Automatically Run Build Script When Switching Branchs Using Git Hooks

This was originally published over on frankcorso.dev.

I have recently been working on a project where we had multiple different JavaScript-heavy features being developed. So, there were many times where I switched branches to test out features.

But, I would constantly spend countless time figuring out why the feature wasn't working until I realized I had once again forgotten to build the assets.

So, I started looking into git hooks to see what my options were.

What are git hooks?

Within git, there are 17 events that fire upon different actions being taken. During each event, there is a "hook" that anyone can hook scripts into.

Some of these hooks include:

• post-checkout: Ran after checking out a branch
• post-merge: Ran after a git pull is run
• pre-commit: Ran prior to a commit being created
• post-commit: Ran after the commit is created

Each git repo has a .git/hooks directory that can contain scripts to be hooked into any of the hooks. You can drop a script that matches a hook's name into this folder.

Since this was a simple gulp command, I used a bash script. But, you could also use other languages, including Ruby or Python.

Hooking into post-checkout

In order to run code upon switching branches, we need the post-checkout hook. This hook fires after you checkout a branch or a file.

Inside the .git/hooks directory, I added a new file named post-checkout.

The post-checkout script gets called with three parameters:

1. The previous HEAD
2. The new HEAD
3. A flag for if it's a branch that is being checked out. 1 for branch and 0 for not.

Since this also gets called when you check out a file, such as rolling back a file, this can cause a cycle where you roll back a built file, and then it autogenerates the file again. To get around this, I check for the third parameter to make sure it's a '1'.

Inside my if, I then added our gulp task, which builds all of our assets, gulp prebuild.

#!/bin/sh
#
# Builds our assets upon checkout
#
# Args passed to this are:
# $1 - Previous HEAD
# $2 - New HEAD
# $3 - 1 if checking out a branch, 0 if checking out something else, such as a file (rollbacks)
#
if [ '1' == $3 ]
then
    echo 'Branch checkout detected. Building assets...'
    gulp prebuild
fi

Next Steps

Now that I have a rough idea of how git hooks work, I could see having scripts for validating commit messages, creating notifications, or automating other build tasks. I also came across articles on automating simple deployments using the post-receive hook, which I could see one used to build simple websites or even docker containers automatically.