The Linux kernel is a free and open-source Unix-like kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the kernel for the GNU operating system (OS), which was created to be a free replacement for Unix. Since the late 1990s, it has been included in many operating system distributions, many of which are called Linux. One such Linux kernel operating system is Android, which is used in many mobile and embedded devices. Most of the kernel code is written in C as supported by the GNU Compiler Collection (GCC), which has extensions beyond standard C. The code also contains assembly code for architecture-specific logic such as optimizing memory use and task execution. The kernel has a modular design such that modules can be integrated as software components – including dynamically loaded. The kernel is monolithic in an architectural sense since the entire OS kernel runs in kernel space. Linux is provided under the GNU General Public License version 2, although it contains files under other compatible licenses.
History
In 1991, Linus Torvalds was a computer science student enrolled at the University of Helsinki. During his time there, he began to develop an operating system as a side-project inspired by UNIX, for a personal computer. He started with a task switcher in Intel 80386 assembly language and a terminal driver. On 25 August 1991, Torvalds posted the following to comp.os.minix, a newsgroup on Usenet:
I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since April, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things).I've currently ported bash(1.08) and gcc(1.40), and things seem to work. This implies that I'll get something practical within a few months [...]Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT protable [sic] (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(. On 17 September 1991, Torvalds prepared version 0.01 of Linux and put on the "ftp.funet.fi" – FTP server of the Finnish University and Research Network (FUNET). It was not even executable since its code still needed Minix to compile and test it. On 5 October 1991, Torvalds announced the first "official" version of Linux, version 0.02.
[As] I mentioned a month ago, I'm working on a free version of a Minix-lookalike for AT-386 computers. It has finally reached the stage where it's even usable (though may not be depending on what you want), and I am willing to put out the sources for wider distribution. It is just version 0.02...but I've successfully run bash, gcc, gnu-make, gnu-sed, compress, etc. under it. At the time, the GNU Project had completed many components for its free UNIX replacement, GNU, but its kernel, the GNU Hurd, was incomplete. The project adopted the Linux kernel for its OS. Torvalds labeled the kernel with major version 0 to indicate that it was not yet intended for general use. Version 0.11, released in December 1991, was the first version to be self-hosted; compiled on a computer running the Linux kernel. When Torvalds released version 0.12 in January 1992, he adopted the GNU General Public License version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution. GPL took effect as of 1 February 1992. In contrast to Unix, all Linux source files are freely available, including device drivers. The initial success of Linux was driven by programmers and testers across the world. Because Linux implemented Unix-like system call semantics and exposed the standard POSIX APIs through an implementation of the C standard library, Linux could run software and applications that had been originally developed for Unix.
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