git bisect

Description:
Use binary search to find the commit that introduced a bug. Efficiently narrows down problematic commits in large histories.

Syntax:

bash

git bisect start
git bisect bad [commit]
git bisect good [commit]
git bisect reset
git bisect run <script>

Examples:

bash

# Start bisect session
git bisect start

# Mark current commit as bad
git bisect bad

# Mark last known good commit
git bisect good v1.0

# Git will checkout a middle commit
# Test it, then mark as good or bad
git bisect good
# or
git bisect bad

# After finding the culprit
git bisect reset

# Automated bisect with test script
git bisect start HEAD v1.0
git bisect run npm test

Detailed Explanation:

Binary search - Divides commit history in half each step (log n complexity)
Manual testing - You test each commit and mark as good/bad
Automated testing - Use bisect run with test script for automation
Finds first bad commit - Identifies exact commit that introduced the bug

Bisect Workflow:

bash

# 1. Start bisect
git bisect start

# 2. Mark current broken state
git bisect bad

# 3. Mark last known working commit
git bisect good v2.0.1

# Output: Bisecting: 15 revisions left to test after this

# 4. Test current checkout
# Run your app, run tests, check functionality

# 5. Mark result
git bisect good  # if it works
# or
git bisect bad   # if broken

# 6. Repeat until Git finds the culprit
# Output: abc123 is the first bad commit

# 7. End bisect session
git bisect reset

Automated Bisect Example:

Create test script test.sh:

bash

#!/bin/bash
npm test
exit $?

Run automated bisect:

bash

git bisect start HEAD v1.0.0
git bisect run ./test.sh

Advanced Usage:

bash

# Skip commits you can't test (e.g., won't compile)
git bisect skip

# Visualize bisect progress
git bisect visualize

# Terms other than good/bad
git bisect start --term-old=working --term-new=broken
git bisect working
git bisect broken

Pro Tips:

Use tags or commit hashes for good/bad boundaries
Automated bisect requires exit code 0 (good) or 1-127 except 125 (bad)
Exit code 125 means "skip this commit"
~10 tests needed for 1000 commits (2^10 = 1024)
Clean build between tests: git bisect run sh -c "make clean && make test"

Related Commands:

git log – View commit history
git blame – Find who changed each line
git reflog – View reference logs

git bisect ​

git bisect