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 contributed code to 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.

A 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 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