How to use Git Bisect to Find the Commit that introduced a Bug

Made this meme myself, characters are from Spongebob

What is Git Bisect ?

Git Bisect — use binary search to find the commit that introduced a bug.

Git Bisect is a handy tool used for debugging, it uses Binary Search to go through the previous commits to find out the exact commit that introduced the regression/bug in the code.

How to use Git Bisect

TL;DR version:

Start - Git Bisect you’ll need to provide it with a last known Good commit and a Bad commit. ( you can ignore the SHA for bad commit if the current HEAD is corrupted )

$ git bisect start
$ git bisect bad 
$ git bisect good <sha_of_goodcommit>

Git Bisect has now begun and you’ll see a message that tells you roughly how many steps are remaining until you find the bad commit.

Bisecting: X revisions left to test after this (roughly Y steps)

Repeat - On every step you’ll need to tell git whether the bug still persists.
If Bug still persists, $ git bisect bad
If Code works fine, $ git bisect good
When all the steps are done, git will show the message with the SHA of the first bad commit

<sha_of_badcommit> is the first bad commit

Done - After you found the commit that introduced the issue/bug you can reset the git bisect using

$ git bisect reset

Longer Version:

Let’s understand how Git Bisect works visually,

Consider that there were 5 commits on the branch and one of the commit (say Commit 4 ) has introduced a breaking change in the code, when we start Git Bisect our initial state would be like this

git bisect figure — 1

Then, git automatically moves to a commit between our good and bad versions using binary search, we will then have to check whether this new state still has buggy code or not
If still buggy - $ git bisect bad
If it’s not - $ git bisect good
We repeat this process until we find the first commit that introduced the breaking change

git bisect figure — 2

Now that we’ve finally found the breaking commit, we can revert our code back to it’s original latest commit by

$ git bisect reset

which brings it back to Initial state ( as represented in git bisect figure — 2 )

If you have test script which can tell if the current source code is either good or bad then you can use $ git bisect run <script_file> command.
Learn more about Git Bisect and its arguments from here.