A beginner's guide to GIT BISECT - The process of elimination

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The slowest, most tedious way of finding a bad git commit is something we've all done before. You checkout some old commit, make sure the broken code isn't there, then checkout a slightly newer commit, check again, and repeat over and over until you find the flawed commit.

Using git bisect is a much better way. It's like a little wizard that walks you through recent commits, asks you if they are good or bad, and narrows down the broken commit. In this blog post, I encourage you to create a fresh git repository and walk through each step. Hopefully, you'll gain an intrinsic understanding of git bisect by the end of the exercise.

Setting up your playground

1. Create an experimental git repository.

   Copy and paste the following lines:

   mkdir git_bisect_tests
   cd git_bisect_tests
   git init

2. Create a history of commits to play with.

   Copy and paste the following lines. This will create a commit history for a file called ‘test.txt’ that contains the opening lyrics to the childrens song “Row Row Row Your Boat”.

   echo row > test.txt
   git add -A && git commit -m "Adding first row"
   echo row >> test.txt
   git add -A && git commit -m "Adding second row"
   echo row >> test.txt
   git add -A && git commit -m "Adding third row"
   echo your >> test.txt
   git add -A && git commit -m "Adding the word 'your'"
   echo boat >> test.txt
   git add -A && git commit -m "Adding the word 'boat'"
   echo gently >> test.txt
   git add -A && git commit -m "Adding the word 'gently'"
   sed -i -e 's/boat/car/g' test.txt 
   git add -A && git commit -m "Changing the word 'boat' to 'car'"
   echo down >> test.txt
   git add -A && git commit -m "Adding the word 'down'"
   echo the >> test.txt
   git add -A && git commit -m "Adding the word 'the'"
   echo stream >> test.txt
   git add -A && git commit -m "Adding the word 'stream'"

   Output:
   

3. Find the bug in ‘test.txt’ file.

   Look at the contents of test.txt. There is a glaring error: the word ‘car’ is where the word ‘boat’ should be. That happened during our commit history – at some point, the commit of the word ‘boat’ was overwritten by the commit for the word ‘car’.
   

The Experiment

1. Use git log to find a good commit, and a bad commit.

   First off, let’s try to find a commit that still had the word ‘boat’, and not the word ‘car’. Check out your git log:

   
   We know that the newest commit in the log is bad, so we will classify this as our bad commit. For the sake of simplicity, let’s say our good commit is the first commit we made for the word 'boat':
   

2. With a good commit, and a bad commit, we’re ready for the process of elimination – git bisect

   Git bisect is like a wizard: it guides you step by step through a process of elimination until you find the commit that broke your code. Once you type git bisect start, you have started the wizard and it won’t end until you type git bisect reset.

3. Start up the git bisect wizard

   

4. Let the git bisect wizard know of a good commit.

   In this case, it was the ‘Adding the word boat’ commit we picked above.

5. Let the git bisect wizard know of a bad commit.

   In this case, let’s use the very last commit in the repository, since we know that had the word ‘car’ in it.

6. Look at the contents of test.txt

   When we typed git bisect bad in this step, git bisect checked out an old commit for us – the commit halfway between the latest bad commit and the known good commit. This is how bisect works – it cuts the commit history down in halves until it finds the original bad commit.
   

7. The word 'car' is still there = BAD!

   This commit is still bad, so we tell bisect about how bad it is: git bisect bad
   

8. Look at the contents of test.txt again.

   Once again, git bisect checked out a new commit halfway between the latest bad commit and the known good commit.

9. The word 'boat' is there = GOOD!

   Now we see the word ‘boat’, so this is a good commit. Let’s let git bisect know how good it is: git bisect good

10. ALL DONE - The bad commit has been found!

    Plain as day, git reports back to you: “I found the first bad commit”

    You are now free to investigate the problem, notify the author, write patches, and take care of business.

11. To end your git bisect wizard, simply type git bisect reset

    

With git bisect, I've been able to narrow down broken code within a few seconds on repositories with a dozen developers adding several dozen commits per hour. I almost never use any other technique to find broken code.