TAR with --to-command

Question

I'd like to calculate MD5 for all files in a tar archive. I tried tar with --to-command.
tar -xf abc.tar --to-command='md5sum'
it outputs like below.
cb6bf052c851c1c30801ef27c9af1968 -
f509549ab4eeaa84774a4af0231cccae -

Then I want to replace '-' with file name.
tar -xf abc.tar --to-command='md5sum | sed "s#-#$TAR_FILENAME#"'
it reports error.
md5sum: |: No such file or directory md5sum: sed: No such file or directory md5sum: s#-#./bin/busybox#: No such file or directory tar: 23255: Child returned status 1

Answer 1

You don't have a shell so this won't work (you also might see that the | gets to md5sum as an argument). one way could be to invoke the shell yourself, but there is some hassle with nested quotes:

tar xf some.tar --to-command 'sh -c "md5sum | sed \"s|-|\$TAR_FILENAME|\""'

Answer 2

At first, it's better to avoid using sed, not only because it's slow, but because $TAR_FILENAME can contain magic chars to be interpreted by sed (you already noticed that, having to use # instead of / for substitution command, didnt you?). Use deadproof solution, like head, followed by echoing actual filename.

Then, as Patrick mentions in his answer, you can't use complex commands without having them wrapped with shell, but for convenience I suggest to use built-it shell escapement ability, for bash it's printf '%q' "something", so the final command be like:

tar xf some.tar \
    --to-command="sh -c $(printf '%q' 'md5sum | head -c 34 && printf "%s\n" "$TAR_FILENAME"')"

"34" is number of bytes before file name in md5sum output format; && instead of ; to allow md5sum's error code (if any) reach tar; printf instead of echo used because filenames with leading "-" may be interpreted by echo as options.