Finding Bad Commits Using 'git bisect'

Simply put, git bisect:

git-bisect - Use binary search to find the commit that introduced a bug

I'll go through an example to better demonstrate how it works.

Let's create an empty Git project, and add the following commits:

git commit --allow-empty -m 'good commit 1'
git commit --allow-empty -m 'good commit 2'
git commit --allow-empty -m 'good commit 3'
git commit --allow-empty -m 'good commit 4'
git commit --allow-empty -m 'good commit 5'
git commit --allow-empty -m 'good commit 6'
git commit --allow-empty -m 'BAD COMMIT'
git commit --allow-empty -m 'good commit 7'
git commit --allow-empty -m 'good commit 8'
git commit --allow-empty -m 'good commit 9'
git commit --allow-empty -m 'good commit 10'

We have just discovered a bug, but we're unsure which commit has introduced it. Git bisect for the rescue!

We first need to run git bisect start in order to start the wizard:

git bisect start

Now we'll need to tell Git what was the last commit known to us that introduced the bug. Assuming we've discovered the bug on the last commit:

git bisect bad HEAD

And of course, the oldest commit known to be good:

git bisect good HEAD~9
Bisecting: 4 revisions left to test after this (roughly 2 steps)
[375c587c8da1c90d07fd58a2402fd8bcf842ffd2] good commit 6

Now Git will perform the binary search on our history, and will checkout a commit that we'll need to decide whether it was good or bad.

We're now on "good commit 6", a commit before the bug was introduced

git bisect good
Bisecting: 2 revisions left to test after this (roughly 1 step)
[f14653c6730ef0537dbf8ec541997d7e9deb5e2a] good commit 7

This commit is not good, as the code already includes the "BAD COMMIT". So we need to tell Git that it's bad:

git bisect bad
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[421dab9fc7f885dfadb3c7a37d70fa7dfbb6f3d1] BAD COMMIT

And finally, the bad commit:

git bisect bad
421dab9fc7f885dfadb3c7a37d70fa7dfbb6f3d1 is the first bad commit

git log will now reveal that it's actually the "BAD COMMIT" that introduced the bug:

git log | grep "BAD COMMIT" -B 5
commit 421dab9fc7f885dfadb3c7a37d70fa7dfbb6f3d1
Author: MarounMaroun <maroun email>
Date:   Sun Jan 19 11:28:03 2020 +0200

    BAD COMMIT

That's all!