udev
UDEV(8) Linux Administrator's Manual UDEV(8)
NAME
udev - Linux configurable dynamic device naming support
SYNOPSIS
udev
DESCRIPTION
udev provides a dynamic device directory containing only the files for
actually present devices. It creates or removes device node files usu-
ally located in the /dev directory, or it renames network interfaces.
As part of the hotplug subsystem, udev is executed if a kernel device
is added or removed from the system. A list of rules is used to match
against specific device attributes.
On device addition, udev matches its configured rules against the
available device attributes to uniquely name the device. udev main-
tains its own database for devices present on the system. This database
can be queried for the relationship of the kernel device path and the
name of the device file.
On device removal, udev queries its database for the name of the device
file to be deleted.
After the device node handling, a list of collected programs specific
to this device is executed.
CONFIGURATION
All udev configuration files consist of a set of lines of text. All
empty lines or lines beginning with '#' will be ignored.
udev expects its main configuration file at /etc/udev/udev.conf. The
file consists of a set of variables and values allowing the user to
override default udev values. The following variables can be overridden
in this file:
udev_root
Indicates where to place the device nodes in the filesystem. The
default value is /dev/.
udev_db
The name and location of the udev database. The default value is
/dev/.udevdb.
udev_rules
The name of the udev rules file or directory to look for files
with the suffix .rules. All rule files are read in lexical
order. The default value is /etc/udev/rules.d/.
udev_log
The logging priority which can be set to err ,info or the corre-
sponding numerical syslog(3) value. The default value is err.
A sample udev.conf file might look like this:
# Where in the filesystem to place the device nodes
udev_root="/dev"
# The name and location of the udev database.
udev_db="/dev/.udevdb"
# The name and location of the udev rules file(s).
udev_rules="@configdir@/rules.d"
# The syslog(3) priority: "err", "info", or the numerical value.
udev_log="err"
The rules for device naming are read from the files located in the
/etc/udev/rules.d/ directory, or at the location specified by the
udev_rules value in the /etc/udev/udev.conf file.
Every line in the rules file defines the mapping between device
attributes and the device name. One or more keys are specified to match
a rule with the current device. If all keys are matching, the rule will
be applied and the name is used to name the device file or the network
interface.
If no matching rule is found, the default kernel device name is used.
Every rule consists of a list of comma separated key value fields:
key ,[key ,...]
The following key names can be used to match against device properties:
BUS Match the bus type of the device. (The sysfs device bus must be
able to be determined by a "device" symlink.)
KERNEL Match the kernel device name.
SUBSYSTEM
Match the kernel subsystem name.
ACTION Match the kernel action name.
DRIVER Match the kernel driver name.
ID Match the device number on the bus, like PCI bus id.
SYSFS{filename}
Match sysfs device attribute like vendor and product id's, USB
serial number or the SCSI disk model number. Up to 5 different
sysfs files can be checked, with all of the values being
required to match the rule.
Trailing whitespace characters in the sysfs attribute value are
ignored, if the key doesn't have any trailing whitespace charac-
ters by itself.
ENV{variable}
Match an environment variable. Up to 5 different environment
variables can be checked, with all of the values being required
to match the rule.
PROGRAM
Call external program. This key is valid if the program returns
successful. The environment variables of udev are also avail-
able to the program.
The string returned by the program may be additionally matched
with the RESULT key in the same or any later rule.
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.
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.
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 over-
writes the compiled-in default value.
RUN Add a program to the list of programs to be executed for a spe-
cific device.
OPTIONS
last_rule stops further rules application. No later rules will
have any effect.
ignore_device will ignore this device. No node will be created
or program executed.
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 parti-
tions of a blockdevice. This may be useful for removable media
devices which do not detect a media change.
Multiple attributes may be separated by comma.
The NAME, SYMLINK, PROGRAM, OWNER and GROUP fields support simple
printf-like string substitutions:
%n The "kernel number" of the device. For example, 'sda3' has a
"kernel number" of '3'.
%k The "kernel name" for the device.
%p The devpath for the device.
%M The kernel major number for the device.
%m The kernel minor number for the device.
%b The bus id for the device.
%c The string returned by the external program, specified in PRO-
GRAM (This does not work within the PROGRAM field for the obvi-
ous reason.)
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+}
%N The name of a created temporary device node to provide access to
the device from a external program.
%P The node name of the parent device.
%s{filename}
The content of a sysfs attribute.
%r The udev_root value.
%e If a device node already exists with the name, the smallest pos-
itive decimal integer N is substituted such that the resulting
name doesn't match an existing device node. Otherwise nothing is
substituted. This can be used to create compatibility symlinks
and enumerate devices of the same type originating from differ-
ent kernel subsystems.
Note: The use of the enumeration facility is unreliable outside
of udevstart where the node creation is serialized and pre-
dictable. The returned numbers rely on the order devices are
probed on the system. If more than one device requests an enu-
meration for the same name at the same time, it may be possible
that both requests receive the same name back from the database.
The use of enumerations in todays setups where device can come
and go at any time is not recomended.
%% The '%' character itself.
The count of characters to insert may be limited by specifying the for-
mat length value. For example, '%3s{file}' will only insert the first
three characters of the sysfs attribute.
A sample udev.rules file might look like this:
# if /sbin/scsi_id returns "OEM 0815", the device will be called disk1
BUS=="scsi", PROGRAM=="/sbin/scsi_id", RESULT=="OEM 0815", NAME="disk1"
# USB printer to be called lp_color
BUS=="usb", SYSFS{serial}=="W09090207101241330", NAME="lp_color"
# SCSI disk with a specific vendor and model number will be called boot
BUS=="scsi", SYSFS{vendor}=="IBM", SYSFS{model}=="ST336", NAME="boot%n"
# sound card with PCI bus id 00:0b.0 to be called dsp
BUS=="pci", ID=="00:0b.0", NAME="dsp"
# USB mouse at third port of the second hub to be called mouse1
BUS=="usb", ID=="2.3", NAME="mouse1"
# ttyUSB1 should always be called pda with two additional symlinks
KERNEL=="ttyUSB1", NAME="pda", SYMLINK="palmtop handheld"
# multiple USB webcams with symlinks to be called webcam0, webcam1, ...
BUS=="usb", SYSFS{model}=="XV3", NAME=="video%n", SYMLINK="webcam%n"
A number of different fields in the above configuration files support a
simple form of shell style pattern matching. It supports the following
pattern characters:
* Matches zero, one, or more characters.
? Matches any single character, but does not match zero charac-
ters.
[ ] 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 follow-
ing the '[' is a '!', any characters not enclosed are matched.
After device node creation, removal, or network device renaming, udev
executes the programs located in the directory tree under /etc/dev.d/.
The name of a program must have the suffix .dev to be recognized.
In addition to the kernel provided hotplug environment variables,
UDEV_LOG is set and contains the numerical priority value, if udev is
configured to use syslog(3). Executed programs may want to follow that
setting. DEVNAME is exported to make the name of the created node, or
the name the network device is renamed to, available to the executed
program. The programs in every directory are sorted in lexical order,
while the directories are searched in the following order:
/etc/dev.d/$(DEVNAME)/*.dev
/etc/dev.d/$(SUBSYSTEM)/*.dev
/etc/dev.d/default/*.dev
ENVIRONMENT
The following variables are read from the environment:
ACTION add or remove signifies the addition or the removal of a device.
DEVPATH
The sysfs devpath of the device without the mountpoint but a
leading slash.
SUBSYSTEM
The subsystem the device belongs to. Alternatively the subsystem
may be passed as the first argument.
UDEV_CONFIG_FILE
Overrides the default location of the udev config file.
UDEV_LOG
Overrides the log priority specified in the config file.
UDEV_RUN
If set to "0", it disables the execution of programs added by
rules.
UDEV_NO_DEVD
The default behavior of udev is to execute programs in the
/etc/dev.d/ directory after device handling. If set, udev will
skip this step.
FILES
/sbin/udev udev program
/etc/udev/* udev config files
/etc/dev.d/* programs invoked by udev
SEE ALSO
udevinfo(8), udevd(8), hotplug(8)
Web resources:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html
http://linux-hotplug.sourceforge.net/
AUTHORS
udev was developed by Greg Kroah-Hartman <greg@kroah.com> with much
help from Dan Stekloff <dsteklof@us.ibm.com>, Kay Sievers <kay.siev-
ers@vrfy.org>, and many others.
October 2003 UDEV(8)