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 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. 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 maintains 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 corresponding 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 ,...]

Each key has the following format:

name op value

There are distinct key operation types, depending on the type of the key, it does a comparison or an assignment.

Comparison operators are:

==  Compare for equality.

!=  Compare for non-equality.

Assignment operators are:

+=  Add the value to a key that holds a list of entries.

:=  Assign a value to a key finally; disallow any later changes, which is useful to prevent changes by any later rules.

=   Asign a value to a key. Keys that represent a list, are reset and only this single value is assigned. While this operator still works inplicitely as comparison on keys that can't get a value assigned, its usage as an comparison operator is deprecated.

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 characters 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 available 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. Only one rule can set the a name, all later rules with a NAME key will be ignored.

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 overwrites the compiled-in default value.

RUN Add a program to the list of programs to be executed for a specific 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 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.

%r  The udev_root value.

%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.

    Note: The use of the enumeration facility is unreliable outside of udevstart where the node creation is serialized and predictable. The returned numbers rely on the order devices are probed on the system. If more than one device requests an enumeration 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 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", 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 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 specified by the RUN key.
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 programs.

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.

FILES
/sbin/udev                           udev program
/etc/udev/*                          udev config files

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.sievers@vrfy.org>, and many others.