133 lines
4 KiB
Markdown
133 lines
4 KiB
Markdown
# Modularity {#sec-modularity}
|
|
|
|
The NixOS configuration mechanism is modular. If your
|
|
`configuration.nix` becomes too big, you can split it into multiple
|
|
files. Likewise, if you have multiple NixOS configurations (e.g. for
|
|
different computers) with some commonality, you can move the common
|
|
configuration into a shared file.
|
|
|
|
Modules have exactly the same syntax as `configuration.nix`. In fact,
|
|
`configuration.nix` is itself a module. You can use other modules by
|
|
including them from `configuration.nix`, e.g.:
|
|
|
|
```nix
|
|
{ config, pkgs, ... }:
|
|
|
|
{ imports = [ ./vpn.nix ./kde.nix ];
|
|
services.httpd.enable = true;
|
|
environment.systemPackages = [ pkgs.emacs ];
|
|
...
|
|
}
|
|
```
|
|
|
|
Here, we include two modules from the same directory, `vpn.nix` and
|
|
`kde.nix`. The latter might look like this:
|
|
|
|
```nix
|
|
{ config, pkgs, ... }:
|
|
|
|
{ services.xserver.enable = true;
|
|
services.xserver.displayManager.sddm.enable = true;
|
|
services.xserver.desktopManager.plasma5.enable = true;
|
|
environment.systemPackages = [ pkgs.vim ];
|
|
}
|
|
```
|
|
|
|
Note that both `configuration.nix` and `kde.nix` define the option
|
|
[](#opt-environment.systemPackages). When multiple modules define an
|
|
option, NixOS will try to *merge* the definitions. In the case of
|
|
[](#opt-environment.systemPackages), that's easy: the lists of
|
|
packages can simply be concatenated. The value in `configuration.nix` is
|
|
merged last, so for list-type options, it will appear at the end of the
|
|
merged list. If you want it to appear first, you can use `mkBefore`:
|
|
|
|
```nix
|
|
boot.kernelModules = mkBefore [ "kvm-intel" ];
|
|
```
|
|
|
|
This causes the `kvm-intel` kernel module to be loaded before any other
|
|
kernel modules.
|
|
|
|
For other types of options, a merge may not be possible. For instance,
|
|
if two modules define [](#opt-services.httpd.adminAddr),
|
|
`nixos-rebuild` will give an error:
|
|
|
|
```plain
|
|
The unique option `services.httpd.adminAddr' is defined multiple times, in `/etc/nixos/httpd.nix' and `/etc/nixos/configuration.nix'.
|
|
```
|
|
|
|
When that happens, it's possible to force one definition take precedence
|
|
over the others:
|
|
|
|
```nix
|
|
services.httpd.adminAddr = pkgs.lib.mkForce "bob@example.org";
|
|
```
|
|
|
|
When using multiple modules, you may need to access configuration values
|
|
defined in other modules. This is what the `config` function argument is
|
|
for: it contains the complete, merged system configuration. That is,
|
|
`config` is the result of combining the configurations returned by every
|
|
module [^1] . For example, here is a module that adds some packages to
|
|
[](#opt-environment.systemPackages) only if
|
|
[](#opt-services.xserver.enable) is set to `true` somewhere else:
|
|
|
|
```nix
|
|
{ config, pkgs, ... }:
|
|
|
|
{ environment.systemPackages =
|
|
if config.services.xserver.enable then
|
|
[ pkgs.firefox
|
|
pkgs.thunderbird
|
|
]
|
|
else
|
|
[ ];
|
|
}
|
|
```
|
|
|
|
With multiple modules, it may not be obvious what the final value of a
|
|
configuration option is. The command `nixos-option` allows you to find
|
|
out:
|
|
|
|
```ShellSession
|
|
$ nixos-option services.xserver.enable
|
|
true
|
|
|
|
$ nixos-option boot.kernelModules
|
|
[ "tun" "ipv6" "loop" ... ]
|
|
```
|
|
|
|
Interactive exploration of the configuration is possible using `nix
|
|
repl`, a read-eval-print loop for Nix expressions. A typical use:
|
|
|
|
```ShellSession
|
|
$ nix repl '<nixpkgs/nixos>'
|
|
|
|
nix-repl> config.networking.hostName
|
|
"mandark"
|
|
|
|
nix-repl> map (x: x.hostName) config.services.httpd.virtualHosts
|
|
[ "example.org" "example.gov" ]
|
|
```
|
|
|
|
While abstracting your configuration, you may find it useful to generate
|
|
modules using code, instead of writing files. The example below would
|
|
have the same effect as importing a file which sets those options.
|
|
|
|
```nix
|
|
{ config, pkgs, ... }:
|
|
|
|
let netConfig = hostName: {
|
|
networking.hostName = hostName;
|
|
networking.useDHCP = false;
|
|
};
|
|
|
|
in
|
|
|
|
{ imports = [ (netConfig "nixos.localdomain") ]; }
|
|
```
|
|
|
|
[^1]: If you're wondering how it's possible that the (indirect) *result*
|
|
of a function is passed as an *input* to that same function: that's
|
|
because Nix is a "lazy" language --- it only computes values when
|
|
they are needed. This works as long as no individual configuration
|
|
value depends on itself.
|