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.

       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:

           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
       /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

       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

           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.

           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 this key for
           devices which are bound to a driver at the time the event is

           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.

           Search the devpath upwards for a matching device name.

           Search the devpath upwards for a device with matching tag.

           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 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.

           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:

           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
           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.

           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
           neither stored in the database nor exported to events or external
           tools (run by, say, the PROGRAM match key).

           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 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.

           A named label to which a GOTO may jump.

           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
               Import a single property from the kernel command line. For
               simple flags the value of the property is 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).

           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.

           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 waits for operations to finish
               before giving up and terminating 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 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

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

               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 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.

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

           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.

udev                              11/07/2011                           UDEV(7)
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