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What is a gem?
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Learn what a gem can do for your Ruby application, and what's inside of one.

Introduction

A RubyGem is a software package, commonly called a "gem". Gems contain a packaged Ruby application or library. The RubyGems software itself allows you to easily download, install, and manipulate gems on your system.

Each gem has a name, version, and platform. For example, the rake gem has a 0.8.7 version. Rake's platform is ruby, which means it works on any platform Ruby runs on. Other platforms include java (like nokogiri) and mswin32 (like sqlite-ruby).

Gems can be used to extend or modify functionality within a Ruby application. Commonly, they're used to split out reusable functionality that others can use in their applications as well. Many gems also provide command line utilities to help automate tasks and speed up workflows. As of Ruby 1.9.2, RubyGems is now included when you install the programming language, so gems are both ubiquitous and extremely useful.

For information installing RubyGems, please visit the Downloads page.

Structure of a Gem

Gems contain three components:

  • Code
  • 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, we see the 3 major components: code, in the lib directory, hopefully along with some tests as well. Tests appear in test or spec, depending on the test framework used. A gem usually has a Rakefile, which the rake program uses to help automate running tests, generating code, and more. This gem also includes an executable file in the bin directory, which will loaded onto your PATH once 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 YARD docs are also nice as well.

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.

Requiring code

RubyGems manages your Ruby load path, or how your Ruby code is found by the require statement. When you require a gem, really you’re just placing that gem’s lib directory onto your $LOAD_PATH. Let’s try this out in irb and get some help from the pretty_print library included with Ruby. Passing -r to irb will automatically require a library when loaded.

% irb -rpp
>> pp $LOAD_PATH
[".../lib/ruby/site_ruby/1.8",
 ".../lib/ruby/site_ruby",
 ".../lib/ruby/vendor_ruby/1.8",
 ".../lib/ruby/vendor_ruby",
 ".../lib/ruby/1.8",
 "."]

By default we have just a few system directories on our load path and the Ruby standard libraries. If we were to run require 'rake' right now, it would fail, because RubyGems isn’t loaded yet.

% irb -rpp
>> require 'rake'
LoadError: no such file to load -- rake
        from (irb):2:in `require'
        from (irb):2
>> require 'rubygems'
=> true
>> require 'rake'
=> true
>> pp $LOAD_PATH[0..1]
[".../gems/rake-0.8.7/bin",
 ".../gems/rake-0.8.7/lib"]

Once we’ve required rake, then RubyGems automatically drops the bin and lib directories onto the $LOAD_PATH. The bin directory is used for creating executables that use your gem’s code, such as rake. These are completely optional and you could have multiple per gem if you wanted.

That’s basically it for what’s in a gem. Drop Ruby code into lib, name a Ruby file the same as your gem (so for freewill, freewill.rb) and it’s loaded by RubyGems.

The lib directory normally contains only one .rb file on the top directory, and then another folder with the same name as the gem with more code in it. For example:

% tree freewill/
freewill/
|-- lib/
|   |-- freewill/
|   |   |-- core_ext/
|   |   |   |-- array.rb
|   |   |   `-- string.rb
|   |   |-- user.rb
|   |   |-- widget.rb
|   |   `-- ...
|   |-- freewill.rb

The Gemspec

Your application, your gem's users, and you 6 months from now need to know who wrote a gem, when, and what it does. The gemspec tells you this information and is your guide to understanding what a gem contains for you.

Here's an example of one. The next tutorial covers how to make a gem.

% cat freewill.gemspec
Gem::Specification.new do |s|
  s.name        = 'freewill'
  s.version     = '1.0.0'
  s.date        = '2010-04-27'
  s.summary     = "Freewill!"
  s.description = "I will choose Freewill!"
  s.authors     = ["Nick Quaranto"]
  s.email       = 'nick@quaran.to'
  s.homepage    = 'http://example.com'
  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.