Error handling with tar and keep-old-files option

Question

I am unpacking tar files with the --keep-old-files option so that I don't clobber files that are already there. The annoying thing is that this causes tar to produce an error code #2 (fatal error) when a file is already there.

The reason it's important to me that this should not be considered an error is because my script logs when a file is successfully unpacked and then uses this log in order to not re-unpack that tar file.

I could just ignore this error completely, not log, and trust that no files will be overwritten but there are two problems with this: it generates a lot of error messages, and there are many files, so it's inefficient for me to open tar once for every file.

Is there any way I can use --keep-old-files, ignore the errors generated by existing files, and not ignore other errors?

Answer 1

This is apparently by design - from the documentation:

To be even more cautious and prevent existing files from being replaced, use the --keep-old-files (-k) option. It causes tar to refuse to replace or update a file that already exists, i.e., a file with the same name as an archive member prevents extraction of that archive member. Instead, it reports an error.

The git repository does contain a patch that introduces a --skip-old-files parameter, so this could be solved in a future GNU tar version:

+A new option --skip-old-files is introduced, which acts exactly as
+--keep-old-files, except that it does not treat existing files as
+errors.  Instead it just silently skips them.  An additional level of
+verbosity can be obtained by using the option --warning=existing-file
+together with this option.

The patch hasn't made it into an official release yet, though.