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| Chapter 46 Shell Script Debugging and Gotchas |

## 46.7 Quoting and Command-Line Parameters

Q: I need to pass a shell script some arguments with multiple words. I thought that putting quotes (8.14) around command-line arguments would group them. The shell script seems to ignore the quoting, somehow. Here's a simple example:

Q:

>     $ cat script
>        ...
>     for arg in $*
>     do
>         echo "Argument is $arg"
>     done
>     $ script '1 2 3' 4
>        ...
>     Argument is 1
>     Argument is 2
>     Argument is 3
>     Argument is 4

A: This is the way `$*` is defined to work. `$*` expands to:

A:

>     $1 $2

A: [not `<">$1<"> <">$2<">`-JP ] if there are two arguments. Hence the for loop reads:

A:

>     for arg in 1 2 3 4

A: Note that the quotes are gone. What you wanted the shell to see was:

A:

>     for arg in '1 2 3' 4

A: You cannot get that, but you can get something that is Good Enough:

A:

    "$@"
    for arg in "$@"

A: In effect, `$@` expands to:

A:

>     $1" "$2

A: Putting `""`s around `$@`, the effect is:

A:

>     for arg in "$1" "$2"

A: Shell quoting is unnecessarily complex. The C shell actually has the right idea (variables can be set to "word lists" (47.5); argv is such a list), but its defaults and syntax for suppressing them make for an artless programming language:

A:

>     foreach arg ($argv:q)      # colon q ?!?

A: For the special case of iterating a shell variable over the argument list as it stands at the beginning of the iteration, the Bourne shell provides the construct `for arg do` [i.e., no `in` `list`-JP ]:

A:

>     for arg
>     do echo "Argument is $arg"
>     done

A: produces:

A:

>     Argument is 1 2 3
>     Argument is 4

A: `"$@"` is still needed for passing argument lists to other programs. Unfortunately, since `$@` is defined as expanding to:

A:

>     $1" "$2...$n-1" "$n

A: (where `n` is the number of arguments), when there are no arguments:

A:

>     "$@"

A: expands to:

A:

>     ""

A: and `""` produces a single argument. [Many UNIX vendors considered this a bug and changed it so that it produces no arguments. -JP ] The best solution for this is to use, for example:

A:

>     % cat bin/okeeffe
>     #! /bin/sh
>     exec rsh okeeffe.berkeley.edu -l torek ${1+"$@"}
>     %

A: The construct `${1+"$@"}` means "expand `$1`, but if `$1` is defined, use `"$@"` instead." [You don't need this on Bourne shells with the "bug fix" I mentioned. -JP ] Hence, if there are no arguments, we get `$1` (which is nothing and produces no arguments), otherwise we get `"$@"` (which expands as above). `${``var``+``instead``}` is one of several sh \*(lqexpansion shortcuts\*(rq (45.12). Another more generally useful one is `${``var-default``}`, which expands to `$``var`, but if var is not set, to `default` instead. All of these can be found in the manual for sh, which is worth reading several times, experimenting as you go.

- CT in comp.unix.questions on Usenet, 18 March 1988

46.6 Watch Out for Bourne Shell -e Bug |  | 46.8 Test Built-In Commands for Failure