The Unix operating system was conceived and implemented in the 1960s and first released in 1970. Its wide availability and portability meant that it was widely adopted, copied and modified by academic institutions and businesses, with its design being influential on authors of other systems.

One so-called Unix-like system was GNU, started in 1984, which had the goal of creating a POSIX-compatible operating system made entirely of free software. In 1985, Richard Stallman created the Free Software Foundation and developed the GNU General Public License (GNU GPL), in order to spread software freely. Many of the programs required in an OS (such as libraries, compilers, text editors, a Unix shell, and a windowing system) were completed by the early 1990s, although low level elements such as device drivers, daemons, and the kernel were stalled and incomplete.Linus Torvalds has said that if the GNU kernel had been available at the time, he would not have decided to write his own.

MINIX
MINIX, a Unix-like system intended for academic use, was released by Andrew S. Tanenbaum in 1987. While source code for the system was available, modification and redistribution were restricted. In addition, MINIX's 16-bit design was not well adapted to the 32-bit design of the increasingly cheap and popular Intel 386 architecture for personal computers.
In 1991, Linus Torvalds began to work on a non-commercial replacement for MINIX while he was attending the University of Helsinki. This eventually became the Linux kernel.
Linux was dependent on the Minix userspace at first. 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. Linux and GNU developers worked to integrate GNU components with Linux to make a fully functional and free operating system.

Source : Wikipedia