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Options for Program Settings
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 ofconfigFile
can be an auto-generated file, with convenient options for controlling the contents. For example an option of typeattrsOf str
can be used for representing environment variables which generates a section likeexport FOO="foo"
. Often it can also be useful to also include anextraConfig
option of typelines
to allow arbitrary text after the autogenerated part of the file.
Nix-representable Formats (JSON, YAML, TOML, INI, ...)
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
andgenerate
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
andgenerate
as specified below. pkgs.formats.ini
{listsAsDuplicateKeys
? false,listToValue
? null, ... }-
A function taking an attribute set with values
listsAsDuplicateKeys
-
A boolean for controlling whether list values can be used to represent duplicate INI keys
listToValue
-
A function for turning a list of values into a single value.
It returns a set with INI-specific attributes
type
andgenerate
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
andgenerate
as specified below. pkgs.formats.elixirConf { elixir ? pkgs.elixir }
-
A function taking an attribute set with values
elixir
-
The Elixir package which will be used to format the generated output
It returns a set with Elixir-Config-specific attributes
type
,lib
, andgenerate
as specified below.The
lib
attribute contains functions to be used in settings, for generating special Elixir values:mkRaw elixirCode
-
Outputs the given string as raw Elixir code
mkGetEnv { envVariable, fallback ? null }
-
Makes the configuration fetch an environment variable at runtime
mkAtom atom
-
Outputs the given string as an Elixir atom, instead of the default Elixir binary string. Note: lowercase atoms still needs to be prefixed with
:
mkTuple array
-
Outputs the given array as an Elixir tuple, instead of the default Elixir list
mkMap attrset
-
Outputs the given attribute set as an Elixir map, instead of the default Elixir keyword list
::: {#pkgs-formats-result} These functions all return an attribute set with these values: :::
type
-
A module system type representing a value of the format
lib
-
Utility functions for convenience, or special interactions with the format. This attribute is optional. It may contain inside a
types
attribute containing types specific to this format. generate
filename jsonValue
-
A function that can render a value of the format to a file. Returns a file path.
::: {.note} This function puts the value contents in the Nix store. So this should be avoided for secrets. :::
::: {#ex-settings-nix-representable .example}
::: {.title}
Example: Module with conventional settings
option
:::
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; };
# ...
};
}
:::
Option declarations for attributes
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.
::: {#ex-settings-typed-attrs .example}
::: {.title}
Example: Declaring a type-checked settings
attribute
:::
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.
'';
};
:::