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91 lines
3 KiB
XML
91 lines
3 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-boot-problems">
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<title>Boot Problems</title>
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<para>
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If NixOS fails to boot, there are a number of kernel command line parameters
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that may help you to identify or fix the issue. You can add these parameters
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in the GRUB boot menu by pressing “e” to modify the selected boot entry
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and editing the line starting with <literal>linux</literal>. The following
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are some useful kernel command line parameters that are recognised by the
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NixOS boot scripts or by systemd:
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<variablelist>
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<varlistentry>
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<term>
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<literal>boot.shell_on_fail</literal>
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</term>
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<listitem>
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<para>
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Start a root shell if something goes wrong in stage 1 of the boot process
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(the initial ramdisk). This is disabled by default because there is no
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authentication for the root shell.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<literal>boot.debug1</literal>
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</term>
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<listitem>
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<para>
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Start an interactive shell in stage 1 before anything useful has been
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done. That is, no modules have been loaded and no file systems have been
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mounted, except for <filename>/proc</filename> and
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<filename>/sys</filename>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<literal>boot.trace</literal>
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</term>
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<listitem>
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<para>
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Print every shell command executed by the stage 1 and 2 boot scripts.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<literal>single</literal>
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</term>
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<listitem>
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<para>
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Boot into rescue mode (a.k.a. single user mode). This will cause systemd
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to start nothing but the unit <literal>rescue.target</literal>, which
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runs <command>sulogin</command> to prompt for the root password and start
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a root login shell. Exiting the shell causes the system to continue with
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the normal boot process.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<literal>systemd.log_level=debug systemd.log_target=console</literal>
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</term>
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<listitem>
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<para>
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Make systemd very verbose and send log messages to the console instead of
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the journal.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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For more parameters recognised by systemd, see <citerefentry>
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<refentrytitle>systemd</refentrytitle>
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<manvolnum>1</manvolnum></citerefentry>.
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</para>
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<para>
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If no login prompts or X11 login screens appear (e.g. due to hanging
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dependencies), you can press Alt+ArrowUp. If you’re lucky, this will start
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rescue mode (described above). (Also note that since most units have a
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90-second timeout before systemd gives up on them, the
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<command>agetty</command> login prompts should appear eventually unless
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something is very wrong.)
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</para>
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</section>
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