Day 4/30 - Git Bisect Explained: Find the Commit That Broke Your Code
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Ruqaiya Beguwala

Ruqaiya Beguwala @ruqaiya_beguwala

About: 👨‍💻 | Software Developer | Open-Source Enthusiast | JavaScript & NodeJS | Think and write about code every single minute

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Day 4/30 - Git Bisect Explained: Find the Commit That Broke Your Code

Publish Date: May 25
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Git bisect is a powerful tool that helps you find the exact commit where a bug was introduced in your codebase. It uses a binary search algorithm to quickly narrow down the problematic commit.

How Git Bisect Works

Imagine you have a project with 100 commits. You know that:

  • At commit #1, the feature worked fine.
  • At commit #100, the feature is broken.

Git bisect will:

  1. Check out the middle commit (#50).
  2. Ask you to test if the bug exists there.
  3. Based on your answer, it will then check either #25 or #75.
  4. Repeat this process until it finds the exact commit where the bug appeared.

Basic Usage Example

Let's walk through a practical example:

  • Start the bisect session: 
git bisect start
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  • Mark the current commit as bad (where the bug exists): 
git bisect bad
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  • Mark an older commit as good (where the bug didn't exist): 
git checkout <good-commit-hash>
git bisect good
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Or combine steps 2 and 3 in one command:

git bisect start <bad-commit> <good-commit>
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  • Git will now checkout a commit in the middle. Test your code to see if the bug exists:

If the bug exists:

git bisect bad
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If the bug doesn't exist:

git bisect good
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  • Repeat step 4 until git identifies the first bad commit.

  • When finished, reset your repository:

git bisect reset
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Behind the Scenes

Best Tips and Tricks

  • Automate testing: Instead of manually testing each commit, you can provide a script:

Create a test script:

#!/usr/bin/env python3
import sys
from calculator import calculate  # Our function to test

# Test case - should return 42
result = calculate(40, 2)
expected = 42

if result == expected:
    sys.exit(0)  # Success - mark as good
else:
    sys.exit(1)  # Failure - mark as bad
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Make it executable:

chmod +x test-script.py
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Run the script:

git bisect run ./test-script.py
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Note: The script should exit with 0 for good commits and non-zero for bad commits.

  • Skip commits: If a commit can't be tested (maybe it doesn't compile), skip it: 
git bisect skip
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  • Visualize progress: 
git bisect log
git bisect visualize
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  • Narrow down faster: Try to pick good and bad commits that are as far apart as possible initially.

  • Use tags: If you know approximate versions where it worked/didn't work:

git bisect start v2.0.0 v1.9.0
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  • Check the diff: Once you find the bad commit, examine what changed: 
git show <bad-commit>
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  • Document findings: After finding the problematic commit, consider adding a test case to catch similar issues in the future.

Key Takeaways:

✅ Fast & Efficient – Instead of checking every commit, git bisect smartly halves the search space each time.
✅ Flexible – Works with manual testing or automated scripts (git bisect run).
✅ Precise – Pinpoints the exact commit, making it easier to understand and fix the issue.

Up Next: Day 5: git worktree – Work on multiple branches simultaneously

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