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       udev - Linux dynamic device management


       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 information 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 are provided by the library libudev.


       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. 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

       Rule files must end in .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 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 rename a network interface, add symlinks pointing
       to the device node, or run a specified program as part of the event

       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.

           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.

           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.

           Match against a device property value.

           Match against a device tag.

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

           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, a network interface should be renamed to. Or as a
           temporary workaraound, the name a device node should be named.
           Usually the kernel provides the defined node name, or even 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 will be 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. Symlink names must never conflict
           with device node names, it will result in unpredictable behavior.

           The name of a symlink targeting the node. Every matching rule will
           add 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
           will always point to the device with the highest link_priority. If
           the current device goes away, the links will be re-evaluated and
           the device with the next highest link_priority will own the link.
           If no link_priority is specified, the order of the devices, and
           which one of them will own the link, is undefined. Claiming the
           same name for a symlink, which is or might be used for a device
           node, may result in unexpected behavior and is not supported.

           The permissions for the device node. Every specified value
           overwrites the compiled-in default value.

           The value that should be written to a sysfs attribute of the event

           Set a device property value. Property names with a leading '.' are
           not stored in the database or exported to external tool or events.

           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.

           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 no absolute path is given, the program is expected to live in
           /lib/udev, otherwise the absolute path must be specified. Program
           name and arguments are separated by spaces. Single quotes can be
           used to specify arguments with spaces.

           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.
               Path specification, command/argument separation, and quoting
               work like in RUN.

               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.

           Wait for a file to become available or until a 10 seconds timeout

           Rule and device options:

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

               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.

               Apply the permissions specified in this rule to a 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, and allow to trigger automatic kernel
               module on-demand loading.

               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

           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, use the attribute from that parent
           device. 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.

           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)