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)