Git Notes

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Git Bisect - Find Bugs with Binary Search

Learn how to use git bisect to efficiently find the commit that introduced a bug using binary search. Master automated bisect with scripts and practical debugging workflows.

Git bisect uses binary search to efficiently find the commit that introduced a bug. Instead of checking every commit linearly, bisect cuts the search space in half with each step, finding the problematic commit in logarithmic time.

How Bisect Works

Example

100 commits to search: Linear: check 100 commits (worst case) Bisect: check ~7 commits (log2(100) ≈ 7) Search visualization: [good]────────────────────────────────────────────[bad] ↓ check middle [good]──────────────────[?]──────────────────────[bad] ↓ mark as good [good]──────────────────[bad] ↓ check middle [good]────[?]───────────[bad] ↓ mark as bad [good]────[bad] ↓ check middle [good][?][bad] ↓ FOUND IT!

Basic Bisect Workflow

Step 1: Start Bisect

bash example

git bisect start

Step 2: Mark the Bad Commit (Has the Bug)

bash example

# Current commit has the bug
git bisect bad

# Or specify a specific commit
git bisect bad HEAD
git bisect bad abc1234

Step 3: Mark a Good Commit (No Bug)

bash example

# Specify a commit where the bug did NOT exist
git bisect good v1.0
# or
git bisect good a1b2c3d

Output

Bisecting: 25 revisions left to test after this (roughly 5 steps)
[f4e3d2c1b0a9] Add user authentication module

Step 4: Test Each Commit

Git checks out a commit in the middle. Test it and mark accordingly:

bash example

# If the bug IS present at this commit
git bisect bad

# If the bug is NOT present at this commit
git bisect good

Output

Bisecting: 12 revisions left to test after this (roughly 4 steps)
[e3d2c1b0a9f8] Refactor database queries

Step 5: Repeat Until Found

Continue marking commits as good or bad until Git identifies the first bad commit:

Output

e3d2c1b0a9f8e7d6c5b4a3 is the first bad commit
commit e3d2c1b0a9f8e7d6c5b4a3
Author: John Doe <john@example.com>
Date:   Mon Jan 15 14:30:00 2024

    Refactor database queries

 src/db/queries.js | 45 ++++++++++++++++++++++++---------------------
 1 file changed, 24 insertions(+), 21 deletions(-)

Step 6: End Bisect

bash example

git bisect reset

Output

Previous HEAD position was e3d2c1b... Refactor database queries
Switched to branch 'main'

Complete Example

bash example

# Bug: Login page crashes on submit (works in v2.0, broken now)
git bisect start
git bisect bad HEAD          # Current state has the bug
git bisect good v2.0         # v2.0 was working fine

Output

Bisecting: 50 revisions left to test after this (roughly 6 steps)
[abc1234] Update form validation

bash example

# Test the application at this commit
npm test  # or manually test

# Tests pass - this commit is fine
git bisect good

Output

Bisecting: 25 revisions left to test after this (roughly 5 steps)
[def5678] Add new login dependencies

bash example

# Test again - this one fails!
npm test
git bisect bad

Output

Bisecting: 12 revisions left to test after this (roughly 4 steps)

bash example

# Continue until found...
git bisect good  # (after testing)
git bisect bad   # (after testing)
git bisect good  # (after testing)

Output

def5678abc is the first bad commit
commit def5678abc
Author: Jane Smith <jane@example.com>
Date:   Wed Jan 17 09:15:00 2024

    Add new login dependencies

 package.json | 3 +++
 src/login.js | 8 ++++++++
 2 files changed, 11 insertions(+)

bash example

# Done! Reset to go back to where you were
git bisect reset

Automated Bisect with Scripts

Instead of manually testing each commit, provide a script:

bash example

git bisect start HEAD v2.0
git bisect run npm test

Output

running 'npm test'
...
Tests passed
Bisecting: 25 revisions left...
running 'npm test'
...
Tests FAILED
...
abc1234 is the first bad commit
bisect run success

Custom Test Script

bash example

# Create a test script
cat > /tmp/test-bisect.sh << 'EOF'
#!/bin/bash
# Return 0 (good) if test passes, non-zero (bad) if it fails
make build && ./run-specific-test
EOF
chmod +x /tmp/test-bisect.sh

# Run automated bisect
git bisect start HEAD v1.0
git bisect run /tmp/test-bisect.sh

Exit Codes for bisect run

Exit Code	Meaning
0	Good commit (no bug)
1-124, 126-127	Bad commit (has bug)
125	Skip (cannot test this commit)
128+	Abort bisect

Handling Untestable Commits

bash example

# If a commit cannot be compiled or tested
git bisect skip

Bisect with Specific Terms

Use custom terms instead of good/bad:

bash example

# Find when a performance regression was introduced
git bisect start --term-old=fast --term-new=slow
git bisect slow HEAD
git bisect fast v1.0

# Mark commits
git bisect fast   # This commit is fast (no regression)
git bisect slow   # This commit is slow (has regression)

Bisect Log and Replay

View Bisect Progress

bash example

git bisect log

Output

git bisect start
# status: waiting for both good and bad commits
git bisect bad abc1234
# status: waiting for good commit
git bisect good def5678
# status: bisecting
git bisect good ghi9012
git bisect bad jkl3456

Save and Replay a Bisect Session

bash example

# Save the log
git bisect log > bisect-session.log

# Replay later
git bisect replay bisect-session.log

Practical Tips

Bisect with Merge Commits

bash example

# Skip merge commits that might not build
git bisect start
git bisect bad HEAD
git bisect good v1.0
# If a merge commit doesn't build:
git bisect skip

Combine with git log

bash example

# After finding the bad commit, investigate
git show <bad-commit>
git log --oneline <good-commit>..<bad-commit>

Interview Questions

What is git bisect and how does it work?

Git bisect uses binary search to find the commit that introduced a bug. You mark a known good and bad commit, and Git repeatedly checks out the middle point for testing, halving the search space each time until the exact commit is found.

How does automated bisect work?

Use git bisect run <script>. The script must return exit code 0 for good commits and non-zero for bad commits. Git automatically marks each tested commit based on the script result.

What exit code 125 means in bisect run?

Exit code 125 tells Git to skip the current commit (it cannot be tested, perhaps because it does not compile). Git will try an adjacent commit instead.

How many steps does bisect take to find a bug in 1000 commits?

Approximately log2(1000) ≈ 10 steps. Each step halves the remaining commits, making bisect extremely efficient for large histories.

How do you go back to normal after bisecting?

Run git bisect reset. This returns HEAD to where it was before bisect started and cleans up the bisect state.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Git Bisect - Find Bugs with Binary Search.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Git & GitHub topic.

Search Terms

git-github, git & github, git, github, advanced, bisect, git bisect - find bugs with binary search

Related Git & GitHub Topics

Continue learning this concept

Advanced GitGit Cherry-Pick - Apply Specific Commits to Any BranchLearn how to use git cherry-pick to apply individual commits from one branch to another. Master cherry-pick options, conflict resolution, and best practices.Advanced GitGit Hooks - Automate Workflows with Git EventsLearn to use Git hooks for automating code quality checks, commit message validation, pre-push testing, and deployment triggers. Covers client-side and server-side hooks.Advanced GitGit Internals - Understanding How Git Works Under the HoodDeep dive into Git internals including objects (blobs, trees, commits), the SHA-1 hash system, packfiles, refs, and how Git stores data as a content-addressable filesystem.Advanced GitGit Stash - Temporarily Save Uncommitted ChangesMaster git stash to save work-in-progress changes without committing. Learn stash operations, named stashes, partial stashing, and workflow patterns.Advanced GitGit Submodules - Manage External Dependencies as Git ReposLearn how to use Git submodules to include external repositories within your project. Cover adding, updating, cloning with submodules, and alternatives.Advanced GitGit Tag - Mark Important Points in HistoryLearn how to create, list, and manage Git tags for releases and milestones. Understand lightweight vs annotated tags, semantic versioning, and tag workflows.

PreviousSSH Keys - Secure Git Authentication Without Passwords NextGit Cherry-Pick - Apply Specific Commits to Any Branch

Git Notes

Git Bisect - Find Bugs with Binary Search

Learn how to use git bisect to efficiently find the commit that introduced a bug using binary search. Master automated bisect with scripts and practical debugging workflows.

How Bisect Works

Example

Basic Bisect Workflow

Step 1: Start Bisect

bash example

git bisect start

Step 2: Mark the Bad Commit (Has the Bug)

bash example

# Current commit has the bug
git bisect bad

# Or specify a specific commit
git bisect bad HEAD
git bisect bad abc1234

Step 3: Mark a Good Commit (No Bug)

bash example

# Specify a commit where the bug did NOT exist
git bisect good v1.0
# or
git bisect good a1b2c3d

Output

Bisecting: 25 revisions left to test after this (roughly 5 steps)
[f4e3d2c1b0a9] Add user authentication module

Step 4: Test Each Commit

Git checks out a commit in the middle. Test it and mark accordingly:

bash example

# If the bug IS present at this commit
git bisect bad

# If the bug is NOT present at this commit
git bisect good

Output

Bisecting: 12 revisions left to test after this (roughly 4 steps)
[e3d2c1b0a9f8] Refactor database queries

Step 5: Repeat Until Found

Continue marking commits as good or bad until Git identifies the first bad commit:

Output

e3d2c1b0a9f8e7d6c5b4a3 is the first bad commit
commit e3d2c1b0a9f8e7d6c5b4a3
Author: John Doe <john@example.com>
Date:   Mon Jan 15 14:30:00 2024

    Refactor database queries

 src/db/queries.js | 45 ++++++++++++++++++++++++---------------------
 1 file changed, 24 insertions(+), 21 deletions(-)

Step 6: End Bisect

bash example

git bisect reset

Output

Previous HEAD position was e3d2c1b... Refactor database queries
Switched to branch 'main'

Complete Example

bash example

# Bug: Login page crashes on submit (works in v2.0, broken now)
git bisect start
git bisect bad HEAD          # Current state has the bug
git bisect good v2.0         # v2.0 was working fine

Output

Bisecting: 50 revisions left to test after this (roughly 6 steps)
[abc1234] Update form validation

bash example

# Test the application at this commit
npm test  # or manually test

# Tests pass - this commit is fine
git bisect good

Output

Bisecting: 25 revisions left to test after this (roughly 5 steps)
[def5678] Add new login dependencies

bash example

# Test again - this one fails!
npm test
git bisect bad

Output

Bisecting: 12 revisions left to test after this (roughly 4 steps)

bash example

# Continue until found...
git bisect good  # (after testing)
git bisect bad   # (after testing)
git bisect good  # (after testing)

Output

def5678abc is the first bad commit
commit def5678abc
Author: Jane Smith <jane@example.com>
Date:   Wed Jan 17 09:15:00 2024

    Add new login dependencies

 package.json | 3 +++
 src/login.js | 8 ++++++++
 2 files changed, 11 insertions(+)

bash example

# Done! Reset to go back to where you were
git bisect reset

Automated Bisect with Scripts

Instead of manually testing each commit, provide a script:

bash example

git bisect start HEAD v2.0
git bisect run npm test

Output

running 'npm test'
...
Tests passed
Bisecting: 25 revisions left...
running 'npm test'
...
Tests FAILED
...
abc1234 is the first bad commit
bisect run success

Custom Test Script

bash example

# Create a test script
cat > /tmp/test-bisect.sh << 'EOF'
#!/bin/bash
# Return 0 (good) if test passes, non-zero (bad) if it fails
make build && ./run-specific-test
EOF
chmod +x /tmp/test-bisect.sh

# Run automated bisect
git bisect start HEAD v1.0
git bisect run /tmp/test-bisect.sh

Exit Codes for bisect run

Exit Code	Meaning
0	Good commit (no bug)
1-124, 126-127	Bad commit (has bug)
125	Skip (cannot test this commit)
128+	Abort bisect

Handling Untestable Commits

bash example

# If a commit cannot be compiled or tested
git bisect skip

Bisect with Specific Terms

Use custom terms instead of good/bad:

bash example

# Find when a performance regression was introduced
git bisect start --term-old=fast --term-new=slow
git bisect slow HEAD
git bisect fast v1.0

# Mark commits
git bisect fast   # This commit is fast (no regression)
git bisect slow   # This commit is slow (has regression)

Bisect Log and Replay

View Bisect Progress

bash example

git bisect log

Output

git bisect start
# status: waiting for both good and bad commits
git bisect bad abc1234
# status: waiting for good commit
git bisect good def5678
# status: bisecting
git bisect good ghi9012
git bisect bad jkl3456

Save and Replay a Bisect Session

bash example

# Save the log
git bisect log > bisect-session.log

# Replay later
git bisect replay bisect-session.log

Practical Tips

Bisect with Merge Commits

bash example

# Skip merge commits that might not build
git bisect start
git bisect bad HEAD
git bisect good v1.0
# If a merge commit doesn't build:
git bisect skip

Combine with git log

bash example

# After finding the bad commit, investigate
git show <bad-commit>
git log --oneline <good-commit>..<bad-commit>

Interview Questions

What is git bisect and how does it work?

How does automated bisect work?

Use git bisect run <script>. The script must return exit code 0 for good commits and non-zero for bad commits. Git automatically marks each tested commit based on the script result.

What exit code 125 means in bisect run?

Exit code 125 tells Git to skip the current commit (it cannot be tested, perhaps because it does not compile). Git will try an adjacent commit instead.

How many steps does bisect take to find a bug in 1000 commits?

Approximately log2(1000) ≈ 10 steps. Each step halves the remaining commits, making bisect extremely efficient for large histories.

How do you go back to normal after bisecting?

Run git bisect reset. This returns HEAD to where it was before bisect started and cleans up the bisect state.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Git Bisect - Find Bugs with Binary Search.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Git & GitHub topic.

Search Terms

git-github, git & github, git, github, advanced, bisect, git bisect - find bugs with binary search

Related Git & GitHub Topics

Continue learning this concept

PreviousSSH Keys - Secure Git Authentication Without Passwords NextGit Cherry-Pick - Apply Specific Commits to Any Branch