gistfile1.rb

def write_file
  begin
    File.open("/test", "w+") do |f|
      f.write("hello!\n")
      f.close
    end
    puts "wrote test file"
  rescue Errno::EACCES
    puts  "could not write test file"
  end
end

puts "ok, set uid to nobody"
Process.euid = 65534

puts "going to try to write to / now..."

write_file

puts "restoring back to root"

Process.euid = 0

puts "now writing file"

write_file