What does Unix like mean?

I used the phrase Unix like operating system in a discussion the other day and quickly realized and not everyone knew what I meant. So what does Unix like mean?

What Unix means

This is a Unix-like operating system called Coherent, which originated in the 1980s.

Unix was an operating system developed at AT&T around 1970, originally on a Digital PDP 11 mini computer. Unix was notable for a number of things, but one of them was being written in a high level language that made it relatively easy to compile and run on other computer systems.

That’s why you still hear about Unix today, even though you may have never even heard of a PDP 11, let alone seen or used one. For various reasons, AT&T could not sell UNIX directly, but they were able to license it. And it wasn’t long before a number of companies were interested. Several large and famous companies sold their own variant of Unix, tailored for their specific hardware. One example was in the movie Jurassic Park, which showed a few snippets of Silicon Graphics’ implementation of Unix, Irix. In a memorable scene, there was an odd 3D graphical user interface, and one of the characters exclaimed, oh this is a Unix system, I know this!

It was, in fact, an accurate representation of an the file manager fsn, a component of Irix, but famously not typical of all Unix systems. In 1993, a more typical experience was a command line interface with cryptic commands like ls and cat running on a terminal. And the typical grade school or high school didn’t have Silicon Graphics workstations. They could be scarce even in college and university environments. We had some at the University of Missouri when I was there, but I’m not sure if we had them in 1993 yet. And it was much easier to get time on an IBM PS/2 running DOS or Apple Macintosh running System 7.

Why Unix clones came about

Unix was popular but expensive. It wasn’t just the software that was expensive, but the hardware tended to be really expensive too. It may or may not have been overpriced, but it was built to survive a war, and had a price tag to match. And that could be problematic for someone who wanted to learn Unix on their own time. Home computers were expensive, but Unix workstations were more than a couple levels above that. Ten thousand US dollars wasn’t unusual as a starting point for hardware from the likes of Sun Microsystems or Silicon Graphics.

So there was demand for less expensive alternatives that ran on less exotic hardware. IBM PS/2s and Apple Macs of the early 90s had a well deserved reputation for being overpriced, but compared to Suns and SGIs they were downright affordable.

Unix clones

is open source software safe to use
Linux is the most famous Unix-like operating system, but it was far from the first. Yggdrasil was the first Linux distribution you could buy and install on your home PC.

The situation led to any number of Unix clones coming into existence. The most famous of these is Linux, but it is by no means the only one. If it hadn’t been for a piece of software called Minix, it is entirely possible Linux might not have been able to come into being. At least not the way we know it today. Minix is a minimal Unix clone initially written by Andrew Tanenbaum as a teaching aid.

A computer science student named Linus Torvalds decided he wanted something a little less minimal. He used it as the environment to write a replacement kernel called Linux. Eventually Linux grew to a point where it could stand on its own and could run the GNU userspace, giving him a complete operating system to use on a pedestrian PC clone. By the early 90s, you could get a 386sx-based PC for around $1,000. You could download Linux from the Internet, or do what thousands of people did, and buy an early Linux CD from the likes of Yggdrasil or Slackware at the college bookstore and install it.


Of course the GNU project likes to get pedantic and say that if it weren’t for them, Linux never would have come into being either. Because Linux is a kernel, and GNU provided the rest of the operating system. GNU intended to create an entire Unix-like operating system, but their significant delays in producing a stable and usable kernel led to Linux essentially stealing the show from them. You don’t hear much about GNU Hurd anymore.


But they weren’t the only ones. A company called Mark Williams produced a clone called Coherent that ran on various architectures, including Motorola 68K, Intel x86, and Zilog Z8000. Coherent was initally closed source commercial software.

Famously, Coherent was supposed to be the basis of a workstation from Commodore called the Commodore 900. It’s okay if you’ve never heard of that workstation. Commodore canceled that project in favor of something called the Amiga, another historical footnote. Partnering with Commodore probably seemed like a good idea at the time, given the company had 1/3 of the microcomputer market, but I don’t think very many people will argue with me if I say Commodore wouldn’t have known how to market a Unix like workstation.

Coherent ended up running on x86 more than anything else. It was relatively inexpensive, but it’s hard to compete with free. In time, the free competition became more complete, and Coherent lost commercial viability.

It became open source once it was no longer commercially viable.

And then there was BSD.

The curious case of BSD

If you want to have a fun discussion, ask your Unix or Linux administrator if the BSD family of operating systems is Unix or if it’s Unix like. The University of California at Berkeley legally obtained source code from AT&T and adapted it for its own use. In the process, they pretty much ended up rewriting all of it. Some people argue BSD is every bit as much Unix as IBM AIX is. Others say because it is a rewrite, it’s Unix-like. It wasn’t a clean room implementation because it didn’t have to be, but it’s not Unix.

Mac OS X is a fun case. It’s based on a FreeBSD core and directly derived from NeXTStep, which was based on Unix. But it deviates from Unix standards enough that most call it Unix-like, regardless of their stance on BSD.

There were some legal issues surrounding BSD in the early 90s and that uncertainty opened a door for Linux. FreeBSD certainly has a cult following, and its BSD cousins have smaller niche following, but are certainly dedicated. But if it hadn’t been for those delays due to the legal issues, it could be Linux that would be the niche operating system, and FreeBSD could be the free UNIX or Unix like operating system of choice.

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2 thoughts on “What does Unix like mean?

  • February 2, 2022 at 8:21 am

    For me UNIX-like means “a command line interface with cryptic commands like ls and cat running on a terminal” but also includes the typical file structure “/usr” etc, the way tasks are scheduled under the hood and so on. In that way Linux is absolutely UNIX-like, Lets face it Microsoft’s latest product is still called Windows, its a long way form Windows 3.1 but does share the key attributes and is to some extent compatible. The jaw-dropper is when you consider modern IT is based on technology designed in the 1970s. Which, it is.

    • February 5, 2022 at 10:47 am

      Good call on the filesystem, I meant to mention that and never got around to writing that paragraph. I should go back and add it. Apple doesn’t follow that filesystem rule consistently and that inconsistency is definitely one reason I don’t like using OS X.

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