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, systemd-udevd.service(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.

       The udev rules are read from the files located in the system rules
       directory /usr/lib/udev/rules.d, the volatile runtime directory
       /run/udev/rules.d and the local administration directory
       /etc/udev/rules.d. All rules files are collectively sorted and
       processed in lexical order, regardless of the directories in which they
       live. However, files with identical file names replace each other.
       Files in /etc have the highest priority, files in /run take precedence
       over files with the same name in /lib. This can be used to override a
       system-supplied rules file with a local file if needed; a symlink in
       /etc with the same name as a rules file in /lib, pointing to /dev/null,
       disables the rules file entirely.

       Rule files must have the extension .rules; other extensions are

       Every line in the rules file contains at least one key-value pair.
       Except for empty lines or lines beginning with '#', which are ignored.
       There are two kinds of keys: match and assignment. If all match keys
       match against their values, the rule gets applied and the assignment
       keys get the specified values 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 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.

       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 a 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 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 unless the specified match value itself
           contains trailing whitespace.

           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.

           This can only be used for very short-running foreground tasks. For
           details see RUN.

           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:

           The name to use for a network interface. The name of a device node
           cannot be changed by udev, only additional symlinks can be created.

           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.

           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 after
           processing all the rules for a specific event, depending on type:

               Execute an external program specified as the assigned value. If
               no absolute path is given, the program is expected to live in
               /usr/lib/udev, otherwise the absolute path must be specified.

               This is the default if no type is specified.

               As program, but use one of the built-in programs rather than an
               external one.

           The program name and following arguments are separated by spaces.
           Single quotes can be used to specify arguments with spaces.

           This can only be used for very short-running foreground tasks.
           Running an event process for a long period of time may block all
           further events for this or a dependent device.

           Starting daemons or other long running processes is not appropriate
           for udev; the forked processes, detached or not, will be
           unconditionally killed after the event handling has finished.

           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
               work like in RUN.

               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).

           This can only be used for very short-running foreground tasks. For
           details see RUN.

           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 node
               creation can be requested by kernel modules. These nodes might
               not have a corresponding kernel device at the time
               systemd-udevd is started; they can trigger automatic kernel
               module loading.

               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,
           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, 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. 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.

       $devnode, %N

systemd 204                                                            UDEV(7)
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