Hunting buggy commits with Git Bisect

Managing and debugging big projects with contributions from multiple people is hard. There might be parts of code that you haven’t explored or worked on. But still when the time comes to debug a high priority issue, your only focus is to track the bug and fix it whether you are familiar with the code or not.

In these scenarios when you are clueless about why things are breaking, one good solution is to track down the commit that introduced the bug and to get help from the person who submitted that commit.

Git Bisect

Git bisect is feature of git that helps you track buggy commits from a known working commit, and the latest broken commit. It is a binary search algorithm that you are following when you use this tool. So pinpointing the commit is faster than just checking out each commit one by one.

Procedure

You start by changing git to bisect mode:

git bisect start

After getting into bisect mode, you assign a good commit and bad commit. You can also specify tags/versions instead of commits.

git bisect bad              # sets current version as bad
git bisect good v0.10.2     # sets v0.10.2 as last known good version

After setting the good and bad versions, running git bisect executes a checkout to the commit in the middle of these two commits.

After testing the current state, you should assign whether this commit is good or bad.

git bisect bad     # sets current version as bad
git bisect good    # sets current version as good

Then run git bisect again to check out to the commit in between the two known commits.

This way, you pinpoint the exact commit that introduced the bug way faster than checking out random commits to test. After finding out the commit, you need to exit bisect mode.

git bisect reset

Git bisect is an effective and useful feature that everyone should know about so. There are several other features that git provides and I am still exploring them one by one when I have the necessity.