nixpkgs/doc/manual/troubleshooting.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink">

<title>Troubleshooting</title>


<section><title>Boot problems</title>

<para>If NixOS fails to boot, there are a number of kernel command
line parameters that may help you to identify or fix the issue.  You
can add these parameters in the GRUB boot menu by pressing “e” to
modify the selected boot entry and editing the line starting with
<literal>linux</literal>.  The following are some useful kernel command
line parameters that are recognised by the NixOS boot scripts or by
systemd:

<variablelist>

  <varlistentry><term><literal>boot.shell_on_fail</literal></term>
    <listitem><para>Start a root shell if something goes wrong in
    stage 1 of the boot process (the initial ramdisk).  This is
    disabled by default because there is no authentication for the
    root shell.</para></listitem>
  </varlistentry>

  <varlistentry><term><literal>boot.debug1</literal></term>
    <listitem><para>Start an interactive shell in stage 1 before
    anything useful has been done.  That is, no modules have been
    loaded and no file systems have been mounted, except for
    <filename>/proc</filename> and
    <filename>/sys</filename>.</para></listitem>
  </varlistentry>

  <varlistentry><term><literal>boot.trace</literal></term>
    <listitem><para>Print every shell command executed by the stage 1
    and 2 boot scripts.</para></listitem>
  </varlistentry>

  <varlistentry><term><literal>single</literal></term>
    <listitem><para>Boot into rescue mode (a.k.a. single user mode).
    This will cause systemd to start nothing but the unit
    <literal>rescue.target</literal>, which runs
    <command>sulogin</command> to prompt for the root password and
    start a root login shell.  Exiting the shell causes the system to
    continue with the normal boot process.</para></listitem>
  </varlistentry>

  <varlistentry><term><literal>systemd.log_level=debug systemd.log_target=console</literal></term>
    <listitem><para>Make systemd very verbose and send log messages to
    the console instead of the journal.</para></listitem>
  </varlistentry>

</variablelist>

For more parameters recognised by systemd, see
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>

<para>If no login prompts or X11 login screens appear (e.g. due to
hanging dependencies), you can press Alt+ArrowUp.  If you’re lucky,
this will start rescue mode (described above).  (Also note that since
most units have a 90-second timeout before systemd gives up on them,
the <command>agetty</command> login prompts should appear eventually
unless something is very wrong.)</para>

</section>


<section>

<title>Maintenance mode</title>

<para>You can enter rescue mode by running:

<screen>
$ systemctl rescue</screen>

This will eventually give you a single-user root shell.  Systemd will
stop (almost) all system services.  To get out of maintenance mode,
just exit from the rescue shell.</para>

</section>


<section>

<title>Rolling back configuration changes</title>

<para>After running <command>nixos-rebuild</command> to switch to a
new configuration, you may find that the new configuration doesn’t
work very well.  In that case, there are several ways to return to a
previous configuration.</para>

<para>First, the GRUB boot manager allows you to boot into any
previous configuration that hasn’t been garbage-collected.  These
configurations can be found under the GRUB submenu “NixOS - All
configurations”.  This is especially useful if the new configuration
fails to boot.  After the system has booted, you can make the selected
configuration the default for subsequent boots:

<screen>
$ /run/current-system/bin/switch-to-configuration boot</screen>

</para>

<para>Second, you can switch to the previous configuration in a running
system:

<screen>
$ nixos-rebuild switch --rollback</screen>

This is equivalent to running:

<screen>
$ /nix/var/nix/profiles/system-<replaceable>N</replaceable>-link/bin/switch-to-configuration switch</screen>

where <replaceable>N</replaceable> is the number of the NixOS system
configuration.  To get a list of the available configurations, do:

<screen>
$ ls -l /nix/var/nix/profiles/system-*-link
<replaceable>...</replaceable>
lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
</screen>

</para>

</section>


</chapter>