UDEV(7)udevUDEV(7)NAMEudev - dynamic device management
DESCRIPTIONudev provides a dynamic device directory containing only the files for
actually present devices. It creates or removes device node files in
the /dev directory, or it renames network interfaces.
Usually udev runs as udevd(8) and receives uevents directly from the
kernel if a device is added or removed form the system.
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.
CONFIGURATION
All udev configuration files are placed in /etc/udev/*. Every file
consists of a set of lines of text. All empty lines or lines beginning
with '#' will be ignored.
Configuration file
udev expects its main configuration file at /etc/udev/udev.conf. It
consists of a set of variables allowing the user to override default
udev values. The following variables can be set:
udev_root
Specifies where to place the device nodes in the filesystem. The
default value is /dev.
udev_rules
The name of the udev rules file or directory to look for files
with the suffix .rules. Multiple rule files are read in lexical
order. The default value is /etc/udev/rules.d.
udev_log
The logging priority. Valid values are the numerical syslog
priorities or their textual representations: err, info and
debug.
Rules files
The udev rules are read from the files located in the /etc/udev/rules.d
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.
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:
== Compare for equality.
!= Compare for non-equality.
= Asign a value to a key. Keys that represent a list, are reset
and only this single value is assigned.
+= Add the value to a key that holds a list of entries.
:= Assign a value to a key finally; disallow any later changes,
which may be used to prevent changes by any later rules.
The following key names can be used to match against device properties:
ACTION Match the name of the event action.
KERNEL Match the name of the device.
DEVPATH
Match the devpath of the device.
SUBSYSTEM
Match the subsystem of the device.
BUS Search the devpath upwards for a matching device subsystem name.
DRIVER Search the devpath upwards for a matching device driver name.
ID Search the devpath upwards for a matching device name.
SYSFS{filename}
Search the devpath upwards for a device with matching sysfs
attribute values. Up to five SYSFS 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.
ENV{key}
Match against the value of an environment variable. Up to five
ENV keys can be specified per rule. This key can also be used to
export a variable to the environment.
PROGRAM
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.
RESULT 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.
Most of the fields support a shell style pattern matching. The
following pattern characters are supported:
* Matches zero, or any number of characters.
? Matches any single character.
[] 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.
The following keys can get values assigned:
NAME 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.
SYMLINK
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.
OWNER, GROUP, MODE
The permissions for the device node. Every specified value
overwrites the compiled-in default value.
ENV{key}
Export a variable to the environment. This key can also be used
to match against an environment variable.
RUN 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.
LABEL Named label where a GOTO can jump to.
GOTO Jumps to the next LABEL with a matching name
IMPORT{type}
Import the printed result or the value of a file in environment
key format into the event environment. program will execute an
external program and read its output. file will inport a text
file. If no option is given, udev will determine it from the
executable bit of of the file permissions.
WAIT_FOR_SYSFS
Wait for the specified sysfs file of the device to be created.
Can be used to fight against kernel sysfs timing issues.
OPTIONS
last_rule stops further rules application. No later rules will
have any effect. ignore_device will ignore this event
completely. ignore_remove will ignore any later remove event
for this device. This may be useful as a workaround for broken
device drivers. all_partitions will create device nodes for all
available partitions of a block device. This may be useful for
removable media.
The NAME, SYMLINK, PROGRAM, OWNER, GROUP and RUN fields support simple
printf-like string substitutions. The RUN 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:
$kernel, %k
The kernel name for this device.
$number, %n
The kernel number for this device. For example, 'sda3' has
kernel number of '3'
$devpath, %p
The devpath of the device.
$id, %b
The name of the device matched while searching the devpath
upwards for BUS, IDDRIVER and SYSFS.
$sysfs{file}, %s{file}
The value of a sysfs attribute found at the current or a parent
device.
$env{key}, %E{key}
The value of an environment variable.
$major, %M
The kernel major number for the device.
$minor %m
The kernel minor number for the device.
$result, %c
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: %c{N}. If the number is followed by the '+' char this
part plus all remaining parts of the result string are
substituted: %c{N+}
$parent, %P
The node name of the parent device.
$root, %r
The udev_root value.
$tempnode, %N
The name of a created temporary device node to provide access to
the device from a external program before the real node is
created.
%% The '%' character itself.
$$ The '$' character itself.
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
AUTHOR
Written by Greg Kroah-Hartman <greg@kroah.com> and Kay Sievers
<kay.sievers@vrfy.org>. With much help from Dan Stekloff
<dsteklof@us.ibm.com> and many others.
SEE ALSOudevd(8), udevinfo(8), udevmonitor(8)udev August 2005 UDEV(7)
