NixOS

NixOS is a Linux distribution built on top of the Nix package manager. Its declarative configuration allows reliable system upgrades via several official channels. NixOS has tools dedicated to DevOps and deployment tasks.
Features that distinguish NixOS from other Linux distributions include:
Abstraction: The software packages making up a system can be configured using the Nix language syntax.
Reproducible systems: A replica of a system can be created on another machine with one configuration file.
Atomic upgrades: System upgrades involve less risk of breakage, and if something does go wrong, it is simple to roll back to the previous state.
Immutability: The software making up a given system configuration cannot be changed once it has been built, preventing accidental or malicious modifications.
Nix package manager: Packages can be installed without affecting the rest of the system, and can be tested without installing.
History
In 2003, Eelco Dolstra started NixOS as a research project. In 2015, the Stichting NixOS (limited-liability organization) was founded in the Netherlands, aiming to support projects like NixOS that implement the purely functional deployment model.
Versions
NixOS publishes releases biannually around May and November. Version numbers follow the format "YY.MM" – for instance, "20.03" was released in March 2020. Each version of NixOS also has a name, such as "Markhor" for the 20.03 release.
Features
Declarative configuration model
In NixOS, the entire operating system – including the kernel, applications, system packages, configuration files, and so on – is built by the Nix package manager from a description in a functional build language. This means that building a new configuration cannot overwrite previous configurations.
A NixOS system is configured by writing a specification of the functionality that the user wants on their machine in a global configuration file. For instance, here is a minimal configuration of a machine running an SSH daemon:
{
boot.loader.grub.device = "/dev/sda";
fileSystems."/".device = "/dev/sda1";
services.sshd.enable = true;
}
After changing the configuration file, the system can be updated using the nixos-rebuild switch command. This command does everything necessary to apply the new configuration, including downloading and compiling packages, and generating configuration files.
Reliable upgrades
Since Nix files are pure and declarative, evaluating them will always produce the same result, regardless of what packages or configuration files are on the system. Thus, upgrading a system is as reliable as reinstalling from scratch.
Atomic upgrades
NixOS has a transactional approach to configuration management, making configuration changes such as upgrades atomic. This means that if the upgrade to a new configuration is interrupted – say, the power fails half-way through – the system will still be in a consistent state: it will either boot in the old or the new configuration. In other systems, a machine might end up in an inconsistent state, and possibly be unable to boot.
Rollbacks
If, after a system update, the new configuration is undesirable, it can be rolled back using a special command (nixos-rebuild switch --rollback). Every system configuration version automatically shows up in the system boot menu. If the new configuration crashes or does not boot properly, an older version can be selected. Rollbacks are lightweight operations that do not involve files being restored from copies.
Reproducible system configurations
NixOS's declarative configuration model makes it easy to reproduce a system configuration on another machine. Copying the configuration file to the target machine and running the system update command generates the same system configuration (kernel, applications, system services, and so on) except for parts of the system not managed by the package manager, such as user data.
Source-based model with binary cache
The Nix build language used by NixOS specifies how to build packages from source. This makes it easy to adapt the system to user needs. However, building from source being a slow process, the package manager automatically downloads pre-built binaries from a cache server when they are available. It is possible to disable the binary cache and force building from source by using --option substitute false as an argument. This gives the flexibility of a source-based package management model, with the efficiency of a binary model.
Consistency
The Nix package manager ensures that the running system is consistent with the logical specification of the system, meaning that it will rebuild all packages that need to be rebuilt. For instance, if the kernel is changed, then the package manager will ensure that external kernel modules will be rebuilt. Similarly, when a library is updated, it ensures that all the system packages use the new version, even packages statically linked to it.
Multi-user package management
There is no need for special privileges to install software in NixOS. In addition to the system-wide profile, every user has a dedicated profile in which they can install packages. Nix also allows multiple versions of a package to coexist, so different users can have different versions of the same package installed in their respective profiles. If two users install the same version of a package, only one copy will be built or downloaded. Nix's security model ensures that this is secure, because only the users explicitly trusted by the system configuration are allowed to use build parameters that would allow them to control the content of a derivation's output (such as adding impurities to the sandbox, or using an untrusted substituter). Without those parameters, paths can only be substituted from a substituter trusted by the system, or a local sandboxed build which is implicitly trusted.
Implementation
NixOS is based on the Nix package manager, which stores all packages in isolation from each other in the package store.
Installed packages are identified by a cryptographic hash of all input used for their build. Changing the build instructions of a package modifies its hash, and that will result in a different package being installed in the package store. This system is also used to manage configuration files, ensuring that newer configurations do not overwrite older ones.
An implication of this is that NixOS does not follow the Filesystem Hierarchy Standard. The only exceptions are that a /bin/sh symlink is created to the version of bash in the Nix store (e.g. /nix/store/s/5rnfzla9kcx4mj5zdc7nlnv8na1najvg-bash-4.3.43/), and while NixOS does have an /etc directory to keep system-wide configuration files, most files in that directory are symlinks to generated files in /nix/store, such as /nix/store/s2sjbl85xnrc18rl4fhn56irkxqxyk4p-sshd_config. Not using global directories such as /bin is part of what allows multiple versions of a package to coexist.
Reception
Jesse Smith, reviewing NixOS 15.09 for DistroWatch Weekly in 2015, wrote:
DistroWatch Weekly also has a 2017 review of NixOS 17.03, written by Ivan Sanders.
A 2022 review of NixOS 21.11 "Porcupine" in Full Circle magazine concluded:
See also
GNU Guix System – an operating system built on GNU Guix that is inspired by Nix
References
External links
Independent Linux distributions
Linux distributions
Operating system security
Source-based Linux distributions
X86-64 Linux distributions