UDEV(8) Linux Administrator's Manual UDEV(8)
udev - Linux configurable dynamic device naming support
udev creates or removes device node files usually located in the /dev
directory. Its goal is to provide a dynamic device directory that con-
tains only the files for devices that are actually present.
As part of the hotplug subsystem, udev is executed if a kernel device
is added or removed from the system. On device creation, udev reads
the sysfs directory of the given device to collect device attributes
like label, serial number or bus device number. These attributes are
treated as a key to determine a unique name for device file creation.
udev maintains a database for devices present on the system.
On device removal, udev queries the internal database for the name of
the device file to be deleted.
All udev configuration files consist of a set of lines of text. All
empty lines, and lines beginning with a '#' will be ignored.
udev expects its main configuration file at /etc/udev/udev.conf. The
file consists of a set of variables and values that allow the user to
override default udev values. The current set of variables that can be
overridden in this file is:
This is the where in the filesystem to place the device nodes.
The default value for this is /udev/
The name and location of the udev database. The default value
for this is /udev/.udev.tdb
This is the location of the udev rules file. The default value
for this is /etc/udev/udev.rules
This is the location of the udev permission file. The default
value for this is /etc/udev/udev.permissions
This is the default mode for all nodes that have no explicit
match in the permissions file. The default value for this is
A sample udev.conf might look like this:
# udev_root - where in the filesystem to place the device nodes
# udev_db - The name and location of the udev database.
# udev_rules - The name and location of the udev rules file
# udev_permissions - The name and location of the udev permission file
# default_mode - set the default mode for all nodes that have no
# explicit match in the permissions file
The rules for udev to use when naming devices may specified at
/etc/udev/udev.rules or specified by the udev_rules value in the
Every line in the rules file define the mapping between device
attributes and the device file name. It starts with a keyword defining
the method used to match, followed by one ore more keys to compare and
the filename for the device. If no matching configuration is found, the
default kernel device name is used.
The line format is:
method, key,[key,...] name
where valid methods with corresponding keys are:
calling external program, that returns a string to match
keys: BUS, PROGRAM, ID
LABEL device label or serial number, like USB serial number, SCSI UUID
or file system label
keys: BUS, sysfsattribute
NUMBER device number on the bus, like PCI bus id
keys: BUS, ID
device position on bus, like physical port of USB device
keys: BUS, PLACE
string replacement of the kernel device name
The methods are applied in the following order: CALLOUT , LABEL , NUM--
BER , TOPOLOGY , REPLACE
The NAME and PROGRAM fields support simple printf-like string subtitu-
%%nn the "kernel number" of the device for example, 'sda3' has
a "kernel number" of '3'
%%MM the kernel major number for the device
%%mm the kernel minor number for the device
%%bb the bus id for the device
%%cc the CALLOUT program returned string (this does not work
within the PROGRAM field for the obvious reason.)
%%DD Use the devfs style disk name for this device. For par-
titions, this will result in 'part%n' If this is not a
partition, it will result in 'disk'
A sample udev.rules might look like this:
# if /sbin/scsi_id returns "OEM 0815" device will be called disk1
CALLOUT, BUS="scsi", PROGRAM="/sbin/scsi_id", ID="OEM 0815", NAME="disk1"
# USB printer to be called lp_color
LABEL, BUS="usb", serial="W09090207101241330", NAME="lp_color"
# sound card with PCI bus id 00:0b.0 to be called dsp
NUMBER, BUS="pci", ID="00:0b.0", NAME="dsp"
# USB mouse at third port of the second hub to be called mouse1
TOPOLOGY, BUS="usb", PLACE="2.3", NAME="mouse1"
# ttyUSB1 should always be called pda
REPLACE, KERNEL="ttyUSB1", NAME="pda"
# USB webcams to be called webcam0, webcam1, ...
LABEL, BUS="usb", model="WebCam Version 3", NAME="webcam%n"
Permissions and ownership for the created device files may specified at
/etc/udev/udev.permissions or specified by the udev_permission value in
the /etc/udev/udev.conf file.
Every line lists a device name followed by owner, group and permission
mode. All values are separated by colons. The name field may contain a
wildcard to apply the values to a whole class of devices.
If udev was built using klibc or is used before the user database is
accessible (e.g. initrd ), only numeric owner and group values may be
A sample udev.permissions might look like this:
A number of different fields in the above configuration files support a
simple form of wildcard matching. This form is based on the fnmatch(3)
style, and supports the following fields:
** Matches zero, one, or more characters.
?? Matches any single character, but does not match zero
[[ ]] Matches any single character specified within the brack-
ets. 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.
/sbin/udev udev program
/etc/udev/* udev config files
/etc/hotplug.d/default/udev.hotplug hotplug symlink to udev program
The http://linux-hotplug.sourceforge.net/ web site.
udev was developed by Greg Kroah-Hartman <email@example.com> with much
help from Dan Stekloff <firstname.lastname@example.org>, Kay Sievers
<email@example.com>, and many others.
October 2003 UDEV(8)
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