MINIX

MINIX was an inexpensive minimal Unix-like operating system, designed for education in computer science, written by Andrew S. Tanenbaum. As of version 3, MINIX is free and redesigned also for “serious” use.

In 1991 while attending the University of Helsinki, Torvalds, curious about the workings of operating system kernels and frustrated by the licensing of MINIX limiting it to educational use only (which prevented any commercial use) began to work on his own kernel which eventually became the Linux kernel.

Torvalds began the development of Linux on MINIX and applications written for MINIX were also used under Linux. Later Linux matured and it became possible for Linux to be developed under itself.Also GNU applications replaced all MINIX ones because, with code from the GNU system freely available, it was advantageous if this could be used with the fledgling OS. Code licensed under the GNU GPL can be used in other projects, so long as they also are released under the same or a compatible license. In order to make the Linux kernel compatible with the components from the GNU Project, Torvalds initiated a switch from his original license (which prohibited commercial redistribution) to the GNU GPL.Developers worked to integrate GNU components with Linux to make a fully functional and free operating system.

source:wikipedia