What versions of Unix can I run on my Macintosh-compatible computer?

Depending on the model of your computer and the kind of solution you are looking for, you have several options for running Unix on your Macintosh-compatible computer:

Mac OS X and Mac OS X Server

Mac OS X and Mac OS X Server are Apple's newest client and server operating systems, respectively. They are based on OPENSTEP, which in turn was a BSD implementation running on top of the Mach microkernel. They can run their own applications, as well as classic Mac OS applications and many Unix programs. Their system requirements are fairly steep, requiring at least a PowerPC G3 processor.

Darwin

Darwin forms the foundation of Mac OS X. It doesn't have Mac OS X's Aqua interface and won't run Mac OS applications, but it will run many Unix programs, and a graphical interface in the form of the X Window System is available for it. Darwin is open source and is available free of charge for anyone who signs Apple's license agreement. It requires a PowerPC G3 or better.

BSD

For those who prefer BSD Unix but can't or don't want to run OS X or Darwin, NetBSD and OpenBSD have been ported to both 68k and PowerPC computers. Based on BSD 4.4-lite, NetBSD and OpenBSD are freeware.

Linux

Several Linux distributions are available for Macintosh-compatible computers, most of them based on the so-called "monolithic" Linux/PPC kernel. There is also a Mach-derived version of Linux for PowerPC-equipped computers and a Linux/68k port for older Macintoshes. For more information, see For Macintosh-compatible computers, what versions of Linux are available?

A/UX

A/UX was Apple's first Unix implementation. It is a System V Unix that will run on certain models equipped with 68030 and 68040 processors. It is a true Unix, but can also run most 68k Mac OS applications. A/UX is no longer being developed and is not compatible with PowerPC-equipped computers.

MachTen

MachTen is not a stand-alone operating system, but works instead as a Mac OS application. It is, nevertheless, a true Unix. Though it is not as robust as the other options, it works on all but the oldest Mac OS computers and allows users to simultaneously run Mac OS. MachTen is not intended for use in OS X, and the MachTen project has been largely abandoned.