Quoting and Command-Line Parameters

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

$ 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:

$1 $2

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

for arg in 1 2 3 4

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

for arg in '1 2 3' 4

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

"$@" 

for arg in "$@"

A: In effect, $@ expands to:

$1" "$2

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

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" (); argv is such a list), but its defaults and syntax for suppressing them make for an artless developing language:

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 ]:

for arg do echo "Argument is $arg" done

A: produces:

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:

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

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

"$@"

A: expands to:

""

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:

% 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 (). 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