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So, there are lots of different versions of Unix out there: HP-UX, AIX, BSD, etc. Linux is considered a Unix clone rather than an implementation of Unix. Are all the "real" Unices actual descendants of the original? If not, what separates Linux from Unix?

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Related: Why isn't GNU/Linux SUS v3+ compliant? –  Gilles Jun 4 '11 at 21:28

5 Answers 5

up vote 116 down vote accepted

That depends on what you mean by “Unix”, and by “Linux”.


  • Strictly speaking, Linux is an operating system kernel that is designed like Unix's kernel.

  • Linux is most commonly used as a name of Unix-like operating systems that use Linux as their kernel. As many of the tools outside the kernel are part of the GNU project, such systems are often known as GNU/Linux. All major Linux distributions consist of GNU/Linux and other software.

  • There are Linux-based Unix-like systems that don't use many GNU tools, especially in the embedded world, but I don't think any of them does away with GNU development tools, in particular GCC.

  • There are operating systems that have Linux as their kernel but are not Unix-like. The most well-known is Android, which doesn't have a Unix-like user experience (though you can install a Unix-like command line) or administrator experience or (mostly) programmer experience (“native” Android programs use an API that is completely different from Unix).

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And let's not forget that GNU stands for "GNU is Not Unix"! –  alex Nov 16 '10 at 7:05
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"Gnu is not Unix" means Gnu isn't based on Unix original code, not that Gnu doesn't want to implement Unix specs. Quoting rms: "Free Unix! Starting this Thanksgiving I am going to write a complete Unix-compatible software system called GNU (for Gnu's Not Unix), and give it away free(1) to everyone who can use it." –  jlliagre Nov 16 '10 at 8:08
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If a Linux distro wanted to get certified as a UNIX, it wouldn't have to change very much and it could be so. But I haven't heard of anyone paying all that money for what would, at this point, be worth so little. –  Kevin Cantu Nov 17 '10 at 21:32
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@Kevin: Caldera (now infamously known through another name) once worked on it, though they didn't go all the way (they seem to have abandoned the idea and pushed for LSB instead well before they suddenly forgot they'd ever heard of Linux). –  Gilles Nov 17 '10 at 22:57
    
Hahahah. I also have some vague memory of someone providing a Linux for government / military procurement that did go through that. This list, though, doesn't include dead companies: opengroup.org/openbrand/register –  Kevin Cantu Nov 18 '10 at 17:46

Linux is a Unix-Like Operating System developed by Linus Torvalds and thousands of others.
BSD is a UNIX operating system that for legal reasons must be called Unix-Like.
OS X is a graphical UNIX Operating System developed by Apple Inc.

Linux is the most prominent example of a "real" Unix OS. It runs on anything and supports way more hardware than BSD or OS X. An interesting quote I found when I was comparing BSD and Linux:

Linux is what you get when a bunch of PC hackers sit down and try to write a Unix system for the PC. BSD is what you get when a bunch of UNIX hackers sit down and try to port a Unix system to the PC.

BSD is more like a Unix OS than Linux is. Also notable is that Apple makes use of BSD and Linux components. Apple Uses APT from Debian and Ubuntu on the iOS and OS X platforms. And it is based on BSD. (The kernel though is Darwin, which is it's own kernel. Beastie the platypus is the Darwin mascot because he is a mix between Beastie from BSD and a Platypus.)

If you want a "real" Unix operating system (One that runs on anything and supports lots of hardware), try Linux.
If you want lower-end hardware support and headaches (I know I'll get a ton of hate but I don't care), use BSD.
If you want to spend $1000+, use OS X and iOS. (Again I'll probably get a ton of hate.)

I'm a long-time Linux User, having used it off and on from the 90's to early 2000's and then quit using it for awhile but started using it again around mid 2012 as my permanent OS so I can recommend it to anyone who wants to try something other than Windoze.

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As Linux is changing recently with Wayland/systemd/etc, it's removing itself more and more from being a Unix-like system and becoming its own entity. It is making itself less compatible with true Unix and Unix-like systems to the point where these real systems have developed a "don't care" attitude toward Linux. Your "recommendations" are way off base and makes me agree with you that you haven't touched any of this stuff in 15 years. –  Rob Feb 15 '13 at 12:58
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Linux is not an OS, but a kernel. –  Martin Schröder May 1 '13 at 13:29

For all intents and purposes, a typical modern Linux distribution (Ubuntu, Debian, Red Hat, Fedora, Slackware, etc) is a Unix, but strictly speaking, no system can claim to be Unix without being certified, so instead people say they are Unix-like. They are inspired by Unix, and carry on its culture.

This also applies to BSD systems.

Mac OS X is certified Unix, so it's Unix both in name and indeed. (and it's actually based on BSD).

It should be noted that since Linux itself is just a kernel, it can be used to build non-unix-like systems (such as Android).

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Note: the Mac OS X kernel is not based on any BSD kernel. The user mode part is based on FreeBSD and in parts NetBSD to my knowledge. –  0xC0000022L Mar 11 '11 at 19:38
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Today's kernel might be rather different from the BSD kernel, but I think it was initially (at least partly) based on it. –  hasenj Mar 13 '11 at 11:20
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nope, it wasn't. And I'm tempted to downvote, but please read up about the Mach kernel first and its involvement in Mac OS X, perhaps you will correct your statement then ;) –  0xC0000022L Mar 13 '11 at 14:44
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It should be noted that FreeBSD is a direct descendant of ATT Unix and once contained ATT Unix code. It was not just "inspired" by Unix. –  Rob Aug 15 '13 at 11:55
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@0xC0000022L, well, the fact is "OS X is based upon the Mach kernel. Certain parts from FreeBSD's and NetBSD's implementation of Unix were incorporated in NeXTSTEP, the core of Mac OS X." Agree? –  poige Oct 28 '13 at 8:42

The choosen answer explains it quite exhaustively, but you would have to watch a whole documentary to know all the details between the UNIX initial developement, and how Linux went out.

First you have to consider that Linux is the kernel, which was made by linus and other programmers. He chose to release it with the GPL, which by the time was a license made by the FSF, (RMS and other folks), who were also developping their own kernel and GNU.

What we usually call Linux is, at first, the kernel, plus all other tools originally coming from the GNU project. Those two project are historically different, because at the time, the kernel from the FSF folks was abandonned to favor the Linux kernel, which was much better.

I remember there is a documentary, you should really watch it, I find it important for your programming culture.

Another thing you have to consider when thinking about "*nix", is also everying involving POSIX and other architecture-standardised stuff. It's subject of OS design/research, but it defines precisely how the involved OSes works, and is crucial when you have a kernel working well with its tools.

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Actually POSIX is now called SUS (Single Unix Specification), which was mentioned. –  0xC0000022L Mar 11 '11 at 19:39
    
@STATUS POSIX was the basis for SUSv2. The Austin Group was later formed as a joint working group between the IEEE who created POSIX and The Open Group who own the UNIX trademark and the UNIX specification. They released POSIX:2001 also known as SUSv3. They are one in the same as far as I can tell. POSIX is still being developed with POSIX:2004 and POSIX:2008 which is the core of SUSv4. I don't think POSIX is dead yet. en.wikipedia.org/wiki/Single_UNIX_Specification –  penguin359 Apr 26 '11 at 4:20
    
I understand that SUS is POSIX with some additions. –  vonbrand Jan 23 '13 at 14:57

Linux is more 'Unix-like' so yes simplistically you could call it a clone, the same is true for BSDs (although admittedly BSDs could be considered closer to Unix than Linux).

The main thing that gives Linux the Unix-like title is the fact that it is nearly fully compliant w/ POSIX (Portable Operating System Interface [for Unix]) standards that have built up over time.

The other key thing, is the inheritance of code etc, as demonstrated on Wikipedia, Linux does not actually originate from Unix sources, instead it is pretty much built from scratch (this is disputed however).

So essentially, the main thing that separates Unix from Linux is the ancestry and the standards that it meets.

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There's also the Single Unix Specification to consider. –  Dennis Williamson Nov 15 '10 at 23:37
    
I'd definitely say the BSDs are closer to Unix than Linux. Due to BSD's more liberal license and it's close development with AT&T UNIX, a lot of BSD source code and it's APIs are used in commercial Unix. The UFS filesystem in Solaris and some other commercial UNIX came straight from BSD. Also, the Berkely Sockets API (not necessarily source) used by everything including Linux was developed on BSD. –  penguin359 Apr 26 '11 at 4:16
    
After the whole SCO vs IBM mess, it has become clear that the Linux kernel contains no Unix code at all. –  vonbrand Jan 23 '13 at 14:55

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