Textbook of Linux

Also known as: GNU Project

GNU ("GNU's Not Unix", a recursive acronym) is a free software project launched by Richard Stallman in 1983 with the goal of creating a complete Unix-like operating system composed entirely of free software. By 1991, GNU had produced a C compiler (GCC), an editor (Emacs), a shell (bash), and a large collection of utilities (coreutils, binutils, findutils, grep, sed, tar)—but its own kernel, Hurd, was incomplete.

Linus Torvalds's Linux kernel filled that gap. Combined with the GNU tools, it produced the first practical, fully free Unix-like operating system. For this reason many insist on the name "GNU/Linux" for the overall system: the kernel is Linux, but most of the commands a user types—ls, cp, cat, bash—come from GNU. Distributions such as Debian officially call themselves GNU/Linux.

GNU also produced the GNU General Public License (GPL), a copyleft licence requiring that derivative works remain under the same terms. The GPL is arguably GNU's most enduring contribution: it creates a legal commons in which code, once freed, cannot be re-enclosed by downstream distributors. Stallman founded the Free Software Foundation in 1985 to steward GNU and advocate for software freedom.