Cheatsheet | How to use git bisect

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In a nutshell

git bisect start
git bisect bad
git bisect checkout last_known_good_commit
git bisect good
# keep running git bisect good|bad until you find the commit that introduced the bug
# alternatively, find the commit automatically
git bisect run <command-that-checks-if-commit-is-good-or-bad>

Intro

git-bisect is an amazing tool that can help you find a specific change or commit.

Whether you are trying to find a commit that indroduced a bug, a broken CSS style or an upgrade of a third-party library git-bisect is a powerful tool to assist you in finding it.

The power of git-bisect comes from a binary search algorithm which can process massive ammounts of commits in very few iterations.

Practice

In this practice, you suspect that a bug was introduced when you upgraded React to version 16 and you want to find this commit.

Most likely you can just run git-log on the package.json and scan through commits, but in your scenario package.json has dozens of commits and it would take a long time to find the commit.

Also, you love to automate things and be efficient so why not use git-bisect?

Prerequisites

• bash
• git
• yarn
• grep

For practice purposes, let’s create a sample Git repo and Yarn package.

# all commands are executed in bash terminal
$ mkdir git-bisect-practice && cd git-bisect-practice
$ yarn init --yes
$ git init
$ echo "node_modules" >> .gitignore
$ git add .
$ git commit -am "Initial commit"

Now that you have our initial Repo let’s create a few dummy commits to simulate a repository under heavy development.

To create a dummy commit you will run yarn version in a loop, which creates a git commit if called inside a git repository.

$ N=10 # Number of dummy commits to create
$ for i in {1..$N}; do yarn version --minor; done

Now you will install React version 16.

$ yarn add react@^16
$ git commit -am "Install React v16"
$ git tag reactV16 # tag it for easy navigation later on

Simulate more heavy development by creating more dummy commits.

$ for i in {1..$N}; do yarn version --minor; done

Upgrade React to version 17.

$ yarn upgrade react@^17
$ git commit -am "Upgrade React v17"

Simulate more heavy development

$ for i in {1..$N}; do yarn version --minor; done

Now stop for a moment and look back.

You have a repository with at least a number of 3N commits. On N-commit, you installed React version 16. On 2N-commit you upgraded React to version 17. Now you find ourselves on 3N-commit and you realize that a bug was introduced when you upgraded to React version 17. How can you find that commit?

There are different ways of finding the commit which has React version upgrade but you are here to practice git-bisect , so dive in.

# Start bisecting
$ git bisect start
# Mark current commit as having a new change
$ git bisect new # alternatively - `git bisect bad`
# Checkout a known commit that has an old version of React v16
$ git checkout reactV16
# Mark that commit as old
$ git bisect old # alternatively - `git bisect good`

These are the required arrangements forgit-bisect to launch the search.

Now you have two ways to tell git-bisect to hunt down the commit you need.

The manual way. You mark a commit good or bad depending on which React version is installed. Every time you mark a commit, git-bisect checks out a new commit picked up using the binary-search algorithm.

First, you want to write a command that will detect if React version is 16 or 17. The utilitygrep is well suited for that purpose.

$ grep '"react":\s*"\^16' package.json

This command will determine if the current commit contains React version 16.

It would look like so:

# Step 1
$ grep '"react":\s*"\^16' package.json
# Step 2
# If React version 16 is installed, run
$ git bisect good # git-bisect checks out the next commit
# Otherwise, run
$ git bisect bad # git-bisect checks out the next commit
# Step 3
# Go to Step 1

But doing things manually could be time-consuming. You could speed up the search with the help of git-bisect run .

The fast way. You can call git bisect run , with the grepcommand from the previous step. That will trigger the manual work from the previous section for you.

# Automate search by using `bisect run`
$ git bisect run grep '"react":\s*"\^16' package.json

If everything worked well, you will see an output with the commit that Upgraded React to version 17.

Lastly, remember to reset the state of your git repository:

$ git bisect reset

And this is it. Congrats, hopefully, now you have a new tool under your belt, and it will help you hunt down bugs faster and easier.

Useful Links

https://git-scm.com/docs/git-bisect