Who are the authors of the pure Linux kernel from scratch, which was integrated with GNU tools and formed the full GNU/Linux Operating system in the 1990s? I have read some wiki articles but I haven't got any clear cut idea on the history.
The wikipedia page has a fairly clear history. Linus Torvalds, then a student, wrote his own kernel in the summer of 1991 because he was unhappy with the available Unix kernels: Unix itself (with the Bell Labs code) was extremely expensive (even PC unices such as Xenix), there was Andrew Tanenbaum's MINIX but it was only available to purchasers of Tanenbaum's book, and Torvalds was unaware of the effort led by Berkeley University to produce a free Unix (BSD), and BSD didn't run on PCs yet at the time.
Since then, thousands of people have contributed to the kernel, most of them in the form of drivers.
Richard Stallman, father of the GNU Project
The real story is :
Year 1991 :
DOS brought by Bill Gates was reigning the world of personal computers. The other player in the personal computer world was UNIX by Bell Labs, but it was extremely expensive and the source was not publicly available.
Then there was the MINIX by Andrew Tanenbaum, which was not a superb OS but made the source code was publicly available. Tanenbaum captured the souls of computer science with the elaborate and lively discussion of the art of creating a working OS. Students of Computer Science all over the world went through the book, reading through the codes to understand the very system that runs their computer, and one of them was Linus Torvalds.
The GNU project created a lot of the tools line GCC, etc. but still there was no OS.
For rest of the story and how Linux was written please read through the following link.
Linux History Time line :
I have had the privilege to hear RMS (Richard M. Stallman) and Linus Torvalds. In RMS's own words, Linus made the kernel and from the e-mail that is now well-known on the comp.unix.os newsgroup http://www.linux.com/news/software/linux-kernel/734956-linuss-famous-email was to make it a toy project. The similarities between the kernel project and the GNU utilities project is that both use a derivative of the GPL license, the monolithic kernel being mostly a GPL2 with some parts in GPLv3 while most GNU projects are strictly GPL2.
Off-topic to the question but still relevant - the GNU project has its own 'Hurd' kernel project where the user is never supposed to touch the core kernel and would just be touching the various sub-systems to do whatever he/she desires.This is supposed to increase security and is bit more modular and can be experienced nowadays via a VM. Once we see it in actual devices (maybe forever) we will really know what the real-world performance is like.
I am also not sure what Linus made (kernel 0.1) should be termed as an OS. Typically an OS would having a kernel, having one or more compilers, bunch of utilities and a user-facing interface which essentially is what a GNU/Linux distribution is.
Just my 2 paise.