Bash: Quotation in command substitution

Question

In short, I want to use directories listed by a command in a find command:

find $(produces_dir_names --options...) -find-options...

The problem comes with white space in the directory names. I thought quoting them in the output of the producing command (which I can change) would be enough:

"a" "a b" "a b c"

but bash complains:

find: ‘"a"’: No such file or directory
find: ‘"a’: No such file or directory
find: ‘b"’: No such file or directory
find: ‘"a’: No such file or directory
find: ‘b’: No such file or directory
find: ‘c"’: No such file or directory

As you can, see bash will split the output of the command on spaces, even with the quotation marks. I tried to fiddle with IFS and set it to \n, but my understanding of it seems too limited to get it working.

The only workaround I found was in this Stack Overflow question: bash command substitution remove quotation, namely putting an eval in front of it, but this seems kind of ugly.

My Questions:

Is there an easy way and how would it look like to write this substitution, without the eval?

Are the quotations even necessary?

Example (producing the same output):

find $(echo '"a" "a b" "a b c"')

Answer 1

Maybe in two lines

IFS=$'\n' DIRS=( $(produces_dir_names --options...) ) 
find "${DIRS[@]}" -find-options...

Example:

$ mkdir -p "/tmp/test/a b/foo" "/tmp/test/x y/bar"

$ IFS=$'\n' DIRS=( $(printf "/tmp/test/a b\n/tmp/test/x y\n") )
$ find "${DIRS[@]}" -mindepth 1
/tmp/test/a b/foo
/tmp/test/x y/bar

But in general this is no good style. For example you will be in trouble if your DIRS contain newlines. Better fix your "produces_dir_names" to print null byte terminated strings. Regarding my example this would be something like:

$ printf "/tmp/test/a b\0/tmp/test/x y\0" | xargs -0 -I '{}' find '{}' -mindepth 1
/tmp/test/a b/foo
/tmp/test/x y/bar

If you can't fix "produces_dir_names", regarding my last comment, the most general solution would look like this:

produces_dir_names --options... | tr '\n' '\0' | xargs -0  -I '{}' find '{}' -find-options...

Still problems with "newlines" unless you fix "produces_dir_names" to avoid tr.

Answer 2

rudimeier’s answer is good — specifically, the part about modifying produces_dir_names to print null-terminated strings — but it might not be obvious from his answer that it executes find once for each directory.  If this is good enough, than fine.  But, of course, it is possible to invoke find with multiple starting points; e.g.,

find  dir1 dir2 dir3  -find-options...

and it appears from the question that that’s what you want.  This can be done as follows:

printf "a\0a b\0a b c" | xargs -0 sh -c 'find "$@" -find-options...' sh

This causes xargs to invoke sh -c once, with all the directory names appended to the command.  The shell will then expand "$@" to a list of those directory names.

P.S. If produces_dir_names lists too many directory names to put on one command line, then xargs will be forced to spawn a few commands.  Use xargs --verbose to see what commands xargs is spawning.

Answer 3

Just to clear up the mystery of the error messages you are getting:

find: ‘"a"’: No such file or directory
find: ‘"a’: No such file or directory
find: ‘b"’: No such file or directory
find: ‘"a’: No such file or directory
find: ‘b’: No such file or directory
find: ‘c"’: No such file or directory

The answer is that Bash quote removal doesn't remove quotes that resulted from command substitution.

From LESS='+/^ *Quote Removal' man bash

Quote Removal
    After the preceding expansions, all unquoted occurrences of the charac-
    ters  \,  ', and " that did not result from one of the above expansions
    are removed.

The "preceding expansions" referenced include:

EXPANSION
   Brace Expansion
   Tilde Expansion
   Parameter Expansion
   Command Substitution
   Arithmetic Expansion
   Process Substitution
   Word Splitting
   Pathname Expansion
   Quote Removal