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Is Linux a Unix?

Kind of confused by the two terms. Is there any difference between the two terms?

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A world of difference. –  Ignacio Vazquez-Abrams Nov 26 '11 at 11:57
    
@Mat The FAQ says that it is the question that must be an exact duplicate to qualify. –  Chris Down Nov 26 '11 at 15:15
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marked as duplicate by Mat, rozcietrzewiacz, phunehehe, Stéphane Gimenez, Gilles Nov 26 '11 at 15:43

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This really depends on what you mean by "Unix". Unix has come to mean various things in modern times (and even at the creation point of Linux, it meant multiple things).

In general, Unix is not a particular system, but a specification for systems calling themselves "Unix-like". When people say "Unix" they do not necessarily mean "the proprietary operating system owned by AT&T/Novell/Cisco/whoever now owns it when you're reading this", rather, they usually are referring to the whole spectrum of Unix-like OSes, like AIX, HP-UX, Linux, BSD, Solaris, etc. To this degree, Linux is a FOSS, Unix-like kernel. It is not a direct fork of the original Unix codebase, but it shares many similarities.

Another reason that many people regard Linux to be Unix-like is the fact that it is mostly POSIX-compliant (which is very important for compatibility with other Unix-like systems). Some also associate Linux with Unix because of the initial history of the project -- Linux was largely inspired by (but was not a fork of) MINIX, which is, and was, widely regarded to be an attempt to create a FOSS Unix clone. Many Linux distributions also often implement many tools (or clones/approximations of tools) from Unix, often in the form of GNU Coreutils. Nowadays these tools have been changed a lot (some would argue for the worse, GNU Coreutils is notorious for feature creep), but usually still maintain portability with their original counterparts.

Linux is also indisputably free, open-source software under the GPL, whereas the licensing of the original Unix codebase often depends on who you're asking, and when.

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Re: the last sentence: Not really depending on who you ask, but depending on which version you're looking at. BSD for example is "real" UNIX by its source code heritage, and is also indisputably free, although a different kind of free than GNU's free. –  nitro2k01 Nov 26 '11 at 15:40
    
@nitro2k01 There are still people that will tell you it belongs to SCO. –  Chris Down Nov 26 '11 at 15:43
    
@ChrisDown SCO owns the code formerly developed at Bell Labs (AT&T) and formerly called Unix. Novel currently owns the Unix trademark. Some of the code in SCO's System V, Unixware, and Openserver products are BSD code (derived from BSD 4.3). AT&T certainly believed at one point in time that BSD 4.3 was a variation of Unix; a point settled out of court. Based on that assumption the current BSD flavors can claim Unix heritage the other unix-compatible OSes can not. In any case the point is moot, as you state in your answer people imply the collection of standards implemented in Unix, not the OS. –  Chris S Nov 27 '11 at 4:16
    
My point is that it isn't completely clear cut. Certainly the licensing of Linux is a lot more stable by any approximation. –  Chris Down Nov 27 '11 at 4:26
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