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UDEV(7)                              udev                              UDEV(7)



NAME
       udev - dynamic device management

DESCRIPTION
       udev provides a dynamic device directory containing only the files for actually
       present devices. It creates or removes device node files in the /dev directory, or
       it renames network interfaces.

       Usually udev runs as udevd(8) and receives uevents directly from the kernel if a
       device is added or removed from the system.

       If udev receives a device event, it matches its configured rules against the
       available device attributes provided in sysfs to identify the device. Rules that
       match may provide additional device information or specify a device node name and
       multiple symlink names and instruct udev to run additional programs as part of the
       device event handling.

CONFIGURATION
       udev configuration files are placed in /etc/udev/ and /lib/udev/. All empty lines,
       or lines beginning with ?#? will be ignored.

   Configuration file
       udev expects its main configuration file at /etc/udev/udev.conf. It consists of a
       set of variables allowing the user to override default udev values. The following
       variables can be set:

       udev_root
           Specifies where to place the device nodes in the filesystem. The default value
           is /dev.

       udev_log
           The logging priority. Valid values are the numerical syslog priorities or their
           textual representations: err, info and debug.

   Rules files
       The udev rules are read from the files located in the default rules directory
       /lib/udev/rules.d/, the custom rules directory /etc/udev/rules.d/ and the temporary
       rules directory /dev/.udev/rules.d/. All rule files are sorted and processed in
       lexical order, regardless in which of these directories they live.

       Rule files are required to have a unique name, duplicate file names are ignored.
       Files in /etc/udev/rules.d/ have precedence over files with the same name in
       /lib/udev/rules.d/. This can be used to ignore a default rules file if needed.

       Every line in the rules file contains at least one key value pair. There are two
       kind of keys, match and assignment keys. If all match keys are matching against its
       value, the rule gets applied and the assign keys get the specified value assigned.

       A matching rule may specify the name of the device node, add a symlink pointing to
       the node, or run a specified program as part of the event handling. If no matching
       rule is found, the default device node name is used.

       A rule consists of a list of one or more key value pairs separated by a comma. 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.

       :=
           Assign a value to a key finally; disallow any later changes, which may be used
           to prevent changes by any later rules.

       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 the node or 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 for devices which are bound
           to a driver at the time the event is generated.

       SECLABEL{module}
           Applies the specified Linux Security Module label to the device node. Note that
           at this time only selinux is supported.

       ATTR{filename}
           Match sysfs attribute values of the event device. Trailing whitespace in the
           attribute values is ignored, if the specified match value does not contain
           trailing whitespace itself.

       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, if the
           specified match value does not contain trailing whitespace itself.

       ENV{key}
           Match against a device property value.

       TEST{octal mode mask}
           Test the existence of a file. An octal mode mask can be specified if needed.

       PROGRAM
           Execute a program. 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 output printed to stdout, is available in the RESULT
           key.

       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 a shell style pattern matching. The following pattern
       characters are supported:

       *
           Matches zero, or any number of 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 within this match with the ?-? character. For example, to match on
           the range of all digits, the pattern [0-9] would be used. If the first
           character following the ?[? is a ?!?, any characters not enclosed are matched.

       The following keys can get values assigned:

       NAME
           The name of the node to be created, or the name the network interface should be
           renamed to.

       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 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.

           Symlink names must never conflict with the kernels default device node names,
           as that would result in unpredictable behavior.

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

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

       ENV{key}
           Set a device property value. Property names with a leading ?.? are not stored
           in the database or exported to external tool or events.

       RUN
           Add a program to the list of programs to be executed for a specific device.
           This can only be used for very short running tasks. Running an event process
           for a long period of time may block all further events for this or a dependent
           device. Long running tasks need to be immediately detached from the event
           process itself. If the option RUN{fail_event_on_error} is specified, and the
           executed program returns non-zero, the event will be marked as failed for a
           possible later handling.

           If the specified string starts with socket:path, all current event values will
           be passed to the specified socket, as a message in the same format the kernel
           sends an uevent. If the first character of the specified path is an @
           character, an abstract namespace socket is used, instead of an existing socket
           file.

       LABEL
           Named label where a GOTO can jump to.

       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 import its
               output, which must be in environment key format.

           file
               Import a text file specified as the assigned value, 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 commandline. For simple flags the
               value of the property will be 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-style pattern matching
               used for comparisons).

           If no option is given, udev will choose between program and file based on the
           executable bit of the file permissions.

       WAIT_FOR
           Wait for a file to become available.

       OPTIONS
           Rule and device options:

           ignore_device
               Ignore this event completely.

           ignore_remove
               Do not remove the device node when the device goes away. This may be useful
               as a workaround for broken device drivers.

           link_priority=value
               Specify the priority of the created symlinks. Devices with higher
               priorities overwrite existing symlinks of other devices. The default is 0.

           all_partitions
               Create the device nodes for all available partitions of a block device.
               This may be useful for removable media devices where media changes are not
               detected.

           event_timeout=
               Number of seconds an event will wait for operations to finish, before it
               will terminate itself.

           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.

           watch
               Watch the device node with inotify, when closed after being opened for
               writing, a change uevent will be synthesised.

           nowatch
               Disable the watching of a device node with inotify.

       The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE and RUN fields support simple
       printf-like string substitutions. The RUN format chars gets applied after all rules
       have been processed, right before the program is executed. It allows the use of
       device properties set by earlier matching rules. For all other fields,
       substitutions are applied 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 of ?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, follow
           the chain of parent devices and use the value of the first attribute that
           matches. 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 ?+? char 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 node. If not changed by a rule, it is the name
           of the kernel device.

       $links
           The current list of symlinks, separated by a space character. The value is only
           set if an earlier rule assigned a value, or during a remove events.

       $root, %r
           The udev_root value.

       $sys, %S
           The sysfs mount point.

       $tempnode, %N
           The name of a created temporary device node to provide access to the device
           from a external program before the real node is created.

       %%
           The ?%? character itself.

       $$
           The ?$? character itself.

AUTHOR
       Written by Greg Kroah-Hartman greg AT kroah.com and Kay Sievers kay.sievers AT vrfy.org.
       With much help from Dan Stekloff and many others.

SEE ALSO
       udevd(8), udevadm(8)



udev                              09/06/2016                           UDEV(7)

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