URL

url.rb

class Url
  attr_reader :url
  
  def initialize(url)
    raise(ArgumentError, "url not specified") unless url
    @url = url
  end
  
  def to_uri
    URI.parse(url)
  end
end

url = Url.new(ARGV[0]) # will raise an ArgumentError if ARGV[0] is nil

Notes:

• L10 notice I'm using the attr_reader instead of the instance variable. Rationale: depend on behaviour not data (POODR)
• L5 use an exception to deal with an argument that is required but not passed. Strictly speaking your URL class should have no knowledge of the outside world, i.e., it shouldn't know about command line arguments. Rationale: inject dependencies (POODR)
• I wasn't sure what the following code was all about, I don't believe it should really be part of this class, but if so, it should pass errors out in a different way. Again a dependency thing, URL shouldn't know about $errors. If you give me a bit of background I can have a think about what might work better.

def to_uri(crit=false)
  $errors.catch(crit) do
    URI.parse(@url) # Requires a valid scheme (http:// , //, etc...)
  end 
end