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What is Linux? The free operating system, explained

Linux is a free, open-source operating system kernel that Linus Torvalds began in 1991. Packaged with other software, it runs most web servers, every Android phone and the world's supercomputers.

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An AI model, Claude Opus 5.5, wrote it for Graphite from the sources at its end.

· 7 min read

This note and the comments marked the Graphite mark are Graphite’s: an AI model, Claude Opus 5.5, wrote them on 5 November 2026, following Graphite’s own instructions, from the sources at the note’s end, and checked them against those sources. Check them as you would anyone’s.

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Contents · 7 sections

Linux is a free, open-source operating system kernel: the core program that manages a computer's processor, memory, storage and devices so that other software can run. The Finnish student Linus Torvalds started it in 1991. Combined with other free software into packages called distributions, Linux is a complete operating system, and it runs most of the world's web servers, every Android phone and all of the fastest supercomputers.

In short

  • Strictly, Linux is a kernel, the part of an operating system that talks to the hardware; a usable system adds tools, a desktop and apps on top.
  • It is released under the GPL, a license that lets anyone use, study, change and share it, as long as they pass on the same freedoms.
  • Linux is rare on desktop PCs but runs much of the internet, the cloud and Android.

What is a kernel, and how is it different from an operating system?

A kernel is the one program that has complete control of the machine. It decides which program gets the processor and for how long, hands out memory, keeps programs from reading each other's data, and talks to disks, screens, keyboards and networks through drivers. Every other program has to ask the kernel for these things, through requests called system calls. As kernel.org puts it, the kernel is "just a component in a working Linux system."

An operating system is the kernel plus everything a person needs to use the computer: a command shell, basic tools for copying files and managing users, libraries other programs build on, and usually a graphical desktop. How computers work covers the hardware underneath; the kernel is the layer that turns that hardware into something programs can share.

Worked example: what happens when you show a file

Type cat notes.txt in a Linux terminal and the file's text appears. Behind that, the work splits between user programs and the kernel:

  1. The shell reads what you typed. It is an ordinary program, usually Bash, from the GNU project.
  2. The shell asks the kernel to start a new process running the cat program. The kernel sets aside memory for it and schedules it on the processor.
  3. cat asks the kernel to open the file. The kernel checks that you're allowed to read it, finds it on the disk through its file-system code, and hands back a number that stands for the open file.
  4. cat asks the kernel to read it. The kernel's disk driver fetches the data, or takes it from memory if it was read recently, and copies the bytes into cat's memory.
  5. cat asks the kernel to write the bytes to the terminal, which draws them on the screen.
  6. cat exits and the kernel takes back its memory. Meanwhile it has been sharing the processor among hundreds of other processes, many times a second.

The kernel never decides what to show you; it only makes it possible, safely, for programs that do. Linux is that kernel. Bash, cat and the desktop are other software.

Where did Linux come from?

Linux grew out of two older projects. From 1969, Bell Labs built Unix, an operating system whose design, small tools combined into larger tasks, many later systems copied. In 1983 the programmer Richard Stallman announced the GNU project to build a free Unix-like system, in which "free" meant users' freedom to run, study, change and share the software. By 1991 GNU had a compiler, a shell and many tools, but its own kernel wasn't ready.

On 25 August 1991, Torvalds, a 21-year-old student at the University of Helsinki, posted to an online newsgroup that he was writing a free operating system kernel for PCs, "just a hobby, won't be big and professional like gnu". The first version came out that September. In 1992 he released it under GNU's General Public License (GPL), version 2, which allows anyone to use and change the code but requires anyone who distributes a changed version to share its source code too. The combination of the Linux kernel with the GNU tools gave the world a complete free operating system.

The name came from Ari Lemmke, who ran the university server that hosted the files and named the folder "linux"; Torvalds had wanted to call it Freax. Torvalds still leads kernel development, now working for the Linux Foundation, while thousands of developers, many paid by technology companies, contribute changes.

Still argued

What to call the whole system is a long-running argument. The Free Software Foundation holds that it should be "GNU/Linux," because the GNU tools are a large part of it and calling it Linux erases the free-software ideas behind it. Most users, companies and Torvalds himself just say Linux, partly because Android and many devices use the Linux kernel without the GNU tools at all.

What is a Linux distribution?

A distribution, or distro, is a complete operating system built around the Linux kernel: the kernel, the GNU tools or alternatives, a package manager for installing and updating software, a desktop, and default settings. Different groups make different choices for different users. Debian, begun in 1993, is run by volunteers. Red Hat Enterprise Linux is a commercial product aimed at businesses. Ubuntu, from the company Canonical since 2004, is built on Debian. Fedora, Arch, Mint and hundreds of others make other trade-offs.

Software that runs on one distribution usually runs on the others, because they share the kernel and most libraries. The differences are mostly in how software is installed, how often it updates and who supports it.

Where does Linux run?

Linux is everywhere that people don't see it. On 5 November 2026, the survey company W3Techs found Linux on 63.3% of websites whose operating system it could identify, and Unix-family systems, Linux among them, on 92.2%. Most cloud computing runs on Linux servers. On the twice-yearly TOP500 list of the fastest supercomputers, every one of the 500 systems had run Linux on every list since November 2017, Network World reported in 2020.

Android, the world's most widely used phone operating system, is built on a modified Linux kernel, according to Android's own documentation, though it uses its own software on top instead of GNU's. Linux also runs Chromebooks, many home Wi-Fi routers, smart TVs, cars' entertainment systems and the many small devices of the Internet of Things. On desktop and laptop PCs, by contrast, it has only a small share.

How does Linux compare with Windows and macOS?

Linux Windows macOS
Who makes it Kernel by thousands of contributors led by Torvalds; distributions by communities and companies Microsoft Apple
License Free and open source; kernel under GPL version 2 Proprietary; a paid license, usually bundled with the PC Proprietary, included with Apple computers; parts of its core are open source
Kernel Linux Windows NT XNU, which combines Mach and BSD code
Relation to Unix Unix-like, written from scratch A separate lineage Built on BSD Unix code; most versions since 2007 certified as UNIX by The Open Group
Where it dominates Servers, cloud, supercomputers, phones (via Android), embedded devices Desktop and laptop PCs Apple's Mac computers

The biggest practical differences for a user are software and support. Microsoft Office and many games run natively on Windows, and many creative apps on macOS; Linux has free alternatives for most tasks. What Linux offers is control: the source is open, the system can be changed down to the kernel, and most of it costs nothing.

Is Linux only for programmers?

No, though it is often thought so. Anyone with an Android phone uses the Linux kernel every day without typing a command. Modern desktop distributions install from a USB stick and work with a mouse, much like Windows or macOS. It is true that Linux rewards people who want to look under the hood, and that most servers are run from the command line, which is one reason developers learning languages such as Python meet it early. For running websites, Linux is the default: much of the internet is served from it.

Check yourself

  • Without looking back, explain what a kernel does that a shell or a desktop doesn't.
  • What does the GPL require of someone who distributes a changed version of Linux?
  • Why can Android be built on Linux and still look nothing like a Linux desktop?

Sources

Published by Graphite on , set the way Graphite publishes a note.

This note and the comments marked the Graphite mark are Graphite’s: an AI model, Claude Opus 5.5, wrote them on 5 November 2026, following Graphite’s own instructions, from the sources at the note’s end, and checked them against those sources. Check them as you would anyone’s.

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Graphite

An AI model, Claude Opus 5.5, wrote it for Graphite from the sources at its end.

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