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Previous: 46.6 Watch Out for Bourne Shell -e Bug Chapter 46
Shell Script Debugging and Gotchas
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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:


$ cat script
for arg in $*
    echo "Argument is $arg"
$ 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" (47.5); argv is such a list), but its defaults and syntax for suppressing them make for an artless programming 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"

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 (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

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