Many programs have configuration files where program-specific settings can be declared. File formats can be separated into two categories: Nix-representable ones: These can trivially be mapped to a subset of Nix syntax. E.g. JSON is an example, since its values like {"foo":{"bar":10}} can be mapped directly to Nix: { foo = { bar = 10; }; }. Other examples are INI, YAML and TOML. The following section explains the convention for these settings. Non-nix-representable ones: These can't be trivially mapped to a subset of Nix syntax. Most generic programming languages are in this group, e.g. bash, since the statement if true; then echo hi; fi doesn't have a trivial representation in Nix. Currently there are no fixed conventions for these, but it is common to have a configFile option for setting the configuration file path directly. The default value of configFile can be an auto-generated file, with convenient options for controlling the contents. For example an option of type attrsOf str can be used for representing environment variables which generates a section like export FOO="foo". Often it can also be useful to also include an extraConfig option of type lines to allow arbitrary text after the autogenerated part of the file.

By convention, formats like this are handled with a generic settings option, representing the full program configuration as a Nix value. The type of this option should represent the format. The most common formats have a predefined type and string generator already declared under pkgs.formats: pkgs.formats.json { } A function taking an empty attribute set (for future extensibility) and returning a set with JSON-specific attributes type and generate as specified below. pkgs.formats.yaml { } A function taking an empty attribute set (for future extensibility) and returning a set with YAML-specific attributes type and generate as specified below. pkgs.formats.ini { listsAsDuplicateKeys ? false, ... } A function taking an attribute set with values listsAsDuplicateKeys A boolean for controlling whether list values can be used to represent duplicate INI keys It returns a set with INI-specific attributes type and generate as specified below. pkgs.formats.toml { } A function taking an empty attribute set (for future extensibility) and returning a set with TOML-specific attributes type and generate as specified below. These functions all return an attribute set with these values: type A module system type representing a value of the format generate filename jsonValue A function that can render a value of the format to a file. Returns a file path. This function puts the value contents in the Nix store. So this should be avoided for secrets. The following shows a module for an example program that uses a JSON configuration file. It demonstrates how above values can be used, along with some other related best practices. See the comments for explanations. { options, config, lib, pkgs, ... }: let cfg = config.services.foo; # Define the settings format used for this program settingsFormat = pkgs.formats.json {}; in { options.services.foo = { enable = lib.mkEnableOption "foo service"; settings = lib.mkOption { # Setting this type allows for correct merging behavior type = settingsFormat.type; default = {}; description = '' Configuration for foo, see <link xlink:href="https://example.com/docs/foo"/> for supported settings. ''; }; }; config = lib.mkIf cfg.enable { # We can assign some default settings here to make the service work by just # enabling it. We use `mkDefault` for values that can be changed without # problems services.foo.settings = { # Fails at runtime without any value set log_level = lib.mkDefault "WARN"; # We assume systemd's `StateDirectory` is used, so we require this value, # therefore no mkDefault data_path = "/var/lib/foo"; # Since we use this to create a user we need to know the default value at # eval time user = lib.mkDefault "foo"; }; environment.etc."foo.json".source = # The formats generator function takes a filename and the Nix value # representing the format value and produces a filepath with that value # rendered in the format settingsFormat.generate "foo-config.json" cfg.settings; # We know that the `user` attribute exists because we set a default value # for it above, allowing us to use it without worries here users.users.${cfg.settings.user} = { isSystemUser = true; }; # ... }; }

Some settings attributes may deserve some extra care. They may need a different type, default or merging behavior, or they are essential options that should show their documentation in the manual. This can be done using . We extend above example using freeform modules to declare an option for the port, which will enforce it to be a valid integer and make it show up in the manual. settings = lib.mkOption { type = lib.types.submodule { freeformType = settingsFormat.type; # Declare an option for the port such that the type is checked and this option # is shown in the manual. options.port = lib.mkOption { type = lib.types.port; default = 8080; description = '' Which port this service should listen on. ''; }; }; default = {}; description = '' Configuration for Foo, see <link xlink:href="https://example.com/docs/foo"/> for supported values. ''; };