<refnamediv>
<refname>udev</refname>
- <refpurpose>dynamic device management</refpurpose>
+ <refpurpose>Linux dynamic device management</refpurpose>
</refnamediv>
<refsect1><title>DESCRIPTION</title>
- <para>udev provides a dynamic device directory containing only the files for
- actually present devices. It creates or removes device node files in the
- <filename>/dev</filename> directory, or it renames network interfaces.</para>
-
- <para>Usually udev runs as <citerefentry><refentrytitle>udevd</refentrytitle>
- <manvolnum>8</manvolnum></citerefentry> and receives uevents directly from the
- kernel if a device is added or removed from the system.</para>
-
- <para>If udev receives a device event, it matches its configured rules
- against the available device attributes provided in sysfs to identify the device.
- Rules that match may provide additional device information or specify a device
- node name and multiple symlink names and instruct udev to run additional programs
- as part of the device event handling.</para>
+ <para>udev supplies the system software with device events, manages permissions
+ of device nodes and may create additional symlinks in the <filename>/dev</filename>
+ directory, or renames network interfaces. The kernel usually just assigns unpredictable
+ device names based on the order of discovery. Meaningful symlinks or network device
+ names provide a way to reliably identify devices based on their properties or
+ current configuration.</para>
+
+ <para>The udev daemon <citerefentry><refentrytitle>udevd</refentrytitle>
+ <manvolnum>8</manvolnum></citerefentry> receives device uevents directly from
+ the kernel whenever a device is added or removed from the system, or it changes its
+ state. When udev receives a device event, it matches its configured set of rules
+ against various device attributes to identify the device. Rules that match, may
+ provide additional device information to be stored in the udev database, or information
+ to be used to create meaningful symlink names.</para>
+
+ <para>All device information udev processes, is stored in the udev database and
+ sent out to possible event subscribers. Access to all stored data and the event
+ sources are provided by the library libudev.</para>
</refsect1>
<refsect1><title>CONFIGURATION</title>
If all match keys are matching against its value, the rule gets applied and the
assign keys get the specified value assigned.</para>
- <para>A matching rule may specify the name of the device node, add a symlink
- pointing to the node, or run a specified program as part of the event handling.
- If no matching rule is found, the default device node name is used.</para>
+ <para>A matching rule may rename a network interface, add symlinks
+ pointing to the device node, or run a specified program as part of
+ the event handling.</para>
<para>A rule consists of a list of one or more key value pairs separated by
a comma. Each key has a distinct operation, depending on the used operator. Valid
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>SYMLINK</option></term>
+ <listitem>
+ <para>Match the name of a symlink targeting the node. It can
+ be used once a SYMLINK key has been set in one of the preceding
+ rules. There may be multiple symlinks; only one needs to match.
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><option>SUBSYSTEM</option></term>
<listitem>
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>TAG</option></term>
+ <listitem>
+ <para>Match against a device tag.</para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><option>TEST{<replaceable>octal mode mask</replaceable>}</option></term>
<listitem>
<varlistentry>
<term><option>NAME</option></term>
<listitem>
- <para>The name of the node to be created, or the name the network interface
- should be renamed to.</para>
+ <para>The name, a network interface should be renamed to. Or as
+ a temporary workaraound, the name a device node should be named.
+ Usually the kernel provides the defined node name, or even creates
+ and removes the node before udev even receives any event. Changing
+ the node name from the kernel's default creates inconsistencies
+ and is not supported. If the kernel and NAME specify different names,
+ an error will be logged. Udev is only expected to handle device node
+ permissions and to create additional symlinks, not to change
+ kernel-provided device node names. Instead of renaming a device node,
+ SYMLINK should be used. Symlink names must never conflict with
+ device node names, it will result in unpredictable behavior.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>SYMLINK</option></term>
<listitem>
- <para>The name of a symlink targeting the node. Every matching rule can add
- this value to the list of symlinks to be created along with the device node.
- Multiple symlinks may be specified by separating the names by the space
- character.</para>
+ <para>The name of a symlink targeting the node. Every matching rule will add
+ this value to the list of symlinks to be created. Multiple symlinks may be
+ specified by separating the names by the space character. In case multiple
+ devices claim the same name, the link will always point to the device with
+ the highest link_priority. If the current device goes away, the links will
+ be re-evaluated and the device with the next highest link_priority will own
+ the link. If no link_priority is specified, the order of the devices, and
+ which one of them will own the link, is undefined. Claiming the same name for
+ a symlink, which is or might be used for a device node, may result in
+ unexpected behavior and is not supported.
+ </para>
</listitem>
</varlistentry>
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>TAG</option></term>
+ <listitem>
+ <para>Attach a tag to a device. This is used to filter events for users
+ of libudev's monitor functionality, or to enumerate a group of tagged
+ devices. The implementation can only work efficiently if only a few
+ tags are attached to a device. It is only meant to be used in
+ contexts with specific device filter requirements, and not as a
+ general-purpose flag. Excessive use might result in inefficient event
+ handling.</para>
+ </listitem>
+ </varlistentry>
+
<varlistentry>
<term><option>RUN</option></term>
<listitem>
<option>RUN{<replaceable>fail_event_on_error</replaceable>}</option> is
specified, and the executed program returns non-zero, the event will be
marked as failed for a possible later handling.</para>
- <para>If the specified string starts with
- <option>socket:<replaceable>path</replaceable></option>, all current event
- values will be passed to the specified socket, as a message in the same
- format the kernel sends an uevent. If the first character of the specified path
- is an @ character, an abstract namespace socket is used, instead of an existing
- socket file.</para>
+ <para>If no absolute path is given, the program is expected to live in
+ <filename>/lib/udev</filename>, otherwise the absolute path must be
+ specified. Program name and arguments are separated by spaces. Single quotes
+ can be used to specify arguments with spaces.</para>
</listitem>
</varlistentry>
<term><option>program</option></term>
<listitem>
<para>Execute an external program specified as the assigned value and
- import its output, which must be in environment key format.</para>
+ import its output, which must be in environment key
+ format. Path specification, command/argument separation,
+ and quoting work like in <option>RUN</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
environment key format.</para>
</listitem>
</varlistentry>
+ <varlistentry>
+ <term><option>db</option></term>
+ <listitem>
+ <para>Import a single property specified as the assigned value from the
+ current device database. This works only if the database is already populated
+ by an earlier event.</para>
+ </listitem>
+ </varlistentry>
<varlistentry>
<term><option>parent</option></term>
<listitem>
<varlistentry>
<term><option>WAIT_FOR</option></term>
<listitem>
- <para>Wait for a file to become available.</para>
+ <para>Wait for a file to become available or until a 10
+ seconds timeout expires.</para>
</listitem>
</varlistentry>
<listitem>
<para>Rule and device options:</para>
<variablelist>
- <varlistentry>
- <term><option>last_rule</option></term>
- <listitem>
- <para>Stops further rules application. No later rules will have
- any effect.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><option>ignore_device</option></term>
- <listitem>
- <para>Ignore this event completely.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><option>ignore_remove</option></term>
- <listitem>
- <para>Do not remove the device node when the device goes away. This may be
- useful as a workaround for broken device drivers.</para>
- </listitem>
- </varlistentry>
<varlistentry>
<term><option>link_priority=<replaceable>value</replaceable></option></term>
<listitem>
priorities overwrite existing symlinks of other devices. The default is 0.</para>
</listitem>
</varlistentry>
- <varlistentry>
- <term><option>all_partitions</option></term>
- <listitem>
- <para>Create the device nodes for all available partitions of a block device.
- This may be useful for removable media devices where media changes are not
- detected.</para>
- </listitem>
- </varlistentry>
<varlistentry>
<term><option>event_timeout=</option></term>
<listitem>
<term><option>watch</option></term>
<listitem>
<para>Watch the device node with inotify, when closed after being opened for
- writing, a change uevent will be synthesised.</para>
+ writing, a change uevent will be synthesised.</para>
</listitem>
</varlistentry>
</variablelist>