What Is Linux? The Kernel, Distributions and Their History

Linux runs most of the internet's servers, every Android phone, almost every container, all of the world's 500 fastest supercomputers, and a large share of routers, TVs and cars. If you work in DevOps, cloud or backend development, Linux is the system your code ends up on, even when your laptop runs macOS or Windows.

This part explains what "Linux" actually means, what a distribution adds, where Linux came from, and how to pick a distribution. The rest of the guide is hands-on.

What Linux Is

Strictly speaking, Linux is a kernel: the program that starts first, talks to the hardware, and shares the CPU, memory, disks and network between all other programs. Linus Torvalds started it in 1991, and thousands of developers and companies maintain it today.

A kernel alone is not usable. You also need a shell, basic commands (ls, cp, grep), a C library, an init system that starts services, a package manager, and often a graphical desktop. A Linux distribution ("distro") puts all of that together, tests it, and ships updates. When people say "I run Linux", they mean a distribution such as Ubuntu, Debian, Fedora or Red Hat Enterprise Linux.

What the Kernel Does

• Processes: it starts programs, decides which one runs on which CPU and for how long, and keeps them apart (Part 7).
• Memory: each process gets its own address space; the kernel maps it to physical RAM and swap, and uses spare RAM as a disk cache.
• Files: it provides one directory tree on top of many file systems and devices (Part 4).
• Networking: interfaces, routing, firewalling and the TCP/IP stack live in the kernel (Part 8).
• Hardware: drivers for disks, network cards, USB, GPUs and everything else, most of them in the kernel source tree.
• Isolation: namespaces and cgroups, the features containers are built on.

Programs ask the kernel for all of this through system calls, such as open, read, fork and connect. Code that runs in the kernel is in kernel space; everything else, including root's programs, runs in user space.

You can ask a running system what it is:

aarch64 is 64-bit ARM, as on a Raspberry Pi, AWS Graviton or an Apple Silicon Mac running a Linux VM; most PCs and cloud servers show x86_64. uname -r prints the kernel version.

GNU/Linux, Android and Other Combinations

Most of the classic command-line tools on a Linux distribution come from the GNU project, which Richard Stallman started in 1983 to build a free Unix-like system. That is why some people say GNU/Linux and why uname -o prints it. The combination is not fixed, though:

• Alpine Linux, popular for small container images, uses BusyBox and the musl C library instead of the GNU tools and glibc.
• Android uses the Linux kernel with a completely different user space.
• Ubuntu 25.10 and later replaced most of the GNU core utilities with uutils, a rewrite in Rust (all of them from 26.10 on), and sudo with sudo-rs.

A Short History

Year	Event
1969	Ken Thompson and Dennis Ritchie create Unix at Bell Labs
1983	Richard Stallman announces the GNU project
1987	Andrew Tanenbaum publishes MINIX, a small Unix-like teaching system
1991	Linus Torvalds, a student in Helsinki, announces his kernel on 25 August and releases version 0.01 in September
1992	Linux moves to the GNU General Public License (GPL) version 2, which it still uses
1993	Slackware and Debian, two of the oldest distributions still maintained, appear
1994	Linux 1.0
2003 to 2004	Red Hat splits into Fedora (community) and Red Hat Enterprise Linux; Ubuntu's first release
2008	The first Android phone ships
2013	Docker makes Linux containers mainstream
2017	Every system on the TOP500 supercomputer list runs Linux, as they all have since
2024	CentOS Linux reaches its end of life; Rocky Linux and AlmaLinux continue as free RHEL rebuilds
2026	Linux 7.0 (April); Linux 7.3 is the current release as of 18 October 2026

The jump from 6.19 to 7.0 did not mean a big change. Torvalds raises the first number when the second one gets large. A new kernel version comes out every nine to ten weeks, and selected versions become long-term support (LTS) kernels with fixes for several years; 6.12 and 6.18 are current examples. Distributions ship their own tested kernel builds and backport security fixes, so a server's kernel version is usually older than the newest release on kernel.org, and that is expected.

Distributions

Every distribution makes its own choices:

• Package format and manager: .deb with apt, .rpm with dnf or zypper, pacman, apk (Part 5).
• Release model: fixed releases (a new version every six months to two years, with only fixes in between) or rolling releases (continuous updates to the newest versions).
• Support lifetime: from about a year to ten years or more.
• Defaults: desktop environment, file system, firewall tool, security framework (AppArmor or SELinux).

Family	Distributions	Notes
Debian	Debian, Ubuntu, Linux Mint, Pop!_OS, Raspberry Pi OS	The largest family. Ubuntu is the most common cloud image
Red Hat	RHEL, Fedora, CentOS Stream, Rocky Linux, AlmaLinux, Oracle Linux, Amazon Linux	Standard in many enterprises. Fedora is the upstream for RHEL; CentOS Stream previews the next RHEL minor release
SUSE	SUSE Linux Enterprise, openSUSE Leap and Tumbleweed	Common in Europe and SAP environments
Arch	Arch Linux, Manjaro, EndeavourOS	Rolling releases, do-it-yourself installation
Independent	Alpine (containers), NixOS (declarative configuration), Gentoo (compiled from source)	Specialised use cases

Support Lifetimes of the Main Server Distributions

Release	Released	Security updates until
Ubuntu 24.04 LTS	April 2024	May 2029 (to 2034 with Ubuntu Pro)
Ubuntu 26.04 LTS	April 2026	May 2031 (to 2036 with Ubuntu Pro)
Debian 13 "trixie"	August 2025	August 2028, then Debian LTS until June 2030
Debian 12 "bookworm"	June 2023	In Debian LTS since July 2026, until June 2028
RHEL 10 (and Rocky, Alma 10)	May 2025	May 2035
Fedora	every six months	about 13 months per release
Amazon Linux 2023	March 2023	June 2029 (Amazon Linux 2 ended in June 2026)

Run a release that still gets security updates, and plan the upgrade before support ends.

Which Distribution Should You Use?

• Learning Linux for DevOps and cloud work: Ubuntu Server 24.04 LTS or 26.04 LTS. It is the most common image on every cloud, and most tutorials assume it. This guide uses Ubuntu 24.04 LTS for its examples, with Debian 13 and Fedora or RHEL 10 where they differ.
• Servers where the vendor or your company requires support: RHEL, SUSE Linux Enterprise or Ubuntu Pro. For free RHEL-compatible servers: Rocky Linux or AlmaLinux.
• Stable servers without a vendor: Debian.
• Desktop or laptop: Ubuntu, Fedora Workstation or Linux Mint.
• Container images: Debian slim or Ubuntu for compatibility, Alpine or distroless images when size matters more than convenience.

The skills transfer. Package commands and a few configuration paths differ between families; the shell, files, processes, users, networking and systemd work the same.

Where Linux Runs

• Web servers: W3Techs counts Linux on 63.2% of the websites whose operating system it can identify, and Unix-like systems (Linux plus servers it can only identify as "Unix") on 92.2% (23 October 2026).
• Supercomputers: all 500 systems on the TOP500 list have run Linux since November 2017.
• Phones: Android uses the Linux kernel.
• Cloud and containers: most cloud VMs run Linux, and the containers you build with Docker or run on Kubernetes are Linux containers, built from Linux kernel features. Docker Desktop on macOS and Windows runs a small Linux VM to provide them, and WSL 2 on Windows is a real Linux kernel in a VM.
• Embedded devices: routers, TVs, cars, set-top boxes and single-board computers.

Open Source and Licenses

The kernel's source code is public and licensed under the GPL version 2: anyone may use, study, change and redistribute it, and whoever distributes a changed version must publish the source under the same license. Much of the rest of a distribution uses the GPL too, and many other projects use permissive licenses such as MIT, BSD or Apache 2.0, which do not require that.

For you as a user, this means you can read exactly what the system does, fix or work around problems yourself, and choose between many distributions built from the same code.

Thinking the Linux Way

A few ideas run through the whole system and through the rest of this guide:

• Small tools that do one job, combined with pipes into bigger ones (Part 3).
• Everything is a file: devices, processes and kernel settings appear as files under /dev, /proc and /sys (Part 4).
• Configuration is plain text in /etc, so you can read it, diff it, keep it in Git and change it with scripts.
• Automate what you repeat: a command you typed twice belongs in a script (Part 9) or a configuration tool.
• Least privilege: work as a normal user and use sudo only when needed (Part 6).

Common Misconceptions

• "Linux is hard to use." Modern desktop distributions install in minutes. The command line takes practice, which is what this guide is for.
• "Linux and Unix are the same." Linux is a Unix-like clone written from scratch; macOS and the BSDs are closer to the original Unix family. The commands are similar, but options and paths often differ.
• "Containers mean I do not need Linux skills." A Linux container is a Linux process isolated with namespaces and cgroups. Debugging one needs exactly the skills in Parts 3 to 8.
• "The newest kernel is always better." Servers run the kernel their distribution tests and patches.

Summary

• Linux is a kernel; a distribution adds the tools, libraries, init system, package manager and updates.
• The kernel manages processes, memory, files, networking and hardware; programs use it through system calls.
• Distributions differ in package manager, release model and support lifetime. Ubuntu LTS, Debian and RHEL-compatible systems dominate servers.
• Linux is licensed under the GPL version 2 and developed in the open.

Next: Getting Started with Linux, where you get a Linux system to practise on.

Keep Learning

• Try commands in the browser: Linux Terminal simulator
• Linux Fundamentals flashcards
• Linux Command Line Quiz