__FILE__と$0の違い

current_file.rb

require_relative 'require_file'
include RequireFile

$0                             # => "current_file.rb"
__FILE__                     # => "current_file.rb"
__LINE__                     # => 6
File.expand_path(__FILE__)  # => "path/to/current_file.rb"

stdout_method
  # $0                         # => "current_file.rb"
  # __FILE__                   # => "path/to/require_file.rb"
  # __LINE__                   # => 5
  # File.expand_path(__FILE__) # => "path/to/require_file.rb"

require_file.rb

module RequireFile
  def stdout_method
    p $0
    p __FILE__
    p __LINE__
    p File.expand_path(__FILE__)
  end
end