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>
users.extraUsers.your-user.initialPassword = "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>