97b0ae26f7
While the word 'simply' is usually added to encourage readers, it often has the opposite effect and may even appear condescending, especially when the reader runs into trouble trying to apply the suggestions from the documentation. It is almost always an improvement to simply drop the word from the sentence. (there are more possible improvements like this, we can apply those in separate PRs)
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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.:
{ 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:
{ 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
. When multiple modules define an
option, NixOS will try to merge the definitions. In the case of
the lists of packages will 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:
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 ,
nixos-rebuild will give an error:
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:
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. (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.)
For example, here is a module that adds some packages to
only if
is set to true somewhere else:
{ 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:
$ 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:
$ 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.
{ config, pkgs, ... }:
let netConfig = hostName: {
networking.hostName = hostName;
networking.useDHCP = false;
};
in
{ imports = [ (netConfig "nixos.localdomain") ]; }