How to Batch Update Git Commit Messages

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At Mozilla we're mostly strict about receiving contributions via git/GitHub.  Aside from requiring tests and enforcing code quality, one basic of submitting a commit (or several) is that the commit message begins with "bug #######", which is a reference to its Bugzilla bug.  The message requirement makes sense but I hate going back to contributors and asking them to do "the paperwork" of changing their commit message to match our standard, so I generally try to make the update for them.

Recently MDN received a contribution which upgraded CKEditor and did so via approximately 60 commits, none of which referenced the bug number for the feature update.  I badly wanted to do the commit message updates for this contributor, but I wanted to do so in an automated way -- a batch update to save me the time updating each message one by one.  Welcome to filter-branch and --msg-filter!

Prepending to Commit Messages

To prepend text to every commit message in a given range, you'd execute a message like:

git filter-branch --msg-filter 'echo "bug ###### - \c" && cat' master..HEAD

The combination of filter-branch and --msg-filter will allow you to recurse through every commit message.  The echo piece allows you to build the new string.  The last piece is the range whose commit messages should be targeted -- in this case, I'm prepending the text to every commit in the feature branch.

You can also sed to achieve this:

git filter-branch -f --msg-filter 'sed "s/^/bug ###### - /"' master..HEAD

Appending to Commit Messages

The case for appending to commit messages could be where you want to add the reviewer name(s) to the message.  Appending is roughly the same:

git filter-branch -f --msg-filter 'cat && echo "[Reviewer Walsh]"' master..HEAD

The difference here is the position of the cat command.  Everything else can be the same.

I have a love/hate relationship with git -- I love git because it's so easy for 90% of my needs but hate it when I need the other 10%.  Hopefully these examples help you if you run into this need in the future!

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  1. Colly

    I like to keep up to date as much as possible. Helpful for shorthand commands to open/create things remotely. I know there are other pre-built tools out there, but this works for me

  2. Peter
    git filter-branch -f --msg-filter 'sed "s/^/bug ###### - /"' master..HEAD

    This doesn’t work for multiline commit messages.
    It will prepend to every single line since sed works on line level.

  3. You can fix the sed example like this:

    git filter-branch -f --msg-filter 'sed "1 s/^/bug ###### - /"' master..HEAD

    (Note the “1” there to limit sed to the first line)

  4. …and you can ignore the -f if you choose; that’s just a leak from me running this various times.

  5. Patrick

    How can you parameterize the prepended label in a git alias?

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!