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           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.


           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:
               Specifies where to place the device nodes in the filesystem. The
               default value is /dev.
               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
               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.
               Match the name of the event action.
               Match the devpath of the event device.
               Match the name of the event device.
               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.
               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.
               Match the subsystem of the event device.
               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.
               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.
               Search the devpath upwards for a matching device name.
               Search the devpath upwards for a matching device subsystem name.
               Search the devpath upwards for a matching device driver name.
               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.
               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:
               The name of the node to be created, or the name the network
               interface should be renamed to.
               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
               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.
               Named label where a GOTO can jump to.
               Jumps to the next LABEL with a matching name
               Import a set of variables as device properties, depending on type:
                   Execute an external program specified as the assigned value and
                   import its output, which must be in environment key format.
                   Import a text file specified as the assigned value, which must
                   be in environment key format.
                   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.
                   Import a single property from the kernel commandline. For
                   simple flags the value of the property will be set to ?1?.
                   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.
                   higher priorities overwrite existing symlinks of other devices.
                   The default is 0.
                   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.
                   Number of seconds an event will wait for operations to finish,
                   before it will terminate itself.
                   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 the device node with inotify, when closed after being
                   opened for writing, a change uevent will be synthesised.
                   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
           $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.
               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
           $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.
               The current name of the device node. If not changed by a rule, it
               is the name of the kernel device.
               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.


           Written by Greg Kroah-Hartman and Kay Sievers
  With much help from Dan Stekloff and many others.


           udevd(8), udevadm(8)

    udev 12/16/2015 UDEV(7)


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