How to Gem like Jem

2 minute read

I’ve been looking around for a super quick start guide to first gem using bundler step-by-step. I haven’t found one so I decided to document my own process. If you want the canonical and more detailed guide look here: RubyGems Guides: Make your own gem.

First things first if you don’t have bundler install it.

$ gem install bundler

Bundler is super awesome and will let us run a single command to generate the directory and all of the files we will need for creating our gem with the following command:

$ bundle gem [gemname]

I called my gem: Jem.


The next thing you want to do is peek into the generated [gemname].gemspec that was created when you generated your gem. You probably want to modify the description, summary, and homepage.

# -*- encoding: utf-8 -*-
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'jem/version' do |gem|          = "jem"
  gem.version       = Jem::VERSION
  gem.authors       = ["Lindsey Bieda"]         = ["[email protected]"]
  gem.description   = %q{TODO: Write a gem description}
  gem.summary       = %q{TODO: Write a gem summary}
  gem.homepage      = ""

  gem.files         = `git ls-files`.split($/)
  gem.executables   = gem.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
  gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
  gem.require_paths = ["lib"]


This file also notes what the dependencies for the gem is. RubyGems Guides: Declaring Dependencies will give you more detail into what these mean, but the gist is you want to specify runtime dependencies with add_dependency. These dependencies are anything that your code needs to run. Meanwhile, for gem development purposes you would specify those dependencies with add_development_dependency.

PRO-TIP: add_dependency is just an alias for add_runtime_dependency

spec.add_dependency "rails"
spec.add_development_dependency "pry"

Actual Gem Code

If your gem is super simple you can probably just dump all you code inside of the lib/[gemname].rb file. However, if you wanted multiple classes you would put those inside of the folder named for your gem inside the lib folder (e.g. lib/jem/) and then make sure to require them from inside your lib/[gemname].rb file.

If I have the file lib/jem/the_holograms.rb then my lib/jem.rb file would look something like:

require 'jem/version'
require 'jem/the_holograms'

lib/jem/the_holograms.rb should look something kind of like this, depending on what you are doing, but the key thing to notice is the module line:

module Jem
  SONG = "Truly, Truly, Truly Outrageous"

Note: that require 'jem/version' is just requiring a file that’s setting the version constant. See the first place we are using that constant? Right in the .gemspec file: gem.version = Jem::VERSION.

Once you’ve gotten your gem where you want it you should commit your code and move on the playing with it locally.

Using the Gem Locally

If you want to test out your gem locally the easiest way to is create a new ruby (or rails) project and specify your gem inside the Gemfile like so:

gem 'gem', :path => "/path/to/jem"

Then bundle to install the gem.

Going Further