guard-rackup is an alternative of shotgun

Install

$ gem i guard-rackup

Or add it your Gemfile:

$ gem "guard-rackup"
$ bundle install

And then add a basic setup to your Guardfile:

$ guard init rackup

Usage

Please reada Guard document

Guardfile

guard "rackup" do
  watch(%r{^app/.*\.rb$})
  watch(%r{^lib/.*\.rb$})
  ...
end

Or

guard "rackup",
  :command => "unicorn ./config.ru -E development",
  :start => proc{ Process.spawn(command) },
  :stop => proc{|pid| Process.kill("QUIT", pid)},
  :reload => proc{ Process.kill("HUP", @pid) }
  do
  watch ...
end