A Ruby script to wait for a device to return to the network, and then trigger some action

return_of_the_mac.rb

#!/usr/bin/env ruby

raise "run with rvmsudo" unless `whoami`.strip == "root"

class ThingDoer
  def self.do_something
    # TODO
  end
end

macs = ENV["MACS"]

#
# Return of the MAC
#

loop do
  nmap =
    `nmap -sn 192.168.1.0/24 | grep -Eio "([0-9A-F]{2}:){5}[0-9A-F]{2}"`.split(
      "\n"
    )

  detected = nmap.any? { |n| macs.include?(n.downcase) }

  if detected
    ThingDoer.do_something
    exit
  else
    sleep 2
  end
end