nixpkgs/nixos/doc/manual/configuration/config-file.xml

<section xmlns="http://docbook.org/ns/docbook"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:xi="http://www.w3.org/2001/XInclude"
    version="5.0"
    xml:id="sec-configuration-file">
 <title>NixOS Configuration File</title>

 <para>
  The NixOS configuration file generally looks like this:
<programlisting>
{ config, pkgs, ... }:

{ <replaceable>option definitions</replaceable>
}
</programlisting>
  The first line (<literal>{ config, pkgs, ... }:</literal>) denotes that this
  is actually a function that takes at least the two arguments
  <varname>config</varname> and <varname>pkgs</varname>. (These are explained
  later, in chapter <xref linkend="sec-writing-modules" />) The function returns
  a <emphasis>set</emphasis> of option definitions (<literal>{
  <replaceable>...</replaceable> }</literal>). These definitions have the form
  <literal><replaceable>name</replaceable> =
  <replaceable>value</replaceable></literal>, where
  <replaceable>name</replaceable> is the name of an option and
  <replaceable>value</replaceable> is its value. For example,
<programlisting>
{ config, pkgs, ... }:

{ <xref linkend="opt-services.httpd.enable"/> = true;
  <xref linkend="opt-services.httpd.adminAddr"/> = "alice@example.org";
  <link linkend="opt-services.httpd.virtualHosts">services.httpd.virtualHosts.localhost.documentRoot</link> = "/webroot";
}
</programlisting>
  defines a configuration with three option definitions that together enable
  the Apache HTTP Server with <filename>/webroot</filename> as the document
  root.
 </para>

 <para>
  Sets can be nested, and in fact dots in option names are shorthand for
  defining a set containing another set. For instance,
  <xref linkend="opt-services.httpd.enable"/> defines a set named
  <varname>services</varname> that contains a set named
  <varname>httpd</varname>, which in turn contains an option definition named
  <varname>enable</varname> with value <literal>true</literal>. This means that
  the example above can also be written as:
<programlisting>
{ config, pkgs, ... }:

{ services = {
    httpd = {
      enable = true;
      adminAddr = "alice@example.org";
      virtualHosts = {
        localhost = {
          documentRoot = "/webroot";
        };
      };
    };
  };
}
</programlisting>
  which may be more convenient if you have lots of option definitions that
  share the same prefix (such as <literal>services.httpd</literal>).
 </para>

 <para>
  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 <emphasis>option declaration</emphasis>),
  <command>nixos-rebuild</command> will give an error like:
<screen>
The option `services.httpd.enable' defined in `/etc/nixos/configuration.nix' does not exist.
</screen>
  Likewise, values in option definitions must have a correct type. For
  instance, <option>services.httpd.enable</option> must be a Boolean
  (<literal>true</literal> or <literal>false</literal>). Trying to give it a
  value of another type, such as a string, will cause an error:
<screen>
The option value `services.httpd.enable' in `/etc/nixos/configuration.nix' is not a boolean.
</screen>
 </para>

 <para>
  Options have various types of values. The most important are:
  <variablelist>
   <varlistentry>
    <term>
     Strings
    </term>
    <listitem>
     <para>
      Strings are enclosed in double quotes, e.g.
<programlisting>
<xref linkend="opt-networking.hostName"/> = "dexter";
</programlisting>
      Special characters can be escaped by prefixing them with a backslash
      (e.g. <literal>\"</literal>).
     </para>
     <para>
      Multi-line strings can be enclosed in <emphasis>double single
      quotes</emphasis>, e.g.
<programlisting>
<xref linkend="opt-networking.extraHosts"/> =
  ''
    127.0.0.2 other-localhost
    10.0.0.1 server
  '';
</programlisting>
      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
      <literal>"</literal> and <literal>\</literal> are not special (making it
      more convenient for including things like shell code). See more info
      about this in the Nix manual
      <link
      xlink:href="https://nixos.org/nix/manual/#ssec-values">here</link>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     Booleans
    </term>
    <listitem>
     <para>
      These can be <literal>true</literal> or <literal>false</literal>, e.g.
<programlisting>
<xref linkend="opt-networking.firewall.enable"/> = true;
<xref linkend="opt-networking.firewall.allowPing"/> = false;
</programlisting>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     Integers
    </term>
    <listitem>
     <para>
      For example,
<programlisting>
<xref linkend="opt-boot.kernel.sysctl"/>."net.ipv4.tcp_keepalive_time" = 60;
</programlisting>
      (Note that here the attribute name
      <literal>net.ipv4.tcp_keepalive_time</literal> is enclosed in quotes to
      prevent it from being interpreted as a set named <literal>net</literal>
      containing a set named <literal>ipv4</literal>, and so on. This is
      because it’s not a NixOS option but the literal name of a Linux kernel
      setting.)
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     Sets
    </term>
    <listitem>
     <para>
      Sets were introduced above. They are name/value pairs enclosed in braces,
      as in the option definition
<programlisting>
<xref linkend="opt-fileSystems"/>."/boot" =
  { device = "/dev/sda1";
    fsType = "ext4";
    options = [ "rw" "data=ordered" "relatime" ];
  };
</programlisting>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     Lists
    </term>
    <listitem>
     <para>
      The important thing to note about lists is that list elements are
      separated by whitespace, like this:
<programlisting>
<xref linkend="opt-boot.kernelModules"/> = [ "fuse" "kvm-intel" "coretemp" ];
</programlisting>
      List elements can be any other type, e.g. sets:
<programlisting>
swapDevices = [ { device = "/dev/disk/by-label/swap"; } ];
</programlisting>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     Packages
    </term>
    <listitem>
     <para>
      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 <varname>pkgs</varname>. Typical uses:
<programlisting>
<xref linkend="opt-environment.systemPackages"/> =
  [ pkgs.thunderbird
    pkgs.emacs
  ];

<xref linkend="opt-services.postgresql.package"/> = pkgs.postgresql_10;
</programlisting>
      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
      <xref linkend="sec-custom-packages"/>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </para>
</section>