Why do the md5 hashes of two tarballs of the same file differ?

Question

I can run:

echo \"asdf\" > testfile
tar czf a.tar.gz testfile
tar czf b.tar.gz testfile
md5sum *.tar.gz

and it turns out that a.ta

Answer 1

tar czf outfile infiles is equivalent to

tar cf - infiles | gzip > outfile

The reason the files are different is because gzip puts its input filename and modification time into the compressed file. When the input is a pipe, it uses an empty string as the filename and the current time as the modification time.

But it also has a --no-name option, which tells it not to put the name and timestamp into the file. So if you write the expanded command explicitly, instead of using the -z option to tar, you can make use of this option.

tar cf - testfile | gzip --no-name > a.tar.gz
tar cf - testfile | gzip --no-name > b.tar.gz

I tested this on OS X 10.6.8 and it works.