nixpkgs/nixos/doc/manual/man-nixos-option.xml

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 <refmeta>
  <refentrytitle><command>nixos-option</command>
  </refentrytitle><manvolnum>8</manvolnum>
  <refmiscinfo class="source">NixOS</refmiscinfo>
<!-- <refmiscinfo class="version"><xi:include href="version.txt" parse="text"/></refmiscinfo> -->
 </refmeta>
 <refnamediv>
  <refname><command>nixos-option</command>
  </refname><refpurpose>inspect a NixOS configuration</refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <cmdsynopsis>
   <command>nixos-option</command> 
   <arg>
    <option>-I</option> <replaceable>path</replaceable>
   </arg>
    
   <arg>
    <option>--verbose</option>
   </arg>
    
   <arg>
    <option>--xml</option>
   </arg>
    
   <arg choice="plain">
    <replaceable>option.name</replaceable>
   </arg>
  </cmdsynopsis>
 </refsynopsisdiv>
 <refsection>
  <title>Description</title>
  <para>
   This command evaluates the configuration specified in <filename>/etc/nixos/configuration.nix</filename> and returns the properties of the option name given as argument.
  </para>
  <para>
   When the option name is not an option, the command prints the list of attributes contained in the attribute set.
  </para>
 </refsection>
 <refsection>
  <title>Options</title>
  <para>
   This command accepts the following options:
  </para>
  <variablelist>
   <varlistentry>
    <term>
     <option>-I</option> <replaceable>path</replaceable>
    </term>
    <listitem>
     <para>
      This option is passed to the underlying <command>nix-instantiate</command> invocation.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <option>--verbose</option>
    </term>
    <listitem>
     <para>
      This option enables verbose mode, which currently is just the Bash <command>set</command> <option>-x</option> debug mode.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <option>--xml</option>
    </term>
    <listitem>
     <para>
      This option causes the output to be rendered as XML.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsection>
 <refsection>
  <title>Environment</title>
  <variablelist>
   <varlistentry>
    <term>
     <envar>NIXOS_CONFIG</envar>
    </term>
    <listitem>
     <para>
      Path to the main NixOS configuration module. Defaults to <filename>/etc/nixos/configuration.nix</filename>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsection>
 <refsection>
  <title>Examples</title>
  <para>
   Investigate option values:
<screen><prompt>$ </prompt>nixos-option boot.loader
This attribute set contains:
generationsDir
grub
initScript

<prompt>$ </prompt>nixos-option boot.loader.grub.enable
Value:
true

Default:
true

Description:
Whether to enable the GNU GRUB boot loader.

Declared by:
  "/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/nixos/modules/system/boot/loader/grub/grub.nix"

Defined by:
  "/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/nixos/modules/system/boot/loader/grub/grub.nix"
</screen>
  </para>
 </refsection>
 <refsection>
  <title>Bugs</title>
  <para>
   The author listed in the following section is wrong. If there is any other bug, please report to Nicolas Pierron.
  </para>
 </refsection>
</refentry>