Chunk NixOS manual

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+<section xmlns="http://docbook.org/ns/docbook"
+        xmlns:xlink="http://www.w3.org/1999/xlink"
+        xmlns:xi="http://www.w3.org/2001/XInclude"
+        version="5.0"
+        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:
+
+<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>