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