell operator $< . It reads keyboard input and waits for a RETURN. In this case, you can probably ignore the input; you'll use $< to make the loop wait. For example, to make the loop above prompt before each command line:

set
 

% foreach file (/usr/fran/report /usr/rob/file{3,21})


? echo -n "Press RETURN to see $file-"


? set x="$<"


? cat -t -v $file | pg


? end


Press RETURN to see /usr/fran/report- [RETURN]

   Shell runs cat -t -v /usr/fran/report | pg
...
Press RETURN to see /usr/rob/file3- [RETURN]

   Shell runs cat -t -v /usr/rob/file3 | pg
...
Press RETURN to see /usr/rob/file21- [RETURN]

   Shell runs cat -t -v /usr/rob/file21 | pg
...

The loop parameters don't need to be filenames. For instance, you could send a personalized mail (1.33 ) message to five people this way: [1]

[1] If you're sending lots of mail messages with a loop, your system mailer may get overloaded. In that case, it's a good idea to put a command like sleep  5 (40.2 ) on a separate line before the end . That will give the mailer five seconds to send each message.

cat -
 

% foreach person (John Cathy Agnes Brett Elma)


? echo "Dear $person," | cat - formletter | mail $person


? end

The first line of the first letter will be "Dear John,"; the second letter "Dear Cathy,"; and so on.

Want to take this idea further? It's a part of shell programming (44.1 ) . I usually don't recommend (47.2 ) shell programming with the C shell, but this is a handy technique to use interactively.