lfs-book/chapter07/symlinks.xml

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<sect1 id="ch-scripts-symlinks">
  <?dbhtml filename="symlinks.html"?>

  <title>Creating Custom Symlinks to Devices</title>

  <sect2>

    <title>CD-ROM symlinks</title>

    <para>Some software that you may want to install later (e.g., various
    media players) expect the /dev/cdrom and /dev/dvd symlinks to exist.
    Also, it may be convenient to put references to those symlinks into
    <filename>/etc/fstab</filename>. For each of your CD-ROM devices,
    find the corresponding directory under
    <filename class="directory">/sys</filename> (e.g., this can be
    <filename class="directory">/sys/block/hdd</filename>) and
    run a command similar to the following:</para>

<screen role="nodump"><userinput>udevtest /block/hdd</userinput></screen>

    <para>Look at the lines containing the output of various *_id programs.</para>

    <para>There are two approaches to creating symlinks. The first one is to
    use the model name and the serial number, the second one is based on the
    location of the device on the bus. If you are going to use the first
    approach, create a file similar to the following:</para>

<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/70-persistent-cd.rules &lt;&lt; EOF
<literal>
# Custom CD-ROM symlinks
SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", \
    ENV{ID_REVISION}=="PS05", ENV{GENERATED}="1", SYMLINK+="cdrom"
SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", \
    ENV{ID_SERIAL}=="5VO1306DM00190", ENV{GENERATED}="1", SYMLINK+="cdrom1 dvd"
</literal>
EOF</userinput></screen>

    <note>
      <para>Be aware that Udev does not recognize the backslash for line
      continuation.  The examples in this book work properly because both
      the backslash and newline are ignored by the shell.  This makes the
      shell send each rule to cat on only one line.  (The shell ignores
      this sequence because the EOF string used in the here-document
      redirection is not enclosed in either double or single quotes.  For
      more details, see the bash(1) manpage, and search it for "Here
      Documents".)</para>
      <para>If modifying Udev rules with an editor, be sure to leave each
      rule on one physical line.</para>
    </note>

    <para>This way, the symlinks will stay correct even if you move the drives
    to different positions on the IDE bus, but the
    <filename>/dev/cdrom</filename> symlink won't be created if you replace
    the old SAMSUNG CD-ROM with a new drive.</para>
<!-- The symlinks in the first approach survive even the transition
     to libata for IDE drives, but that is not for the book. -->

    <para>The SUBSYSTEM==&quot;block&quot; key is needed in order to avoid
    matching SCSI generic devices. Without it, in the case with SCSI
    CD-ROMs, the symlinks will sometimes point to the correct
    <filename>/dev/srX</filename> devices, and sometimes to
    <filename>/dev/sgX</filename>, which is wrong.</para>

    <para>The ENV{GENERATED}="1" key is needed to prevent the Udev
    75-cd-aliases-generator.rules file from overriding your custom
    rules.</para>

    <para>The second approach yields:</para>

<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/70-persistent-cd.rules &lt;&lt; EOF
<literal>
# Custom CD-ROM symlinks
SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
    ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", \
    ENV{GENERATED}="1", SYMLINK+="cdrom"
SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
    ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", \
    ENV{GENERATED}="1", SYMLINK+="cdrom1 dvd"
</literal>
EOF</userinput></screen>

    <para>This way, the symlinks will stay correct even if you replace drives
    with different models, but place them to the old positions on the IDE
    bus. The ENV{ID_TYPE}==&quot;cd&quot; key makes sure that the symlink
    disappears if you put something other than a CD-ROM in that position on
    the bus.</para>

    <para>Of course, it is possible to mix the two approaches.</para>

  </sect2>

  <sect2>

    <title>Dealing with duplicate devices</title>

    <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in
    which devices with the same function appear in
    <filename class="directory">/dev</filename> is essentially random.
    E.g., if you have a USB web camera and a TV tuner, sometimes
    <filename>/dev/video0</filename> refers to the camera and
    <filename>/dev/video1</filename> refers to the tuner, and sometimes
    after a reboot the order changes to the opposite one.
    For all classes of hardware except sound cards and network cards, this is
    fixable by creating udev rules for custom persistent symlinks.
    The case of network cards is covered separately in
    <xref linkend="ch-scripts-network"/>, and sound card configuration can
    be found in <ulink url="&blfs-root;">BLFS</ulink>.</para>

    <para>For each of your devices that is likely to have this problem
    (even if the problem doesn't exist in your current Linux distribution),
    find the corresponding directory under
    <filename class="directory">/sys/class</filename> or
    <filename class="directory">/sys/block</filename>.
    For video devices, this may be
    <filename
    class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
    Figure out the attributes that identify the device uniquely (usually,
    vendor and product IDs and/or serial numbers work):</para>

<screen role="nodump"><userinput>udevinfo -a -p /sys/class/video4linux/video0</userinput></screen>

    <para>Then write rules that create the symlinks, e.g.:</para>

<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/83-duplicate_devs.rules &lt;&lt; EOF
<literal>
# Persistent symlinks for webcam and tuner
KERNEL=="video*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", \
    SYMLINK+="webcam"
KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{vendor}=="0x109e", \
    SYMLINK+="tvtuner"
</literal>
EOF</userinput></screen>

    <para>The result is that <filename>/dev/video0</filename> and
    <filename>/dev/video1</filename> devices still refer randomly to the tuner
    and the web camera (and thus should never be used directly), but there are
    symlinks <filename>/dev/tvtuner</filename> and
    <filename>/dev/webcam</filename> that always point to the correct
    device.</para>

    <para>More information on writing Udev rules can be found in
    <filename>/usr/share/doc/udev-&udev-version;/index.html</filename>.</para>

 </sect2>

</sect1>