"'Unix (all-caps UNIX for the trademark) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, developed in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others." (Wikipedia)

Apple systems have been based on Unix (apparently in one way or another). (Finley)

FreeBSD? and OpenSolaris? "stem off of actual Unix operating systems but have the proprietary bits taken out and replaced with open source ones." Linux "code was written from complete scratch so that the system would act very much like a Unix system, but wouldn’t contain any Unix code." (Stieben)

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