config.ru

# hello_world.rb
require 'rack'
require 'rack/server'

class HelloWorldApp
  def self.call(env)
    [ 200, {}, ['Hello World'] ]
  end
end

run HelloWorldApp

my_rack_app.rb

require 'rack'
require 'rack/server'

class HelloWorldApp
  def self.call(env)
    [
      200,            #Status Code
      {},             #Headers
      ['Hello World'] #Body
    ]
  end
end


Rack::Server.start :app => HelloWorldApp, Port: 9999