What is __FILE__ in Ruby?

While reading open source Ruby code, I often come across this mysterious looking __FILE__ variable (among others) that I never bothered to look into. After coming across it umpteenth time this evening, I decided to read up on it and here's everything I learned.

__FILE__ is a magic variable that contains the filename with extension of the current file that's being executed. For example, if you're running code in a file named main.rb, __FILE__ will return "main.rb".

# scratch/main.rb

puts __FILE__  # main.rb

You can use __FILE__ with File.expand_path to get the absolute path of the current file.

puts File.expand_path __FILE__

# ~/Software/rails/learn/scratch/main.rb

Here're some interesting use cases I found for this variable while browsing the Rails source code.

• Get the name of the current directory

# actionmailbox/lib/action_mailbox/mail_ext.rb

File.expand_path(File.dirname(__FILE__))

• To set the text for error messages in class_eval or module_eval functions, which evaluate the given string or block in the context of the module.

class_eval <<-RUBY, __FILE__, __LINE__ + 1
  def #{sym}(...)
    custom(Mime[:#{sym}], ...)
  end
RUBY

module_eval <<-CODE, __FILE__, __LINE__ + 1
  def #{option_name}
    options[:#{option_name}]
  end
  def #{option_name}=(value)
    options[:#{option_name}] = value
  end
CODE

Note: As you might have guessed, the __LINE__ variable returns the current line number.