UDEV(7) udev UDEV(7)
NAME
udev - dynamic device management
DESCRIPTION
udev 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 configuration file.
Every line in the rules file contains at least one key value pair.
There are two kind of keys, match and assignment 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.
=
Assign 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.
DEVPATH
Match the devpath of the event device.
KERNEL
Match the name of the event device.
SUBSYSTEM
Match the subsystem of the event device.
ATTR{filename}
Match sysfs attribute values of the event device. Up to five ATTR
keys can be specified per rule. Trailing whitespace in the
attribute values is ignored, if the specified match value does not
contain trailing whitespace itself. Depending on the type of
operator, this key is also used to set the value of a sysfs
attribute.
KERNELS
Search the devpath upwards for a matching device name.
SUBSYSTEMS
Search the devpath upwards for a matching device subsystem name.
DRIVERS
Search the devpath upwards for a matching device driver name.
ATTRS{filename}
Search the devpath upwards for a device with matching sysfs
attribute values. Up to five ATTRS 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. Depending on the type of operator,
this key is also 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 programs 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. For
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.
ATTR{key}
The value that should be written to a sysfs attribute of the event
device. Depending on the type of operator, this key is also used to
match against the value of a sysfs attribute.
ENV{key}
Export a variable to the environment. Depending on the type of
operator, this key is also 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 a set of variables into the event environment, depending on
type:
program
Execute an external program specified as the assigned value
and import its output, which must be in environment key
format.
file
Import a text file specified as the assigned value, which
must be in environment key format.
parent
Import the stored keys from the parent device by reading
the database entry of the parent device. The value assigned
to IMPORT{parent} is used as a filter of key names to
import (with the same shell-style pattern matching used for
comparisons).
If no option is given, udev will choose between program and
file based on 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 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.
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 SUBSYSTEMS, KERNELS, DRIVERS and ATTRS.
$attr{file}, %s{file}
The value of a sysfs attribute found at the device, where all keys
of the rule have matched. If the matching device does not have such
an attribute, all devices along the chain of parents are searched
for a matching attribute. If the attribute is a symlink, the last
element of the symlink target is returned as the value.
$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 and Kay Sievers
. With much help from Dan Stekloff
and many others.
SEE ALSO
udevd(8), udevinfo(8), udevmonitor(8)
udev August 2005 UDEV(7)
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