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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