How do I get a list of old->new rewritten commit SHAs from 'git filter-branch'?

Question

After running git filter-branch, how do I get a list of old commit SHAs as rewritten by filter-branch to their new corresponding commit SHAs?

For example, something similar to:

b19fd985746c1f060f761d42d353e387bec243fb -> c8ab40ef9bae3b58642a8d1e5b90720d093a60b5
c5ebba1eeb92ca76c0effa32de14178ec7f07db6 -> 4d5a9958b98dbcfa47ce1354bb2af4cc77904639
705f71543235b872ca3e1067538e36d14044429d -> d2aafbd6e5b91955b62dee34f4a0abf0171ba016

Where the left column is the list of original SHAs, and the right column are the new SHAs after being rewritten by filter-branch.

I see that the man page for filter-branch mentions a map function, but I don't understand whether that's useful here, or if it is, how to use it.

Thanks for your help!

Answer 1

I had to go poking around the source for git-filter-branch to work this one out. It's not documented (as far as I can tell), but the old commit ID is explicitly exported as $GIT_COMMIT. This worked for me:

$ git filter-branch --your-filters-here --commit-filter 'echo -n "${GIT_COMMIT}," >>/tmp/log; git commit-tree "$@" | tee -a /tmp/log' your-branch-here
[...]
$ cat /tmp/log
70d609ba7bc58bb196a2351ba26afc5db0964ca6,d9071b49743701c7be971f76ddc84e76554516c7
0d1146dcabc00c45fb9be7fe923c955f7b6deb50,cb6813f9aca5e5f26fcc85007c5bb71552b91017
[...]

(That file, of course, has the format <original commit hash>,<new commit hash>.)

I'm kind of curious what your intentions are with using this though. It doesn't seem like information you'd need to typically know if you're using filter-branch the "right" way (i.e., not manipulating existing published history).