Kay Sievers kay.sievers@vrfy.org 2005 Kay Sievers

August 2005 udev udev 8 udev dynamic device management udev provides a dynamic device directory containing only the files for actually present devices. It creates or removes device node files in the /dev directory, or it renames network interfaces. Usually udev runs as udevd 8 and receives uevents directly from the kernel if a device is added or removed form the system. The program udev itself may be used as an event handler in situations, where running the daemon is not appropriate, like in initramfs. If udev receives a device event, it matches its configured rules against the available device attributes provided in sysfs to identify the device. Rules that match, may provide additional device information or specify a device node name and multiple symlink names and instruct udev to run additional programs as part of the device event handling. All udev configuration files are placed in /etc/udev/*. Every file 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. 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_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. 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+} %e, $enum If a device node already exists with the name, the smallest next free number is used. 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 for events that request a number at the same time. The use of enumerations in todays setups where devices can come and go at any time is not recomended. %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 ACTION add or remove signifies the addition or the removal of a device. DEVPATH The sysfs devpath without the mountpoint but a leading slash. SUBSYSTEM The kernel subsystem the device belongs to. UDEV_LOG Overrides the syslog priority specified in the config file. Written by Greg Kroah-Hartman greg@kroah.com and Kay Sievers kay.sievers@vrfy.org. With much help from Dan Stekloff dsteklof@us.ibm.com and many others. udev8 , udevinfo8 , udevd8 , udevmonitor8