Amending

Important

You see how Jujutsu takes git commit --amend and makes it universal.

Found a typo in your last commit?

A---B---C     <- main (HEAD)

Easy: git commit --amend. Voilà!

A---B---C'    <- main (HEAD)

This is an elegant abstraction. Commits are immutable, so amend actually creates a new commit with the same parent, then resets the branch:

A---B---C
     \
      C'      <- main (HEAD)

Git pulls a fast one, giving you the impression the last commit was editable. Under the hood, git commit --amend is equivalent to something like:

git reset --hard HEAD^
git cherry-pick -n ORIG_HEAD
git add .
git commit -C ORIG_HEAD

But you don’t need to know! This is the beauty of amend: it captures your intention and hides the mechanics. Neat!

Leaky abstraction

How do you amend the second-to-last commit? There’s no git commit --amend HEAD~1. You need a terrifying sequence of checkouts, cherry-picks, and branch resets. The abstraction tears, revealing the gears beneath.

What about amending Y here?

A---B---C---D---E---F
     \         /
      X---Y---Z

Oh beautiful abstraction, where art thou?

More consistent abstraction

When you amend, your intention is to edit a commit. The underlying metaphor is: The last commit is editable.

Can we make this universal? Imagine jj edit modifying any point in history.

• Want to amend C? jj edit C.
• Want to amend Y? jj edit Y.

We return to intuitive UX. jj edit captures the what and manages the internal how. Editing Y cascades changes to Z, E, and F automatically.

In practice

• Install Jujutsu.
• Run jj git init in your Git repo. It safely coexists with Git.
• Need to amend a commit deep in history?

jj edit <SHA1>
# Do your changes
git checkout <your-branch>

I’m lying slightly. You might hit conflicts, so keep reading. But I hope you just had your first a-ha moment.