systemd/man/hwdb.xml

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<refentry id="hwdb" conditional="ENABLE_HWDB">
  <refentryinfo>
    <title>hwdb</title>
    <productname>systemd</productname>
    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Kay</firstname>
        <surname>Sievers</surname>
        <email>kay@vrfy.org</email>
      </author>
      <author>
        <contrib>Developer</contrib>
        <firstname>Tom</firstname>
        <surname>Gundersen</surname>
        <email>teg@jklm.no</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>hwdb</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>hwdb</refname>
    <refpurpose>Hardware Database</refpurpose>
  </refnamediv>

  <refsect1><title>Description</title>
    <para>The hardware database is a key-value store for associating modalias-like keys to
    udev-property-like values. It is used primarily by udev to add the relevant properties
    to matching devices, but it can also be queried directly.</para>
  </refsect1>

  <refsect1><title>Hardware Database Files</title>
      <para>The hwdb files are read from the files located in the
      system hwdb directory <filename>/usr/lib/udev/hwdb.d</filename> and
      the local administration directory <filename>/etc/udev/hwdb.d</filename>.
      All hwdb files are collectively sorted and processed in lexical order,
      regardless of the directories in which they live. However, files with
      identical filenames replace each other. Files in <filename>/etc</filename>
      have the highest priority and take precedence over files with the same
      name in <filename>/usr/lib</filename>. This can be used to override a
      system-supplied hwdb file with a local file if needed;
      a symlink in <filename>/etc</filename> with the same name as a hwdb file in
      <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
      disables the hwdb file entirely. hwdb files must have the extension
      <filename>.hwdb</filename>; other extensions are ignored.</para>

      <para>The hwdb file contains data records consisting of matches and
      associated key-value pairs. Every record in the hwdb starts with one or
      more match strings, specifying a shell glob to compare the database
      lookup string against. Multiple match lines are specified in additional
      consecutive lines. Every match line is compared individually, and they are
      combined by OR. Every match line must start at the first character of
      the line.</para>

      <para>The match lines are followed by one or more key-value pair lines, which
      are recognized by a leading space character. The key name and value are separated
      by <literal>=</literal>. An empty line signifies the end
      of a record. Lines beginning with <literal>#</literal> are ignored.</para>

      <para>The content of all hwdb files is read by
      <citerefentry><refentrytitle>systemd-hwdb</refentrytitle><manvolnum>8</manvolnum></citerefentry>
      and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>,
      or alternatively <filename>/usr/lib/udev/hwdb.bin</filename> if you want ship the compiled
      database in an immutable image.
      During runtime, only the binary database is used.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry>
        <refentrytitle>systemd-hwdb</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>
    </para>
  </refsect1>
</refentry>