How to share variables across my .rb files?

Ruby will never share local variables between files. You can wrap them in a Module though:

module SharedVaribles
  @test_var="Hello, World"

  def self.test_var
    return @test_var
  end

  def self.test_var=(val)
    @test_val=val;
  end
end

Put that in settings.rb, require it into all your files, and use SharedVaribles.test_var and SharedVaribles.test_var= to access the variable. Remember, Ruby's require is nothing like C's #include, it is much more complex. It executes the file, then imports all constants, modules, and classes to the requireer.

module Foo
  attr_accessor :test_var
  def initialize
    @test_var = "hello world"
  end
end

Create the module with your variable or variables in your config file and then include the module in whatever class you intend to use.

require_relative 'foomod.rb'
class Bar
  include Foo
end

foobar = Bar.new
foobar.test_var = "goodbye"
puts foobar.test_var

Each instance of the class will initialize with whatever value you would like.

1. Constants (which include modules and classes) are added to the shared global environment:

   phrogz$ cat constants1.rb 
   TEST_VARIABLE = "test"
   
   phrogz$ cat constants2.rb 
   require_relative 'constants1'
   p TEST_VARIABLE
   
   phrogz$ ruby constants2.rb 
   "test"
   

2. Instance variables declared in main are all part of the same main:

   phrogz$ cat instance1.rb 
   @test_variable = "test"
   
   phrogz$ cat instance2.rb 
   require_relative 'instance1'
   p @test_variable
   
   phrogz$ ruby instance2.rb 
   "test"
   

3. Global variables are also all part of the same environment (tested in 1.8.6, 1.8.7, and 1.9.2):

   phrogz$ cat global1.rb 
   $test_variable = "test"
   
   phrogz$ cat global2.rb 
   require_relative 'global1'
   p $test_variable, RUBY_DESCRIPTION
   
   phrogz$ ruby global2.rb 
   "test"
   "ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.7.0]"