Overview of the NixOS Linux distribution

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, although it can additionally support imperative package and user management. In NixOS, all components of the distribution — including the kernel, installed packages and system configuration files — are built by Nix from purely functions called Nix expressions.

Since Nix uses binary caching, this provides a unique compromise between the binary-oriented approach used by distributions such as Debian and the source-oriented approach used by distributions such as Gentoo. Binaries can be used for standard components, and custom-built packages and modules can be used automatically when a prebuilt binary is not available.

Stable NixOS releases are delivered biannually. 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, see the Installation chapter of the NixOS manual.

Most users will install NixOS via one of the ISO images. Both "graphical" and "minimal" ISO variants are available for each supported architecture; the "graphical" images are suitable for users intending to install a desktop environment, and the "minimal" images are suitable for users intending to install NixOS in a server role or desiring a smaller ISO image. The ISO images are hybrid images which can be burnt to optical media or copied raw to a USB drive and booted as-is. See the installation guide for details.

In addition to the ISO images, the download page provides a number of alternative methods for installing NixOS. These include:

Virtual appliances in OVA format (compatible with VirtualBox);
Amazon EC2 AMIs;
Microsoft Azure blobs.

Additionally, many existing Linux installations can be converted into NixOS installations using nixos-infect or nixos-in-place; this is particularly useful for installing NixOS on hosting providers which do not natively support NixOS.

For information on installing NixOS on various ARM devices, 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 specifies the entire system state, including which packages should be installed and all the various system settings and options. This configuration file is normally located at /etc/nixos/configuration.nix (although another location may be specified using the environment variable NIX_PATH); after the configuration file is modified, the new configuration is then made active by running nixos-rebuild switch. The switch is atomic and can be rolled back if necessary. The configuration files under /etc/nixos may even be kept in a version control system such as Git if desired.

Conventional distributions require users to manually modify configuration files, but these changes are not tracked. If distributions change the default contents of configuration files, these changes often have to be manually merged by users if they have previously modified the file, or the distribution modifications may not be integrated at all, leading to undesired configuration drift. Configuration settings and changes are rarely recorded in a version control system. These shortcomings are often rectified after-the-fact if at all by configuration management solutions such as Puppet or Chef. These tools reconcile system configuration with a description of the expected state. However, these tools are not integrated into the operating system design and are simply layered on top, and OS configuration may still vary where an aspect of OS configuration has not been specified in the description of expected state.

By comparison, NixOS's declarative configuration system provides a fully integrated facility for OS configuration management. Failure to specify any given item of configuration results in that item having a well-defined state, rather than being allowed to drift unmonitored. Because the full system configuration is captured in the NixOS configuration system, this also makes NixOS highly suited to the automatic deployment of configuration in environments such as automated server farms; tools such as NixOps make this easy.

The following is an example /etc/nixos/configuration.nix:

{ 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

While NixOS is typically configured declaratively as much as possible, these are a few domains where imperative operations are still necessary; these include user environment management and channel management.

User Environments

In addition to declarative system configuration, NixOS users can utilize Nix's imperative nix-env command to install packages at the user level, without changing the system state. See the user environments section of the Nix article for more information.

Channels

Nix channels are mechanisms for distributing Nix expressions as well as the associated binaries for them; a Nix channel corresponds to a repository in a conventional package management system. Official Nix channels are automatically updated once 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.

NixOS is distributed through a number of channels. These NixOS channels can be broadly categorised into stable and unstable channels, and large and small channels:

Stable/unstable:
- Stable channels (nixos-17.09) provide conservative updates for fixing bugs and security vulnerabilities, but do not receive major updates after initial release. New stable channels are released every six months.
- Unstable channels (nixos-unstable, nixos-unstable-small) correspond to the main development branch of Nixpkgs, delivering the latest tested updates on a rolling basis.
Large/small:
- Large channels (nixos-17.09, nixos-unstable) provide binary builds for the full breadth of Nixpkgs.
- Small channels (nixos-17.09-small, nixos-unstable-small) are identical to large channels, but contain fewer binaries. This means they update faster, but require more to be built from source.

Most users will want the stable/large channel, currently nixos-17.09.

Like packages installed via nix-env, channels are managed at user-level. NixOS uses the channels set for the root user to update the system-wide configuration; channels set for other users control only the user environment for that user. If you wish to change the channel used by the system-level configuration (/etc/nixos/configuration.nix), ensure you run the correct nix-channel command as root:

Common nix-channel commands
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 updating channels won't cause a rebuild in itself; if you want to update channels and rebuild, you can run nixos rebuild --upgrade to do both in one step.

Internals

Comparison with traditional Linux Distributions

Main Article: Nix vs. Linux Standard Base

The main difference between NixOS and other Linux distributions is that NixOS does not follow the Linux Standard Base file system structure. On LSB-compliant systems software is stored under /{,usr}/{bin,lib,share} and configuration is generally stored in /etc. Software binaries are available in the user environment if they are placed in one of the LSB's /bin directories. When a program references dynamic libraries it will search for the required libraries in the LSB folders (/lib, /usr/lib).

In NixOS however /lib and /usr/lib do not exist. Instead all system libraries, binaries, kernels, firmware and configuration files are placed in the Nix store. The files and directories in /nix/store are named by hashes of the information describing the built data. All of the files and directories placed in the Nix store are immutable. /bin and /usr/bin are almost absent: they contain only /bin/sh and /usr/bin/env respectively, to provide minimal compatibility with existing scripts using shebang lines. User-level environments are implemented using a large number of symbolic links to all required packages and auxiliary files. These environments are called profiles and are stored in /nix/var/nix/profiles, each user having their own profiles. Structuring the system in this way is how NixOS obtains its key advantages over conventional Linux distributions, such as atomicity and rollback support.

Usage of the Nix store

A lot of confusion for newcomers arises from the fact that configuration is stored in the read-only /nix/store tree along with all the installed packages. This fact makes it impossible to manually edit system configuration; all configuration changes must be performed by editing the /etc/nixos/configuration.nix file and executing nixos-rebuild switch. NixOS provides the modules system for editing all required configurations. Users should first use the option search tool to check if the option they need exists before attempting to manually add files or configuration via low-level NixOS features like activation scripts.

The system purity makes it possible to keep system configuration in a central place, without the need to edit multiple files. This configuration can be distributed or version controlled as desired. It also provides for determinism; if you provide the same inputs, the same version of Nixpkgs and the same /etc/nixos/configuration.nix you will get the exact same system state.

Modules

The NixOS module system as defined in Nixpkgs provides the means necessary to customize the configuration of the OS. It is used to enable and customize services such as nginx, enable firmware and customize the kernel.

All module configuration is generally performed by adding options to /etc/nixos/configuration.nix. Most of the examples in the wiki show how this file can be used to configure the OS.

The NixOS module system implements a typing system which allows typechecking of option settings. It also enables options defined in multiple places to be merged automatically. This allows you to spread your configuration over multiple files, and the options you set across all of those files will be merged together:

# /etc/nixos/configuration.nix
{
  imports = [
    ./basic-webserver.nix
    ./blog.nix
  ];
}

# /etc/nixos/basic-webserver.nix
{
  services.nginx.enable = true;
  services.nginx.virtualHosts."example.com" = {
    root = "/var/www/example.com";
  };
}

# /etc/nixos/blog.nix
{
  services.nginx.virtualHosts."blog.example.com" = {
    root = "/var/www/blog.example.com";
  };
}

See the Modules section of the NixOS Manual for more details.

Generations

Every time the system state is rebuilt using nixos-rebuild switch, a new generation is created. You can revert to the previous generation at any time, which is useful if a configuration change (or system update) turns out to be detrimental.

You can roll back via:

$ nix-env --rollback               # roll back a user environment
$ nixos-rebuild switch --rollback  # roll back a system environment

NixOS also places entries for previous generations in the bootloader menu, so as a last resort you can always revert to a previous configuration by rebooting.

Because NixOS keeps previous generations of system state available in case rollback is desired, old package versions aren't deleted from your system immediately after an update. You can delete old generations manually:

# delete generations older than 30 days
$ nix-collect-garbage --delete-older-than 30d

# delete ALL previous generations - you can no longer rollback after running this
$ nix-collect-garbage -d

You can configure automatic garbage collection by setting the nix.gc options in /etc/nixos/configuration.nix. This is recommended, as it keeps the size of the Nix store down.