Git: Modifying Previous Commits with Fixup

Scenario: Amending a Commit in the Middle of Your History

If you've already made several commits and realize you need to modify a file in an earlier commit, you can use git commit --fixup combined with an interactive rebase.

Example History:

abc1234 feat: add configuration system
def5678 feat: add monitoring setup ← you want to modify this one
ghi9012 chore: update documentation
jkl3456 feat: add new feature (HEAD)

Steps:

1. Make your changes to the file:

   Edit the file you want to modify:

   nano your-file.txt

2. Stage the changes:

   git add your-file.txt

3. Create a fixup commit:

   git commit --fixup <commit-hash>

   Example:

   git commit --fixup def5678

   This creates a new commit with the message fixup! feat: add monitoring setup.

4. View your current history:

   git log --oneline

   You'll see something like:

   mno7890 fixup! feat: add monitoring setup
   jkl3456 feat: add new feature
   ghi9012 chore: update documentation
   def5678 feat: add monitoring setup
   abc1234 feat: add configuration system

5. Perform an interactive rebase with autosquash:

   git rebase -i --autosquash <parent-commit>

   The <parent-commit> is the commit before the one you want to modify:

   git rebase -i --autosquash abc1234

   Alternative: Use HEAD~4 to rebase the last 4 commits:

   git rebase -i --autosquash HEAD~4

6. Git will automatically arrange the commits:

   The editor will open showing:

   pick def5678 feat: add monitoring setup
   fixup mno7890 fixup! feat: add monitoring setup
   pick ghi9012 chore: update documentation
   pick jkl3456 feat: add new feature

   Notice that:

   • The fixup! commit is placed right after the original commit
   • It's already set to fixup (not pick)

7. Save and close the editor:

   Git will automatically merge the fixup commit into the original commit.

8. Verify the result:

   git log --oneline

   You should now see a clean history without the fixup commit:

   pqr4567 feat: add new feature
   stu8901 chore: update documentation
   vwx2345 feat: add monitoring setup ← your changes are here
   abc1234 feat: add configuration system

9. Push the updated branch (if already pushed):

   Since you've rewritten history, you need to force push:

   git push --force-with-lease origin <branch-name>

---

Important Notes:

• Parent Commit: When using git rebase -i, always specify the commit before the one you want to modify
• Commit Hashes Change: After rebasing, all affected commits will have new hashes
• Force Push Warning: Only use --force-with-lease on branches where you're the sole contributor, or coordinate with your team first
• Autosquash: The --autosquash flag automatically arranges fixup commits correctly

Tips:

Enable autosquash by default:

git config --global rebase.autosquash true

Then you can simply run:

git rebase -i abc1234

Multiple fixups for the same commit:

You can create multiple fixup commits for the same original commit. They will all be squashed together during rebase.

Fixup vs. Squash:

• --fixup: Merges changes without prompting for a new commit message
• --squash: Merges changes but allows you to edit the commit message

Example with squash:

git commit --squash def5678