trying to make this only run only if a file does not exisit

config.rb

if File.exists?('user-data.master') && ARGV[0].eql?('up') || File.exists?('user-data.master.yml')
  require 'open-uri'

  token = open('https://discovery.etcd.io/new').read

  file_names = ['user-data.master']

  file_names.each do |file_name|
    text = File.read(file_name)
    new_contents = text.gsub(/AUTO_REPLACED_WITH_DISCOVERY_URL/, token )

    # To merely print the contents of the file, use:
    puts token

    # To write changes to the file, use:
    File.open(file_name + '.yaml', "w") {|file| file.puts new_contents }
  end
end