Linux kernel

Linux is an operating system kernel used by a group of Unix-like operating systems. These are known as Linux operating systems.
The Linux kernel is released with the GNU General Public License version 2 (GPLv2) and developed by programmers worldwide.
History
In April 1991, Linus Torvalds, then 21 years old, started working on some simple ideas for an operating system. Then, on 25 August 1991, Torvalds posted to comp.os.minix:
After the post, many people made code for the project. By September 1991, Linux version 0.01 was released. It had 10,239 lines of code. In October 1991, Linux version 0.02 was released.
The newsgroup alt.os.linux was started, and on January 19, 1992, the first post to alt.os.linux was made. On March 31, 1992, alt.os.linux became comp.os.linux.
The X Window System was soon moved to Linux. In March 1992, Linux version 0.95 was the first to be capable of running X. This large version number jump (from 0.1x to 0.9x) was because of a feeling that a version 1.0 with no major missing parts would be released soon.
On March 14, 1994, Linux 1.0.0 was released, with 176,250 lines of code. In March 1995, Linux 1.2.0 was released (310,950 lines of code).
Linus decided, on May 9, 1996, to adopt Tux the penguin as the mascot for Linux.
Version 2 of Linux, released on June 9, 1996, was a landmark. Strong development continued:
January 25, 1999 - Linux 2.2.0 was released (1,800,847 lines of code).
December 18, 1999 - IBM mainframe patches for 2.2.13 were published, allowing Linux to be used on enterprise-class machines.
January 4, 2001 - Linux 2.4.0 was released (3,377,902 lines of code).
December 17, 2003 - Linux 2.6.0 was released (5,929,913 lines of code).
April 16, 2008 - Linux 2.6.25 was released (9,232,484 lines of code).
May 31, 2019 - Linux 5.1.6 was released.
Currently a new kernel version is released in every 2 3/4 months.
Legal aspects
Licensing terms
In the start, Torvalds released Linux under a license which did not allow anyone to sell it. This was soon changed to the GNU General Public License (GPL), during version 0.12. This license allows distribution and sale of modified and unmodified versions of Linux, but requires that all those copies be released under the same license, and be accompanied by the complete corresponding source code.
Torvalds has said licensing Linux under the GPL as the "best thing I ever did".
Related pages
menuconfig
Git (software)
Modprobe
System.map
User space
Userspace header
References
Sources:
(audio tape, abridged ed., read by David Diamond) - on the beginnings of the Linux kernel
Nikolai Bezroukov, Portraits of Open Source Pioneers. Ch 4: A benevolent dictator (Softpanorama e-book).
FreeBSD/Linux kernel source code cross-reference. Browsable Linux (and other operating system) kernel source cross-reference.
LXR: The Linux Cross-Reference project . Browsable Linux kernel source.
KernelHQ - a browsable kernel source tree - with all versions present, and with browsable diffs
The LWN 2001 Linux Timeline
Everyone's Favorite Linux Mascot
Linux Timeline
History of Linux
Upgrade to 2.6 kernel
The Wonderful World of Linux 2.6 ; Joseph Pranevich; December 2003
; Josh Aas, 17 February 2005
LinuxChanges
Seminar Paper on Linux Kernel 2.6
Linux Device Drivers, 3rd Edition
Understanding the Linux Kernel, 3rd Edition (Book)
Linux: The GPL And Binary Modules
Anatomy of the Linux kernel
Other websites
kernel.org - The Linux Kernel Archives, the official kernel repository
Email lists for linux kernel developers
kernelnewbies.org - information for (beginning) Linux kernel developers
Interactive Linux Kernel Map and poster gives you a graphical view of the kernel source
Linux Kernel in a Nutshell - a book about kernel configuration and building
KernelTrap
LWN.net kernel page
Linux Kernel Janitor
Anatomy of the Linux kernel
http://LinuxDriverProject.org/
Linux Kernel Development (April 2008)
Linux