Pinpoint Bugs with Git Bisect | Blog • Soorria Saruva

Recently at work, I encountered a couple issues where our frontend was failing to load in our development environment, but we had no clue about the cause, or which commit introduced the bug! I could have manually checked each commit until I found the offending one, but that would have been time-consuming and more importantly boring. Instead, I used git bisect and its subcommands to quickly find the causal commit, and diagnose the issue!

How to use it

Get started by running the following:
console
```
git bisect start
```
This will start the binary search of your git history,
Checkout a branch, or commit where you're 100% sure is buggy.
console
```
git checkout <branch or commit hash>
```
There's a decent chance it's the current branch you're on if the bug exists your latest code, and if so you can just skip this step.

Now, mark the branch as "bad":
console
```
git bisect bad
```
Find a commit you're sure is not buggy. A good way to do this is to find a time without the bug, and use git log to find a commit around that time. It's okay if the commit is too old - since git bisect does a binary search you won't have that many extra iterations! Mark this commit as "good":
console
```
git bisect good <commit hash>
```
Now, we're ready to binary search! After you run the command above, you should see something like the following in your console:
console
```
Bisecting: 311 revisions left to test after this (roughly 8 steps)
[d0f8d96ef806c440d4d2bce0bb56244540fd292f] Do a thing with code :)
```

Check if the bug exists in the current commit - run unit / integration / E2E tests, or just manually check if the bug exists. If the bug exists, run

console

git bisect bad

Otherwise, if the bug doesn't exist in the current commit, run

console

git bisect good

Repeat this step until you see a message like

console

14ff3ae7ab1403ec630e831d68cd8307e9ce982a is the first bad commit
commit 14ff3ae7ab1403ec630e831d68cd8307e9ce982a
Author: Soorria Saruva <soorria.ss@gmail.com>
Date:   Mon Aug 29 13:33:37 2022 +1000

    add bugs

path/to/real-file.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Fix the bug! Git checks out the commit for you

Automating the boring bits away

Step 4 above is the most time-consuming and boring part of this process. If you can check if the bug exists without manually checking functionality (e.g. running tests), git bisect can run the command for you and automatically mark commits as "good" and "bad". To do this, run

console

git bisect run <command that checks if bug exists>

Info

The command must return a non-zero exit code if the commit should be marked as "bad". If you're using a testing framework, it probably already does this.
Do not quote the whole command. For example, if you want to run npm run test, your git bisect command should be
console
```
git bisect run npm run test
```
and not
console
```
git bisect run "npm run test"
```

Found a mistake, or want to suggest an improvement? Source on GitHub here
and see edit history here