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UDEV(7)				     udev			       UDEV(7)

NAME
       udev - dynamic device management

DESCRIPTION
       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 from the system.

       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.

CONFIGURATION
       udev configuration files are placed in /etc/udev/ and /lib/udev/. All
       empty lines, or lines beginning with ´#´ will be 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
       /dev/.udev/rules.d/. All rule files are sorted and processed in lexical
       order, regardless in which of these directories they live.

       Rule files are required to have a unique name, duplicate file names are
       ignored. Files in /etc/udev/rules.d/ have precedence over files with
       the same name in /lib/udev/rules.d/. This can be used to ignore a
       default rules file if needed.

       Every line in the rules file contains at least one key value pair.
       There are two kind of keys, match and assignment 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 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 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, which
	   may be used to prevent changes by any later rules.

       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 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, if the specified
	   match value does not contain trailing whitespace itself.

       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, if the specified match value does not
	   contain trailing whitespace itself.

       ENV{key}
	   Match against a device property value.

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

       PROGRAM
	   Execute a program. 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 output printed
	   to 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 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. 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.

       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.

       SYMLINK
	   The name of a symlink targeting the node. Every matching rule adds
	   this value to the list of symlinks to be created.

	   The set of characters to name a symlink is limited. Allowed
	   characters are [0-9A-Za-z#+-.:=@_/], valid utf8 character
	   sequences, and "\x00" hex encoding. All other characters are
	   replaced by a _ character.

	   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.

	   Symlink names must never conflict with the kernels 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
	   not stored in the database or exported to external tool or events.

       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 the
	   option RUN{fail_event_on_error} is specified, and the executed
	   program returns non-zero, the event will be marked as failed for a
	   possible later handling.

	   If the specified string starts with socket:path, all current event
	   values will be passed to the specified socket, as a message in the
	   same format the kernel sends an uevent. If the first character of
	   the specified path is an @ character, an abstract namespace socket
	   is used, instead of an existing socket file.

       LABEL
	   Named label where a GOTO can jump to.

       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.

	   file
	       Import a text file specified as the assigned value, 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 commandline. For
	       simple flags the value of the property will be 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 will choose between program and file
	   based on the executable bit of the file permissions.

       WAIT_FOR
	   Wait for a file to become available.

       OPTIONS
	   Rule and device options:

	   ignore_device
	       Ignore this event completely.

	   ignore_remove
	       Do not remove the device node when the device goes away. This
	       may be useful as a workaround for broken device drivers.

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

	   all_partitions
	       Create the device nodes for all available partitions of a block
	       device. This may be useful for removable media devices where
	       media changes are not detected.

	   event_timeout=
	       Number of seconds an event will wait for operations to finish,
	       before it will terminate 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.

	   watch
	       Watch the device node with inotify, when closed after being
	       opened for writing, a change uevent will be synthesised.

	   nowatch
	       Disable the watching of a device node with inotify.

       The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE and RUN fields support
       simple printf-like string substitutions. The RUN format chars gets
       applied after all rules have been processed, right before the program
       is executed. It allows the use of device properties set by earlier
       matching rules. For all other fields, substitutions are applied 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, follow the chain of parent devices and use the value
	   of the first attribute that matches. 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 ´+´ char 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
	   The current list of symlinks, separated by a space character. The
	   value is only set if an earlier rule assigned a value, or during a
	   remove events.

       $root, %r
	   The udev_root value.

       $sys, %S
	   The sysfs mount point.

       $tempnode, %N
	   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.

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				  11/25/2013			       UDEV(7)
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