# NixOS Configuration File {#sec-configuration-file} The NixOS configuration file generally looks like this: ```nix { config, pkgs, ... }: { option definitions } ``` The first line (`{ config, pkgs, ... }:`) denotes that this is actually a function that takes at least the two arguments `config` and `pkgs`. (These are explained later, in chapter [](#sec-writing-modules)) The function returns a *set* of option definitions (`{ ... }`). These definitions have the form `name = value`, where `name` is the name of an option and `value` is its value. For example, ```nix { config, pkgs, ... }: { services.httpd.enable = true; services.httpd.adminAddr = "alice@example.org"; services.httpd.virtualHosts.localhost.documentRoot = "/webroot"; } ``` defines a configuration with three option definitions that together enable the Apache HTTP Server with `/webroot` as the document root. Sets can be nested, and in fact dots in option names are shorthand for defining a set containing another set. For instance, [](#opt-services.httpd.enable) defines a set named `services` that contains a set named `httpd`, which in turn contains an option definition named `enable` with value `true`. This means that the example above can also be written as: ```nix { config, pkgs, ... }: { services = { httpd = { enable = true; adminAddr = "alice@example.org"; virtualHosts = { localhost = { documentRoot = "/webroot"; }; }; }; }; } ``` which may be more convenient if you have lots of option definitions that share the same prefix (such as `services.httpd`). NixOS checks your option definitions for correctness. For instance, if you try to define an option that doesn't exist (that is, doesn't have a corresponding *option declaration*), `nixos-rebuild` will give an error like: ```plain The option `services.httpd.enable' defined in `/etc/nixos/configuration.nix' does not exist. ``` Likewise, values in option definitions must have a correct type. For instance, `services.httpd.enable` must be a Boolean (`true` or `false`). Trying to give it a value of another type, such as a string, will cause an error: ```plain The option value `services.httpd.enable' in `/etc/nixos/configuration.nix' is not a boolean. ``` Options have various types of values. The most important are: Strings : Strings are enclosed in double quotes, e.g. ```nix networking.hostName = "dexter"; ``` Special characters can be escaped by prefixing them with a backslash (e.g. `\"`). Multi-line strings can be enclosed in *double single quotes*, e.g. ```nix networking.extraHosts = '' 127.0.0.2 other-localhost 10.0.0.1 server ''; ``` The main difference is that it strips from each line a number of spaces equal to the minimal indentation of the string as a whole (disregarding the indentation of empty lines), and that characters like `"` and `\` are not special (making it more convenient for including things like shell code). See more info about this in the Nix manual [here](https://nixos.org/nix/manual/#ssec-values). Booleans : These can be `true` or `false`, e.g. ```nix networking.firewall.enable = true; networking.firewall.allowPing = false; ``` Integers : For example, ```nix boot.kernel.sysctl."net.ipv4.tcp_keepalive_time" = 60; ``` (Note that here the attribute name `net.ipv4.tcp_keepalive_time` is enclosed in quotes to prevent it from being interpreted as a set named `net` containing a set named `ipv4`, and so on. This is because it's not a NixOS option but the literal name of a Linux kernel setting.) Sets : Sets were introduced above. They are name/value pairs enclosed in braces, as in the option definition ```nix fileSystems."/boot" = { device = "/dev/sda1"; fsType = "ext4"; options = [ "rw" "data=ordered" "relatime" ]; }; ``` Lists : The important thing to note about lists is that list elements are separated by whitespace, like this: ```nix boot.kernelModules = [ "fuse" "kvm-intel" "coretemp" ]; ``` List elements can be any other type, e.g. sets: ```nix swapDevices = [ { device = "/dev/disk/by-label/swap"; } ]; ``` Packages : Usually, the packages you need are already part of the Nix Packages collection, which is a set that can be accessed through the function argument `pkgs`. Typical uses: ```nix environment.systemPackages = [ pkgs.thunderbird pkgs.emacs ]; services.postgresql.package = pkgs.postgresql_14; ``` The latter option definition changes the default PostgreSQL package used by NixOS's PostgreSQL service to 10.x. For more information on packages, including how to add new ones, see [](#sec-custom-packages).