All modern operating systems have their roots in 1969 when Dennis Ritchie and
Ken Thompson developed the C language and the Unix operating system at AT&T
Bell Labs. They shared their source code (yes, there was open source back in the
Seventies) with the rest of the world, including the hippies in Berkeley California. By
1975, when AT&T started selling Unix commercially, about half of the source code
was written by others. The hippies were not happy that a commercial company sold
software that they had written; the resulting (legal) battle ended in there being two
versions of Unix in the Seventies : the official AT&T Unix, and the free BSD Unix.
In the Eighties many companies started developing their own Unix: IBM created
AIX, Sun SunOS (later Solaris), HP HP-UX and about a dozen other companies did
the same. The result was a mess of Unix dialects and a dozen different ways to do the
same thing. And here is the first real root of Linux, when Richard Stallman aimed
to end this era of Unix separation and everybody re-inventing the wheel by starting
the GNU project (GNU is Not Unix). His goal was to make an operating system that
was freely available to everyone, and where everyone could work together (like in the
Seventies). Many of the command line tools that you use today on Linux or Solaris
are GNU tools.
The Nineties started with Linus Torvalds, a Swedish speaking Finnish student,
buying a 386 computer and writing a brand new POSIX compliant kernel. He put
the source code online, thinking it would never support anything but 386 hardware.
Many people embraced the combination of this kernel with the GNU tools, and the
rest, as they say, is history.
Today more than 90 percent of supercomputers (including the complete top 10), more
than half of all smartphones, many millions of desktop computers, around 70 percent
of all web servers, a large chunk of tablet computers, and several appliances (dvdplayers,
washing machines, dsl modems, routers, …) run Linux. It is by far the most
commonly used operating system in the world.
Linux kernel version 3.2 was released in January 2012. Its source code grew by almost
two hundred thousand lines (compared to version 3.1) thanks to contributions of over
4000 developers paid by about 200 commercial companies including Red Hat, Intel,
Broadcom, Texas Instruments, IBM, Novell, Qualcomm, Samsung, Nokia, Oracle,
Google and even Microsoft.
http://www.levenez.com/unix/ (a huge Unix history poster)