From: Peter Hickman <peter@...>
Date: 2006-02-22T00:23:12+09:00
Subject: Re: FILE test

You will see this sort of thing in other scripting languages. __FILE__ 
contains the name of the file source file and $0 is the name of the 
currently executing script. So a file called hello.rb

class Hello
    def initialize(name)
        @name = name
    end

    def greet
        puts "Hello #{@name}"
    end
end

if __FILE__ == $0 then
    h = Hello.new('World')
    h.greet
end

you can then run this file with ruby hello.rb which will run the code at 
the bottom. However with tom.rb

require 'hello'

h = Hello.new('tom')
h.greet

when you run tom.rb the __FILE__ == $0 part of hello does not run as the 
file hello.rb but the currently executing script is tom.rb. This allows 
you to have a ruby file hold the class for inclusion by other scripts 
and also be a utility script in it's own right.