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systemd/udev.xml
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<?xml version='1.0'?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<article>
<section>
<title>udev</title>
<refentry>
<refentryinfo>
<title>udev</title>
<date>August 2005</date>
<productname>udev</productname>
</refentryinfo>
<refmeta>
<refentrytitle>udev</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo class="version"></refmiscinfo>
</refmeta>
<refnamediv>
<refname>udev</refname>
<refpurpose>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 form 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>
</refsect1>
<refsect1><title>CONFIGURATION</title>
<para>All udev configuration files are placed in <filename>/etc/udev/*</filename>.
Every file consists of a set of lines of text. All empty lines or lines beginning
with '#' will be ignored.</para>
<refsect2><title>Configuration file</title>
<para>udev expects its main configuration file at <filename>/etc/udev/udev.conf</filename>.
It consists of a set of variables allowing the user to override default udev values.
The following variables can be set:</para>
<variablelist>
<varlistentry>
<term><option>udev_root</option></term>
<listitem>
<para>Specifies where to place the device nodes in the filesystem.
The default value is <filename>/dev</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>udev_rules</option></term>
<listitem>
<para>The name of the udev rules file or directory to look for files
with the suffix <filename>.rules</filename>. Multiple rule files are
read in lexical order. The default value is
<filename>/etc/udev/rules.d</filename>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>udev_log</option></term>
<listitem>
<para>The logging priority. Valid values are the numerical syslog priorities
or their textual representations: <option>err</option>, <option>info</option>
and <option>debug</option>.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect2>
<refsect2><title>Rules files</title>
<para>The udev rules are read from the files located in the
<filename>/etc/udev/rules.d</filename> directory or at the location specified
value in the configuraton file. Every line in the rules file contains at least
one key value pair. There are two kind of keys, match and assignement keys.
If all match keys are matching against its value, the rule gets applied and the
assign keys get the specified value assigned. 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 rule may 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
operators are:</para>
<variablelist>
<varlistentry>
<term><option>==</option></term>
<listitem>
<para>Compare for equality.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>!=</option></term>
<listitem>
<para>Compare for non-equality.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>=</option></term>
<listitem>
<para>Assign a value to a key. Keys that represent a list, are reset
and only this single value is assigned.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>+=</option></term>
<listitem>
<para>Add the value to a key that holds a list of entries.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>:=</option></term>
<listitem>
<para>Assign a value to a key finally; disallow any later changes,
which may be used to prevent changes by any later rules.</para>
</listitem>
</varlistentry>
</variablelist>
<para>The following key names can be used to match against device properties:</para>
<variablelist>
<varlistentry>
<term><option>ACTION</option></term>
<listitem>
<para>Match the name of the event action.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>KERNEL</option></term>
<listitem>
<para>Match the name of the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>DEVPATH</option></term>
<listitem>
<para>Match the devpath of the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>SUBSYSTEM</option></term>
<listitem>
<para>Match the subsystem of the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>BUS</option></term>
<listitem>
<para>Search the devpath upwards for a matching device subsystem name.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>DRIVER</option></term>
<listitem>
<para>Search the devpath upwards for a matching device driver name.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>ID</option></term>
<listitem>
<para>Search the devpath upwards for a matching device name.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>SYSFS{<replaceable>filename</replaceable>}</option></term>
<listitem>
<para>Search the devpath upwards for a device with matching sysfs attribute values.
Up to five <option>SYSFS</option> keys can be specified per rule. All attributes
must match on the same device. Trailing whitespace in the attribute values is ignored,
if the specified match value does not contain trailing whitespace itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>ENV{<replaceable>key</replaceable>}</option></term>
<listitem>
<para>Match against the value of an environment variable. Up to five <option>ENV</option>
keys can be specified per rule. This key can also be used to export a variable to
the environment.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>PROGRAM</option></term>
<listitem>
<para>Execute external program. The key is true, if the program returns
without exit code zero. The whole event environment is available to the
executed program. The program's output printed to stdout is available for
the RESULT key.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>RESULT</option></term>
<listitem>
<para>Match the returned string of the last PROGRAM call. This key can
be used in the same or in any later rule after a PROGRAM call.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Most of the fields support a shell style pattern matching. The following
pattern characters are supported:</para>
<variablelist>
<varlistentry>
<term><option>*</option></term>
<listitem>
<para>Matches zero, or any number of characters.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>?</option></term>
<listitem>
<para>Matches any single character.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>[]</option></term>
<listitem>
<para>Matches any single character specified within the brackets.
example, the pattern string 'tty[SR]' would match either 'ttyS' or 'ttyR'.
Ranges are also supported within this match with the '-' character.
For example, to match on the range of all digits, the pattern [0-9] would
be used. If the first character following the '[' is a '!', any characters
not enclosed are matched.</para>
</listitem>
</varlistentry>
</variablelist>
<para>The following keys can get values assigned:</para>
<variablelist>
<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. Only one rule can set the node name, all later rules with
a NAME key will be ignored.</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>
</listitem>
</varlistentry>
<varlistentry>
<term><option>OWNER, GROUP, MODE</option></term>
<listitem>
<para>The permissions for the device node. Every specified value overwrites
the compiled-in default value.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>ENV{<replaceable>key</replaceable>}</option></term>
<listitem>
<para>Export a variable to the environment. This key can also be used to match
against an environment variable.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>RUN</option></term>
<listitem>
<para>Add a program to the list of programs to be executed for a specific
device. This can only be used for very short running tasks. Running an
event process for a long period of time may block all further events for
this or a dependent device. Long running tasks need to be immediately
detached from the event process itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>LABEL</option></term>
<listitem>
<para>Named label where a GOTO can jump to.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>GOTO</option></term>
<listitem>
<para>Jumps to the next LABEL with a matching name</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>IMPORT{<replaceable>type</replaceable>}</option></term>
<listitem>
<para>Import the printed result or the value of a file in environment key
format into the event environment. <option>program</option> will execute an
external program and read its output. <option>file</option> will inport a
text file. If no option is given, udev will determine it from the executable
bit of of the file permissions.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>WAIT_FOR_SYSFS</option></term>
<listitem>
<para>Wait for the specified sysfs file of the device to be created. Can be used
to fight against kernel sysfs timing issues.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>OPTIONS</option></term>
<listitem>
<para><option>last_rule</option> stops further rules application. No later rules
will have any effect.
<option>ignore_device</option> will ignore this event completely.
<option>ignore_remove</option> will ignore any later remove event for this
device. This may be useful as a workaround for broken device drivers.
<option>all_partitions</option> will 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>
</variablelist>
<para>The <option>NAME</option>, <option>SYMLINK</option>, <option>PROGRAM</option>,
<option>OWNER</option>, <option>GROUP</option> and <option>RUN</option>
fields support simple printf-like string substitutions. The <option>RUN</option>
format chars gets applied after all rules have been processed, right before the program
is executed. It allows the use of the complete environment set by earlier matching
rules. For all other fields, substitutions are applied while the individual rule is
being processed. The available substitutions are:</para>
<variablelist>
<varlistentry>
<term><option>$kernel</option>, <option>%k</option></term>
<listitem>
<para>The kernel name for this device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$number</option>, <option>%n</option></term>
<listitem>
<para>The kernel number for this device. For example, 'sda3' has
kernel number of '3'</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$devpath</option>, <option>%p</option></term>
<listitem>
<para>The devpath of the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$id</option>, <option>%b</option></term>
<listitem>
<para>The name of the device matched while searching the devpath upwards for
<option>BUS</option>, <option>ID</option> <option>DRIVER</option> and <option>SYSFS</option>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$sysfs{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
<listitem>
<para>The value of a sysfs attribute found at the current or a parent device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
<listitem>
<para>The value of an environment variable.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$major</option>, <option>%M</option></term>
<listitem>
<para>The kernel major number for the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$minor</option> <option>%m</option></term>
<listitem>
<para>The kernel minor number for the device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$result</option>, <option>%c</option></term>
<listitem>
<para>The string returned by the external program requested with PROGRAM.
A single part of the string, separated by a space character may be selected
by specifying the part number as an attribute: <option>%c{N}</option>.
If the number is followed by the '+' char this part plus all remaining parts
of the result string are substituted: <option>%c{N+}</option></para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$parent</option>, <option>%P</option></term>
<listitem>
<para>The node name of the parent device.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$root</option>, <option>%r</option></term>
<listitem>
<para>The udev_root value.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$tempnode</option>, <option>%N</option></term>
<listitem>
<para>The name of a created temporary device node to provide access to
the device from a external program before the real node is created.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>%%</option></term>
<listitem>
<para>The '%' character itself.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>$$</option></term>
<listitem>
<para>The '$' character itself.</para>
</listitem>
</varlistentry>
</variablelist>
<para>The count of characters to be substituted may be limited by specifying
the format length value. For example, '%3s{file}' will only
insert the first three characters of the sysfs attribute</para>
</refsect2>
</refsect1>
<refsect1><title>AUTHOR</title>
<para>Written by Greg Kroah-Hartman <email>greg@kroah.com</email> and
Kay Sievers <email>kay.sievers@vrfy.org</email>. With much help from
Dan Stekloff <email>dsteklof@us.ibm.com</email> and many others.</para>
</refsect1>
<refsect1>
<title>SEE ALSO</title>
<para><citerefentry>
<refentrytitle>udevd</refentrytitle><manvolnum>8</manvolnum>
</citerefentry>,
<citerefentry>
<refentrytitle>udevinfo</refentrytitle><manvolnum>8</manvolnum>
</citerefentry>,
<citerefentry>
<refentrytitle>udevmonitor</refentrytitle><manvolnum>8</manvolnum>
</citerefentry></para>
</refsect1>
</refentry>
</section>
</article>
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