What does the "$" character mean in Ruby?

Question

Been playing with Ruby on Rails for awhile and decided to take a look through the actual source. Grabbed the repo from GitHub and started looking around. Came across some code that I am not sure what it does or what it references. Can anyone tell me what the dollar sign ($) character does in Ruby?

I saw this code in actionmailer/test/abstract_unit.rb

root = File.expand_path('../../..', __FILE__)
 begin
 require "#{root}/vendor/gems/environment"
 rescue LoadError
 $:.unshift("#{root}/activesupport/lib")
 $:.unshift("#{root}/actionpack/lib")
end

lib = File.expand_path("#{File.dirname(__FILE__)}/../lib")
$:.unshift(lib) unless $:.include?('lib') || $:.include?(lib)

require 'rubygems'
require 'test/unit'

require 'action_mailer'
require 'action_mailer/test_case'

Can someone tell me what $: is referencing?

Answer 1

$: is the global variable used for looking up external files.

From http://www.zenspider.com/Languages/Ruby/QuickRef.html#18

$: Load path for scripts and binary modules by load or require.

Answer 2

To quote the Ruby Forum:

ruby comes with a set of predefined variables

$: = default search path (array of paths)
__FILE__ = current sourcefile

if i get it right (not 100% sure) this adds the lib path to this array of search paths by going over the current file. which is not exactly the best way, i would simply start with RAILS_ROOT (at least for a rails project)

Answer 3

$:.unshift

is the same as

$LOAD_PATH.unshift

. You can also say:

$: <<
$LOAD_PATH <<

They are pretty common Ruby idioms to set a load path.

Answer 4

$ identifies a global variable, as opposed to a local variable, @instance variable, or @@class variable.

Among the language-supplied global variables are $:, which is also identified by $LOAD_PATH