UDEV(8) Linux Administrator's Manual UDEV(8)
NAME
udev - Linux configurable dynamic device naming support
SYNOPSIS
udev hotplug-subsystem
The environment must provide the following variables:
ACTION add or remove signifies the connection or disconnection of a device.
DEVPATH
The sysfs devpath of the device without the mountpoint but a leading slash.
Additional optional environment variables:
UDEV_CONFIG_FILE
Overrides the default location of the udev config file.
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.
DESCRIPTION
udev provides a dynamic device directory containing only the files for actually present
devices. It creates or removes device node files usually 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. 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 may be used as keys to determine a unique name for the device. udev maintains
a database for devices present on the system.
On device removal, udev queries its database for the name of the device file to be
deleted.
CONFIGURATION
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 allowing the user to override default udev values. The follow-
ing 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/.udev.tdb.
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_permissions
The name of the udev permission file or directory to look for files with the suffix
.permissions. All permission files are read in lexical order. The default value is
/etc/udev/permissions.d/.
udev_log
The switch, if udev logs some information for every device handled. The default
value is yes.
default_mode
The default mode for all nodes not explicitely matching in the permissions file.
The default value is 0666.
default_owner
The default owner for all nodes not explicitely matching in the permissions file.
The default value is root.
default_group
The default group for all nodes not explicitely matching in the permissions file.
The default value is root.
A sample udev.conf might look like this:
# udev_root - where to place the device nodes in the filesystem
udev_root="/udev"
# udev_db - The name and location of the udev database
udev_db="/udev/.udev.tdb"
# udev_rules - The name of the udev rules file or directory to look
for files with the suffix .rules
udev_rules="/etc/udev/rules.d/"
# udev_permissions - The name of the udev permission file or directory
to look for files with the suffix .permissions
udev_permissions="/etc/udev/udev.permissions"
# udev_log - set to "yes" if you want logging, else "no"
udev_log="yes"
# default_mode - set the default mode for all nodes not
# explicitely matching in the permissions file
default_mode="0666"
# default_owner - set the default owner for all nodes not
# explicitely matching in the permissions file
default_owner="root"
# default_group - set the default group for all nodes not
# explicitely matching in the permissions file
default_group="root"
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 fields:
key ,[key ,...] name [, symlink]
where fields are:
BUS Match the bus type of the device. (The sysfs device bus must be able to be deter-
mined by a "device" symlink.)
KERNEL Match the kernel device name.
ID Match the device number on the bus, like PCI bus id.
PLACE Match the topological position on bus, like physical port of USB device
SYSFS{filename}
Match sysfs device attribute like label, vendor, USB serial number, SCSI UUID or
file system label. 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 characters by itself.
PROGRAM
Call external program. This key is valid if the program returns successful. The
environment variables of udev are also available for the program.
The string returned by the program may be additionally matched with the RESULT key.
RESULT Match the returned string of the last PROGRAM call. This key may be used in any
following rule after a PROGRAM call.
NAME The name of the node to be created, or the name, the network interface should be
renamed to.
If given with the attribute NAME{all_partitions} it will create all 15 partitions
of a blockdevice. This may be useful for removable media devices.
SYMLINK
The name of a symlink targeting the node. Multiple symlinks may be specified by
separating the names by the space character.
If both the name and the symlink fields are omitted or its values empty, the device
will be ignored and no node will be created.
If only the symlink field is given and the name field is omitted, the rule will not
be applied immediatly, but the symlink field is added to the symlink list of the
rule which will create the node. This makes it possible to specify additional sym-
links in a possibly separate rules file, while the device nodes are maintained by
the distribution provided rules file.
OWNER, GROUP, MODE
The permissions for this device. Every specified value overwrites the value given
in the permissions file.
The NAME ,SYMLINK and PROGRAM fields support simple printf-like string substitution:
%n The "kernel number" of the device. For example, 'sda3' has a "kernel number" of
'3'.
%k The "kernel name" 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 from the execution of PROGRAM (This does not work within the
PROGRAM field for the obvious 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+}
%s{filename}
The content of a sysfs attribute.
%e If a device node already exists with the name, the smallest positive decimal inte-
ger 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 different kernel
subsystems.
%% The '%' character itself.
The count of charcters to insert may be limited by specifying the format length value. For
example, '%3s{file}' will only insert the first three characters of the sysfs attribute.
A sample udev.rules might look like this:
# if /sbin/scsi_id returns "OEM 0815" 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", PLACE="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"
# grouping of optical drives from multiple kernel subsystems
KERNEL="sr*", NAME="%k", SYMLINK="cdrom%e"
KERNEL="scd*", NAME="%k", SYMLINK="cdrom%e"
KERNEL="pcd*", NAME="%k", SYMLINK="cdrom%e"
KERNEL="hd[a-z]", PROGRAM="/bin/cat /proc/ide/%k/media", RESULT="cdrom",
NAME="%k", SYMLINK="cdrom%e"
The permissions and ownership of the created device file is read from the files located in
the /etc/udev/permissions.d/ directory, or at the location specified by the udev_permis-
sion 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 pattern to apply the values to a
whole class of devices.
A sample udev.permissions might look like this:
#name:user:group:mode
input/*:root:root:644
ttyUSB1:0:8:0660
video*:root:video:0660
dsp1:::0666
The value $local can be used instead of a specific username. In that case, udev will
determine the current local user at the time of device node creation and substitute that
username as the owner of the new device node. This is useful, for example, to let hot-
plugged devices, such as cameras, be owned by the user at the current console. Note that
if no user is currently logged in, or if udev otherwise fails to determine a current user,
the default_owner value is used in lieu.
A number of different fields in the above configuration files support a simple form of
shell style pattern matching. It supports the following pattern characters:
* Matches zero, one, or more characters.
? Matches any single character, but does not match zero characters.
[ ] Matches any single character specified within the brackets. For example, the pat-
tern string "tty[SR]" would match either "ttyS" or "ttyR". Ranges are also
supported within this match with the '-' character. For example, to match on the
range of all digits, the pattern [0-9] would be used. If the first character fol-
lowing the '[' is a '!', any character not enclosed is matched.
After device node creation, removal, or network device renaming, udev executes the pro-
grams in the directory tree under /etc/dev.d/. The name of a program must end with .dev
suffix, to be recognized.
In addition to the hotplug environment variables, DEVNAME is exported to make the name of
the created node, or the name the network device is renamed to, available to the executed
program. The programs in every directory are sorted in lexical order, while the directo-
ries are searched in the following order:
/etc/dev.d/$(DEVNAME)/*.dev
/etc/dev.d/$(SUBSYSTEM)/*.dev
/etc/dev.d/default/*.dev
FILES
/sbin/udev udev program
/etc/udev/* udev config files
/etc/hotplug.d/default/udev.hotplug hotplug symlink to udev program
/etc/dev.d/* programs invoked by udev
SEE ALSO
udevinfo(8), udevd(8), hotplug(8)
The http://linux-hotplug.sourceforge.net/ web site.
AUTHORS
udev was developed by Greg Kroah-Hartman <> with much help from Dan Stekloff
<>, Kay Sievers <>, and many others.
October 2003 UDEV(8)
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