git bisect. What you need to know and why you need to know it!

git bisect. What you need to know and why you need to know it!

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Matt Dean
·May 18, 2022·

2 min read

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Table of contents

  • The Intro
  • An overview of what git bisect does
  • How it works - the cheatsheet

The Intro

We had a dilemma today of an issue found in our code that needed to be fixed. After some time hunting I was able to find a distinct "this is good" reference in time and a distinct "this is not good" reference - but had no idea which commit between the two introduced the issue.

I was debating on going one commit at a time, until I learned about git bisect.

git bisect quickly allows you to only focus on verifying if the next checked out commit is good or bad - without worrying about which commit to test next

An overview of what git bisect does

git bisect accepts a "good" commit reference, and a "bad" commit reference and orchestrates a binary search within the limits of the two commits. This quickly allows you (the developer) to only focus on verifying if the next checked out commit is good or bad - without worrying about which commit to test next. At the end it spits out (assuming it works right and you have used it correctly): COMMIT_ID is the first bad commit

How it works - the cheatsheet

Initialize the search

git bisect start

Inform it of the commit instance that works

git bisect good COMMIT_ID

Inform it of the commit instance that does not work

git bisect bad COMMIT_ID

It will load up a new commit, test and check if it is a "good" or a "bad" commit, and let it know.

git bisect [good/bad]

Repeat until the following appears:

COMMIT_ID is the first bad commit (hang on to this commit id)

Close the bisect session

git bisect reset

Investigate the differences in the first bad commit

git diff COMMIT_ID~ COMMIT_ID

Voila! You should now be able to quickly find the instance of the issue!

 
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