git bisect is a powerful tool for quickly determining when a particular bug was introduced into your project. This sample project will show you the basics of using git bisect to identify what commit broke this toy project.
Clone this repository to your local machine like so:
git clone https://github.com/bradleyboy/bisectercise.git
cd bisectercise
Load the index.html file in a browser. Click + or -. Notice how the number always goes down. Oops!
Resist the urge to dig into the code and find the issue. The code in this example is admittedly trivial. Instead, let's practice using git bisect to find the offending commit.
Bisect requires a good and bad commit. We already know the bad commit – HEAD is broken. But when was the last time our project was working properly? In our case, we know the initial commit in the project was working. Give it a try:
git checkout 4d83cf
Now refresh index.html in your browser. The + and - buttons should now work properly. Great, we've found a good commit!
Now that we have a reference to both a good and bad commit, we can use git bisect to quickly find the bad commit. First, let's get back to the main branch:
git checkout main
Now start the bisect:
git bisect start HEAD 4d83cf
Notice we provide our known bad and good commits (in that order) as additional arguments after git bisect start.
Bisect now checks out a commit halfway between the good and bad commit. It will keep doing this until it finds the commit that first caused this bug. Each time, we need to test for the bug in the browser and tell bisect if this is a good or bad commit.
So now that we have started, load index.html in the browser again and see if the functionality is working as expected. If it is, tell bisect this is a good commit:
git bisect good
Otherwise, mark this commit as bad:
git bisect bad
Bisect will now check out another commit. Test again and mark the commit as good or bad using the commands we just used above. Keep repeating this process until bisect dumps out the offending commit, which will look something like this:
a1b2c3 is the first bad commit
Hooray! We found the bad commit.