Find the Git commit that introduced a bug

Git's bisect command is a powerful, yet underutilized tool that can help you find the origin of a bug in your codebase. It uses a binary search algorithm to find the commit that introduced the bug, and you can mark commits as "good" or "bad" manually or automatically to help narrow down the search.

Manually find the commit with a bug

In order to start the process, you need to use git bisect start. You need to first mark a commit as "good", indicating it is known to be bug-free, and another commit as "bad", indicating it has the bug. You can do so using git bisect good <commit> and git bisect bad <commit>.

Having performed these steps, the binary search algorithm will stop at each commit and you'll have to use git bisect (good | bad) to mark each commit as "good" or "bad" depending on whether it has the bug or not.

After running through the commits, the algorithm will print the commit that introduced the bug. Finally, you can reset to the original branch using git bisect reset. You can optionally specify a commit to reset to.

# Syntax:
#  git bisect start
#  git bisect good <commit>
#  git bisect bad <commit>
#  git bisect (good | bad)
#  git bisect reset [<commit>]

git bisect start

git bisect good 3050fc0de
git bisect bad c191f90c7

git bisect good  # Current commit is good
git bisect bad   # Current commit is buggy

# ... some time later the bad commit will be printed
git bisect reset # Goes to the original branch

Automatically find the commit with a bug

If all of that seems like too much work, thankfully you can automate the process using git bisect run. This command will run a given script on each subsequent commit to find which commit introduced the bug.

Apart from that the startup of the process is the same, you need to use git bisect start and mark a commit as "good" and another as "bad". After that, you can use git bisect run <command> to run the given command on each subsequent commit and wait for the bad commit to be printed.

# Syntax:
#  git bisect start
#  git bisect good <commit>
#  git bisect bad <commit>
#  git bisect run <command>
#  git bisect reset [<commit>]

git bisect start

git bisect good 3050fc0de
git bisect bad c191f90c7

git bisect run npm test # Run `npm test` for each commit

# ... some time later the bad commit will be printed
git bisect reset # Goes to the original branch