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

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  <refentrytitle><filename>configuration.nix</filename>
  </refentrytitle><manvolnum>5</manvolnum>
  <refmiscinfo class="source">NixOS</refmiscinfo>
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 <refnamediv>
  <refname><filename>configuration.nix</filename>
  </refname><refpurpose>NixOS system configuration specification</refpurpose>
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 <refsection>
  <title>Description</title>
  <para>
   The file <filename>/etc/nixos/configuration.nix</filename> contains the
   declarative specification of your NixOS system configuration. The command
   <command>nixos-rebuild</command> takes this file and realises the system
   configuration specified therein.
  </para>
 </refsection>
 <refsection>
  <title>Options</title>
  <para>
   You can use the following options in <filename>configuration.nix</filename>.
  </para>
  <xi:include href="./generated/options-db.xml"
            xpointer="configuration-variable-list" />
 </refsection>
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