I always wanted to try Unix but I can't seem to find an ISO file or somewhere to buy it. Is Unix published for the whole wide world to use or is it a special OS only for high-class servers, mainframes and supercomputers?

Can I try it out? Does it comes in distros? This is like the problem I had with Linux: I was going around the Internet, wondering how to install Linux and looking for a Linux ISO when I did not know it came in distros.

  • To get started with Linux (which is a free subfamily of Unix), I recommend starting with the free Introduction to Linux course from the Linux Foundation. It walks you through installing the system, but that's less than 5% of the course. – Wildcard Jan 2 '16 at 7:08
  • Related: What is Unix now. – RedGrittyBrick Nov 1 '16 at 16:02

Unix was originally a product, first developed in AT&T's Bell Labs. But today, the word “Unix”, except in historical context, means a family of operating systems, not a single product (similarly to “Linux” meaning a family of distributions, not a single product). This family has a somewhat complex history (see also Evolution of Operating systems from Unix).

It's difficult to say when this product ended, because the original code was licensed to a number of vendors, some of which still maintain their product. I believe the last product released by AT&T was Unix Time-Sharing System 10 in 1989. However by that time most Unix systems were actually modified versions of the AT&T code maintained by other companies such as Sun (SunOS, later renamed Solaris), Hewlett-Packard (HP-UX), IBM (AIX), etc. These three are still extant today, and they're directly derived from the AT&T code (although after over 25 years there probably isn't much of the AT&T era code remaining).

In addition to unix systems that are derived from the AT&T code, there are systems that don't contain any AT&T code but have a similar design and compatible user and programmer interfaces. The main families of such unix systems are BSD (FreeBSD, OpenBSD, NetBSD, Darwin/macOS etc.), Linux (many distributions) and MINIX.

Just to add to the confusion, UNIXⓇ is a trademark (actually, a family of trademarks: there are several versions) which does not designate a particular product: any product can use it as long as it passes a series of conformance tests. This unusual situation is a consequence of a long legal battle. Basically, a product can lay claim to one of the UNIX trademarks if it complies with the Single UNIX Specification, which describes user and programmer interfaces of the operating system (but not administration interfaces).

If you want to run a product that came directly from AT&T, you can run Unix V5, V6 or V7 on a PDP-11 simulator (the PDP-11 was a popular series of minicomputers in from the early 1970s to the early 1990s).

If you want to run a product based on code from AT&T, you can run OpenIndiana, which is based on the now-discontinued open source edition of Solaris (OpenSolaris). OpenIndiana is free software and runs on a PC. (It might not support as much hardware as Linux does though, but it can run in e.g. VirtualBox.) I believe that you can also download Oracle's Solaris for free for personal use, and it too can run on a PC. As far as I know, it isn't possible to run AIX or HP-UX on easily-available hardware or emulators.

If you want to run a product which has the UNIX brand, you can go through the official list. It includes several versions of Solaris (including PC versions), several versions of macOS, and a few uncommon Linux distributions. In a twist of fate none of the historical Unix products have the UNIX trademark, because they're too old and fail to meet some of the newer requirements for the UNIX brand.

If you want to run a product in the unix family of operating systems, Linux is one (or rather Linux is a subfamily, and each distribution is a Unix-like operating system).

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  • A couple of problems with this answer. First, UNIX has not "ended", is very much alive, and one example of this is Apple's OSX which is a certified UNIX. Second, FreeBSD is a direct descendant of UNIX and once contained ATT code, which was removed and rewritten, while Linux was, at the beginning, an attempt to emulate or imitate UNIX but my opinion is that Linux no longer has that goal and is moving farther away from resembling anything UNIX. If one wants to get closest to UNIX, the easiest and best way is to use Apple's OSX or one of the BSDs. – Rob Jan 2 '16 at 16:03
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    @Rob Please read my answer more carefully instead of jumping to incorrect conclusions. You're confusing AT&T's Unix product with the UNIX trade mark. I took care to state explicitly that I was referring to the product. The trade mark is off-topic for this question. There's no real difference between *BSD and Linux with respect to their closeness to the original Unix product. BSD started out as an add-on for AT&T Unix, but it's been a long time since it was free of AT&T code. – Gilles 'SO- stop being evil' Jan 2 '16 at 16:10
  • BSD could not be an "add-on" to UNIX if it was a standalone OS itself so I don't know what you mean by that. I get your point about the product but if a product is certified as UNIX then that makes it a UNIX product. – Rob Jan 2 '16 at 16:18
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    @Rob On second thoughts, I've discussed the trade mark as well, because the question could be read as “How to run UNIX™”. Regarding BSD, it started out as additional software for AT&T's Unix, then gradually all the AT&T code was replaced, making BSD a standalone operating system. Regarding products certified as UNIX, you can say that they are “UNIX products”, but conversely, most Unix products are not certified, let alone Unix-like products that are practically compliant but haven't passed the formal conformance tests. – Gilles 'SO- stop being evil' Jan 2 '16 at 17:24


FreeBSD is a free Unix-like operating system descended from Research Unix via the Berkeley Software Distribution (BSD). Although for legal reasons FreeBSD cannot use the Unix trademark, it is a direct descendant of BSD, which was historically also called "BSD Unix" or "Berkeley Unix".

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    FREEBSD is much more unix-like than linux. – HashTables Jan 1 '16 at 4:40
  • I know because linux is more minix. – Henry WH Hack v2.1.3 Jan 1 '16 at 4:42
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    @HenryWHHack, Linux has nothing in common with Minix, neither v2 (now discontinued) and much less with the current v3. In it's earliest beginnings it used Minix' filesystem, and was initially developed under Minix. In large part because Linus was extremely dissatisfied with Minix v2 design decisions he started Linux. – vonbrand Jan 2 '16 at 2:23

Today Unix is a trademark, and an operating system can be certified as Unix if it complies to a set of certification procedures. There are several operating systems, not all directly based on original AT&T Unix, that have passed the certification and can call themselves Unix. Others just haven't bothered, but might pass. Linux for one goes it's own way whenever the developers (in the end, Linus Torvalds) deem the relevant standards misguided or directly braindead.

Unix is not just the kernel of the operating system. While Unix specifications cover system calls in detail, the specifications covers a set of libraries and a rich set of utilities and user tools too.

There are literally hundreds of Unix derivatives, and a large number of more or less clones. And the different lines have evolved over the last 40 or so years, together with the machines on which the systems run. Today's machines have little resemblance to the original PDP-7, or even the PDP-11 workhorse, or the very popular VAX-11 of yore.

In summary, "Unix" is a ill-defined term. If you want some feel of what it is all about, use one of the current BSDs or Linux. If you want a more specific version, you'll have to hunt it down, perhaps at The Unix Heritage Society.

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    In fact, Linux Torvalds once said that if BSD was available, he never would have created Linux. (FreeBSD was in court with ATT due to its containing original ATT UNIX code which was subsequently removed and rewritten.) – Rob Jan 2 '16 at 16:13
  • @Rob, as I remember, the "AT&T code rewritten" in the end was a couple of files of a dozen lines. Most of the heavy lifting had been done long before. – vonbrand Jan 2 '16 at 16:25

I'm posting this because whilst the other answers are good, I feel any discussion like this really needs to include a reference to the POSIX standard.

Unix isn't a thing, it's a family. A bit like mammals - a mammal has a certain set of features that make it a mammal - like 4 'legs', warm blooded, endoskeleton, etc.

But there's a big difference between an elephant and a seal. (Or a penguin!)

Unix is much the same - there is no 'true Unix' there are just a bunch of things in the family, that have common features.

From: http://www.levenez.com/unix/

We have a family tree that is quite long and complicated - it starts with Unics in 1969, and ... well, branches a lot from there.

Linux is a descendent of that family tree. It also implements POSIX, and therefore is Unix. You have some other major branches at the moment - the bigger examples are:

  • System V, which is HP-UX and Solaris (and things like AIX too, but that's just a bit of a strange one).
  • BSD - which gives us FreeBSD and is the basis for the current generation of Apple operating systems.
  • Linux - which gives us Android as well.

There's been a degree of crossbreeding - Linux is proving quite a successful beast, thanks to its links to open source. (Unix has always had a cordial relationship with Open Source, but most distributions historically have been vendor supplied/supported/patched.). But BSD ... well, Apple adopting it certainly hasn't hurt any. It's also quite popular in a lot of embedded systems - firewalls and the like (I think NetApp is based on BSD too).

Likewise - historically a lot of the big players had their own hardware/processors and OS.

So the long and short of it is - if you've already done Linux and got a distribution installed that you like - you have already used Unix, and all is good. If you really want you could give some other variants a whirl, and see how they differ (and yet how similar they are too!)

So for example, you can get Solaris for x64 which is a System V, FreeBSD

But you'll have some similarities (POSIX standard, and things that operating systems have 'evolved' from each other) and some differences (AIX keeps 'getting' me with the ODM).

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    UNIX is a thing and there is documentation that must be followed and you can be certified as UNIX (trademarked) opengroup.org/subjectareas/platform/unix. Also, Apple's OSX is certified UNIX but its kernel is not BSD. It borrows a number of elements from FreeBSD but it is not based on FreeBSD or BSD in general. – Rob Jan 2 '16 at 16:11
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    Windows also conforms to POSIX (but it is said that the options have been selected carefully to make the result as useless as possible). At one time, POSIX compliance was mandatory for DoD (or USA government) use, so many not even remotely Unix systems acquired POSIX compatibility of some sort. On the other hand, Android (your smartphone or tablet) has Linux as a kernel, and most emphatically isn't Unix by any stretch. – vonbrand Jan 2 '16 at 16:30

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