This article is from the MachTen & CodeBuilder FAQ, by Jaime Julca firstname.lastname@example.org with numerous contributions by others.
Apple developed a commercial UNIX (A/UX) that ran on a subset of Macs. Apple
no longer supports or sells A/UX, which was an older version of UNIX --
SystemV, version 2 -- and was expensive in terms of memory and required disk
space. MachTen is a more compact UNIX that can run on all Macs and all Power
Macs. A/UX was implemented on the bare Macintosh hardware, which required
Apple to port A/UX to new platforms on a machine-by-machine basis. Unlike
A/UX, MachTen sits on top of the MacOS, which means that you don't need
special device drivers and you don't have to partition your disk.
In 1996, Apple began distributing MkLinux. MkLinux is a public domain
version of Linux that has been ported to a few Power Mac platforms (Nu-bus
based Power Macs). MkLinux, much like A/UX, runs on the bare Power Mac
hardware. Users must use MkLinux-specific device drivers and cannot run Mac
applications while MkLinux is running. The current version of MkLinux is
Developer Release 2.1. It is our understanding that Apple is no longer
supporting new MkLinux development.