class File
  U_R = 0400
  U_W = 0200
  U_X = 0100
  G_R = 0040
  G_W = 0020
  G_X = 0010
  O_R = 0004
  O_W = 0002
  O_X = 0001
end
class File
  A_R = 0444
  A_W = 0222
  A_X = 0111
end
open("my_file", "w") {}
File.chmod(File::U_R | File::U_W | File::G_R | File::O_R, "my_file")
File.chmod(File::A_R | File::U_W, "my_file")
File.chmod(0644, "my_file")                   # Bitmap: 110001001 
File::U_R | File::U_W | File::G_R | File::O_R

File::A_R | File::U_W

0644

File.lstat("my_file").mode & 0777

new_permission = File.lstat("my_file").mode ^ File::O_R
File.chmod(new_permission, "my_file")
File.lstat("my_file").mode & 0777

new_permission = File.lstat("my_file").mode | File::A_R | File::A_W | File::A_X
File.chmod(new_permission, "my_file")
File.lstat("my_file").mode & 0777

new_permission = File.lstat("my_file").mode ^ (File::O_W | File::O_X)
File.chmod(new_permission, "my_file")
File.lstat("my_file").mode & 0777

class File
  def File.fancy_chmod(permission_string, file)
    mode = File.lstat(file).mode
    permission_string.scan(/[ugoa][+-=][rwx]+/) do |setting|
      who = setting[0..0]
      setting[2..setting.size].each_byte do |perm|
        perm = perm.chr.upcase
        mask = eval("File::#{who.upcase}_#{perm}")
      (setting[1] == ?+) ? mode |= mask : mode ^= mask
      end
    end
    File.chmod(mode, file)
  end
end

Give the owning user write access

File.fancy_chmod("u+w", "my_file")
File.lstat("my_file").mode & 0777

File.fancy_chmod("g-x", "my_file")
File.lstat("my_file").mode & 0777

File.fancy_chmod("a+rwx", "my_file")
File.lstat("my_file").mode & 0777

File.fancy_chmod("u+rwxo-x", "my_file")
File.lstat("my_file").mode & 0777