udev expects its main configuration file at /etc/udev/udev.conf. It consists of a set of variables allowing the user to override default udev values. The following variables can be set:

udev_root

Specifies where to place the device nodes in the filesystem. The default value is /dev.

udev_rules

The name of the udev rules file or directory to look for files with the suffix .rules. Multiple rule files are read in lexical order. The default value is /etc/udev/rules.d.

udev_log

The logging priority. Valid values are the numerical syslog priorities or their textual representations: err, info and debug.

The udev rules are read from the files located in the /etc/udev/rules.d directory or at the location specified value in the configuraton file. Every line in the rules file contains at least one key value pair. There are two kind of keys, match and assignement keys. If all match keys are matching against its value, the rule gets applied and the assign keys get the specified value assigned. A matching rule may specify the name of the device node, add a symlink pointing to the node, or run a specified program as part of the event handling. If no matching rule is found, the default device node name is used.

A rule may consists of a list of one or more key value pairs separated by a comma. Each key has a distinct operation, depending on the used operator. Valid operators are:

==

Compare for equality.

!=

Compare for non-equality.

=

Asign a value to a key. Keys that represent a list, are reset and only this single value is assigned.

+=

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

:=

Assign a value to a key finally; disallow any later changes, which may be used to prevent changes by any later rules.

The following key names can be used to match against device properties:

ACTION

Match the kernel action name.

KERNEL

Match the kernel device name

DEVPATH

Match the kernel devpath.

SUBSYSTEM

Match the kernel subsystem name

BUS

Match the typ of bus the device is connected to.

DRIVER

Match the kernel driver name.

ID

Match the device number on the bus.

ENV{key}

Match against the value of an environment key. Depending on the specified operation, this key is also used as a assignment.

SYSFS{filename}

Match the sysfs attribute value. Up to five values can be specified. Trailing whitespace is ignored, if the specified match value does not contain trailing whitespace itself.

PROGRAM

Execute external program. The key is true, if the program returns without exit code zero. The whole event environment is available to the executed program. The program's output printed to stdout is available for the RESULT key.

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.

Most of the fields support a shell style pattern matching. The following pattern characters are supported:

*

Matches zero, or any number of characters.

?

Matches any single character.

[]

Matches any single character specified within the brackets. 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.

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 over writes the compiled-in default value.

ENV{key}

Export the key to the environment. Depending on the specified operation, this key is also used as a match.

RUN

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

LABEL

Named label where a GOTO can jump to.

GOTO

Jumps to the next LABEL with a matching gname

IMPORT{type}

Import the printed result or the content of a file in environment key format into the event environment. program will execute an external program and read its output. file will inport a text file. If no option is given, udev will determine it from the executable bit of of the file permissions.

WAIT_FOR_SYSFS

Wait for the specified sysfs file of the device to be created. May be used to fight agains timing issues wth the kernel.

OPTIONS

last_rule stops further rules application. No later rules will have any effect. ignore_device will ignore this event completely. 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 block device. This may be useful for removable media.

The NAME, SYMLINK, PROGRAM, OWNER and GROUP fields support simple printf-like string substitutions:

%k, $kernel

The kernel name for this device.

%b, $id

The kernel bus id for this device.

%n, $number

The kernel number for this device. For example, 'sda3' has kernel number of '3'

%p, $devpath

The devpath of the device.

%s{file}, $sysfs{file}

The content of a sysfs attribute.

%E{key}, $env{key}

The value of an environment variable.

%m, $major

The kernel major number for the device.

%M $minor

The kernel minor number for the device.

%c, $result

The string returned by the external program requested with PROGRAM. 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+}

%P, $parent

The node name of the parent device.

%r, $root

The udev_root value.

%N, $tempnode

The name of a created temporary device node to provide access to the device from a external program before the real node is created.

%%

The '%' character itself.

$$

The '$' character itself.

The count of characters to be substituted may be limited by specifying the format length value. For example, '%3s{file}' will only insert the first three characters of the sysfs attribute