A

$ mkdir rack-test
$ cd rack-test
$ vi Gemfile
$ vi config.ru
$ vi unicorn_config.rb
$ bundle install --path=.bundle
$ bundle exec unicorn -c ./unicorn_config.rb

config.ru

class Hello
  def call(env)
    [ 200,
      { 'Content-Type' => 'text/plain' },
      [env.inspect]
    ]
  end
end

run Hello.new

Gemfile

source 'https://rubygems.org'

gem 'rack'
gem 'unicorn'

unicorn_config.rb

worker_processes 1
listen 8000