What is a gem?

Unpack the mystery behind what’s in a RubyGem.

Structure of a Gem

Each gem has a name, version, and platform. For example, the rake gem has a 13.0.6 version (from Jul 2021). Rake’s platform is ruby, which means it works on any platform Ruby runs on.

Platforms are based on the CPU architecture, operating system type and sometimes the operating system version. Examples include “x86-mingw32” or “java”. The platform indicates the gem only works with a ruby built for the same platform. RubyGems will automatically download the correct version for your platform. See gem help platform for full details.

Inside gems are the following components:

• Code (including tests and supporting utilities)
• Documentation
• gemspec

Each gem follows the same standard structure of code organization:

% tree freewill
freewill/
├── bin/
│   └── freewill
├── lib/
│   └── freewill.rb
├── test/
│   └── test_freewill.rb
├── README
├── Rakefile
└── freewill.gemspec

Here, you can see the major components of a gem:

• The lib directory contains the code for the gem
• The test or spec directory contains tests, depending on which test framework the developer uses
• A gem usually has a Rakefile, which the rake program uses to automate tests, generate code, and perform other tasks.
• This gem also includes an executable file in the bin directory, which will be loaded into the user’s PATH when the gem is installed.
• Documentation is usually included in the README and inline with the code. When you install a gem, documentation is generated automatically for you. Most gems include RDoc documentation, but some use YARD docs instead.
• The final piece is the gemspec, which contains information about the gem. The gem’s files, test information, platform, version number and more are all laid out here along with the author’s email and name.

More information on the gemspec file

Building your own gem

The Gemspec

The gemspec specifies the information about a gem such as its name, version, description, authors and homepage.

Here’s an example of a gemspec file. You can learn more in how to make a gem.

% cat freewill.gemspec
Gem::Specification.new do |s|
  s.name        = 'freewill'
  s.version     = '1.0.0'
  s.summary     = "Freewill!"
  s.description = "I will choose Freewill!"
  s.authors     = ["Nick Quaranto"]
  s.email       = 'nick@quaran.to'
  s.homepage    = 'http://example.com/freewill'
  s.files       = ["lib/freewill.rb", ...]
end

For more information on the gemspec, please check out the full Specification Reference which goes over each metadata field in detail.

Credits

This guide was adapted from Gonçalo Silva’s original tutorial on docs.rubygems.org and from Gem Sawyer, Modern Day Ruby Warrior.