Linux Kernel (1991) 4 Linux

Linux, originally created by the Finnish programmer Linus Torvalds, is a Unix-like operating system loosely inspired by Minix. The pivotal difference, however, was how it was licensed. Unlike Unix operating systems that remained expensive and proprietary, Linux and its source code were licensed for free distribution, with the provision that all changes or additions to the source code must also be released for free.

• 0.x

Xenix (1984) 37 Linux

Xenix was the variant of UNIX originally published by Microsoft, later sold to SCO. It added a variety of technical enhancements over System V Unix, including a menu-driven business shell. It was ported to many different platforms from a PDP-11 including the Altos 8600 (First x86 port), IBM PC, Intel System 86, TRS-80 Model 16, SCP Gazelle II, and Apple Lisa. The Xenix Software Development System, Microsoft Multiplan 2.x and Microsoft Multiplan 3.x, Microsoft Word 3.0 and Microsoft Word 5.x, and FoxBase Plus 1.00

• IBM PC 2.00