How to use git bisect

Today I was briefly looking over my website and realized that some of my CSS transition animations weren’t working correctly. Instead of easing in and out of the animation, the transition was happening immediately. I wasn’t totally sure when this bug was introduced, so how do I figure out when it happened so I can fix it?

Table of contents

The Solution

This is a great use case for git bisect. In order to use git bisect, you must have a general idea of when the bug did not exist. In my case, I felt pretty confident that the bug did not exist a week ago, which was only about 20-30 commits, which isn’t bad at all to bisect through.

Once you know that, we can start running the commands. These are the three main commmands:

git bisect start         # start the bisect process
git bisect bad           # tell git that our current commit has a bug
git bisect good <commit> # tell git which commit we know did not have the bug

After we run the third command, git will start “bisecting” the commits between the bad commit and the good commit. This means that git bisect will checkout a commit between the bad commit and the good commit for us to test if the bug exists in that commit. If it does, then we tell git bisect that the bug exists with the git bisect bad command. If the bug does not exist in that commit, then we use the git bisect good command. Then we repeat the process until we find the exact commit in which the bug was introduced.

Example

We’ll run through my example. First, I start the process with the following commands:

git bisect start
git bisect bad

Now, I look through my commit history and make a guess on when I think the bug did not exist, and throw that commit in the git bisect good command.

git bisect good 2762d7c18981f0f79c1afd574960ed560072bcbe

git bisect will now checkout a commit between the good commit and the bad commit, and I get the following prompt in my terminal which tells me which commit I currently have checked out:

Bisecting: 13 revisions left to test after this (roughly 4 steps)
[2870f3b77b95ccbfbd86846ae48ecb1288aaf524] Add tests for home page

Now I can check my website at this commit and see if the bug still exists. In my case, the bug did not exist at this commit, so I can let git bisect know that the commit is good with the following command:

git bisect good

I once again am prompted with a similar message from last time, and I am checked out to the middle commit between the commit that was just checked out and the bad commit. I rinse and repeat the previous steps of checking to see if the bug exists, typing either git bisect good or git bisect bad in the terminal, until we identify the exact commit in which the bug was introduced.

Resetting

If you ever need to stop the git bisect, you can run the following command to reset back to your original HEAD position.

git reset