The cool git: Git Bisect

TL;DR: Use binary search to find the commit that introduced a bug.

For humans: Take 2 commits and start removing halves until you find the error.

The command git bisect has been my ally in those times that the bug is a mistery. It has allowed me to go back in history and see the commit that broke something. Usually it's me being a detective for my own murder.

As simple as it sounds, it takes a "correct" commit and a broken one and with binary search, it allows you to find the offending one fast.

This is probably as far as I can go on explaning so let's see it with an example, using my favorite song.

This code will go to your home directory and create a folder with a git history for us to experiment. Read it first, and go ahead and run it.

cd 
mkdir wm-git-bisect
cd wm-git-bisect
git init
echo 'It might not be the right time' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 1'
echo 'I might not be the right one' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 2'
echo 'But there´s something about us I want to say' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 3'
echo 'Cause there´s something between us anyway' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 4'
echo 'I might not be the right one' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 5'
echo 'It might not be the right time' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 6'
echo 'But there´s something about us I´ve got to do' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 7'
echo 'Some kind of secret I will share with you' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 9'
echo 'I need you more than anythin´ in my life' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 10'
sed -i -e 's/something/nothing/g' wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'change something for nothing'
echo 'I want you more than anythin´ in my life' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 11'
echo 'I´ll miss you more than anyone in my life' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 12'
echo 'I love you more than anyone in my life' >> wm-git-bisect.txt
git add wm-git-bisect.txt && git commit -m 'Add line 13'

As you can see, after adding line 10, we change a word in line 4. This is where our code broke, but we continued working without noticing for a while, just like the real world.

And just like the real world, a user reported and now it's on us to fix it, let's do it:

git log

This will bring up our git history, where we'll find the offending commit, but this will be way harder in a real timeline e.g. if you squash your commits, your commit names are not standarized, or your history is longer.

Now, you know tht the latest commit ("Add line 13") is broken, this will be the bad commit. You need a commit that is considered good, so we'll take the original one where we added the line ("Add line 4"). This probably will take longer, skip back a lot and test.

Your commit IDs will be different, remember to use the output on your console.

commit 778d62a21ae08ff38db34bf9465db423f7e990fb (HEAD -> master)                                                                     
Add line 13    
commit c0ac1bdd88d6e89b2a07d5356dd50c3720fc9ace
Add line 12    
commit a2fa117307aacd11ddb502a1059cecdd3d8da78f
Add line 11    
commit e1249a7f0d86a8dca982289aa35e3892fe189ae2
change something for nothing  
commit c93b967c5d5e7a601e83c3adda2b3ab814ad10f1
Add line 10    
commit 219e531c50779854789dbae70a911dbfb2c652e8
Add line 9     
commit 2ab752fcb792a742ac0fda9ddb4d65a97045090d
Add line 7                             
...

We're in luck, git bisect is interactive, so let's let it guide us through this rabbit hole. Once you start you will be in bisect mode untill you type git bisect reset.

git bisect

This command has no output. Let's give it some input.

You can use short commit IDs but we already have the long ones.

git bisect bad 778d62a21ae08ff38db34bf9465db423f7e990fb
git bisect good 6e83eec542f915bdaeb9565fe5dbd1c6cfaec4d6

And it will output a useful number of revisions and a calculated distance. With longer histories you'll see the power of binary search.

Bisecting: 4 revisions left to test after this (roughly 2 steps)
[219e531c50779854789dbae70a911dbfb2c652e8] Add line 9

Now, the second part is the interesting one. Notice how a commit that we didn't input was printed? Well, now it is checked out and we can test it to know if it's a good or bad one.

cat wm-git-bisect.txt

We can see that this specific commit, even if incomplete, has the correct lyrics, so we'll tell git this one is good.

git bisect good

And now the output will be similar, also checking out another commit.

Bisecting: 2 revisions left to test after this (roughly 1 step)
[e1249a7f0d86a8dca982289aa35e3892fe189ae2] change something for nothing

We check again.

cat wm-git-bisect.txt

Aha! We have found the error, or bug in a real development environment.

git bisect bad

Output tells us that this is the last step.

Bisecting: 0 revisions left to test after this (roughly 0 steps)
[c93b967c5d5e7a601e83c3adda2b3ab814ad10f1] Add line 10

We check for the last time.

cat wm-git-bisect.txt

No error so...

git bisect good

And now git tells us the offending commit has been found, the commit author, date, commit message and lists all changed files.

e1249a7f0d86a8dca982289aa35e3892fe189ae2 is the first bad commit
commit e1249a7f0d86a8dca982289aa35e3892fe189ae2
Author: estebandalelr <estebandalelr@gmail.com>
Date:   Mon Mar 7 11:07:06 2022 -0600
 
    change something for nothing
 
:100644 100644 ebecac7931b48bb4aaa63e1e2a8624712d959eb2 ffc445f53b954c6ad8c05a8a860920df064388d1 M      wm-git-bisect.txt

So we close the case, it was me (as it's common), we can fix it commit and close the Jira ticket.

git bisect reset

Now we are in the commit we were in (usually HEAD) when we started, and know what broke our app.

Phew, that was a lot. Coding as a team is hard, but if you do it right it doesn't have to be crazy. The right tools help, and that's why we built Watermelon (opens in a new tab), a VSCode extension that gets the historical context of your code to make it easier to understand.

This blog post was originally released for Watermelon. Find it on our blog! (opens in a new tab)

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