External Dependencies in Homebrew External Commands

The Homebrew package manager uses external commands to extend its functionality. These are either shell scripts or ruby scripts that can be added on top of the existing brew infrastructure via the brew tap command.

When creating the license external command, I needed to add an external ruby dependency of the octokit gem to facilitate attempting to fetch a formula’s licensing information from the Github API. It was obvious that I needed to add

    require 'octokit'

to my brew-license.rb script, but it wasn’t obvious how I could trigger that gem being installed on a user’s machine if it wasn’t already present.

The solution that I settled on was to create a Gemfile where I defined my dependencies:

source 'https://rubygems.org'
gem 'octokit' 

Then, at the beginning of my brew-license.rb script, which is what is executed when a user types brew license in their shell, I needed my ruby script to invoke bundler if the octokit gem wasn’t installed locally on the user’s machine. This can be accomplished with the following:

    REPO_ROOT = Pathname.new "#{File.dirname(__FILE__)}/.."
    VENDOR_RUBY = "#{REPO_ROOT}/vendor/ruby".freeze
    BUNDLER_SETUP = Pathname.new "#{VENDOR_RUBY}/bundler/setup.rb"
    unless BUNDLER_SETUP.exist?
	    Homebrew.install_gem_setup_path! "bundler"
      REPO_ROOT.cd do
        safe_system "bundle", "install", "--standalone", "--path", "vendor/ruby"
    require "rbconfig"
    ENV["GEM_HOME"] = ENV["GEM_PATH"] = "#{VENDOR_RUBY}/#{RUBY_ENGINE}/#{RbConfig::CONFIG["ruby_version"]}"
    require_relative BUNDLER_SETUP

Now, the first time that a user executes brew license, this bit of code will ensure that the octokit gem is installed locally.

You can see this in action by installing and using brew license, and if you’re interested in adding licensing information for your favorite Homebrew formulae, please do feel free to submit issues or PRs!