Simple Rack app as a Ruby file instead of config.ru.

hello_rack_app.rb

# Start server from command line with 'rackup hello_rack_app.rb'
# Make a GET request to the localhost port with cURL or your browser

require 'rack'

class Hello
  def self.call(env)
    [
      '200',
      { 'Content-type' => 'text/html' },
      ['<h1>You made a Rack app!<h1>']
    ]
  end
end

Rack::Handler::WEBrick.run Hello