How to Find Bugs Using Git Bisect: Step-by-Step Guide

Use git bisect to find the commit that introduced a bug by marking a known good commit and a bad commit, then testing commits in between. Git will guide you through a binary search to isolate the exact commit causing the issue.

Syntax

The basic git bisect workflow involves starting the bisect session, marking commits as good or bad, and letting Git narrow down the culprit commit.

• git bisect start: Begin the bisect process.
• git bisect bad: Mark the current commit as bad (where the bug exists).
• git bisect good <commit>: Mark a known good commit (where the bug did not exist).
• git bisect reset: End the bisect session and return to the original branch.

bash

git bisect start
# Mark current commit as bad
git bisect bad
# Mark known good commit
git bisect good <commit>
# After testing each commit, mark it good or bad
# Repeat until Git finds the first bad commit
# End bisect session
git bisect reset

Example

This example shows how to find the commit that introduced a bug using git bisect. Assume the current commit is bad and abc1234 is a known good commit.

bash

git bisect start
# Mark current commit as bad
git bisect bad
# Mark known good commit
git bisect good abc1234

# Git checks out a commit in the middle
# Test the code manually or run tests
# If bug exists, mark bad
git bisect bad
# If bug does not exist, mark good
git bisect good

# Repeat until Git finds the first bad commit

# When done, reset to original branch
git bisect reset

Output

Bisecting: 3 revisions left to test after this (roughly 2 steps)
[def5678] Fix typo in README
Bisecting: 1 revision left to test after this (roughly 1 step)
[789abcd] Add new feature
abc1234 is the first bad commit

Common Pitfalls

• Not marking commits correctly: Always mark the current commit as good or bad after testing.
• Using wrong good commit: The good commit must be before the bug appeared.
• Not resetting bisect: Always run git bisect reset to return to your original branch.
• Automate tests: Manual testing slows down bisect; use scripts if possible.

bash

git bisect start
# Wrong: marking good commit as bad
git bisect bad
# Correct:
git bisect bad
git bisect good abc1234

Quick Reference

Command	Description
git bisect start	Start bisect session
git bisect bad	Mark current commit as bad
git bisect good	Mark known good commit
git bisect reset	End bisect and return to original branch

Key Takeaways

Start bisect with a known bad and good commit to isolate the bug.

Mark each tested commit as good or bad to guide Git's search.

Always reset bisect to return to your original branch.

Automate testing during bisect to speed up bug finding.

Choose the good commit carefully to avoid confusion.