Git Bisect: Find the Commit That Introduced a Bug

What Does git bisect Do?

git bisect performs a binary search through your commit history to find the exact commit that introduced a bug. Instead of checking every commit one by one, bisect halves the search space with each step, making it incredibly efficient even across thousands of commits.

How It Works

1. You tell Git which commit is "bad" (has the bug) and which is "good" (does not).
2. Git checks out a commit halfway between the two.
3. You test that commit and mark it as "good" or "bad".
4. Git narrows the range and checks out another midpoint.
5. This repeats until the offending commit is found.

Step-by-Step Example

# Start bisecting
git bisect start

# Mark the current commit as bad
git bisect bad

# Mark a known good commit (e.g., from last week)
git bisect good v2.1.0

# Git checks out a midpoint. Test it, then:
git bisect good   # if the bug is NOT present
# or
git bisect bad    # if the bug IS present

# Repeat until Git identifies the culprit
# Git will print: "<hash> is the first bad commit"

# End the bisect session
git bisect reset

Automating Bisect

If you have a test script that exits with 0 for good and non-zero for bad:

git bisect start
git bisect bad HEAD
git bisect good v2.1.0
git bisect run ./test-script.sh

Git will automatically run the script at each midpoint and find the offending commit without manual intervention.

Tips

Bisect works best when commits are small and atomic. If a single commit introduces thousands of changes, the result is less actionable. Pair bisect with git blame and git log to understand the context once you find the culprit.