Linux

Strictly speaking, Linux is not the whole operating system but the kernel — the program that allocates a machine's resources to the other programs a user runs.¹ Used together, they make a system that is, in essence, GNU plus a Linux kernel — GNU/Linux.¹ Every distribution commonly called Linux is, by the same logic, a distribution of GNU/Linux.¹ Richard Stallman asks that the whole system not be called Linux, since that means attributing the GNU Project's work to someone else.²

It is the combination that users actually run: pairing Linux with the then not-quite-complete GNU system yielded a complete free operating system.² When the kernel was re-released under the GNU GPL in 1992, GNU and Linux together made it possible for the first time to run a PC without non-free software.³ The kernel travelled far beyond the desktop: by his 1999 essay, Torvalds could note Linux in embedded systems, controlling robotic devices, and flying on the space shuttle.⁴ That breadth rests on flexibility: the operating system is widely used in diverse environments because its kernel can be configured flexibly.⁵

By 2000 it was the dominant choice for Internet servers and ISPs, for academia, for hobbyists and for computer-science research.⁶ Free systems such as Debian GNU/Linux ship with literally thousands of applications.⁶ The name itself is protected: it is registered as a trademark controlled by Linus Torvalds, with use governed by the Linux Trademark Institute.⁷ The mascot has its own terms: Larry Ewing created Tux the penguin with GIMP, and it may be used even commercially, as long as he receives proper credit.⁷

The story starts earlier: in September 1983 Richard Stallman announced a plan for GNU, a free software, Unix-like operating system.³ The name is a recursive joke: GNU stands for Gnu's Not Unix, the complete Unix-compatible system he set out to give away free to anyone who could use it.⁸ Work began in earnest in January 1984, when Stallman resigned from MIT and started writing GNU software.² The departure was deliberate: had he stayed on the MIT staff, the institute could have claimed ownership of the work and imposed its own distribution terms.²

Distribution terms were the point: the GNU Manifesto puts GNU in a protected commons — everyone may modify and redistribute it, but no distributor may restrict further redistribution, and proprietary modifications are ruled out.⁸ The goal was to give users freedom, not merely to be popular, so the project needed terms that would prevent GNU software from being turned into proprietary software; the method is called copyleft.² Its central idea is that everyone may run, copy, modify and distribute the program, but no one may add restrictions of their own.² The specific implementation for most GNU software is the GNU General Public License, or GNU GPL.²

The Free Software Foundation followed in 1985, founded as a tax-exempt charity for free software development.² Software counts as free when its users hold the four essential freedoms.⁹ Freedom 1 covers studying how the program works and changing it so it does your computing as you wish, with access to the source code as a precondition.⁹ Freedom 3 covers distributing copies of modified versions so the whole community can benefit from them.⁹ None of that means noncommercial: a free program must be available for commercial use, commercial development and commercial distribution.⁹

By 1990 the GNU system had only one major piece missing: the kernel.² The intended replacement, the Hurd, is a collection of servers that run on top of Mach and do the various jobs of the Unix kernel.² It started working reliably only in 2001, still a long way from ready for general use.¹ Linux had already filled the gap: Torvalds' Unix-compatible kernel, written in 1991, started out proprietary and became free software in 1992.² The result of joining Linux to that almost-finished GNU system was a complete free operating system.²

The kernel ships under version 2 of the GNU GPL, a licence whose own history runs from 1989 to 2007.³ Version 3 arrived on 29 June 2007 with rules against tivoization, and the question of adopting it went to the Linux developers.³ The result was the split described in the box below.³

The founding document of Linux is a Usenet post: on 25 August 1991 Linus Torvalds announced a free operating system for 386(486) AT clones, describing it as just a hobby that would not be big and professional like GNU.¹⁰ The project had been brewing since April and was starting to get ready.¹⁰ By announcement time bash 1.08 and gcc 1.40 had been ported, and things seemed to work.¹⁰ He also stressed that the system was free of any minix code and had a multi-threaded file system.¹⁰

The first numbered release, 0.01, contained no binaries at all — just source code for people curious about what linux looked like.¹⁰ Version 0.02 followed on 5 October 1991, with sources placed at nic.funet.fi in the directory /pub/OS/Linux.¹⁰ It still required minix-386 to be compiled and minix to be set up, so it was not yet a standalone system.¹⁰ Early copyright was also stricter than the GNU copyleft — no money at all could change hands because of linux — a rule relaxed with 0.12.¹⁰

By December 1991 the kernel had reached version 0.11, still in beta yet already with a small following.¹⁰ Version 0.12 arrived on January 5th with major corrections, and proved very stable — no patches were needed for a long time.¹⁰ That was the release that made it: 0.12 is when linux started to spread a lot faster.¹⁰

In early 1992, with the Linux discussion having taken over comp.os.minix, Andrew Tanenbaum decided it was time to comment on Linux.¹¹ His post of 29 January 1992 was titled LINUX is obsolete and argued that, among people who actually design operating systems, the debate was over: microkernels had won.¹¹ He contrasted MINIX, a microkernel-based system, with LINUX, which he called a monolithic style system and a giant step back into the 1970s.¹¹ Torvalds' reply conceded the charge: linux was monolithic, and he agreed that microkernels are nicer.¹¹

Still, Linux was never a copy of anything: it aims to provide a Unix-compatible interface, but the kernel was written from scratch without reference to Unix source code, so it is not a port of Unix.⁴ Portability came slowly — in 1991 Torvalds judged porting impossible, since the kernel leaned on every conceivable feature of the 386 he could find.¹⁰ Later versions changed that, and linux was in fact ported to other architectures.¹⁰ The first serious port went to DEC's Alpha: it started in 1993, took about a year to complete, and established design principles that made later ports easier.⁴

Development kept growing: with the 2.0 kernel Linux grew up a lot, and loadable kernel modules were added at that point.⁴ On licensing, Torvalds decreed that system calls would not count as linking against the kernel, so programs running on top of Linux are not covered by the GPL.⁴ One early reason for the licence choice was the compiler: the GPL was the licence of the GCC compiler.⁴ By 1999 the kernel ran from PalmPilots to Alpha workstations, making it the most widely ported system available for PCs.⁴

The dated record below begins with GNU's announcement in September 1983.³ It runs through the 1991 announcement, the 1992 relicensing and the GPLv3 split to the longterm kernel 6.12.¹² The distribution table that follows shows how that kernel reaches users as products and services.¹³

The release machine is regular: the mainline tree, where new features are introduced, is maintained by Linus Torvalds, and a new mainline kernel appears every 9–10 weeks.¹² Each release opens a 2-week merge window during which new major features are introduced into the kernel.¹² The merge window then closes and a 7-week stabilisation phase begins, publishing a release candidate each week.¹² rc7 is usually the last release candidate — pre-releases carry the -rc suffix and must be compiled from source.¹²

Maintenance is layered: a designated maintainer backports fixes from the mainline tree to the stable kernel.¹² Stable updates appear on an as-needed basis, usually once a week.¹² Longterm kernels take only important bugfixes and therefore do not see very frequent releases, especially the older trees.¹² Their projected EOL dates are not set in stone: each new longterm kernel usually starts with a 2-year projected end of life that can be extended if there is enough interest from industry.¹²

The flagship example is 6.12: Greg Kroah-Hartman and Sasha Levin maintain it, and it was released on 17 November 2024 with a projected end of life in December 2028.¹² Distributions also maintain their own longterm kernels; those are not hosted at kernel.org and carry no support from kernel developers.¹² The site itself is operated by the Linux Kernel Organization, a 501(c)3 nonprofit corporation.¹⁶ One quirk is worth remembering: the major version number means nothing — it is incremented when the number after the dot starts looking too big, and there is literally no other reason.¹²

The kernel sits on the older licence: GNU GPL version 2, which the Free Software Foundation defines as Free Software.⁷ Version 3 took years: Stallman began revising the GPL in 2005, and in January 2006 the FSF opened a systematic public review with legal support from the Software Freedom Law Center.³ The FSF released version 3 on 29 June 2007.³ Its flashpoints were about control rather than features: it does not restrict a program's features and does not prohibit DRM, but it prohibits tivoization and Treacherous Computing used to stop users from changing the software.³

The FSF was clear that the licensing of Linux would be decided by the developers of Linux.³ With them staying on version 2, a GNU/Linux system ends up mixed: GNU packages using GPL version 3 alongside a Linux kernel that remained on GPL version 2.³ Red Hat's filing reads the business meaning: licences such as the GPL impose significant limits on a distributor's ability to license derivative works under more restrictive terms and generally require the distributor to disclose the source code of such works.¹³ A further consequence is firmware: it is not part of Linux and is not executed by the kernel, merely uploaded to a device during the driver initialisation stage.⁷

Firmware blobs now live in their own repository, linux-firmware, moved out of the main Linux tree to keep licensing clear.⁷ A fully free setup stays possible but expensive: Linux runs without non-free firmware binaries, usually at the cost of rendering much hardware inoperable.⁷ The GPL remains the most widely used free software licence; almost three quarters of all packages are distributed under it.³

Red Hat describes itself as a leading global provider of open source software solutions, using a community-powered approach to develop and offer its technologies.¹³ Its flagship, Red Hat Enterprise Linux, is an operating system built with open source software components, including the Linux kernel, and designed expressly for enterprise computing.¹³ The filing credits Torvalds and a relatively small group of engineers, many not employed by Red Hat, with the kernel's development and evolution — the heart of that operating system.¹³ Customers pay for support: they agree to purchase a support subscription for each unit of Red Hat software they deploy.¹³

Entry looks easy, and the filing says why: financial and legal barriers to creating a new Linux distribution are relatively low, since the components are publicly available under licences permitting copying, modification and redistribution.¹³ Competition is constant: Red Hat, SUSE, Amazon and Oracle all ship enterprise Linux kernels, and vendors compete with free options such as CentOS, Debian, Fedora, openSUSE and Ubuntu.¹³ Growth nonetheless: total revenue increased from $2.92 billion in fiscal 2018 to $3.36 billion in fiscal 2019.¹³ In a world where GPL terms limit traditional proprietary models, Red Hat treats its trademarks as possibly its most valuable intellectual property.¹³

The corporate story turned in 2019, when IBM's acquisition of Red Hat closed on 9 July 2019, funded through a combination of debt and cash, with incremental debt issued earlier that year.¹⁵ IBM frames the target in cloud terms: Red Hat is, in its words, the world's leader in open source technology, including Enterprise Linux — the operating system of the cloud — as well as containers and OpenShift.¹⁵ IBM's own forecast is Linux-shaped: as clients move more mission-critical workloads to the cloud, their multi-cloud environments will be based on a foundation of Linux, with Kubernetes to deploy, manage and scale container-based applications.¹⁵ A guide of the same era captured the momentum: Linux, started from scratch not even 10 years earlier, had been the favorite kernel of the Free Software world.⁶

Much of Linux's early history survives in the principals' own words, from Usenet posts to essays and project pages.¹⁰ The exchange below set the tone: a modest announcement, a harsh rebuke, and a reply that conceded the theory.¹¹ The licence and community statements that follow show how the project described itself to the outside world.⁷

Read together, the quotations mark the distance from a hobby announcement to an operating system the industry builds on.¹⁰ The community, in Torvalds' account, matters as much as the code itself.⁴

The figures here are snapshots, each stamped with the source that recorded it.¹⁶ The volatile rows — the current kernel versions — carry their September 2026 dates.¹⁶ Two figures frame the whole article: GNU's share of a distribution's source code against Linux's, and the GPL's reach across free software.¹

Configurability is the engine of that breadth: Linux is widely used in diverse environments because its kernel can be configured flexibly.⁵ A 2025 study in ACM Transactions on Software Engineering and Methodology analysed the kernel's configurability across its feature model's entire history, from 2002 to 2024.⁵ A decade earlier, a 2015 study in Empirical Software Engineering mined all known distributed version control systems to show how the official kernel repository's code history diverges from the complete development history.¹⁷ Those systems — git and mercurial, with Github and Bitbucket — revolutionised collaboration by letting developers exchange and integrate code changes peer-to-peer.¹⁷

Development has not slowed: the latest stable kernel recorded here is 7.2.6, dated 14 September 2026, with mainline 7.3-rc3 dated 13 September 2026.¹⁶ The longterm machinery keeps older lines alive at the same time: 6.12 is projected to be maintained until December 2028.¹² The first open question is non-free firmware, and the price of refusing it is hardware support.⁷ The second is who decides the kernel's licensing — in the FSF's account, that decision belonged to the developers of Linux, and they stayed on GPLv2.³

Market structure is the third: Red Hat's filing lists AWS with Amazon Linux 2, EQT Partners with SUSE Linux Enterprise Server and Oracle with Oracle Linux among the competition, alongside freely available distributions.¹³ The community layer is the counterweight: the Linux Foundation says it helps companies and developers identify and contribute to the projects that matter, with the open source community addressing the challenges of industry and technology for the benefit of society.¹⁸ Torvalds' own 1999 formulation still fits: the strength of Linux owes as much to the community of cooperation behind it as to the code itself.⁴ Even the earliest framing survives: Linux really exists, but it remains only one part of the system people use.¹