Prints a list of used UIDs via dscl

osx_used_uids.rb

#!/usr/bin/env ruby -wKU

users = %x(dscl . -list /Users).split("\n")

uids = []
users.each do |user|
  uids += [%x(dscl . -read /Users/#{user} uid).split[-1].to_i]
end

puts uids.sort