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distributions

(Written by Paul Cobbaut, https://github.com/paulcobbaut/ with contributions by Bert Van Vreckem https://github.com/bertvv/)

This chapter gives a short overview of current Linux distributions.

A Linux distribution is a collection of (usually open source) software on top of a Linux kernel. A distribution (or short, distro) can bundle server software, system management tools, documentation and many desktop applications in a central secure software repository. A distro aims to provide a common look and feel, secure and easy software management and often a specific operational purpose.

Let\'s take a look at some popular distributions.

Linux and GNU

The Linux Kernel project was started by Linus Torvalds in 1991 while he was a computer science student. He wanted to run a UNIX-like operating system on his own PC. Now, a kernel in itself is not a complete operating system. The kernel does not provide a terminal, tools to manage files, etc. However, the GNU project (which stands for GNU's Not UNIX), started by Richard Stallman, had been working on a complete operating system since 1983. The GNU project had a lot of the necessary tools and libraries to make a complete POSIX-compliant operating system, a.o. the GNU Compiler Collection (GCC), the GNU C Library (glibc), the GNU Core Utilities (coreutils), the GNU Bash shell, etc. They were also working on a kernel, called GNU Hurd, but development was prohibitively slow. Indeed, it was not until 2015 that the Hurd kernel was ready to be actually used.

Long story short, the Linux kernel in combination with the GNU tools and libraries made a complete operating system. This is why the operating system is often referred to as GNU/Linux. Both Linux as the GNU projects are open source and released under the GNU General Public License. This made it easy for third parties to redistribute GNU+Linux and add other compatible (open source) software packages to form a complete operating system with everything an end user needs to be productive on the computer. This is what we call a Linux distribution. The oldest still active distribution is Slackware, which was started in 1993 by Patrick Volkerding. Since then, many distributions have been created, each with their own goals and target audience. Some distributions (or distro's in short) are built from the ground up, but others are based on existing distributions, leading to large "families" of like-minded distro's.

Writing a comprehensive overview of all Linux distributions is way beyond the scope of this course, but it is useful to know about some of the main ones. If you want to know more about a specific distribution, you can check out the DistroWatch website, which is a great resource for information about Linux distributions.

Package management

One of the central and identifying components of a Linux Distribution is the default selection of software and the package management system to install, update and remove software. For most applications, there is choice in the open source world, so different distributions will make different decisions on what to include and what to avoid. Sometimes this is regrettably the cause of dispute and drama in the Linux community, but on the other hand, it is also the driver of a lot of innovation and diversity and it empowers the user with a lot of freedom of choice and control.

The package manager was actually one of the most important innovations that Linux pioneered in. It is a system that keeps track of all the software installed on a computer and allows the user to select and install new applications from online package repositories. Hotfixes or new releases of the software included in a distribution are made available in these repositories and can be downloaded and installed with a single command. This makes it very easy to keep a Linux system up to date and secure. When Apple introduced the App Store in 2008, it was actually a latecomer to the concept of a central secure software repository.

The concept of an open source package repository also enables reuse of software and libraries. Applications don't have to write their own code to do things like read and write files, manage memory, etc. They can use libraries that are already available on the system and that are used by other applications. The package manager also takes care of dependencies, which are other software packages that are required for the software to work. This makes it very easy to install complex software with a single command.

The Red Hat family of distributions

Red Hat is one of the first commercial companies that successfully leveraged open source software as a business strategy. They started in 1993 and grew in the next decades to become a billion dollar company. In 2019, Red Hat was acquired by IBM for 34 billion dollars and it still operates as an independent subsidiary.

The flagship product of Red Hat is Red Hat Enterprise Linux, or RHEL in short. RHEL is a commercial Linux distribution, but on release, the source code is made available. The business model of Red Hat is based on selling support contracts.

RHEL is a stable and secure operating system, with long support cycles, which is why it is widely used in enterprise environments where the stability of IT infrastructure is of paramount importance. Enterprise software vendors that target Linux as a platform, usually certify their software to run on RHEL. This is why RHEL is often used in data centers, cloud environments and other mission-critical systems.

In order to innovate on the RHEL platform, Red Hat is also involved in the development of the Fedora distribution. Fedora is a community-driven project that aims to be a cutting-edge, free and open source operating system that showcases the latest in free and open source software. It is used as a testbed for new technologies that will eventually make their way into RHEL. Fedora has a release cycle of 6 months. Where RHEL is particularly suited as a server operating system, Fedora is an excellent choice as a desktop operating system for power users and IT professionals.

Since RHEL is open source, it is in principle possible to create a compatible clone of RHEL, albeit without the support and without Red Hat branding. This is exactly what the CentOS project did for years. CentOS used to be a community driven project that aimed to be 100% (bug-for-bug) compatible with RHEL and based on the released source code of all software included in RHEL. However, in 2014, Red Hat acquired the CentOS project, and later, they announced that CentOS Linux was going to be replaced by CentOS Stream, which is a rolling release distribution "upstream" of RHEL. This means that CentOS Stream now takes the place between Fedora and RHEL, and it is no longer a 100% compatible clone of RHEL anymore.

This incensed many users and organizations that relied on CentOS as a free and compatible alternative to RHEL. The CentOS project was forked, and the Rocky Linux project was started by Gregory Kurtzer, who was also one of the original founders of CentOS. The goal of Rocky Linux is to be a 100% compatible replacement for CentOS Linux. Likewise, AlmaLinux was started by CloudLinux, another company that was involved in the CentOS project. These RHEL-like distributions are sometimes referred to as "Enterprise Linux" or EL.

Distinctive features of the Red Hat family of distributions are:

  • The use of the RPM package format (Red Hat Package Management) and the dnf package manager
  • The systemd init system
  • The firewalld firewall management tool
  • The SELinux security framework
  • The Anaconda installer
  • The Cockpit web-based management interface
  • Their own container runtimes, runc and crun and management tools podman and buildah (instead of Docker)

Oracle Enterprise Linux is Oracle's commercial Linux distribution, put in the market as a direct competitor to RHEL. Scientific Linux was a community driven project that was used by scientific institutions like CERN and Fermilab, but it was discontinued in 2021. The final maintenance window for Scientific Linux 7 is June 30, 2024. After that, users are advised to migrate to AlmaLinux. The Amazon Linux distribution is a RHEL-like distribution that is used as the default operating system for Amazon Web Services (AWS) EC2 instances.

The Debian family of distributions

There is no company behind Debian. Instead there are thousands of well organised developers that elect a Debian Project Leader every two years. Debian is seen as one of the most stable Linux distributions. It is also the basis of every release of the well-known Ubuntu (see below). Debian comes in three versions: stable, testing and unstable. Every Debian release is named after a character in the movie Toy Story.

Canonical, a company founded by South African entrepreneur Mark Shuttleworth, started sending out free compact discs with Ubuntu Linux in 2004 and quickly became popular for home users (many switching from Microsoft Windows). Canonical wants Ubuntu to be an easy to use graphical Linux desktop without need to ever see a command line. Of course they also want to make a profit by selling commercial support for Ubuntu. Ubuntu is known for their Long Term Support (LTS) releases, which are supported for 5 years (or 10 years for a fee). Intermediate releases come out every 6 months (in April and October) and are supported for 9 months. Releases are named after the year and month of the release, e.g. 19.10 for October 2019. LTS releases come out every even year in April, e.g. 22.04 and 24.04. Canonical also has the reputation of going their own way and doing things differently from the rest of the Linux community. For example, they developed their own init system, Upstart (which was later abandoned and replaced by systemd), and their own display server, Mir (which was later replaced by Wayland), a desktop environment (Unity, later replaced with Gnome), etc. Some of these decisions were controversial and have led to a lot of criticism, but the strength of the open source community lies precisely in the freedom to make different choices, which is a driver for innovation.

Distinctive features of the Debian family of distributions are:

  • The use of the deb package format and the apt package manager (Advanced Package Tool)
  • The systemd init system
  • The ufw firewall management tool
  • The AppArmor security framework
  • The Debian-installer installer
  • The Docker container runtime and management tools

Linux Mint, Edubuntu and many other distributions with a name ending on -buntu are based on Ubuntu and thus share a lot with Debian. Kali Linux is another Debian-based distribution that is specifically designed for digital forensics and penetration testing. It comes with a lot of pre-installed tools for hacking and security testing. Kali is not suitable for daily use as a desktop operating system, but it is very popular among security professionals and hobbyists. The popular mini-computer Raspberry Pi has its own Debian-based distribution called Raspberry Pi OS.

Notable "independent" distributions

Apart from the two big families of distributions, i.e. Red Hat and Debian families, there are many other distributions that are not based on either of these. Some of the most notable ones are:

  • Alpine Linux: an independent non-commercial, general purpose distribution with a focus on security and simplicity. Alpine Linux is very small and lightweight, and it is often used in containers.

  • Arch Linux: another independent general purpose distribution. Arch Linux is a rolling release distribution, which means that you install it once and then continuously update individual packages when new versions become available. The distribution itself does not have an overarching (see what I did there?) release cycle. Arch has its own package manager, Pacman. One of the most notable features of Arch Linux is its outstanding documentation, which is very extensive and well written and even quite useful for users of other distributions. Installing Arch Linux is not as straightforward as installing other distributions: you start with a minimal system with the kernel and a shell, and then you build up the system to your own liking. This is not for novice users, but it is a great way to learn about the inner workings of a Linux system.

  • openSUSE: a general purpose community driven distribution that is sponsored by SUSE, a German company that also offers commercial support for derivative distro's SUSE Linux Enterprise Server (SLES) and Desktop (SLED). openSUSE is known for its YaST (Yet another Setup Tool) configuration tool, which is a central place to configure many aspects of the system. openSUSE comes in two flavours: Leap and Tumbleweed. Leap is a regular release distribution with a fixed release cycle, while Tumbleweed is a rolling release distribution.

Which to choose?

If you ask 10 people what the best Linux distribution is, chances are that you will get 20 different answers. Posting it as a question on a forum may lead to a discussion that goes on for weeks or months, if not years. You will get a lot of passionate and sometimes even insightful opinions, but in the end you won't be none the wiser. So giving good advice that is universally applicable is very hard, indeed.

Below are some very personal opinions (albeit informed by experience) on some of the most popular Linux distributions. Keep in mind that any of the below Linux distributions can be a stable server and a nice graphical desktop client.

Distribution name Reason(s) for using
AlmaLinux You want a stable Red Hat-like server OS without commercial support contract.
Arch You want to know how Linux really works and want to take your time to learn.
Debian An excellent choice for servers, laptops, and any other device.
Fedora You want a Red Hat-like OS on your laptop/desktop.
Kali You want a pointy-clicky hacking interface.
Linux Mint You want a personal graphical desktop to play movies, music and games.
RHEL You are a manager and need good commercial support.
RockyLinux You want a stable Red Hat-like server OS without commercial support contract.
Ubuntu Desktop Very popular, suited for beginners and based on Debian.
Ubuntu Server (LTS particulary) You want a Debian-like OS with commercial support.

When you are new to Linux, and are looking for a distribution with a graphical desktop and all the tools that you need as a daily driver, check out the latest Linux Mint (suitable for computer novices and experienced computer users alike) or Fedora (recommended for power users and IT professionals).

If you only want to practice the Linux command line, or are interested in the use of Linux as a server, then install a VM with the latest release of either Debian stable and/or AlmaLinux (without graphical interface)1.

As you gain experience, you can try out other distributions and see what you like best. Good luck on your journey and enjoy the ride!

  1. Remark that this advice was originally written in 2015 and basically still holds in 2024. The only amendment is that AlmaLinux has taken the place of CentOS as a recommendation for a server OS. ↩