Section A.

                         INTRODUCTION TO UNIX

                         The Operating System

       Unix is a multiuser, timesharing/multi-tasking, and remarkably
       open system that allows for simple file sharing capability among
       users.  All input and output (I/O) is streams (sequences of
       characters followed by the end-of-text character), rather than
       records.  Files are named streams and are randomly addressable.  A
       particular type of file, a directory, provides mapping between
       names of ordinary files and their physical existence on disk.  The
       filesystem forms an inverted tree structure, the root directory
       (/) being the topmost level, with sub-directories branching below.
       Files may reside at any level of the directory tree.  Links to a
       file by the same or different names may exist in several
       directories.  Devices (terminals/printers) have associated with
       them a special file, usually found in the directory /dev, and I/O
       is done exactly as with ordinary files, except that the device is
       also activated.  Pipes (|) allow directing output from one command
       to another via a buffer (a temporary storage area) without making
       any changes to the command.

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