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Handling Command-Line Arguments with a for Loop

Sometimes you want a script that will step through the command-line arguments one by one. (The $@ parameter gives you all of them at once.) The Bourne shell for loop can do this. The for loop looks like this:

for arg in list
do
    ...handle $arg...
done

If you omit the in list, the loop steps through the command-line arguments. It puts the first command-line argument in arg (or whatever else you choose to call the shell variable), then executes the commands from do to done. Then it puts the next command-line argument in arg, does the loop... and so on... ending the loop after handling all the arguments.

For an example of a for loop, let's hack on the the zpg script.

#!/bin/sh
# zpg - UNCOMPRESS FILE(S), DISPLAY WITH pg
# Usage: zpg [pg options] file [...files]
stat=1  # DEFAULT EXIT STATUS; RESET TO 0 BEFORE NORMAL EXIT
temp=/tmp/zpg$$
trap 'rm -f $temp; exit $stat' 0
trap 'echo "`basename $0`: Ouch! Quitting early..." 1>&2' 1 2 15

files=  switches=
for arg
do
    case "$arg" in
    -*) switches="$switches $arg" ;;
    *)  files="$files $arg" ;;
    esac
done

case "$files" in
"") echo "Usage: `basename $0` [pg options] file [files]" 1>&2 ;;
*)  for file in $files
    do gzcat "$file" | pg $switches
    done
    stat=0
    ;;
esac

We added a for loop to get and check each command-line argument. For example, let's say that a user typed:

% zpg -n afile ../bfile

The first pass through the for loop, $arg is -n. Because the argument starts with a minus sign (-), the case treats it as an option. Now the switches variable is replaced by its previous contents (an empty string), a space, and -n. Control goes to the esac and the loop repeats with the next argument.

The next argument, afile, doesn't look like an option. So now the files variable will contain a space and afile.

The loop starts over once more, with ../bfile in $arg. Again, this looks like a file, so now $files has afile ../bfile. Because ../bfile was the last argument, the loop ends; $switches has the options and $files has all the other arguments.

Next, we added another for loop. This one has the word in followed by $files, so the loop steps through the contents of $files. The loop runs gzcat on each file, piping it to pg with any switches you gave.

Note that $switches isn't quoted. This way, if $switches is empty, the shell won't pass an empty argument to pg. Also, if $switches has more than one switch, the shell will break the switches into separate arguments at the spaces and pass them individually to pg.

You can use a for loop with any space-separated (actually, IFS-separated) list of words -- not just filenames. You don't have to use a shell variable as the list; you can use command substitution (backquotes), shell wildcards, or just "hardcode" the list of words:

for person in Joe Leslie Edie Allan
do
   echo "Dear $person," | cat - form_letter | lpr
done

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