nixpkgs/nixos/doc/manual/installation/changing-config.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         version="5.0"
         xml:id="sec-changing-config">
 <title>Changing the Configuration</title>
 <para>
  The file <filename>/etc/nixos/configuration.nix</filename> contains the
  current configuration of your machine. Whenever you’ve
  <link linkend="ch-configuration">changed something</link> in that file, you
  should do
<screen>
# nixos-rebuild switch</screen>
  to build the new configuration, make it the default configuration for
  booting, and try to realise the configuration in the running system (e.g., by
  restarting system services).
 </para>
 <warning>
  <para>
   These commands must be executed as root, so you should either run them from
   a root shell or by prefixing them with <literal>sudo -i</literal>.
  </para>
 </warning>
 <para>
  You can also do
<screen>
# nixos-rebuild test</screen>
  to build the configuration and switch the running system to it, but without
  making it the boot default. So if (say) the configuration locks up your
  machine, you can just reboot to get back to a working configuration.
 </para>
 <para>
  There is also
<screen>
# nixos-rebuild boot</screen>
  to build the configuration and make it the boot default, but not switch to it
  now (so it will only take effect after the next reboot).
 </para>
 <para>
  You can make your configuration show up in a different submenu of the GRUB 2
  boot screen by giving it a different <emphasis>profile name</emphasis>, e.g.
<screen>
# nixos-rebuild switch -p test </screen>
  which causes the new configuration (and previous ones created using
  <literal>-p test</literal>) to show up in the GRUB submenu “NixOS - Profile
  'test'”. This can be useful to separate test configurations from
  “stable” configurations.
 </para>
 <para>
  Finally, you can do
<screen>
$ nixos-rebuild build</screen>
  to build the configuration but nothing more. This is useful to see whether
  everything compiles cleanly.
 </para>
 <para>
  If you have a machine that supports hardware virtualisation, you can also
  test the new configuration in a sandbox by building and running a QEMU
  <emphasis>virtual machine</emphasis> that contains the desired configuration.
  Just do
<screen>
$ nixos-rebuild build-vm
$ ./result/bin/run-*-vm
</screen>
  The VM does not have any data from your host system, so your existing user
  accounts and home directories will not be available unless you have set
  <literal>mutableUsers = false</literal>. Another way is to temporarily add
  the following to your configuration:
<screen>
<link linkend="opt-users.users._name__.initialHashedPassword">users.users.your-user.initialHashedPassword</link> = "test";
</screen>
  <emphasis>Important:</emphasis> delete the $hostname.qcow2 file if you have
  started the virtual machine at least once without the right users, otherwise
  the changes will not get picked up. You can forward ports on the host to the
  guest. For instance, the following will forward host port 2222 to guest port
  22 (SSH):
<screen>
$ QEMU_NET_OPTS="hostfwd=tcp::2222-:22" ./result/bin/run-*-vm
</screen>
  allowing you to log in via SSH (assuming you have set the appropriate
  passwords or SSH authorized keys):
<screen>
$ ssh -p 2222 localhost
</screen>
 </para>
</chapter>