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UNIX-like OSes

First started around 1969, UNIX has become many things to many people. It is the backbone of the Internet. It is the foundation upon which many modern operating systems are based. It is the yardstick against which new operating systems are often measured. It is both loved and despised. And, if the present has anything to tell us about the future, UNIX is going to be with us for years to come.

UNIX began at AT&T's Bell Labs, and became a replacement for MULTICS. The following information on the history of UNIX is taken in part from the UNIX FAQ

Ken Thompson and Dennis Ritchie began work on an unused PDP-7 computer in 1969. 1971 marked the release of version 1. In 1973, UNIX was rewritten in the C programming language, making the porting of UNIX to other platforms much easier. Version 6 from 1975 was the first widely available UNIX, and by this time UNIX was finding its way into universities.

After this point, UNIX development began to become more splintered. Both Columbia University and UC Berkeley became important centers of develpment, and many modern UNIX-like OSes are based on BSD UNIX (Berkeley Software Distribution). AT&T's System V provided another development path. In the late 70s Microsoft licenced UNIX from AT&T, creating Xenix—a version to run on Intel chips. Minix was a UNIX clone for x86 used primarily as an educational tool. With Minix as a background, Linus Torvalds began Linux in 1991, today the fastest growing UNIX-like OS available. AT&T sold UNIX to Novell, which then—after having done nothing useful with it—sold it to SCO—the Santa Cruz Operation. The UNIX trademark was transferred to The Open Group.

UNIX traditionally provides a CLI (Command Line Interface), similar to, but significantly more complex and powerful than, DOS. From the get-go, UNIX was designed as a multi-user, multi-tasking OS—hence its use as a server. It is essentially the system upon which the Internet was founded, and the majority of Internet servers still run some UNIX-like OS. Most modern UNIX-like OSes flavors are 32-bit or 64-bit (depending on the hardware platform), with virtual memory and all the other trappings of a modern OS.

Although UNIX-like OSes tend to consist of monolithic kernels, the "Unix philosophy" is based upon small tools: a UNIX based OS consists of lots of simple commands and functions, rather than large, bloated, and even redundant programs (of course, these exist, too). This modularity is one of UNIX's greatest strengths; of course, other argue that UNIX is a huge, sprawling mess.

In the 1984 major UNIX vendors formed X/Open. The primary result is the X Window System, or X, a powerful graphical environment for UNIX, and other OSes. X consists of three parts: client, server, and window manager. One can speak of "UNIX based" or "UNIX-like" (though "UNIX-like" is term The Open Group does not like) operating systems; for the most part it makes sense to refer to these OSes as POSIX systems. The POSIX (the Portable Operating System Interface) standard was heavily influenced by UNIX, and most modern UNIX based OSes aim for POSIX compliance.

UNIX Today

Today there are many UNIX based OSes on the market, from the wildly expensive to the free. Some type of UNIX based OS exists for almost every major hardware platform. Below are some of the major variants:


Tux the Linux penguin

Linux is a UNIX-like operating system begun in 1991 by Linus Torvalds. Since then Linux has grown into a mature, modern, powerful, and free operating system for x86 (Intel, AMD, and Cyrix), Alpha, Sparc, and other platforms. Not only are the files for Linux free; so is the source code. Linux is a UNIX clone, and is not actually based on original UNIX code.

The term "Linux" used in its specific sense refers only to the operating system kernel. "Linux" is then packaged into "distributions" that contain the core operating system, utilities, and a wide variety of applications for end-users. The variety of Linux distributions available is staggering; there are general-purpose distributions (or "distros"), those focused on providing a desktop OS, server distros, embedded Linux distros, distros that fit on one or two floppy disks, etc. Because the Linux kernel itself is but a small part of Linux system, and because most distributions rely on GNU software, Richard Stallman (convincingly) argues that one should more properly refer to it as a "GNU/Linux" system.

A few major Linux distributions include:

FreeBSD, NetBSD, OpenBSD

These three "Unices" all have two things in common: 1) they are based upon BSD UNIX and 2) they are free.

To summarize: FreeBSD focuses on performance and is geared toward the x86, IA-64, Alpha, and UltraSPARC architectures. NetBSD is designed to run on as many platforms as possible, and OpenBSD, a fork of NetBSD, aims for security.


Apple's Mac OS X is based upon BSD UNIX sitting atop a Mach (3.0) microkernel. The Open Source "base" of the OS is known as Darwin. In the late 90s Apple bought Apple-founder Steve Jobs' company NeXT and its UNIX-based OS, NeXTstep. For a while Apple's next-generation OS, based on NeXTstep and including compatability with previous Mac software, was named "Rhapsody" and due in 1998. Rhapsody was planned as a desktop OS, and later released as a server OS. Mac OS X differs from Rhapsody in that, among other things, the Mac APIs are being kept in a modified fashion—the Carbon APIs—which, among other things, pleases developers who found Rhapsody itself to be too radical a shift (in terms of compatability and such).


AIX is a version of UNIX from Big Blue. IBM's version is a combination of BSD and System V from AT&T. AIX is 64-bit and runs IBM's RS/6000. Why would I want to run AIX? That's a good question. The answer is, you probably don't. AIX is commercial and expensive, and is unlikely to run on the computer you own. However, if you get a job in UNIX system administration, you might come across an AIX box or two.

For more information, check out:


Solaris is a commericial UNIX version by Sun, running on UltraSPARC as well as Intel machines. At a price of several hundred dollars (depending on the license arrangement), it is more expensive than Linux and Free-, Net, and OpenBSD, but Sun does provide a free license for non-commercial usage on single processor systems.

For more information, please visit Sun's web site.


SGI—Silcon Graphics Inc.—has their own version of UNIX called Irix (currently the 6.5.x line), the operating system that runs on their wonderful Silicon Graphics workstations. Irix is a 64-bit UNIX OS with good multi-processing support and scalability. The only reason to run Irix is if you are using a Silicon Graphics machine. SGI provides its own desktop environment for Irix, and Irix is compatible with UNIX System V.

For more information, please visit SGI's web site.

Digital UNIX (Tru64)

Digital has a version of UNIX, the 64-bit Digital UNIX (aka Tru64) for their Alpha processor. Digital UNIX was considered by many one of the better commercial Unices available, providing great performance as well as the backing of Digital. However, with Compaq's buyout of Digital, many wondered about the future of DIGITAL UNIX. Then Compaq and HP combined; HP seems committed to Tru64 for now.

For more information, please read the Digital UNIX FAQ and "Secure your Digital UNIX (Tru64) Alpha."


Hewlett Packard's HP-UX is an Enterprise-class UNIX variant for HP's PA-RISC machines, originally released in 1986. HP-UX will not provide an ideal solution for most users, due to the limited hardware it runs on, and the fact that like other commericial UNIX based OSes, it is a proprietary product. However, Hewlett Packard is one of the top suppliers of UNIX work stations and servers. In recent year HP de-emphasized HP-UX, instead shipping machines loaded with Windows NT. Yet, HP looks as if it is re-emphasizing UNIX, and that probably means HP-UX. A recent article states that:

HP's renewed focus on UNIX stemmed from the "realization that the UNIX business was contributing value" to HP and business enterprises and also the "reality that NT wasn't going to be the answer" to industry needs of business-critical applications, he said.

For more informationl please visit HP's web site.


SCO's UnixWare is a UNIX that runs on x86 hardware. AT&T sold UNIX to Novell, which then sold the UNIX code to SCO in 1995. Caldera then bought SCO's operating system divisions in 2001 (the remaining portion of SCO remained independent and was renamed "Tarantella, Inc."). No one particularly likes UnixWare or Caldera, and it makes sense to avoid this product.

For more information, see the Caldera web site.


Before MS-DOS, there was UNIX, and in the late 70s, Microsoft bought a UNIX licence from AT&T. The result became Xenix, Microsoft's brief foray into the world of UNIX. In 1987 Microsoft (which had never sold/licenced Xenix to end users, only OEMs) licensed Xenix to SCO, which then marketed and developed Xenix for a time (though it appears SCO worked as early as 1983 with Microsoft on Xenix). As mentioned above, SCO later bought UNIX from Novell, and Xenix was terminated—it is very difficult to find working copies of the original software. Due to a contract with Microsoft (which owns 11% of SCO), SCO was required to retain particular Microsoft code in UnixWare, supposedly so that older (Xenix) software could be supported—European courts released SCO from this obligation in the late 90s.

Xenix was designed for Intel's x86 architecture, with later versions for the 286 and 386 in particular. The original Xenix was based on System III, but later added features from BSD and System V. Due to its humble origins and intended platform, Xenix has about the lowest level of system requirements among UNIX variants, running nicely on older machines such as a 386 with 2MB of memory (4MB being much better).

For more information, try these links:


As the FAQ says: "UNIX for the rest of us"—A/UX is a UNIX version from Apple (Apple UniX), running on some Macintosh computers. It is based upon an older AT&T release of UNIX—therefore not a UNIX clone like Linux—but sports numerous enhancements (from BSD and elsewhere) and incorporates the Mac interface. It is POSIX compliant, meaning it will run text-based UNIX programs, plus it comes with X, and "incorporates System 7 for the Macintosh allowing for the use of the vast majority of Macintosh applications under A/UX." (see the A/UX FAQ).

Now for the bad news: A/UX does not run on PowerMacs, Apple has dropped it completely, and it is expensive. It won't even run on all m68k Macs. Therefore, if you want UNIX on your Mac, you should look at MkLinux, Yellow Dog Linux fo PowerPC, Linux/m68k, or Apple's OS X (for newer machines). Another option is MachTen...


MachTen by Tenon Intersystems is a UNIX implementation for m68k and PowerPPC based Macs. As is the case with OS X, it is BSD UNIX sitting atop a Mach microkernel. The primary difference between MachTen and A/UX is that A/UX ran the MacOS under UNIX, whereas Mach Ten is run under the MacOS. OS X has made MachTen obsolete in a sense; however, MachTen is the only UNIX that will run on all m68k Macs, something which the Linux ports have not yet achieved, and something which OS X will not even attempt. It appears Tenon is trying to "close it out."


Minix is not your every-day UNIX implementation. It is a UNIX-clone created by Andrew Tanenbaumm, and originally released in 1987. Like Linux, it is a clone, and hence contains no AT&T UNIX code. In fact, it is the "UNIX" which inspired Linux. Unlike Linux, it has a micro-kernel architecture. Minix runs on 286, 386, 486, and Pentium class machines (indeed, it was originally coded for pre-386 PCs). It is not as complete as Linux (or FreeBSD). It is copyrighted, but the source is available, and it provides a starting point for those wanting to learn about UNIX.

The following are a few links to Minix related sites:


UNIX is powerful and stable. Today's desktop are finally able to take advantage of the speed and power of UNIX. There is a lot of software available for UNIX operating systems, and knowing UNIX is an important computer skill.

Free UNIX-like OSes, such as FreeBSD and Linux, provide high quality at an unbeatable price, and are perfect for servers and workstations, and increasingly so for desktop computers. With the large amount of Open Source software available (under the GNU General Public Licence, BSD License, etc.), UNIX is no longer just for businesses.

Why not UNIX?

There are two main complaints about UNIX: 1) it is cryptic, 2) it is old.

The first complaint is based upon a misunderstanding—that UNIX = CLI (command-line interface). While all UNIX based systems provide a command line interface, only is the CLI the only interface. In fact, most UNIX based systems use the X Window System and desktop environments such as Gnome or KDE, and Mac OS X has its own advanced windowing system.

Regarding the second complaint: some of the best OSes available today are based on UNIX principles and the POSIX standards: Linux and OS X are just two. It looks as if UNIX has a long life ahead of it.

However, UNIX is not for everyone. It has a high learning curve. There is little "Edutainment" software available for traditional UNIX based operating systems, and no one buys AIX to play games.


Not everyone needs or wants UNIX. However, it is with us to stay—at least for now. Very few modern OSes—the BeOS is such an exception—bypass UNIX, and even in the case of Be, it drew inspiration from UNIX. There are users who want to play with their OS—free UNIX based systems provide that freedom. Nowhere else can you get the power and flexibility afforded by UNIX.

Below you will find some further UNIX links: