How to determine a tar archive's format

Question

In Linux, how can I determine the format which was used to create a given tarball?

I'd like to detect whether a given archive was created using posix or gnu format.

I've already read the man page for GNU tar, which explains how to control the format during creation. However, I don't see anything about how to view the format of an existing file.

Answer 1

You can use file under Linux to look at the fingerprint of the uncompressed archive (file won't peer beyond the compression layer, so decompress it first):

$ tar --format=posix -cf posix.tar foo      # create test posix archive
$ tar --format=gnu   -cf gnu.tar   foo      # create test gnu archive
$ file gnu.tar posix.tar
gnu.tar:   POSIX tar archive (GNU)
posix.tar: POSIX tar archive

If the archive is compressed, decompress it first:

$ tar --format=posix -czf posix.tar.gz foo  # create test gzip posix archive
$ tar --format=gnu   -czf gnu.tar.gz   foo  # create test gzip gnu archive
$ gunzip posix.tar.gz
$ gunzip gnu.tar.gz
$ file gnu.tar posix.tar
gnu.tar:   POSIX tar archive (GNU)
posix.tar: POSIX tar archive

GNU is based on an older POSIX format, so that is why it says it is both.

For the nitty gritty details, the format is described in the GNU tar manual here and more details here.