The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally written in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU operating system, which was written to be a free (libre) replacement for Unix.

Linux is provided under the GNU General Public License version 2 only, but it contains files under other compatible licenses. Since the late 1990s, it has been included as part of a large number of operating system distributions, many of which are commonly also called Linux.

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A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and often a package management system. Linux users usually obtain their operating system by downloading one of the Linux distributions, which are available for a wide variety of systems ranging from embedded devices (for example, OpenWrt) and personal computers (for example, Linux Mint) to powerful supercomputers (for example, Rocks Cluster Distribution).

A typical Linux distribution comprises a (usually modified) Linux kernel, an init system (such as systemd, OpenRC, or runit), GNU tools and libraries, documentation, and many other software (such as IP network configuration utilities and the getty TTY setup program, among others). Optionally, to provide a desktop experience (most commonly the Mesa userspace graphics drivers) a display server (the most common being the X.org Server, or, more recently, a Wayland compositor such as Sway, KDE's KWin, or GNOME's Mutter), a desktop environment, a sound server (usually either PulseAudio or more recently PipeWire), and other related programs may be included with the distribution or are installable by the user.

In broad terms, Linux distributions may be:

• Commercial or non-commercial

• Designed for enterprise users, power users, or for home users

• Supported on multiple types of hardware, or platform-specific, even to the extent of certification by the platform vendor

• Designed for servers, desktops, or embedded devices

• General purpose or highly specialized toward specific machine functionalities (e.g. firewalls, network routers, and computer clusters)

• Targeted at specific user groups, for example through language internationalization and localization, or through inclusion of many music production or scientific computing packages

• Built primarily for security, usability, portability, or comprehensiveness

• Standard release or rolling release.

Widely used GNU-based or GNU-compatible distributions

• Debian, a non-commercial distribution and one of the earliest, maintained by a volunteer developer community with a strong commitment to free software principles and democratic project management.

  • Ubuntu, a desktop and server distribution derived from Debian, maintained by British company Canonical Ltd.

    • There are several distributions based on Ubuntu that mainly replace the GNOME stock desktop environment, like: Kubuntu based on KDE, Lubuntu based on LXQT, Xubuntu based on XFCE, Ubuntu MATE based on MATE, Ubuntu Budgie based on Budgie. Other official forks have specific uses like: Ubuntu Kylin for Chinese-speaking users, or Ubuntu Studio for media content creators.

    • Linux Mint, a distribution based on and compatible with Ubuntu. Supports multiple desktop environments, among others GNOME Shell forkCinnamon and GNOME 2 fork MATE.

• Fedora Linux, a community distribution sponsored by American company Red Hat and the successor to the company's previous offering, Red Hat Linux. It aims to be a technology testbed for Red Hat's commercial Linux offering, where new open-source software is prototyped, developed, and tested in a communal setting before maturing into Red Hat Enterprise Linux.

  • Red Hat Enterprise Linux (RHEL), a derivative of Fedora Linux, maintained and commercially supported by Red Hat. It seeks to provide tested, secure, and stable Linux server and workstation support to businesses.

• openSUSE, a community distribution mainly sponsored by German company SUSE.

  • SUSE Linux Enterprise, derived from openSUSE, maintained and commercially supported by SUSE

• Arch Linux, a rolling release distribution targeted at experienced Linux users and maintained by a volunteer community, offers official binary packages and a wide range of unofficial user-submitted source packages. Packages are usually defined by a single PKGBUILD text file.

  • Manjaro Linux, a derivative of Arch Linux that includes a graphical installer and other ease-of-use features for less experienced Linux users.

• Gentoo, a distribution targeted at power users, known for its FreeBSD Ports-like automated system for compiling applications from source code

Files and processes

• Everything in UNIX/Linux is either a file or a process.

• A process is an executing program identified by a unique PID (process identifier).

• A file is a collection of data. They are created by users using text editors, running compilers etc.

• Examples of files:

  • a document (report, essay etc.)

  • the text of a program written in some high-level programming language

  • instructions comprehensible directly to the machine and incomprehensible to a casual user, for example, a collection of binary digits (an executable or binary file);

  • a directory, containing information about its contents, which may be a mixture of other directories (subdirectories) and ordinary files.