udev






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 directories /usr/lib/udev/rules.d and
/usr/local/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 under /usr. 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 /usr/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









                             ‐2‐


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.










                             ‐3‐


     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}
     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.

     SYSCTL{kernel parameter}
     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.

     CONST{key}
     Match against a system−wide constant. Supported keys
     are:

          "arch"
          System's architecture. See ConditionArchitecture=
          in systemd.unit(5) for possible values.

          "virt"
          System's virtualization environment. See systemd‐
          detect‐virt(1) for possible values.









                             ‐4‐


     Unknown keys will never match.

     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









                             ‐5‐


     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









                             ‐6‐


     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 /usr/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.

     Note that running programs that access the network or
     mount/unmount filesystems is not allowed inside of udev
     rules, due to the default sandbox that is enforced on
     systemd−udevd.service.

     Please also note that ":=" and "=" are clearing both,
     program and builtin commands.

     In order to activate long−running processes from udev
     rules, provide a service unit, and pull it in from a
     udev device using the SYSTEMD_WANTS device property.
     See systemd.device(5) for details.

     LABEL
     A named label to which a GOTO may jump.

     GOTO
     Jumps to the next LABEL with a matching name.









                             ‐7‐


     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.









                             ‐8‐


          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.

          db_persist
          Set the flag (sticky bit) on the udev database
          entry of the event device. Device properties are
          then kept in the database even when udevadm info
          −−cleanup−db is called. This option can be useful
          in certain cases (e.g. Device Mapper devices) for
          persisting device state on the transition from
          initramfs.

     The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE,
SECLABEL, 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.









                             ‐9‐


     $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.









                            ‐10‐


     %%
     The "%" character itself.

     $$
     The "$" character itself.



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