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Previous: 46.6 Watch Out for Bourne Shell -e Bug Chapter 46
Shell Script Debugging and Gotchas
Next: 46.8 Test Built-In Commands for Failure
 

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


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