UDEV(8)

NAME
udev - Linux configurable dynamic device naming support

SYNOPSIS
udev hotplug-subsystem

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 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 switch to enable/disable logging of udev information The default value is yes.

A sample udev.conf file 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/.udevdb"

# 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_log - set to "yes" if you want logging, else "no"
udev_log="yes"

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.

SUBSYSTEM
    Match the kernel subsystem name.

DRIVER
    Match the kernel driver 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.

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 are 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 immediately, 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 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 the device node. Every specified value overwrites the compiled-in default value.

OPTIONS
    ignore_device will ignore this device. No node will be created.

    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 partitions 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 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+}

%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.

%e  If a device node already exists with the name, the smallest positive 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 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 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", 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"

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 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.

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 hotplug environment variables, UDEV_LOG is set if udev is configured to use the syslog facility. 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_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/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)

Web resources:
http://linux-hotplug.sourceforge.net/
http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html

AUTHORS
udev was developed by Greg Kroah-Hartman <greg@kroah.com> with much help from Dan Stekloff <dsteklof@us.ibm.com>, Kay Sievers <kay.sievers@vrfy.org>, and many others.