Exec allows us to either pass all arguments at once with {} + or to pass them one by one with {} \;

Now let's say I want to rename all jpeg, no problem doing this:

find . \( -name '*.jpg' -o -name '*.jpeg' \) -exec mv '{}' '{}'.new \;

But if I need to redirect output, '{}' isn't accessible after redirection.

find . \( -name '*.jpg' -o -name '*.jpeg' \) -exec cjpeg -quality 80 '{}' > optimized_'{}' \;

This wouldn't work. I'd have to use a for loop, storing find's output into a variable before using it. Let's admit it, it's cumbersome.

for f in `find . \( -name '*.jpg' -o -name '*.jpeg' \)`; do cjpeg -quality 80 $f > optimized_$f; done;

So is there a better way?

  • Isn't there a > missing in the third code sample? – choroba Nov 14 at 11:49
  • 1
    Even your first line is nonstandard and thus non-portable. Try to avoid command lines where {} appears in a longer strings as such strings are typically not expanded. – schily Nov 14 at 12:35
  • That first example does not do what you say. – ctrl-alt-delor Nov 14 at 13:05
  • 1
    You have fixed your question: I would no-longer change it. – ctrl-alt-delor Nov 14 at 13:32
  • 1
    For what it's worth, the primary reason that your redirection does not work as you wanted is that it is handled by the shell from which you launch find, once, and applied to the find command itself. The {} has no special meaning in that context. The redirection is not an argument to find, and it certainly is not part of the -exec clause. – John Bollinger Nov 14 at 19:30
up vote 12 down vote accepted

You could use bash -c within the find -exec command and use the positional parameter with the bash command:

find . \( -name '*.jpg' -o -name '*.jpeg' \) -exec bash -c 'cjpeg -quality 80 "$1" > "$(dirname "$1")/optimized_$(basename "$1")"' sh {} \;

That way {} is provided with $1.

The sh before the {} tells the inner shell its "name", the string used here is used in e.g. error messages. This is discussed more in this answer on stackoverflow.

You have an answer(https://unix.stackexchange.com/a/481687/4778), but here is why.

The redirection >, and also pipes |, and $ expansion, are all done by the shell before the command is executed. Therefore stdout is redirected to optimized_{}, before find is started.

The redirection needs to be quoted to avoid that the present shell interprets it.
But quoting it will also avoid the output of the command to be redirected.
The known solution to this is to call a shell:

find . -name '*.jpg' -exec sh -c 'echo "$1" >"$1".new' called_shell '{}' \;

In this case, the redirection (>) is quoted on the present shell and works correctly inside the called shell. The called_shell is used as the $0 parameter (the name) of the child shell (sh).

That works well if a suffix is added the name of the file, but not if you use a prefix. For a prefix to work you need both to remove the ./ that find prepend to filenames with ${1#./} and to use the -execdir option.

You may (or may not) want to use the -iname option so that files named *.JPG or *.JpG or other variations are also included.

find . \( -iname '*.jpg' -o -iname '*.jpeg' \) -execdir sh -c '
     cjpeg -quality 80 "$1" > optimized_"${1#./}"
     ' called_shell '{}' \;

And, you may (or may not) also want to call the shell once per directory instead of once per file by adding a loop (for f do … ; done) and a + at the end:

find . \( -iname '*.jpg' -o -iname '*.jpeg' \) -execdir sh -c '
     for f; do cjpeg -quality 80 "$f" > optimized_"${f#./}"; done
     ' called_shell '{}' \+

And, finally, as cjpeg is able to directly write to a file, the redirection could be avoided as:

find . \( -iname '*.jpg' -o -iname '*.jpeg' \) -execdir sh -c '
     for f; do cjpeg -quality 80 "$f" -outfile optimized_"${f#./}"; done
     ' called_shell '{}' \+

cjpeg has an option that lets you write to a named file, rather than standard output. If your version of find supports the -execdir option, you can take advantage of that to make the redirection unnecessary.

find . \( -name '*.jpg' -o -name '*.jpeg' \) \
  -execdir cjpeg -quality 80 -outfile optimized_'{}' '{}' \;

Note: this actually assumes the BSD version of find, which appears to strip the leading ./ from the file name when exanding to {}. (Or conversely, GNU find adds ./ to the name. There's no standard to say which behavior is "right".)

  • If your find supports -execdir, you can use that instead of -exec. It causes the command to run in the directory where the file was found, and {} will become aa.jpg instead of ./t2/aa.jpg. – chepner Nov 14 at 15:33
  • Still in error: cjpeg: can't open optimized_./aa.jpg. – Isaac Nov 14 at 15:36
  • Hm, that appears to be a difference between GNU find and BSD find. (The perils of using non-standard extensions.) – chepner Nov 14 at 15:37
  • A filename without a leading ./ seems to be more prone to errors. A reasonable solution is proposed in this answer. – Isaac Nov 14 at 15:50

Create a script cjq80:

#!/bin/bash
cjpeg -quality 80 "$1" > "${1%/*}"/optimized_"${1##*/}"

Make it executable

chmod u+x cjq80

And use it in -exec:

find . \( -name '*.jpg' -o -name '*.jpeg' \) -exec cjq80 '{}' \;
  • I had though about this, but it's not very handy. Especially as I had to incorporate this in a build process – Buzut Nov 14 at 13:25
  • Just add the script to other build scripts, I find it more readable than doubly nested bash -c. – choroba Nov 14 at 13:57
  • Well, it's a matter of taste. Now every option is specified so if someone encounters the same issue, he'll choose for himself :) – Buzut Nov 14 at 14:37
  • @mosvy: Thanks, fixed. – choroba Nov 15 at 20:48

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