What do unix and Linux have in common?

Answer 1

Their source code is available for free.

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Answer 2

Unix and Linux have a common command structure, similar command shell environments, common set of standard utility programs, games, development tools, functions, manual (help) file format. They do not have a common history.

The Unix System Program was original developed at Bell Laboratories by Dennis Ritchie and Ken Thompson in 1969. It was the property of AT&T (or some branch of it), was free with source to educational institutions and was later offered commercially for DEC PDP computers for 12 to 20 thousand dollars and more depending on the installed computers. Unix and BSD (a free derivative of UNIX) were licensed and ported to various other systems vendors such as DEC (ultrix, osf/1, TRU64. etc), SUN (sunos), Microsoft (xenix), IBM (AIX), HP (HPux), Apple (OSx), and many more. BSD itself had a whole bunch of different free implementations under a BSD license.

There was also a lawsuit between BSD and Novell (which had purchased ownership of UNIX) claiming BSD should not be giving away BSD UNIX for free. It ended with BSD allowed to continue its free licensing after making some changes to its software base.

Linux was a result of an independent project by Linus Torvald with a small group of independent hackers that became very popular. He developed a kernel system program that was compatible with UNIX but was written independently of UNIX, had no UNIX code in it, was not subject to a UNIX licensing issues, and was distributed under a GPL license. The Linux kernel together with some independently written programs, together with many free programs borrowed from BSD constitute a Linux distribution.