NixOS modules
NixOS produces a full system configuration by combining smaller, more isolated and reusable components: Modules. A module is a file containing a Nix expression with a specific structure. It declares options for other modules to define (give a value). It processes them and defines options declared in other modules.[1]
For example, /etc/nixos/configuration.nix
is a module. Most other modules are in
.
nixos/modules
Structure
Modules have the following syntax:
{
imports = [
# Paths to other modules.
# Compose this module out of smaller ones.
];
options = {
# Option declarations.
# Declare what settings a user of this module can set.
# Usually this includes a global "enable" option which defaults to false.
};
config = {
# Option definitions.
# Define what other settings, services and resources should be active.
# Usually these depend on whether a user of this module chose to "enable" it
# using the "option" above.
# Options for modules imported in "imports" can be set here.
};
}
There is a shorthand for modules without any declarations:
{
imports = [
# Paths to other modules.
./module.nix
/path/to/absolute/module.nix
];
# Config definitions.
services.othermodule.enable = true;
# ...
# Notice that you can leave out the "config { }" wrapper.
}
Beginners often confuse the modules attribute imports = [./module.nix]
here with the Nix builtins function import module.nix
. The first expects a path to a file containing a NixOS module (having the same specific structure we're describing here), while the second loads whatever Nix expression is in that file (no expected structure). See this post.
Note: imports
provides the same behavior as the obsolete require
. There is no reason to use require
anymore, however it may still linger in some legacy code.
Function
A module can be turned into a function accepting an attribute set.
{ config, pkgs, ... }:
{
imports = [];
# ...
}
It may require the attribute set to contain:
config
- The configuration of the entire system.
options
- All option declarations refined with all definition and declaration references.
lib
- An instance of the nixpkgs "standard library", providing what usually is in
pkgs.lib
. pkgs
- The attribute set extracted from the Nix package collection and enhanced with the
nixpkgs.config
option. modulesPath
- The location of the
module
directory of NixOS.
Passing custom values to modules
The config
, options
, lib
, pkgs
, and modulesPath
arguments are passed automatically to modules, when the module is imported.
For example, in the following Nix flake, the `./configuration.nix` file will be provided with the default set of arguments listed above, plus `extraArg`, which was set in the `specialArgs` argument to the `nixosGenerate` function.
{
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
nixos-generators = {
url = "github:nix-community/nixos-generators";
inputs.nixpkgs.follows = "nixpkgs";
};
xc = {
url = "github:joerdav/xc";
inputs.nixpkgs.follows = "nixpkgs";
};
};
outputs = { nixpkgs, nixos-generators, xc, ... }:
let
pkgsForSystem = system: import nixpkgs {
inherit system;
overlays = [
(final: prev: { xc = xc.packages.${system}.xc; })
];
};
allVMs = [ "x86_64-linux" "aarch64-linux" ];
forAllVMs = f: nixpkgs.lib.genAttrs allVMs (system: f {
inherit system;
pkgs = pkgsForSystem system;
});
in
{
packages = forAllVMs ({ system, pkgs }: {
vm = nixos-generators.nixosGenerate {
system = system;
specialArgs = {
extraArg = "foobar";
};
modules = [
./configuration.nix
];
format = "raw";
};
});
};
}
modulesPath
Some modules use modulesPath
to import nixos libraries
For example nixos/modules/virtualisation/digital-ocean-config.nix
{ config, pkgs, lib, modulesPath, ... }: imports = [ (modulesPath "/profiles/qemu-guest.nix") (modulesPath "/virtualisation/digital-ocean-init.nix") ];
The Nix variable modulesPath
is parsed from the environment variable NIX_PATH
When NIX_PATH
is empty, Nix can throw the error undefined variable 'modulesPath'
NIX_PATH
should look something like this:
echo $NIX_PATH | tr : '\n' nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos nixos-config=/etc/nixos/configuration.nix /nix/var/nix/profiles/per-user/root/channels
Here, the modulesPath
is /nix/var/nix/profiles/per-user/root/channels
When a Nix expression calls import <nixpkgs>
,
then Nix will load /nix/var/nix/profiles/per-user/root/channels/nixos
Imports
Imports are paths to other NixOS modules that should be included in the evaluation of the system configuration. A default set of modules is defined in
. These don't need to be added in the import list.
nixos/modules/module-list.nix
Declarations
Declarations specify a module's external interfaces.
optionName = mkOption {
# ...
}
They are created with mkOption
, a function accepting a set with following attributes:[2][3]
type
The type of the option. It may be omitted, but that’s not advisable since it may lead to errors that are hard to diagnose.
default
The default value used if no value is defined by any module. A default is not required; but if a default is not given, then users of the module will have to define the value of the option, otherwise an error will be thrown.
example
An example value that will be shown in the NixOS manual.
description
A textual description of the option, in DocBook format, that will be included in the NixOS manual.
Rationale
Modules were introduced to allow extending NixOS without modifying its source code.[4] They also allow splitting up configuration.nix
, making the system configuration easier to maintain and to reuse.
Example
To see how modules are setup and reuse other modules in practice put hello.nix
in the same folder as your configuration.nix
:
{ lib, pkgs, config, ... }:
with lib;
let
# Shorter name to access final settings a
# user of hello.nix module HAS ACTUALLY SET.
# cfg is a typical convention.
cfg = config.services.hello;
in {
# Declare what settings a user of this "hello.nix" module CAN SET.
options.services.hello = {
enable = mkEnableOption "hello service";
greeter = mkOption {
type = types.str;
default = "world";
};
};
# Define what other settings, services and resources should be active IF
# a user of this "hello.nix" module ENABLED this module
# by setting "services.hello.enable = true;".
config = mkIf cfg.enable {
systemd.services.hello = {
wantedBy = [ "multi-user.target" ];
serviceConfig.ExecStart = "${pkgs.hello}/bin/hello -g'Hello, ${escapeShellArg cfg.greeter}!'";
};
};
}
The other configuration.nix
module can then import this hello.nix
module
and decide to enable it (and optionally set other allowed settings) as follows:
{
imports = [ ./hello.nix ];
...
services.hello = {
enable = true;
greeter = "Bob";
};
}
Advanced Use Cases
Compatibility Issues with Different Nixpkgs Versions
Module options between Nixpkgs revisions can sometimes change in incompatible ways.
For example, the option services.nginx.virtualHosts.*.port
in nixpkgs-17.03 was replaced by services.nginx.virtualHosts.*.listen
in nixpkgs-17.09. If configuration.nix has to accommodate both variants, options
can be inspected:
{ options, ... }: {
services.nginx.virtualHosts.somehost = { /* common configuration */ }
// (if builtins.hasAttr "port" (builtins.head options.services.nginx.virtualHosts.type.getSubModules).submodule.options
then { port = 8000; }
else { listen = [ { addr = "0.0.0.0"; port = 8000; } ]; });
}
Abstract imports
To import a module that's stored somewhere (but for which you have neither an absolute nor a relative path), you can use NIX_PATH elements or specialArgs
from nixos/lib/eval-config.nix
.
This is useful for e.g. pulling modules from a git repository without adding it as a channel, or if you just prefer using paths relative to a root you can change (as opposed to the current file, which could move in the future).
let
inherit (import <nixpkgs> {}) writeShellScriptBin fetchgit;
yourModules = fetchgit { ... };
in rec {
nixos = import <nixpkgs/nixos/lib/eval-config.nix> {
modules = [ ./configuration.nix ];
specialArgs.mod = name: "${yourModules}/${name}";
};
/* use nixos here, e.g. for deployment or building an image */
}
{ config, lib, pkgs, mod, ... }: {
imports = [
(mod "foo.nix")
];
...
}
Using external NixOS modules
Some external modules provide extra functionality to the NixOS module system. You can include these modules, after making them available as a file system path (e.g. through builtins.fetchTarball
), by using imports = [ `path to module`]
in your configuration.nix
.
- Nixsap - allows to run multiple instances of a service without containers.
- musnix - real-time audio in NixOS.
- nixos-mailserver - full-featured mail server module
- X-Truder Nix-profiles - modules for Nix to quickly configure your system based on application profiles.
Under the hood
The following was taken from a comment by Infinisil on reddit [5].
A NixOS system is described by a single system derivation. nixos-rebuild
builds this derivation with nix-build '<nixpkgs/nixos>' -A system
and then switches to that system with result/bin/switch-to-configuration
.
The entrypoint is the file at '<nixpkgs/nixos>' (./default.nix)
, which defines the system
attribute to be the NixOS option config.system.build.toplevel
. This toplevel
option is the topmost level of the NixOS evaluation and it's what almost all options eventually end up influencing through potentially a number of intermediate options.
As an example:
- The high-level option services.nginx.enable uses the lower-level option systemd.services.nginx
- Which in turn uses the even-lower-level option systemd.units."nginx.service"
- Which in turn uses environment.etc."systemd/system"
- Which then ends up as result/etc/systemd/system/nginx.service in the toplevel derivation
So high-level options use lower-level ones, eventually ending up at config.system.build.toplevel
.
How do these options get evaluated though? That's what the NixOS module system does, which lives in the ./lib
directory (in modules.nix
, options.nix
and types.nix
). The module system can even be used without NixOS, allowing you to use it for your own option sets. Here's a simple example of this, whose toplevel
option you can evaluate with nix-instantiate --eval file.nix -A config.toplevel
:
let
systemModule = { lib, config, ... }: {
options.toplevel = lib.mkOption {
type = lib.types.str;
};
options.enableFoo = lib.mkOption {
type = lib.types.bool;
default = false;
};
config.toplevel = ''
Is foo enabled? ${lib.boolToString config.enableFoo}
'';
};
userModule = {
enableFoo = true;
};
in (import <nixpkgs/lib>).evalModules {
modules = [ systemModule userModule ];
}
The module system itself is rather complex, but here's a short overview. A module evaluation consists of a set of "modules", which can do three things:
- Import other modules (through imports = [ ./other-module.nix ])
- Declare options (through options = { ... })
- Define option values (through |config = { ... }, or without the config key as a shorthand if you don't have imports or options)
To do the actual evaluation, there's these rough steps:
- Recursively collect all modules by looking at all
imports
statements - Collect all option declarations (with
options
) of all modules and merge them together if necessary - For each option, evaluate it by collecting all its definitions (with
config
) from all modules and merging them together according to the options type.
Note that the last step is lazy (only the options you need are evaluated) and depends on other options itself (all the ones that influence it)
More complex usages
The examples below contain:
- a child `mkOption` inherits their default from a parent `mkOption`
- reading default values from neighbouring `mkOption`(s) for conditional defaults
- passing in the config, to read the hostName from a submodule (email system)
- setting default values from attrset (email system)
- generating documentation for custom modules (outside of nixpkgs). See here
Source:
- https://github.com/nixcloud/nixcloud-webservices/blob/master/modules/services/reverse-proxy/default.nix
- https://github.com/nixcloud/nixcloud-webservices/blob/master/modules/services/reverse-proxy/options.nix
- https://github.com/nixcloud/nixcloud-webservices/blob/master/modules/services/TLS/default.nix
- https://github.com/nixcloud/nixcloud-webservices/blob/master/modules/services/email/nixcloud-email.nix#L114
(sorry, dont' have more time to make this into a nice little guide yet, but this links should be pretty good introductions into more advanced module system usages) qknight
Developing modules
To test your module out, you can run the following from a local checkout of nixpkgs with a copy of a configuration.nix
:
nixos-rebuild build-vm --fast -I nixos-config=./configuration.nix -I nixpkgs=.
If you're developing on top of master, this will potentially cause the compilation of lots of packages, since changes on master might not cached on cache.nixos.org yet. To avoid that, you can develop your module on top of the nixos-unstable
channel branch, tracked by the eponymous branch in https://github.com/NixOS/nixpkgs:
git checkout -b mymodule upstream/nixos-unstable
With Flakes
If you're developing a module from nixpkgs, you can try and follow the directions here: https://github.com/Misterio77/nix-starter-configs/issues/28.
If you want to develop a module from a git repo, you can use `--override-input`. For example, if you have an input in your flake called jovian
,, you can use
nixos-rebuild switch --override-input jovian <path-to-url>` --flake <uri>
Of course, it doesn't have to be {{|c|nixos-rebuild}} in particular.
References
See also
- NixOS:extend_NixOS
- NixOS:Properties
- NixOS discourse, "Best resources for learning about the NixOS module system?"
- Debian Config::Model: target configuration upgrades by abstracting the option of the configuration. Each file is a tree structure where leaves are values defined with an interpreted type. The interpreters are defined for each meta-configuration files name *.conf. Configuration files does not seems to interact with each other to make consistent configuration. They provide an UI for editing their configuration file.