The UNIX Time-Sharing System
Dennis M. Ritchie and Ken Thompson - 1974
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Summary: This paper describes the UNIX operating system with a focus
on what the authors believe is its most important role, providing a
file system.

Three kinds of files:

   1. Ordinary: These files can contain any information the user wants
      (e.g., text, binary, etc.).  The system doesn't enforce any
      particular structure, so if a user program requires files with a
      certain structure, that has to be enforced in the the user
      program.

   2. Directory: These files provide structure by mapping names of
      files to the files themselves.  A directory behaves like an
      ordinary file except that it cannot be written on by
      unprivileged programs; thus, the system controls the contents
      (and format) of the these files.

      Linking: The same nondirectory file may appear in several
      directories under possibly different names.  Thus, a file does
      not exist within a particular directory.  The directory entry
      for a file consists of its name and a pointer to the file's
      i-node.  Links to directories are not allowed to simplify the
      writing of programs that visit subtrees of the directory
      structure and to avoid the separation of portions of the
      hierarchy.

   3. Special: These files represent I/O devices.  The advantages of
      treating devices as files are threefold: 1. similar interfaces
      for file and device I/O, 2. file and device names have the same
      syntax and meaning so that a program expecting a file name can
      be passed a device name, and 3. special files are subject to the
      same protection mechanism as normal files.

Miscellaneous points:

   - Mounting file systems: No link may exist between one file system
     hierarchy and another.  This avoids elaborate bookkeeping
     required to assure that links are removed when unmounting a file
     system.

   - Locks: UNIX does not have file locks because they are neither
     necessary nor sufficient.  They are unnecessary because there are
     typically not large single-file databases maintained by
     independent processes.  They are insufficient because locks that
     prevent a user from writing a file which another user is reading
     cannot prevent confusion when, for example, both users are
     editing a file with an editor which makes a copy of the file
     being edited.

   - i-list: When the file is accessed, its i-number is used as an
     index into a system table (i-list) stored in a known part of the
     device on which the directory resides.  This table contains
     i-node information (e.g., last modification, reference count,
     etc.).  This way of organizing the file system permits a simple
     and rapid way for checking file system consistency since only the
     linear i-list needs to be checked.