------------------------------------------------------------ Class: File
      A  File  is an abstraction of any file object accessible by the
      program and is closely associated with class  IO   File  includes
      the methods of module  FileTest  as class methods, allowing you to
      write (for example)  File.exist?("foo") .
      In the description of File methods, _permission bits_ are a
      platform-specific set of bits that indicate permissions of a file.
      On Unix-based systems, permissions are viewed as a set of three
      octets, for the owner, the group, and the rest of the world. For
      each of these entities, permissions may be set to read, write, or
      execute the file:
      The permission bits  0644  (in octal) would thus be interpreted as
      read/write for owner, and read-only for group and other.
      Higher-order bits may also be used to indicate the type of file
      (plain, directory, pipe, socket, and so on) and various other
      special features. If the permissions are for a directory, the
      meaning of the execute bit changes