udev - 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, 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 /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 filenames 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 ignored.

   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.

       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.

       Remove the value from 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 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

   ATTR{filename}, SYSCTL{kernel parameter}
       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.  Match a kernel
       parameter value.

       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.

       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 standard output 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 glob pattern matching and alternate
   patterns. The following special 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.

       Separates alternative patterns. For example, the pattern string
       "abc|x*" would match either "abc" or "x*".

   The following keys can get values assigned:

       The name to use for a network interface. See systemd.link(5) for a
       higher-level mechanism for setting the interface name. 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 UTF-8 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 kernel's default device
       node names, as that would result in unpredictable behavior.

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

       Applies the specified Linux Security Module label to the device

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

   SYSCTL{kernel parameter}
       The value that should be written to kernel parameter.

       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, for example, 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
           /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

           Execute an external program specified as the assigned value
           and, if it returns successfully, import its output, which must
           be in environment key format. Path specification,
           command/argument separation, and quoting work like in RUN.

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

           Import a text file specified as the assigned value, the content
           of 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 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 glob pattern matching used for comparisons).

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

       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.

           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. Also, for every tag
           specified in this rule, create a symlink in the directory
           /run/udev/static_node-tags/tag pointing at the static device
           node with the specified name. Static device node creation is
           performed by systemd-tmpfiles before systemd-udevd is started.
           The static nodes might not have a corresponding kernel device;
           they are used to trigger automatic kernel module loading when
           they are accessed.

           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.

   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

   $kernel, %k
       The kernel name for this device.

   $number, %n
       The kernel number for this device. For example, "sda3" has kernel
       number "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, 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
       The name of the device node.

       The "%" character itself.

       The "$" character itself.


   systemd-udevd.service(8), udevadm(8), systemd.link(5)


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