<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-boot-problems">
  <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:
  </para>
  <variablelist>
    <varlistentry>
      <term>
        <literal>boot.shell_on_fail</literal>
      </term>
      <listitem>
        <para>
          Allows the user to 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
          <literal>/proc</literal> and <literal>/sys</literal>.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>boot.debug1devices</literal>
      </term>
      <listitem>
        <para>
          Like <literal>boot.debug1</literal>, but runs stage1 until
          kernel modules are loaded and device nodes are created. This
          may help with e.g. making the keyboard work.
        </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term>
        <literal>boot.debug1mounts</literal>
      </term>
      <listitem>
        <para>
          Like <literal>boot.debug1</literal> or
          <literal>boot.debug1devices</literal>, but runs stage1 until
          all filesystems that are mounted during initrd are mounted
          (see
          <link linkend="opt-fileSystems._name_.neededForBoot">neededForBoot</link>).
          As a motivating example, this could be useful if you’ve
          forgotten to set
          <link linkend="opt-fileSystems._name_.neededForBoot">neededForBoot</link>
          on a file system.
        </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
          <literal>sulogin</literal> 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</literal>
        <literal>systemd.log_target=console</literal>
      </term>
      <listitem>
        <para>
          Make systemd very verbose and send log messages to the console
          instead of the journal. For more parameters recognised by
          systemd, see systemd(1).
        </para>
      </listitem>
    </varlistentry>
  </variablelist>
  <para>
    In addition, these arguments are recognised by the live image only:
  </para>
  <variablelist>
    <varlistentry>
      <term>
        <literal>live.nixos.passwd=password</literal>
      </term>
      <listitem>
        <para>
          Set the password for the <literal>nixos</literal> live user.
          This can be used for SSH access if there are issues using the
          terminal.
        </para>
      </listitem>
    </varlistentry>
  </variablelist>
  <para>
    Notice that for <literal>boot.shell_on_fail</literal>,
    <literal>boot.debug1</literal>,
    <literal>boot.debug1devices</literal>, and
    <literal>boot.debug1mounts</literal>, if you did
    <emphasis role="strong">not</emphasis> select <quote>start the new
    shell as pid 1</quote>, and you <literal>exit</literal> from the new
    shell, boot will proceed normally from the point where it failed, as
    if you’d chosen <quote>ignore the error and continue</quote>.
  </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
    <literal>agetty</literal> login prompts should appear eventually
    unless something is very wrong.)
  </para>
</section>
