Qwerks of History
Software, as the word itself suggests, is more malleable than hardware, and so one might think it would also be more ephemeral. The history of Unix suggests otherwise.
Unix began as a small-scale, low-budget project of Dennis M. Ritchie and Ken Thompson at Bell Laboratories. It has grown and prospered. Today it is the software of choice for scientific and engineering workstations and for multicomputer grids and clusters; it also runs many Internet servers and other "back office" machines. The Linux variant is installed on several million PCs. Recently the Macintosh has become yet another Unix box. Furthermore, Unix has exerted an unmistakable influence on other operating systems, not least Microsoft Windows. In certain crucial internal structures, such as the organization of files into nested directories, almost all computer systems today have adopted methods that were first introduced in Unix.
The early versions of Unix were written for minicomputers in the PDP series, made by Digital Equipment Corporation. Many of those machines had only a few kilobytes of memory; disk storage was equally scant; the standard means of communicating with the computer was a hard-copy teletypewriter or a text-only video terminal. Looking back from our present abundance of gigabytes and megapixels, it seems remarkable that a complex software system could be made to work at all on such primitive machinery. That it would still be running today—and would be widely regarded as the best of its kind—is astonishing.
Of course the Unix of today is not your grandfather's operating system. If you wish, you can still invoke programs from a 24-by-40-character terminal (and some thoughtful people insist that this "command line" remains the best interface), but there are also glitzy window-and-mouse alternatives. Under the hood, the internals of the system have been rewritten over and over—so much so that Unix has to be seen as a kind of standing wave, a pattern that persists even though all of its substance is constantly in flux. Quite possibly, not one line of code has been preserved from the earliest versions. (In the case of Linux, lawyers are currently wrangling over this very question.) Nevertheless, the essence-of-Unix has remained quite stable through all the transformations.