Things used to work, now they don’t anymore. Somebody pushed a rogue commit, and you want to find the culprit. The solution to this problem is easier than you’d imagine.

To put it simply, git-bisect performs a binary search across a good commit and a bad commit. At each step it tries to reduce the number of revisions that are potentially bad by half.

To use git bisect:

$ git bisect start
$ git bisect bad //this means the current revision
$ git bisect good <good_hash>

After these steps, test your changes. Depending on the tests, mark the current revision as good/bad using:

git bisect good|bad

Eventually there will be no more commits to evaluate, and you’ll find out the dodgy commit. Huzzah!

Here’s a drawing that you can use as a cheat sheet. :)