git commit --amend

git commit --amend does not create a new commit. It replaces the last one. Precisely: it creates a new commit with updated content and moves the branch to it. The old commit remains in .git/objects/ as a dangling object.

What you can do

Fix the message

git commit --amend -m "new message"

# Without -m, the editor opens with the current message.

git commit --amend

Add a forgotten file

git add forgot.js

git commit --amend --no-edit

# --no-edit keeps the old message; the file is folded into the commit.

Fix the content

If you committed and immediately spotted a bug, fix the file, stage it, then --amend. You get a correct commit instead of a separate "fix previous commit" entry.

What physically happens

1. The current index is taken (including new edits, if any).
2. A new commit object is created:
   • tree: from the current index
   • parent: the same as the old commit's parent (not the old commit itself)
   • author/committer/message: from the old commit, plus your changes
3. The current branch moves to the new SHA.
4. The old commit becomes dangling.

The old commit is visible in reflog for about a month. If you made a mistake in the amend, you can recover it with git reflog.

When not to use it

If the commit was already pushed and someone else pulled it, --amend breaks history. You have one SHA locally; others have a different one. A regular push will fail:

! [rejected]        feature -> feature (non-fast-forward)

Options:

• Force-push with --force-with-lease. Safe only for a feature branch that nobody else is watching.
• Do not amend. Create a new commit instead. This is the right approach for main/master and any shared branch.

Rule: amend is allowed before the first push. After push, only on personal branches, only with force-with-lease.

Pitfalls

• Author date. --amend keeps the author date unchanged but updates the committer date. git log --pretty=fuller shows both.
• GPG signatures. If the commit was signed, the signature is dropped after --amend. Re-sign with git commit --amend -S.
• Ordering. git log sorts by committer date by default. After an amend the commit floats to the top, even if the author date is old.