Suppose I have a script that prints out the number of arguments passed to it:
# file: num_args
echo "Number of arguments: $#"
Now my question pertains to the following invocations:
> ./num_args a b c
> Number of arguments: 3 # As I would expect.
> ./num_args "a b c"
> Number of arguments: 1 # As I would expect.
> ./num_args a\ b\ c
> Number of arguments: 1 # As I would expect.
> printf "%q\n" "a b c"
> a\ b\ c # As I would expect.
> ./num_args $(printf "%q" "a b c")
> Number of arguments: 3 # NOT as I would expect.
Given that the printf
man page states that
%q ARGUMENT is printed in a format that can be reused as shell input, escaping non-printable characters with the proposed POSIX $'' syntax.
I am not sure what happens in the last case above.
printf %q
output is escaped correctly to be parsed as source code by a shell interpreter .
However, unquoted expansions aren't parsed as source code; they go only through string-splitting and glob-expansion phases. (This is a Good Thing: Otherwise handling untrusted filenames or other potentially-dangerous content in shell scripts would be nearly impossible).
You can substitute this output into a shell command, as in sh -c "...$foo..."
or eval "...$foo..."
or ssh somehost "...$foo..."
; it can't (shouldn't) just be used unquoted.
Some important references:
eval
(one of the families of workarounds discussed above).
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