Why does Zipping the same content twice gives two files with different SHA1?

Question

I have run into a strange problem with git and zip files. My build script takes a bunch of documentation html pages and zips them into a docs.zip I then check this file into

Answer 1

By default, gzip saves file name and time stamp

%> gzip -help 2>&1 | grep -e '-n'
 -N --name            save or restore original file name and time stamp
 -n --no-name         don't save original file name or time stamp

%> gzip -V
Apple gzip 272

Using -n option:

%> tar cv foo/ | gzip -n > foo.tgz; shasum foo.tgz # sha256sum on Ubuntu

you will consistently get the same hash.

Try above without -n and you should see a different hash each time.