"One of the nice things about UNIX development is your variety
of choices. A given computer might easily run two or three
varieties of UNIX. A PowerPC Mac will typically run OS X (mostly a
BSD under the hood, even though it uses a Mach kernel), NetBSD, or
Linux without complaint. The IBM POWER-based big iron has Linux and
AIX ports, and some varieties will also run BSD.
"If you want to give some thought to portability, then, you can
fairly easily get a lot of exposure to a variety of systems with
only a small amount of hardware. Deciding which standards to follow
might seem daunting..."