A multi-user operating system based on the earlier (1960's) operating system Multics ("MULTiplexed Information and Computing Service"). BellLabs felt Multics was getting bloated, so they wrote their own system called Unix, which was "Multics without the balls" or "only one of whatever Multics was supposed to be a lot of".

There is one important dichotomy in the Unix world: the BSDish systems and the SysVish systems.

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1? There is a must-see graph of the mess that is the history of Unix here.