Git protip: quickly rewrite an old commit message

By Maxime Bréhin • Published on 17 October 2022 • 2 min

Just as I sometimes forget files in a commit, I also sometimes botch commit messages only to find out about it a while later (after a few more commits). My most common oversight is probably forgetting to link the issue the commit refers to.

Later, but not too late!

Don’t tell me you’re thinking: *“Never mind, I’ll pop open the UI and fill it in by hand!” or that you’re giving up! Maybe that’s because you don’t know about interactive rebasing, or are afraid to use it 😨. Honestly, you shouldn’t be, with a little learning this command proves to be a super valuable ally!

Apart from the rebase thing, do you know that you can create a commit that expresses the intent to change the message? I bet you don’t! Fortunately, I’m here 😁 to help. Let me introduce the git commit --fixup reword:<commit-ref-to-fix> command. This is a bit of a mouthful but who cares, we’re going to wrap it with a nice alias that looks kind of like a magic incantation 🔮. Introducing autoreword:

git config --global alias.autoreword '!git commit --fixup reword:$1 && GIT_EDITOR=true && git rebase --autosquash --interactive --rebase-merges $1~1 && echo "autoreword finished"'

What happens if you run this command?

🌀 An evil wormhole opens, releasing the flames of hell that reduce this world to ashes 🔥! (Actually no, we don’t even need to use magic for that 😭)

But seriously, Git will open your editor with a message starting with amend! … followed by the first line of the commit message you want to fix, then a line break and again the (full) commit message.

amend! chore(dx): setup ESLint and Prettier

chore(dx): setup ESLint and Prettier

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# On branch chore/automation

You can then change the message (but keep the very first line as-is).

amend! chore(dx): setup ESLint and Prettier

chore(dx): setup ESLint and Prettier

# Here is the reference to my issue!
Closes #42

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# On branch chore/automation

Now you can save and close the file.

Git will then create the fixup commit and our alias will automatically run the interactive rebase, starting from the faulty commit’s direct ancestor. Then setting GIT_EDITOR environment variable temporary to true tells Git to not open the editor and directly runs the rebase. If you’re curious about what’s happening then, you can remove that GIT_EDITOR=true part so you can see that the fixup commit is already at the right spot in the list of actions to run, prefixed by fixup -C.

pick 3f0714b chore(dx): setup ESLint and Prettier
fixup -C 61eecda amend! chore(dx): setup ESLint and Prettier
pick 9a7cf39 chore(dx): setup Husky, lint-staged, precommit-checks, commitlint

The rebase then runs (without conflict) and you can verify your log to see that the commit message was updated!

You may see in the actions list some keys like label onto and reset onto. This is a because the rebase is called with a --rebase-merges option that preserves your local merges while rebasing. You don’t need to worry about it 😌.