Git protip: quickly rewrite an old commit message

Maxime Bréhin — Mon, 19 Sep 2022 09:43:00 +0000

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 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. It opens your editor one last time with the list of actions tu run so you can see that the fixup commit is already at the right spot in the list 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

You’ve got nothing more to do but verify, save and close the file. 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 😌.

Git "autofixup"

Maxime Bréhin — Thu, 15 Sep 2022 13:54:30 +0000

How many times did you forget to add a file or change to a commit, only to discover your mistake later on (after a few commits)?

Since I'm a strong proponent that atomic commits are key to project quality and automation (see Conventional Commit and Semantic Release, for instance), I do my best to fix these commits using Git interactive rebasing. I strongly recommend you do the same. You don’t have to be afraid of Git rebase, you just have to learn how to use it 😉.

My goal here is to help you do this in a quick, smooth, painless way.

You may already be familiar with the git commit --fixup option (or --squash) that creates a commit with a special-format message about our intent to fix another commit. Using that, when you run a properly-configured interactive rebase, Git will automatically move that commit line as a fixup next to the one you're fixing.

Step by step

Add the missing files to the stage: git add ….

Ask Git to create the fixup commit: git commit --fixup=<commit-ref-to-be-fixed>.

Then run an interactive rebase, starting one commit before the one you fixed: git rebase --autosquash -i -r <fixed-commit-ref>~1.

Git opens your editor with the list of actions to run, but with the fixup already in the right spot. There's nothing more to do except save and close the file, that’s it !

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

You might want to check your log to confirm everything’s fine. You may also want to check that all the intended files were recorded by the fixed commit git show --name-only <new-commit-ref>.

As this is a recurring thing for me, I made a "magic" 🧙‍♂️ alias that calls both commands in sequence (to fix a single commit):

# Don't forget to stage the relevant fixes first
git config --global alias.autofixup '!!GIT_EDITOR=true && git commit --fixup $1 && git rebase --autosquash --interactive --rebase-merges $1~1 && echo "autofixup finished"'

Et voilà !

(The Git config you may love: there are many useful aliases you can add to your Git configuration. You can check out the one we’ve built through years of experience to learn more.)

Forem: Maxime Bréhin

Git protip: quickly rewrite an old commit message

Git "autofixup"

Step by step