UDEV(7)
NAME
udev - Linux 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 udevd(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 information 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 are provided by the library libudev.
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 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 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. - 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.
- 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. 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, a network interface should be renamed to. Or as a
temporary workaraound, the name a device node should be named.
Usually the kernel provides the defined node name, or even creates and removes the node before udev even receives any event. Changing the node name from the kernel's default creates inconsistencies and is not supported. If the kernel and NAME specify different names,
an error will be logged. Udev is only expected to handle device
node permissions and to create additional symlinks, not to change
kernel-provided device node names. Instead of renaming a device
node, SYMLINK should be used. Symlink names must never conflict
with device node names, it will result in unpredictable behavior. - SYMLINK
The name of a symlink targeting the node. Every matching rule will add this value to the list of symlinks to be created. Multiple
symlinks may be specified by separating the names by the space
character. In case multiple devices claim the same name, the link
will always point to the device with the highest link_priority. If the current device goes away, the links will be re-evaluated and
the device with the next highest link_priority will own the link.
If no link_priority is specified, the order of the devices, and
which one of them will own the link, is undefined. Claiming the
same name for a symlink, which is or might be used for a device
node, may result in unexpected behavior and is not supported. - 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.
- 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
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 no absolute path is given, the program is expected to live in
/lib/udev, otherwise the absolute path must be specified. Program
name and arguments are separated by spaces. Single quotes can be
used to specify arguments with spaces. - 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:programExecute an external program specified as the assigned value and import its output, which must be in environment key format.
Path specification, command/argument separation, and quoting
work like in RUN.fileImport a text file specified as the assigned value, which must be in environment key format.dbImport 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.cmdlineImport a single property from the kernel commandline. For
simple flags the value of the property will be set to '1'.parentImport 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 or until a 10 seconds timeout
expires. - OPTIONS
Rule and device options:link_priority=valueSpecify the priority of the created symlinks. Devices with
higher priorities overwrite existing symlinks of other devices. The default is 0.event_timeout=Number of seconds an event will wait for operations to finish, before it will terminate itself.string_escape=none|replaceUsually 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 a static device node with the specified name. Static device nodes might be
provided by kernel modules, or copied from /lib/udev/devices.
These nodes might not have a corresponding kernel device at the time udevd is started, and allow to trigger automatic kernel
module on-demand loading.watchWatch the device node with inotify, when closed after being
opened for writing, a change uevent will be synthesised.nowatchDisable 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@kroah.com and Kay Sievers
kay.sievers@vrfy.org. With much help from Dan Stekloff and many others.