# 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