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UDEV(7) 				       udev					  UDEV(7)

NAME
       udev - Linux dynamic device management

DESCRIPTION
       udev supplies the system software with device events, manages permissions of device nodes
       and may create additional symlinks in the /dev directory, or renames network interfaces.
       The kernel usually just assigns unpredictable device names based on the order of
       discovery. Meaningful symlinks or network device names provide a way to reliably identify
       devices based on their properties or current configuration.

       The udev daemon, udevd(8), receives device uevents directly from the kernel whenever a
       device is added or removed from the system, or it changes its state. When udev receives a
       device event, it matches its configured set of rules against various device attributes to
       identify the device. Rules that match may provide additional device information to be
       stored in the udev database or to be used to create meaningful symlink names.

       All device information udev processes is stored in the udev database and sent out to
       possible event subscribers. Access to all stored data and the event sources is provided by
       the library libudev.

CONFIGURATION
       udev configuration files are placed in /etc/udev/ and /lib/udev/. All empty lines or lines
       beginning with '#' are ignored.

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

   Rules files
       The udev rules are read from the files located in the default rules directory
       /lib/udev/rules.d/, the custom rules directory /etc/udev/rules.d/ and the temporary rules
       directory /run/udev/rules.d/. All rule files are collectively sorted and processed in
       lexical order, regardless of the directories in which they live. However, files in
       /etc/udev/rules.d/ take precedence over files with the same name in /lib/udev/rules.d/;
       this can be used to ignore a default rules file if needed.

       Rule files must have the extension .rules; other extensions are ignored.

       Every line in the rules file contains at least one key-value pair. There are two kind of
       keys: match and assignment. If all match keys are matching against its value, the rule
       gets applied and the assignment keys get the specified value assigned.

       A matching rule may rename a network interface, add symlinks pointing to the device node,
       or run a specified program as part of the event handling.

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

       ==
	   Compare for equality.

       !=
	   Compare for inequality.

       =
	   Assign 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.

       The following key names can be used to match against device properties. Some of the keys
       also match against properties of the parent devices in sysfs, not only the device that has
       generated the event. If multiple keys that match a parent device are specified in a single
       rule, all these keys must match at one and the same parent device.

       ACTION
	   Match the name of the event action.

       DEVPATH
	   Match the devpath of the event device.

       KERNEL
	   Match the name of the event device.

       NAME
	   Match the name of the node or network interface. It can be used once the NAME key has
	   been set in one of the preceding rules.

       SYMLINK
	   Match the name of a symlink targeting the node. It can be used once a SYMLINK key has
	   been set in one of the preceding rules. There may be multiple symlinks; only one needs
	   to match.

       SUBSYSTEM
	   Match the subsystem of the event device.

       DRIVER
	   Match the driver name of the event device. Only set this key for devices which are
	   bound to a driver at the time the event is generated.

       ATTR{filename}
	   Match sysfs attribute values of the event device. Trailing whitespace in the attribute
	   values is ignored unless the specified match value itself contains trailing
	   whitespace.

       KERNELS
	   Search the devpath upwards for a matching device name.

       SUBSYSTEMS
	   Search the devpath upwards for a matching device subsystem name.

       DRIVERS
	   Search the devpath upwards for a matching device driver name.

       ATTRS{filename}
	   Search the devpath upwards for a device with matching sysfs attribute values. If
	   multiple ATTRS matches are specified, all of them must match on the same device.
	   Trailing whitespace in the attribute values is ignored unless the specified match
	   value itself contains trailing whitespace.

       TAGS
	   Search the devpath upwards for a device with matching tag.

       ENV{key}
	   Match against a device property value.

       TAG
	   Match against a device tag.

       TEST{octal mode mask}
	   Test the existence of a file. An octal mode mask can be specified if needed.

       PROGRAM
	   Execute a program to determine whether there is a match; the key is true if the
	   program returns successfully. The device properties are made available to the executed
	   program in the environment. The program's stdout is available in 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 shell-style pattern matching. The following pattern characters
       are supported:

       *
	   Matches zero or more characters.

       ?
	   Matches any single character.

       []
	   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 via
	   the '-' character. For example, to match on the range of all digits, the pattern [0-9]
	   could be used. If the first character following the '[' is a '!', any characters not
	   enclosed are matched.

       The following keys can get values assigned:

       NAME
	   What a network interface should be named.

	   Also, as a temporary workaround, this is what a device node should be named; usually
	   the kernel provides the defined node name or creates and removes the node before udev
	   even receives any event. Changing the node name from the kernel's default creates
	   inconsistencies and is not supported. If the kernel and NAME specify different names,
	   an error is logged. udev is only expected to handle device node permissions and to
	   create additional symlinks, not to change kernel-provided device node names. Instead
	   of renaming a device node, SYMLINK should be used. However, symlink names must never
	   conflict with device node names, as that would result in unpredictable behavior.

       SYMLINK
	   The name of a symlink targeting the node. Every matching rule adds this value to the
	   list of symlinks to be created. Multiple symlinks may be specified by separating the
	   names by the space character. In case multiple devices claim the same name, the link
	   always points to the device with the highest link_priority. If the current device goes
	   away, the links are re-evaluated and the device with the next highest link_priority
	   becomes the owner of the link. If no link_priority is specified, the order of the
	   devices (and which one of them owns the link) is undefined. Also, symlink names must
	   never conflict with the kernel's default device node names, as that would result in
	   unpredictable behavior.

       OWNER, GROUP, MODE
	   The permissions for the device node. Every specified value overwrites the compiled-in
	   default value.

       ATTR{key}
	   The value that should be written to a sysfs attribute of the event device.

       ENV{key}
	   Set a device property value. Property names with a leading '.' are neither stored in
	   the database nor exported to events or external tools (run by, say, the PROGRAM match
	   key).

       TAG
	   Attach a tag to a device. This is used to filter events for users of libudev's monitor
	   functionality, or to enumerate a group of tagged devices. The implementation can only
	   work efficiently if only a few tags are attached to a device. It is only meant to be
	   used in contexts with specific device filter requirements, and not as a
	   general-purpose flag. Excessive use might result in inefficient event handling.

       RUN
	   Add a program to the list of programs to be executed for a specific device. This can
	   only be used for very short running tasks. Running an event process for a long period
	   of time may block all further events for this or a dependent device. Long running
	   tasks need to be immediately detached from the event process itself.

	   If no absolute path is given, the program is expected to live in the directory
	   provided at compile-time to configure via --libexecdir (this is usually /lib/udev),
	   otherwise the absolute path must be specified. The program name and following
	   arguments are separated by spaces. Single quotes can be used to specify arguments with
	   spaces.

       LABEL
	   A named label to which a GOTO may jump.

       GOTO
	   Jumps to the next LABEL with a matching name.

       IMPORT{type}
	   Import a set of variables as device properties, depending on type:

	   program
	       Execute an external program specified as the assigned value and import its output,
	       which must be in environment key format. Path specification, command/argument
	       separation, and quoting work like in RUN.

	   file
	       Import a text file specified as the assigned value, the content of which must be
	       in environment key format.

	   db
	       Import a single property specified as the assigned value from the current device
	       database. This works only if the database is already populated by an earlier
	       event.

	   cmdline
	       Import a single property from the kernel command line. For simple flags the value
	       of the property is set to '1'.

	   parent
	       Import the stored keys from the parent device by reading the database entry of the
	       parent device. The value assigned to IMPORT{parent} is used as a filter of key
	       names to import (with the same shell-style pattern matching used for comparisons).

	   If no option is given, udev chooses between program and file based on the executable
	   bit of the file permissions.

       WAIT_FOR
	   Wait for a file to become available or until a timeout of 10 seconds expires. The path
	   is relative to the sysfs device; if no path is specified, this waits for an attribute
	   to appear.

       OPTIONS
	   Rule and device options:

	   link_priority=value
	       Specify the priority of the created symlinks. Devices with higher priorities
	       overwrite existing symlinks of other devices. The default is 0.

	   event_timeout=
	       Number of seconds an event waits for operations to finish before giving up and
	       terminating itself.

	   string_escape=none|replace
	       Usually control and other possibly unsafe characters are replaced in strings used
	       for device naming. The mode of replacement can be specified with this option.

	   static_node=
	       Apply the permissions specified in this rule to the static device node with the
	       specified name. Static device nodes might be provided by kernel modules or copied
	       from /lib/udev/devices. These nodes might not have a corresponding kernel device
	       at the time udevd is started; they can trigger automatic kernel module loading.

	   watch
	       Watch the device node with inotify; when the node is closed after being opened for
	       writing, a change uevent is synthesized.

	   nowatch
	       Disable the watching of a device node with inotify.

       The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE and RUN fields support simple string
       substitutions. The RUN substitutions are performed after all rules have been processed,
       right before the program is executed, allowing for the use of device properties set by
       earlier matching rules. For all other fields, substitutions are performed while the
       individual rule is being processed. The available substitutions are:

       $kernel, %k
	   The kernel name for this device.

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

       $devpath, %p
	   The devpath of the device.

       $id, %b
	   The name of the device matched while searching the devpath upwards for SUBSYSTEMS,
	   KERNELS, DRIVERS and ATTRS.

       $driver
	   The driver name of the device matched while searching the devpath upwards for
	   SUBSYSTEMS, KERNELS, DRIVERS and ATTRS.

       $attr{file}, %s{file}
	   The value of a sysfs attribute found at the device where all keys of the rule have
	   matched. If the matching device does not have such an attribute, and a previous
	   KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS test selected a parent device, then the
	   attribute from that parent device is used.

	   If the attribute is a symlink, the last element of the symlink target is returned as
	   the value.

       $env{key}, %E{key}
	   A device property value.

       $major, %M
	   The kernel major number for the device.

       $minor, %m
	   The kernel minor number for the device.

       $result, %c
	   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 '+' character, this
	   part plus all remaining parts of the result string are substituted: %c{N+}

       $parent, %P
	   The node name of the parent device.

       $name
	   The current name of the device node. If not changed by a rule, it is the name of the
	   kernel device.

       $links
	   A space-separated list of the current symlinks. The value is only set during a remove
	   event or if an earlier rule assigned a value.

       $root, %r
	   The udev_root value.

       $sys, %S
	   The sysfs mount point.

       $tempnode, %N
	   The name of a temporary device node created to provide access to the device from a
	   external program before the real node is created.

       %%
	   The '%' character itself.

       $$
	   The '$' character itself.

AUTHOR
       Written by Greg Kroah-Hartman greg@kroah.com and Kay Sievers kay.sievers@vrfy.org. With
       much help from Dan Stekloff and many others.

SEE ALSO
       udevd(8), udevadm(8)

udev					    07/30/2011					  UDEV(7)
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