Edit/Fix-up Old Commits in Git

May 3, 2024

In this blog post, we'll walk through the process of creating a "fix-up" commit to amend an old commit identified by a specific hash, <OLD_HASH>.

First, we create the fix-up commit by adding our changes and committing with the --fixup=<OLD_HASH> flag:

git add <changes>
git commit --fixup=<OLD_HASH>

Next, we perform an interactive rebase starting from one commit before <OLD_HASH>:

git rebase --interactive --autosquash <COMMIT_BEFORE_OLD_HASH>

If <COMMIT_BEFORE_OLD_HASH> is the first or root commit, pass --root¹ instead.

Upon executing this command, your default editor opens up (with a text similar to below), allowing you to save and exit. Done.

pick a43f263 Add initial interfaces
pick a643ac3 Add base types
fixup a08d5fa Add implementation
pick 01e156a fix typo in base types

# Rebase fba8887..a08d5fa onto fba8887 (3 commands)
#
# Commands:
# p, pick <commit> = use commit
# r, reword <commit> = use commit, but edit the commit message
# e, edit <commit> = use commit, but stop for amending
# s, squash <commit> = use commit, but meld into previous commit
# f, fixup [-C | -c] <commit> = like "squash" but keep only the previous
#                    commit's log message, unless -C is used, in which case
#                    keep only this commit's message; -c is same as -C but
#                    opens the editor

# [1]

The --autosquash option is crucial here as it automatically arranges fix-up commits immediately after the commit they are intended to fix, streamlining the rebase process.²

However, it's important to note that these operations rewrite your repository's history, which can pose challenges in collaborative environments. Therefore, exercise caution when force-pushing your new commits to the remote repository, using --force-with-lease:

git push --force-with-lease

If the Git at origin successfully completes the exchange, you're all set. Otherwise, you can run git fetch origin to update your local repository, make any necessary adjustments, and attempt the force-push again.

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