Textbook of Linux

Also known as: GNU/Linux, Linux kernel

Linux is, strictly speaking, an operating system kernel—the low-level program that mediates between applications and hardware. Colloquially, "Linux" also refers to the full operating systems built around that kernel, which combine it with GNU userland tools, a C library, an init system, and thousands of other components. Linus Torvalds began the kernel in 1991 as a personal project at the University of Helsinki, announcing it to the comp.os.minix newsgroup with the now-famous line that it was "just a hobby, won't be big and professional like gnu."

Linux today runs on a staggering range of devices: the overwhelming majority of the world's web servers, every Android phone, most supercomputers, embedded devices from routers to smart TVs, scientific instruments, and a small but devoted minority of desktops. Its dominance arises from a combination of technical qualities—portability, stability, performance—and a licensing model (the GPL) that permits anyone to use, modify, and redistribute the code.

A Linux system is typically assembled as a distribution: a curated collection of the kernel plus user-space software, packaging tools, and a release process. Debian, Ubuntu, Fedora, Arch, and Alpine are among the most widely used. Because Linux is POSIX-like and largely compatible with historical Unix, decades of accumulated knowledge and software transfer to it with minimal friction.