Undoing a Commit That Is Already In

The commit is made. Possibly it is on GitHub. Possibly Theo already has it. And it was wrong.

Amending is off the table — that was the rule on the last page. What you want instead is git revert: a command that makes a new commit doing the exact opposite of the old one, leaving the history honest about what happened.

How revert works

git revert 7f4b2a9

Git works out what that commit did and creates a new commit doing the reverse. Lines it added are removed; lines it removed come back. Nothing is deleted from the history: the project gains a step rather than losing one.

Git opens an editor with a proposed message — usually Revert "Add the sightings list". Accept it or improve it; the way out of the editor is the same one from Chapter 2.

9a1c3e5 Revert "Redesign the home page"
7f4b2a9 Redesign the home page

Why adding beats removing

This is the part worth understanding, because it is the reason revert is the right tool for anything shared.

Suppose Theo already pulled the bad commit. If your fix were to remove it from the history, your copy of the project and his would now disagree about a commit he definitely has. Git has no good way to resolve that on its own, and the resulting mess is the subject of a whole section in Chapter 10.

A revert avoids all of it. Theo pulls once more, receives an ordinary new commit, and the undo applies to his copy exactly as it did to yours. Nothing disagrees with anything.

The mistake stays visible

Some people dislike this: the bad commit is still in the log, for anyone to see.

It is a feature. The history is a record of what happened, and what happened is that a change went in and was taken out again. Reading that later tells you something true and occasionally important — that this approach was tried, and that it did not work.

Reverting a revert

Perfectly ordinary, and it works exactly as you would expect. Revert the revert and the original change comes back, as another new commit. This is common when a change is pulled out to unblock something and put back a day later.

The other command, named honestly

You will see git reset --hard suggested for undoing commits. It is worth knowing what it is so that you can decline it.

git reset moves your branch to a different commit. With --hard it also overwrites your working directory to match, discarding uncommitted work in the process. Used deliberately on a local branch it is a real tool with real uses. Copied from a forum into a shared project, it is the origin of most "I lost a day of work" stories.

This book teaches revert for undoing commits and does not use reset anywhere. The full picture — what it does, when it is right — is one of the deep dive's chapters.