Simple rack application. Start with `rackup`

app.rb

class App
  def self.call(env)
    path = env["REQUEST_PATH"]
    case path
    when "/"
      body = "<h1>Hello,</h1><p>world!</p>"
      [200, {}, [body]]
    else
      body = "Couldn't find any page at #{path}"
      [400, {}, [body]]
    end
  end
end

config.ru

$: << "."

require "app"

run App