Automatic git bisect

Posted by Tiago Scolari on Jul 20, 2013
In Development, Git

There are some situations that you found your build broken and need to know at what revision the “brokeness” was introduced.

git bisect is a great tool for that, basically you tell git at which revision the build was good (assuming HEAD is broken), and it does a binary search for the revision where the bug was introduced.

How it works:

git bisect start
git bisect good SOME_WORKING_PAST_REVISION

git will tell your how many revisions are between HEAD and WORKING_PAST_REVISION and how many steps it will take. Then it will walk you through each step (revisions), where you manually mark them good or bad.

If the revision still works, you mark it as good:

git bisect good

If the revision doesn’t work, you mark it as bad:

git bisect bad

When you reach the last step git will tell you at what revision the bug was introduced.

The good stuff

You can automate this process if you have test coverage on the bug. git bisect has a run argument that accepts an external program to automatically flag a revision as good or bad. This makes things very easy:

git bisect start
git bisect good WORKING_PAST_REVISION
git bisect run rspec spec/models/user_spec.rb
...
...
abcde1234def is the first bad commit
commit abc1234def5678
Author: John Snow <john@starks.com>
Date: Thu Dec 23 13:37:00 2010 +0100
    Some stuff

HUGOMORE42

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