To run .rb files:

bundle exec ruby 01-Hello_World.rb

To run .ru files:

bundle exec rackup config.ru

To run .rb files with rackup:

bundle exec rackup -I . -r 01-Hello_World.rb -b "run Application.new"

Please note that the default port is 8080 if you run with ruby and 9292 if you run with rackup.