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RubyGems Basics
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/what-is-a-gem

The gem command allows you to interact with RubyGems.

Ruby 1.9 and newer ships with RubyGems built-in but you may need to upgrade for bug fixes or new features. To upgrade RubyGems or install it for the first time (if you need to use Ruby 1.9) visit the download page.

If you want to see how to require files from a gem, skip ahead to What is a gem

Finding Gems

The search command lets you find remote gems by name. You can use regular expression characters in your query:

$ gem search ^rails

*** REMOTE GEMS ***

rails (4.0.0)
rails-3-settings (0.1.1)
rails-action-args (0.1.1)
rails-admin (0.0.0)
rails-ajax (0.2.0.20130731)
[...]

If you see a gem you want more information on you can add the details option. You'll want to do this with a small number of gems, though, as listing gems with details requires downloading more files:

$ gem search ^rails$ -d

*** REMOTE GEMS ***

rails (4.0.0)
    Author: David Heinemeier Hansson
    Homepage: http:https://www.rubyonrails.org

    Full-stack web application framework.

You can also search for gems on rubygems.org such as this search for rake

Installing Gems

The install command downloads and installs the gem and any necessary dependencies then builds documentation for the installed gems.

$ gem install drip
Fetching: rbtree-0.4.1.gem (100%)
Building native extensions.  This could take a while...
Successfully installed rbtree-0.4.1
Fetching: drip-0.0.2.gem (100%)
Successfully installed drip-0.0.2
Parsing documentation for rbtree-0.4.1
Installing ri documentation for rbtree-0.4.1
Parsing documentation for drip-0.0.2
Installing ri documentation for drip-0.0.2
Done installing documentation for rbtree, drip after 0 seconds
2 gems installed

Here the drip command depends upon the rbtree gem which has an extension. Ruby installs the dependency rbtree and builds its extension, installs the drip gem, then builds documentation for the installed gems.

You can disable documentation generation using the --no-doc argument when installing gems.

Requiring code

RubyGems modifies your Ruby load path, which controls 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.

Tip: Passing -r to irb will automatically require a library when irb is loaded.

% irb -rpp
>> pp $LOAD_PATH
[".../lib/ruby/site_ruby/1.9.1",
 ".../lib/ruby/site_ruby",
 ".../lib/ruby/vendor_ruby/1.9.1",
 ".../lib/ruby/vendor_ruby",
 ".../lib/ruby/1.9.1",
 "."]

By default you have just a few system directories on the load path and the Ruby standard libraries. To add the awesome_print directories to the load path, you can require one of its files:

% irb -rpp
>> require 'ap'
=> true
>> pp $LOAD_PATH.first
".../gems/awesome_print-1.0.2/lib"

Note: For Ruby 1.8 you must require 'rubygems' before requiring any gems.

Once you've required ap, RubyGems automatically places its lib directory on the $LOAD_PATH.

That's basically it for what's in a gem. Drop Ruby code into lib, name a Ruby file the same as your gem (for the gem "freewill" the file should be freewill.rb, see also name your gem) and it's loadable by RubyGems.

The lib directory itself normally contains only one .rb file and a directory with the same name as the gem which contains the rest of the files.

For example:

% tree freewill/
freewill/
└── lib/
    ├── freewill/
    │   ├── user.rb
    │   ├── widget.rb
    │   └── ...
    └── freewill.rb

Listing Installed Gems

The list command shows your locally installed gems:

$ gem list

*** LOCAL GEMS ***

bigdecimal (1.2.0)
drip (0.0.2)
io-console (0.4.2)
json (1.7.7)
minitest (4.3.2)
psych (2.0.0)
rake (0.9.6)
rbtree (0.4.1)
rdoc (4.0.0)
test-unit (2.0.0.0)

(Ruby ships with some gems by default, bigdecimal, io-console, json, minitest, psych, rake, rdoc, test-unit for ruby 2.0.0).

Uninstalling Gems

The uninstall command removes the gems you have installed.

$ gem uninstall drip
Successfully uninstalled drip-0.0.2

If you uninstall a dependency of a gem RubyGems will ask you for confirmation.

$ gem uninstall rbtree

You have requested to uninstall the gem:
	rbtree-0.4.1

drip-0.0.2 depends on rbtree (>= 0)
If you remove this gem, these dependencies will not be met.
Continue with Uninstall? [yN]  n
ERROR:  While executing gem ... (Gem::DependencyRemovalException)
    Uninstallation aborted due to dependent gem(s)

Viewing Documentation

You can view the documentation for your installed gems with ri:

$ ri RBTree
RBTree < MultiRBTree

(from gem rbtree-0.4.0)
-------------------------------------------
A sorted associative collection that cannot
contain duplicate keys. RBTree is a
subclass of MultiRBTree.
-------------------------------------------

You can view the documentation for your installed gems in your browser with the server command:

$ gem server
Server started at http:https://0.0.0.0:8808
Server started at http:https://[::]:8808

You can access this documentation at http:https://localhost:8808

Fetching and Unpacking Gems

If you wish to audit a gem's contents without installing it you can use the fetch command to download the .gem file then extract its contents with the unpack command.

$ gem fetch malice
Fetching: malice-13.gem (100%)
Downloaded malice-13
$ gem unpack malice-13.gem
Fetching: malice-13.gem (100%)
Unpacked gem: '.../malice-13'
$ more malice-13/README

Malice v. 13

DESCRIPTION

A small, malicious library.

[...]
$ rm -r malice-13*

You can also unpack a gem you have installed, modify a few files, then use the modified gem in place of the installed one:

$ gem unpack rake
Unpacked gem: '.../rake-10.1.0'
$ vim rake-10.1.0/lib/rake/...
$ ruby -I rake-10.1.0/lib -S rake some_rake_task
[...]

The -I argument adds your unpacked rake to the ruby $LOAD_PATH which prevents RubyGems from loading the gem version (or the default version). The -S argument finds rake in the shell's $PATH so you don't have to type out the full path.

Further Reading

This guide only shows the basics of using the gem command. For information on what's inside a gem and how to use one you've installed see the next section, What is a gem. For a complete reference of gem commands see the Command Reference.