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| Chapter 44 Shell Programming for the Uninitiated |

## 44.16 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 (44.15) 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 (6.8)), 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 zpg (44.12) script.

    case
    #!/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 `$switches` isn't quoted (8.14). This way, if `$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 (35.21)-separated) list of words - not just filenames. You don't have to use a shell variable as the list; you can use command substitution (9.16) (backquotes), shell wildcards (15.2), or just "hardcode" the list of words:

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

The getopt and getopts (44.18) commands handle command-line arguments in a more standard way than for loops.

- JP

44.15 Handling Command-Line Arguments in Shell Scripts |  | 44.17 Handling Arguments with while and shift