| Mid- to Late-60's | Initial development @ Bell Labs |
| '71 | 1st Edition |
| '73 | Rewritten in C. Portable |
| '75 | Available outside Bell Labs, including UC Berkeley |
| '70s-'80s | Many versions, e.g., Sys V, BSD 4.2, SunOS, HP-UX, Ultrix, etc. |
| '84 | 100,000 installations around the world BSD 4.2, including TCP/IP beginnings of GNU |
| '86 | 250,000 installations |
| '87 | 750,000 installations |
| '88 | POSIX.1 |
| '91 | Linus Torvalds begins work on Linux kernel |
| '94 | Separation of code streams (proprietary vs not) |
| '99 | 30th Anniv of UNIX; Linux 2.2 |