Should I define a main method in my ruby scripts?

Question

I usually use

if __FILE__ == $0
  x = SweetClass.new(ARGV)
  x.run # or go, or whatever
end

So yes, you can. It just depends on what you are doing.

I've always found $PROGRAM_NAME more readable than using $0. Half the time that I see the "Perl-like" globals like that, I have to go look them up.

if __FILE__ == $PROGRAM_NAME
  # Put "main" code here
end

You should put library code in lib/ and executables, which require library code, in bin/. This has the additional advantage of being compatible with RubyGems's packaging method.

A common pattern is lib/application.rb (or preferably a name that is more appropriate for your domain) and bin/application, which contains:

require 'application'
Application.run(ARGV)

My personal rule of thumb is: the moment

if __FILE__ == $0
    <some code>
end

gets longer than 5 lines, I extract it to main function. This holds true for both Python and Ruby code. Without that code just looks poorly structured.