udev

UDEV(7)                               udev                               UDEV(7)



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
       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 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}
           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.

           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.

           Note that multiple PROGRAM keys may be specified in one rule, and
           "=", ":=", and "+=" have the same effect as "==".

       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}
           Specify a program to be executed after processing of all the rules
           for the event. With "+=", this invocation is added to the list, and
           with "=" or ":=", it replaces any previous contents of the list.
           Please note that both "program" and "builtin" types described below
           use a single list, so clearing the list with ":=" and "=" affects
           both types.

           type may be:

           "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.

           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.

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

       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.

           Note that multiple IMPORT{} keys may be specified in one rule, and
           "=", ":=", and "+=" have the same effect as "==". The key is true if
           the import is successful, unless "!=" is used as the operator which
           causes the key to be true if the import failed.

       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.

           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.

       $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)



systemd 246                                                              UDEV(7)