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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-changing-config">
<title>Changing the Configuration</title>
<para>
The file <literal>/etc/nixos/configuration.nix</literal> contains
the current configuration of your machine. Whenever you’ve
<link linkend="ch-configuration">changed something</link> in that
file, you should do
</para>
<programlisting>
# nixos-rebuild switch
</programlisting>
<para>
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>
This command doesn't start/stop
<link xlink:href="options.html#opt-systemd.user.services">user
services</link> automatically. <literal>nixos-rebuild</literal>
only runs a <literal>daemon-reload</literal> for each user with
running user services.
</para>
</warning>
<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
</para>
<programlisting>
# nixos-rebuild test
</programlisting>
<para>
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
</para>
<programlisting>
# nixos-rebuild boot
</programlisting>
<para>
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.
</para>
<programlisting>
# nixos-rebuild switch -p test
</programlisting>
<para>
which causes the new configuration (and previous ones created using
<literal>-p test</literal>) to show up in the GRUB submenu
<quote>NixOS - Profile 'test'</quote>. This can be useful to
separate test configurations from <quote>stable</quote>
configurations.
</para>
<para>
Finally, you can do
</para>
<programlisting>
$ nixos-rebuild build
</programlisting>
<para>
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
</para>
<programlisting>
$ nixos-rebuild build-vm
$ ./result/bin/run-*-vm
</programlisting>
<para>
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:
</para>
<programlisting language="bash">
users.users.your-user.initialHashedPassword = "test";
</programlisting>
<para>
<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):
</para>
<programlisting>
$ QEMU_NET_OPTS="hostfwd=tcp::2222-:22" ./result/bin/run-*-vm
</programlisting>
<para>
allowing you to log in via SSH (assuming you have set the
appropriate passwords or SSH authorized keys):
</para>
<programlisting>
$ ssh -p 2222 localhost
</programlisting>
</chapter>
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