Writing NixOS ModulesNixOS has a modular system for declarative configuration. This
system combines multiple modules to produce the
full system configuration. One of the modules that constitute the
configuration is /etc/nixos/configuration.nix.
Most of the others live in the nixos/modules
subdirectory of the Nixpkgs tree.Each NixOS module is a file that handles one logical aspect of
the configuration, such as a specific kind of hardware, a service, or
network settings. A module configuration does not have to handle
everything from scratch; it can use the functionality provided by
other modules for its implementation. Thus a module can
declare options that can be used by other
modules, and conversely can define options
provided by other modules in its own implementation. For example, the
module pam.nix
declares the option that allows
other modules (e.g. sshd.nix)
to define PAM services; and it defines the option
(declared by etc.nix)
to cause files to be created in
/etc/pam.d.In , we saw the following structure
of NixOS modules:
{ config, pkgs, ... }:
{ option definitions
}
This is actually an abbreviated form of module
that only defines options, but does not declare any. The structure of
full NixOS modules is shown in .Structure of NixOS Modules
{ config, pkgs, ... }:
{
imports =
[ paths of other modules
];
options = {
option declarations
};
config = {
option definitions
};
}The meaning of each part is as follows.
This line makes the current Nix expression a function. The
variable pkgs contains Nixpkgs, while
config contains the full system configuration.
This line can be omitted if there is no reference to
pkgs and config inside the
module.This list enumerates the paths to other NixOS modules that
should be included in the evaluation of the system configuration.
A default set of modules is defined in the file
modules/module-list.nix. These don't need to
be added in the import list.The attribute options is a nested set of
option declarations (described below).The attribute config is a nested set of
option definitions (also described
below). shows a module that handles
the regular update of the “locate” database, an index of all files in
the file system. This module declares two options that can be defined
by other modules (typically the user’s
configuration.nix):
(whether the database should
be updated) and (when the
update should be done). It implements its functionality by defining
two options declared by other modules:
(the set of all systemd services)
and (the list of
commands to be executed periodically by cron).NixOS Module for the “locate” Service
{ config, lib, pkgs, ... }:
with lib;
let locatedb = "/var/cache/locatedb"; in
{
options = {
services.locate = {
enable = mkOption {
type = types.bool;
default = false;
description = ''
If enabled, NixOS will periodically update the database of
files used by the locate command.
'';
};
period = mkOption {
type = types.str;
default = "15 02 * * *";
description = ''
This option defines (in the format used by cron) when the
locate database is updated. The default is to update at
02:15 at night every day.
'';
};
};
};
config = {
systemd.services.update-locatedb =
{ description = "Update Locate Database";
path = [ pkgs.su ];
script =
''
mkdir -m 0755 -p $(dirname ${locatedb})
exec updatedb --localuser=nobody --output=${locatedb} --prunepaths='/tmp /var/tmp /run'
'';
};
services.cron.systemCronJobs = optional config.services.locate.enable
"${config.services.locate.period} root ${config.systemd.package}/bin/systemctl start update-locatedb.service";
};
}