Overview of the NixOS Linux distribution

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NixOS is a Linux distribution based on the Nix Package Manager. It supports reproducible and declarative system-wide configuration management as well as atomic upgrades and rollbacks. Alongside this declarative operation mode, NixOS supports imperative package and user management. In NixOS, all components of the distribution - including the kernel, system packages and configuration files - are built by Nix from purely functional Nix Expressions. Since Nix supports binary caching, this provides a convenient compromise between source-based and binary approaches, allowing the use of a binaries for standard components and custom built packages and modules when needed. Stable NixOS releases are delivered biannually, with the latest stable version being 17.09. NixOS was created by Eelco Dolstra and Armijn Hemel, and initially released in 2003. It is community developed and maintained under the stewardship of the NixOS Foundation.

Usage

Installation

For a full installation guide using the ISOs - including steps for installing with USB stick and on UEFI systems - see the installation chapter of the manual.

In addition to graphical and minimal 64bit and 32bit ISOs, the NixOS download page provides a variety of formats for installing NixOS. These include virtual appliances in OVA format (compatible with Virtual Box), Amazon EC2 AMIs and Microsoft Azure BLOBs. Additionally, one can convert a variety of existing Linux installations into NixOS using nixos-infect and nixos-in-place; this is particularly useful for installing NixOS on hosting providers such as Digital Ocean, which have no NixOS images available out-of-the-box. For instructions on installing NixOS on various ARM boards, see NixOS on ARM.

Declarative Configuration

One of NixOS's most distinguishing features is the ability to declaratively configure the whole system. This is done by specifying a configuration file which defines which packages are installed on the system, which services to run and various other settings and options. This file is normally called configuration.nix and is found by default at /etc/nixos, although another location can be set using the environment variable NIX_PATH. The system configuration is then built with the command nixos-rebuild. The following is an example of a configuration.nix file:

{ config, pkgs, ... }: 

{
    # Import other configuration modules
    # (hardware-configuration.nix is autogenerated upon installation)
    # paths in nix expressions are always relative the file which defines them
    imports =
        [
            ./hardware-configuration.nix
            ./my-dev-tools.nix
            ./my-desktop-env.nix
            ./etc.nix
        ];

    # Name your host machine
    networking.hostName = "mymachine"; 

    # Set your time zone.
    time.timeZone = "Europe/Utrecht";

    # Enter keyboard layout
    services.xserver.layout = "us";
    services.xserver.xkbVariant = "altgr-intl";

    # Define user accounts
    users.users = 
        { 
            myuser = 
            {
                home = "/home/myuser";
                extraGroups = [ "wheel" "networkmanager" ];
                isNormalUser = true;
                uid = 1000;
            };
        };
    
    # Install some packages
    environment.systemPackages = 
            with pkgs; 
            [
                ddate
                testdisk
                zsh
            ]; 
 
    # Enable the OpenSSH daemon
    services.openssh.enable = true;
    
}

For inspiration, a variety of NixOS configuration files made by community members can be found in the Configuration Collection.

Imperative Operations

User Environment Management

In addition to declarative system configuration, NixOS offers imperative commands to manage user specific package management. These operations are managed by the nix-env command line tool. The following is a summary of some common operations that can be performed with it:

Common nix-env Commands:

Searching for packages	nix-env -qaP '.*packagename.*'
Installing a package	nix-env -i packagename
List installed packages	nix-env -q
Uninstall packages	nix-env -e packagename
Upgrade packages	nix-env -u

Channels

Nix channels are mechanisms for distributing Nix expressions alongside the associated binaries for them. Official Nix channels are automatically updated once a certain tests are passed in Nixpkgs' Hydra instance. It is also possible to create one's own Nix channels, but here we focus the official channels. A full list of the available official channels is available at https://nixos.org/channels/, but they can be classified into three main types.

Channel Types:

Stable	nixos-17.09	These receive conservative updates for fixing bugs and security vulnerabilities.
Unstable	nixos-unstable	Corresponds to the main development branch of Nixpkgs, delivering the latest tested updates.
Small	nixos-17.09-small, nixos-unstable-small	Identical to their normal namesakes, but containing fewer binaries. This means they update faster but require more to be built from source.

NixOS will use the root's channels to update the system wide configuration, and user-specific channels to manage the user environment; this means that you must sudo commands intended to manage the channel your configuration.nix should use. The following are common commands to manage channels on NixOS:

Listing current channels	nix-channel --list
Adding a primary channel	nix-channel --add https://nixos.org/channels/channel-name nixos
Adding other channels	nix-channel --add https://some.channel/url my-alias
Remove a channel	nix-channel --remove channel-alias
Updating a channel	nix-channel --update channel-alias
Updating all channels	nix-channel --update

Note that after updating channels, one still has to rebuild with nixos-rebuild switch, but one can also run nixos-rebuild switch --upgrade to update channels and rebuild.

Internals

Nix Store

Nix parses Nix expressions written in the Nix Expression Language; these are pure functions taking dependencies as arguments and producing derivation specifying a reproducible build environment for the package. The package is then built the Nix store, receiving a unique address specified by a cryptographic hash of the build's dependency graph followed by the package name and version, for example /nix/store/nawl092prjblbhvv16kxxbk6j9gkgcqm-git-2.14.1. This allows Nix to simultaneously install different versions of the same package, and even different builds of the same version, for example variants built with different compilers.

See also

• NixOS Modules