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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 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. 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 main-
tains 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 following variables can be overrid-
den 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/permis-
sions.d/.
udev_log
The switch, if udev logs some information for every device han-
dled. 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 determined 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 char-
acters by itself.
PROGRAM
Call external program. This key is valid if the program returns
successful. The environment variables of udev are also avail-
able 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 cre-
ate 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 val-
ues empty, the device will be ignored and no node will be cre-
ated.
If only the symlink field is given and the name field is omit-
ted, the rule will not be applied immediatly, but the symlink
field is added to the symlink list of the rule which will cre-
ate the node. This makes it possible to specify additional
symlinks 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 over-
writes 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 integer N is substituted such that the result-
ing name doesn't match an existing device node. Otherwise noth-
ing 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 for-
mat 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_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
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 $local can be used instead of a specific username. In that
case, udev will determine the current local user at the time of device
node creation and substitute that username as the owner of the new
device node. This is useful, for example, to let hot-plugged devices,
such as cameras, be owned by the user at the current console. Note
that if no user is currently logged in, or if udev otherwise fails to
determine a current user, the default_owner value is used in lieu.
A number of different fields in the above configuration files support
a simple form of shell style pattern matching. It supports the follow-
ing pattern characters:
* Matches zero, one, or more characters.
? Matches any single character, but does not match zero charac-
ters.
[ ] Matches any single character specified within the brackets. For
example, the pattern string "tty[SR]" would match either "ttyS"
or "ttyR". Ranges are also supported within this match with
the '-' character. For example, to match on the range of all
digits, the pattern [0-9] would be used. If the first character
following the '[' is a '!', any character not enclosed is
matched.
After device node creation, removal, or network device renaming, udev
executes the programs in the directory tree under /etc/dev.d/. The
name of a program must end with .dev suffix, to be recognized.
In addition to the hotplug environment variables, DEVNAME is exported
to make the name of the created node, or the name the network device
is renamed to, available to the executed program. The programs in
every directory are sorted in lexical order, while the directories are
searched in the following order:
/etc/dev.d/$(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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