Install it first:

gem install jekyll-git-hash

With Jekyll 2, simply add the gem to your _config.yml gems list:

gems: ['jekyll-git-hash', ... your other plugins]

Or for previous versions, create a plugin file within your Jekyll project's _plugins directory:

# _plugins/jekyll-git-hash.rb
require 'jekyll-git-hash'

Highly recommend to use Bundler. If you're using it, add this line to your Gemfile:

gem "jekyll-git-hash"

The plugin is compatible with Jekyll 3.9.3 and Jekyll 4.3.2.

Use {{ site.data['hash']}} inside your liquid template.

When the site is being generated by Jekyll, the plugin retrieves Git hash of the source code and exposes it as a item in site.data. This feature is very helpful when you want your static resources (CSS, JS, etc.) be reloaded by end users every time you deploy a new version of the site.

For example, in your default.html:

<link rel="stylesheet" href="/css/layout.css?{{ site.data['hash'] }}"/>

The URL will be generated with a suffix at the end. This suffix doesn't change the URL (layout.css will still be accesible by the browser), but it makes the URL unique for the browser when you deploy a new Git revision. All browsers will reload this CSS.