Quick Git Bisect Example

Scenario:

You know a bug was introduced somewhere between two commits. One commit is working fine (good), and the other is failing (bad). You need to find the exact commit that introduced the bug.

Steps:

1. Start the bisect process:
   First, tell Git which commit is good and which one is bad.

   git bisect start
   git bisect bad HEAD         # Mark the current commit (HEAD) as bad
   git bisect good <good_commit_hash>  # Mark a known good commit

   For example:

   git bisect bad HEAD
   git bisect good 7907b1b    # The first commit is known to be good

2. Git will check out a commit halfway between the good and bad commits.
   Test the code to see if the bug still exists.

3. Mark the result:

   • If the bug exists, mark the commit as bad:

     git bisect bad

   • If the bug does not exist, mark the commit as good:

     git bisect good

4. Repeat the process:
   Git will continue narrowing down the range, checking out commits in the middle, until it finds the first bad commit.

5. Once found, Git will tell you the exact commit that introduced the bug:

   <commit_hash> is the first bad commit

6. Finish the bisect process:
   After finding the bad commit, run:

   git bisect reset

   This returns you to the branch you were on.

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Example in Action:

$ git bisect start
$ git bisect bad HEAD
$ git bisect good 7907b1b
# Git checks out a commit halfway between 7907b1b and HEAD
# Test the code, then mark the commit:
$ git bisect bad
# Repeat until Git identifies the bad commit
$ git bisect reset

Automated Testing (optional):

If you have an automated test to detect the bug, you can run git bisect with a test script:

git bisect run ./test-script.sh

This will automatically run your script and mark commits as good or bad based on the test result.

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For a more in-depth explanation and smarter approaches to bug localization with Git Bisect, check out my full article:
Debugging with Git Bisect: A Smarter Approach to Bug Localization

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License

MIT License
Copyright (c) 2024 Vitalii Semianchuk

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