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/ Primary Hierarchy and Root Directory of the entire file system.
/bin Essential user command binaries (executable Programs To Bring The System Up Or Repair It).
/boot Static Files OF The Boot Loader (stores data used before the kernel begins executing user-mode programs). Kernel image place.
/dev Special or Device Files. Contains special block and character files associated with hardware devices. Here are all the physical devices (hardware) of the system.
/etc Contains system-wide Host-specific system configuration files.
/home Contains all directories, saved files, personal settings, etc. of all users, except superuser (administrator, root), inside its own directory in this folder.
/lib Essential Shared Libraries and Kernel Modules.
/media Mount Point For Removable Media Devices.
/mnt Mount Point For Mounting A Filesystem Temporarily. Mount files for temporaty filesystems.
/opt Optional add-on Applications Software Packages.
/proc Process Information (Interface to kernel data structures). Kernel and process information virtual filesystem.
/root Home directory for Root User (Super-user).
/run Data relevant to running processes.
/sbin Essential System Binaries. Commands needed to boot the system, but which are usually not executed by normal users.
/srv Service Data (Site-specific Data Served On System).
/sys Virtual Directory Providing Information About The System (kernel's view of the hardware).
/tmp Temporary Files deleted on bootup.
/usr User Programs (Shareable and Read-only User Files); Second Hierarchy.
/var Variable/Temporary Files (File that is expected to continously change).
See also: GNU/Linux distributions History of GNU/Linux distributions GNU/Linux distributions tree GNU/Linux distributions list

GNU/Linux is a free and open source Unix-like operating system that combines the Linux kernel with the GNU userland.

The GNU part of the name comes from the GNU Project, initiated by Richard Stallman in the early 1980s. The GNU Project aimed to create a free and open source Unix-like operating system, but it lacked a kernel. That’s where Linux kernel came in.

The collaboration between the GNU Project and Linus Torvalds (who initially started the creation of the Linux kernel, which was then improved by the community), led to the development of this powerful and versatile system.

Components of GNU/Linux
  • Kernel: The core component that interacts with hardware and manages system resources.
    • The Linux kernel is responsible for tasks like process scheduling, memory management, and device drivers.
  • GNU Userland: The collection of essential utilities and libraries provided by the GNU Project. These include commands like ls, cp, grep, and many others.
  • Shell: The command-line interface that allows users to interact with the system. Popular shells include Bash, Zsh, and Fish.
  • Windowing system: A windowing system (or window system) is a software suite that manages separately different parts of display screens. These can be:
  • Desktop environment with or without a window manager, or a window manager standalone:
  • Package Managers: Tools for installing, updating, and managing software packages. Common package managers include:
  • Drivers for the correct operation of the hardware of each equipment.


GNU/Linux is typically packaged as a GNU/Linux distribution (distro).


Linux kernel was originally developed for personal computers based on the Intel x86 architecture, but has since been ported to more platforms than any other operating system.

Although GNU/Linux is, as of November 2022, used by only around 2.6 percent of desktop computers, the Chromebook, which runs the Linux kernel based ChromeOS, dominates the US K–12 education market and represents nearly 20 percent of sub-$300 notebook sales in the US. GNU/Linux is the leading operating system on servers (over 96.4% of the top one million web servers operating systems are GNU/Linux), leads other big iron systems such as mainframe computers, and is used on all of the world's 500 fastest supercomputers (as of November 2017, having gradually displaced all competitors).

GNU/Linux also runs on embedded systems, i.e., devices whose operating system is typically built into the firmware and is highly tailored to the system. This includes routers, automation controls, smart home devices, video game consoles, televisions (Samsung and LG Smart TVs), automobiles (Tesla, Audi, Mercedes-Benz, Hyundai, and Toyota), and spacecraft (Falcon 9 rocket, Dragon crew capsule, and the Perseverance rover).

GNU/Linux is one of the most prominent examples of free and open-source software collaboration. The source code may be used, modified, and distributed commercially or non-commercially by anyone under the terms of its respective licenses, such as the GNU General Public License (GPL).

The Linux kernel, for example, is licensed under the GPLv2, with an exception for system calls that allows code that calls the kernel via system calls not to be licensed under the GPL.

GNU Project

The GNU Project is a free software, mass collaboration project announced by Richard Stallman on 27/09/1983.

  • Its goal is to give computer users freedom and control in their use of their computers and computing devices by collaboratively developing and publishing software that gives everyone the rights to freely run the software, copy and distribute it, study it, and modify it.
  • GNU software grants these rights in its license (GPL).
  • In order to ensure that the entire software of a computer grants its users all freedom rights (use, share, study, modify).
  • GNU is an extensive collection of free software, which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operating systems popularly known as GNU/Linux. Most of GNU is licensed under the GNU Project's own General Public License (GPL).
  • GNU is also the project within which the free software concept originated. Richard Stallman, the founder of the project, views GNU as a "technical means to a social end". Relatedly, Lawrence Lessig states in his introduction to the second edition of Stallman's book Free Software, Free Society that in it Stallman has written about "the social aspects of software and how Free Software can create community and social justice".


GNU is a recursive acronym for "GNU's Not Unix!", chosen because GNU's design is Unix-like, but differs from Unix by being free software and containing no Unix code. Stallman chose the name by using various plays on words, including the song The Gnu.


Development of the GNU operating system was initiated by Richard Stallman while he worked at MIT Artificial Intelligence Laboratory. It was called the GNU Project, and was publicly announced on September 27, 1983, on the net.unix-wizards and net.usoft newsgroups by Stallman.

Software development began on January 5, 1984, when Stallman quit his job at the Lab so that they could not claim ownership or interfere with distributing GNU components as free software.

The goal was to bring a completely free software operating system into existence. Stallman wanted computer users to be free to study the source code of the software they use, share software with other people, modify the behavior of software, and publish their modified versions of the software. This philosophy was published as the GNU Manifesto in March 1985.

Richard Stallman's experience with the Incompatible Timesharing System (ITS), an early operating system written in assembly language that became obsolete due to discontinuation of PDP-10, the computer architecture for which ITS was written, led to a decision that a portable system was necessary. It was thus decided that the development would be started using C and Lisp as system programming languages, and that GNU would be compatible with Unix. At the time, Unix was already a popular proprietary operating system. The design of Unix was modular, so it could be reimplemented piece by piece.

Much of the needed software had to be written from scratch, but existing compatible third-party free software components were also used such as the TeX typesetting system, the X Window System, and the Mach microkernel that forms the basis of the GNU Mach core of GNU Hurd (the official kernel of GNU). With the exception of the aforementioned third-party components, most of GNU has been written by volunteers; some in their spare time, some paid by companies, educational institutions, and other non-profit organizations. In October 1985, Stallman set up the Free Software Foundation (FSF). In the late 1980s and 1990s, the FSF hired software developers to write the software needed for GNU.

As GNU gained prominence, interested businesses began contributing to development or selling GNU software and technical support. The most prominent and successful of these was Cygnus Solutions, now part of Red Hat.


The system's basic components include the GNU Compiler Collection (GCC), the GNU C library (glibc), and GNU Core Utilities (coreutils), but also the GNU Debugger (GDB), GNU Binary Utilities (binutils), and the GNU Bash shell. GNU developers have contributed to Linux ports of GNU applications and utilities, which are now also widely used on other operating systems such as BSD variants, Solaris and macOS.

Many GNU programs have been ported to other operating systems, including proprietary platforms such as Microsoft Windows and macOS. GNU programs have been shown to be more reliable than their proprietary Unix counterparts.

As of January 2022, there are a total of 459 GNU packages (including decommissioned, 383 excluding) hosted on the official GNU development site.


The GNU Project maintains two kernels itself, allowing the creation of pure GNU operating systems, but the GNU toolchain is also used with non-GNU kernels. Due to the two different definitions of the term 'operating system', there is an ongoing debate concerning the naming of distributions of GNU packages with a non-GNU kernel.

Kernels maintained by GNU and FSF

Parabola GNU/Linux-libre, an example of an FSF approved distribution that uses a rolling release model

GNU Hurd

The original kernel of GNU Project is the GNU Hurd (together with the GNU Mach microkernel), which was the original focus of the Free Software Foundation (FSF).

With the April 30, 2015 release of the Debian GNU/Hurd 2015 distro, GNU now provides all required components to assemble an operating system that users can install and use on a computer.

However, the Hurd kernel is not yet considered production-ready but rather a base for further development and non-critical application usage.


As of 2012, a fork of the Linux kernel became officially part of the GNU Project in the form of Linux-libre, a variant of Linux with all proprietary components removed. The GNU Project has endorsed Linux-libre distributions, such as Trisquel, Parabola GNU/Linux-libre, PureOS and GNU Guix System.

Non-GNU kernels

Because of the development status of Hurd, GNU is usually paired with other kernels such as Linux or FreeBSD. Whether the combination of GNU libraries with external kernels is a GNU operating system with a kernel (e.g. GNU with Linux), because the GNU collection renders the kernel into a usable operating system as understood in modern software development, or whether the kernel is an operating system unto itself with a GNU layer on top (i.e. Linux with GNU), because the kernel can operate a machine without GNU, is a matter of ongoing debate. The FSF maintains that an operating system built using the Linux kernel and GNU tools and utilities should be considered a variant of GNU, and promotes the term GNU/Linux for such systems (leading to the GNU/Linux naming controversy). This view is not exclusive to the FSF. Notably, Debian, one of the biggest and oldest Linux distributions, refers to itself as Debian GNU/Linux.

Official mascot

Torvalds announced in 1996 that there would be a mascot for Linux, a penguin. This was because when they were about to select the mascot, Torvalds mentioned he was bitten by a little penguin (Eudyptula minor) on a visit to the National Zoo & Aquarium in Canberra, Australia. Larry Ewing provided the original draft of today's well known mascot based on this description. The name Tux was suggested by James Hughes as derivative of Torvalds' UniX, along with being short for tuxedo, a type of suit with color similar to that of a penguin.

Linux kernel

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.

Linux is deployed on a wide variety of computing systems, such as embedded devices, mobile devices (including its use in the Android operating system), personal computers, servers, mainframes, and supercomputers. It can be tailored for specific architectures and for several usage scenarios using a family of simple commands (that is, without the need of manually editing its source code before compilation); privileged users can also fine-tune kernel parameters at runtime. Most of the Linux kernel code is written using the GNU extensions of GCC to the standard C programming language and with the use of architecture-specific instructions (ISA) in limited parts of the kernel. This produces a highly optimized executable (vmlinux) with respect to utilization of memory space and task execution times.

Day-to-day development discussions take place on the Linux kernel mailing list (LKML). Changes are tracked using the version control system git, which was originally authored by Torvalds as a free software replacement for BitKeeper.


Linux began in 1991 as a personal project by Finnish student Linus Torvalds to create a new free operating system kernel. The resulting Linux kernel has been marked by constant growth throughout its history. Since the initial release of its source code in 1991, it has grown from a small number of C files under a license prohibiting commercial distribution to the 4.15 version in 2018 with more than 23.3 million lines of source code, not counting comments, under the GNU General Public License v2 with a syscall exception meaning anything that uses the kernel via system calls are not subject to the GNU GPL.

Events leading to creation

Ken Thompson (left) and Dennis Ritchie (right), creators of the Unix operating system

After AT&T had dropped out of the Multics project, the Unix operating system was conceived and implemented by Ken Thompson and Dennis Ritchie (both of AT&T Bell Laboratories) in 1969 and first released in 1970. Later they rewrote it in a new programming language, C, to make it portable. The availability and portability of Unix caused it to be widely adopted, copied and modified by academic institutions and businesses.

In 1977, the Berkeley Software Distribution (BSD) was developed by the Computer Systems Research Group (CSRG) from UC Berkeley, based on the 6th edition of Unix from AT&T. Since BSD contained Unix code that AT&T owned, AT&T filed a lawsuit (USL v. BSDi) in the early 1990s against the University of California. This strongly limited the development and adoption of BSD.

Onyx Systems began selling early microcomputer-based Unix workstations in 1980. Later, Sun Microsystems, founded as a spin-off of a student project at Stanford University, also began selling Unix-based desktop workstations in 1982. While Sun workstations didn't utilize commodity PC hardware like Linux was later developed for, it represented the first successful commercial attempt at distributing a primarily single-user microcomputer that ran a Unix operating system.

In 1983, Richard Stallman started the GNU project with the goal of creating a free UNIX-like operating system. As part of this work, he wrote the GNU General Public License (GPL). By the early 1990s, there was almost enough available software to create a full operating system. However, the GNU kernel, called Hurd, failed to attract enough development effort, leaving GNU incomplete.[citation needed]

In 1985, Intel released the 80386, the first x86 microprocessor with a 32-bit instruction set and a memory management unit with paging.

In 1986, Maurice J. Bach, of AT&T Bell Labs, published The Design of the Unix Operating System. This definitive description principally covered the System V Release 2 kernel, with some new features from Release 3 and BSD.

In 1987, MINIX, a Unix-like system intended for academic use, was released by Andrew S. Tanenbaum to exemplify the principles conveyed in his textbook, "Operating Systems: Design and Implementation". While source code for the system was available, modification and redistribution were restricted. In addition, MINIX's 16-bit design was not well adapted to the 32-bit features of the increasingly cheap and popular Intel 386 architecture for personal computers. In the early nineties a commercial Unix operating system for Intel 386 PCs was too expensive for private users.

These factors and the lack of a widely adopted, free kernel provided the impetus for Torvalds' starting his project. He has stated that if either the GNU Hurd or 386BSD kernels had been available at the time, he likely would not have written his own.

The creation of Linux

In 1991, while studying computer science at University of Helsinki, Linus Torvalds began a project that later became the Linux kernel. He wrote the program specifically for the hardware he was using and independent of an operating system because he wanted to use the functions of his new PC with an 80386 processor. Development was done on MINIX using the GNU C Compiler.

On July 3, 1991, in an effort to implement Unix system calls in his project, Linus Torvalds attempted to obtain a digital copy of the POSIX standards documentation with a request to the comp.os.minix newsgroup. He was not successful in finding the POSIX documentation, so Torvalds initially resorted to determining system calls from SunOS documentation owned by the university for use in operating its Sun Microsystems server. He also learned some system calls from Tanenbaum's MINIX text that was a part of the Unix course.

As Torvalds wrote in his book Just for Fun, he eventually ended up writing an operating system kernel. On 25 August 1991, he (at age 21) announced this system in another posting to the comp.os.minix newsgroup:

Hello everybody out there using minix -

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since april, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things).

I've currently ported bash(1.08) and gcc(1.40), and things seem to work. This implies that I'll get something practical within a few months, and I'd like to know what features most people would want. Any suggestions are welcome, but I won't promise I'll implement them :-)

Linus (

PS. Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT portable (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(. — Linus Torvalds

According to Torvalds, Linux began to gain importance in 1992 after the X Window System was ported to Linux by Orest Zborowski, which allowed Linux to support a GUI for the first time.


Linus Torvalds had wanted to call his invention "Freax", a portmanteau of "free", "freak", and "x" (as an allusion to Unix). During the start of his work on the system, some of the project's makefiles included the name "Freax" for about half a year. Initially, Torvalds considered the name "Linux" but dismissed it as too egotistical.

To facilitate development, the files were uploaded to the FTP server ( of FUNET in September 1991. Ari Lemmke, Torvalds' coworker at the Helsinki University of Technology (HUT) who was one of the volunteer administrators for the FTP server at the time, did not think that "Freax" was a good name, so he named the project "Linux" on the server without consulting Torvalds. Later, however, Torvalds consented to "Linux".

According to a newsgroup post by Torvalds, the word "Linux" should be pronounced (LIN-uuks) with a short 'i' as in 'print' and 'u' as in 'put'. To further demonstrate how the word "Linux" should be pronounced, he included an audio guide with the kernel source code. However, in this recording, he pronounces Linux as (LEEN-uuks) with a short but close front unrounded vowel, instead of a near-close near-front unrounded vowel as in his newsgroup post.

Linux under the GNU GPL

Torvalds first published the Linux kernel under its own licence, which had a restriction on commercial activity.

The software to use with the kernel was software developed as part of the GNU project licensed under the GNU General Public License, a free software license. The first release of the Linux kernel, Linux 0.01, included a binary of GNU's Bash shell.

In the "Notes for linux release 0.01", Torvalds lists the GNU software that is required to run Linux:

Sadly, a kernel by itself gets you nowhere. To get a working system you need a shell, compilers, a library etc. These are separate parts and may be under a stricter (or even looser) copyright. Most of the tools used with linux are GNU software and are under the GNU copyleft. These tools aren't in the distribution - ask me (or GNU) for more info.

In 1992, he suggested releasing the kernel under the GNU General Public License. He first announced this decision in the release notes of version 0.12. In the middle of December 1992 he published version 0.99 using the GNU GPL. Linux and GNU developers worked to integrate GNU components with Linux to make a fully functional and free operating system. Torvalds has stated, "making Linux GPLed was definitely the best thing I ever did."

Around 2000, Torvalds clarified that the Linux kernel uses the GPLv2 license, without the common "or later clause".

After years of draft discussions, the GPLv3 was released in 2007; however, Torvalds and the majority of kernel developers decided against adopting the new license.