NAME

       udev - 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, 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.

RULES FILES

       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 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.

       "-="
           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.

       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 a 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}, 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.

       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 standard output is available in the RESULT
           key.

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

       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 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:

       NAME
           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.

       SYMLINK
           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.

       OWNER, GROUP, MODE
           The permissions for the device node. Every specified value overrides the compiled-in
           default value.

       SECLABEL{module}
           Applies the specified Linux Security Module label to the device node.

       ATTR{key}
           The value that should be written to a sysfs attribute of the event device.

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

       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, for example, 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{type}
           Add a program to the list of programs to be executed after processing all the rules
           for a specific event, depending on "type":

           "program"
               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.

           "builtin"
               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.

       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, 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.

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

           "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 glob pattern matching used for comparisons).

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

       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.

           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. 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
               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 "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. 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.

       $devnode, %N
           The name of the device node.

       %%
           The "%" character itself.

       $$
           The "$" character itself.

SEE ALSO

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