short_url.rb

require "uri"
require "net/http"

class ShortUrl
  def self.expand(url)
    uri = URI.parse(url)
    response = Net::HTTP.start(uri.host, uri.port) do |request| 
      request.head(uri.path)
    end
    response["Location"]
  rescue
    url
  end
end

%w( tinyurl.com is.gd bit.ly ).each do |provider|
  puts ShortUrl.expand("http://#{provider}/abc")
end