[Practical Git] Diagnose which commit broke something with git bisect

Sometimes you find a bug in your project that has been around for a while without being noticed; it can be hard to track down where that bug was introduced and why just by searching through logs and diffs. Git has a slick tool called git bisect that can be used to find out which commit introduced problem in our code - it creates a binary search where the programmer can mark each search commit as good or bad; by the end of the bisect, Git shows you exactly which commit introduced the issue. In this lesson, we walk through an example of using git bisect from start to finish.

To get started:

git bisect start

First, we need to find out which commit cause the problem, now assume it is the lastest commit, so we need to mark it as 'bad':

git bisect bad

Then , we need to find out which commit works fine, then mark it as good:

git log --oneline // to pick one which you are sure code works fine

# Then get the commit id

git bisect good <commit_id>

Then git will show you the commits which stay between the bad and good commits and will automatically checkout the next commit. Then you can do the testing and find out whether this commit is bad or not, if it is bad then mark as 'bad' or the other way around.

Finnally git bisect will find out the bad commit which introduce the bugs, but we are still in the bisecting state, to back to normal, we need to run:

git bisect reset