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>
<prompt># </prompt>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).
  <warning>
   <para>
    This command doesn't start/stop <link linkend="opt-systemd.user.services">user services</link> automatically. <command>nixos-rebuild</command> only runs a <literal>daemon-reload</literal> for each user with running user services.
   </para>
  </warning>
 </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>
<prompt># </prompt>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>
<prompt># </prompt>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>
<prompt># </prompt>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>
<prompt>$ </prompt>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>
<prompt>$ </prompt>nixos-rebuild build-vm
<prompt>$ </prompt>./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 $hostname.qcow2 file if you have started the virtual machine at least once without the right users, otherwise the changes will not get picked up. You can forward ports on the host to the guest. For instance, the following will forward host port 2222 to guest port 22 (SSH):
<screen>
<prompt>$ </prompt>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>
<prompt>$ </prompt>ssh -p 2222 localhost
</screen>
 </para>
</chapter>