Textbook of Linux

Also known as: GNU GPL, General Public License

The GNU General Public License (GPL) is the foundational copyleft licence of the free software movement. Drafted by Richard Stallman with the help of lawyer Eben Moglen, it grants recipients the rights to use, study, modify, and redistribute the software—but requires that any distributed modified version be licensed under the same terms. This condition, known as copyleft, turns copyright into its own opposite: instead of restricting use, it enforces openness.

Three major versions exist. GPLv1 (1989) established the basic rules. GPLv2 (1991) is the licence under which the Linux kernel is released and remains one of the most widely used free-software licences in the world. GPLv3 (2007) added protections against software patents and "tivoisation" (where hardware prevents users from running modified free software). The kernel has deliberately stayed on GPLv2 because of Torvalds's disagreements with v3.

The GPL's viral or reciprocal character has profound consequences. A company that incorporates GPL code into a product and distributes binaries must also make the corresponding source available to recipients. This has produced an enormous commons of interoperable code but has also led companies to avoid GPL code in favour of more permissive licences like MIT, BSD, or Apache. Variants such as the LGPL (Lesser GPL) weaken the copyleft requirement for library linking.