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 directory /usr/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 /usr/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 /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.
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.
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.
"[]"
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.
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,
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 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.
WAIT_FOR
Wait for a file to become available or until a timeout of 10 seconds expires. The path is relative to the
sysfs device; if no path is specified, this waits for an attribute to appear.
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.
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
$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 219 UDEV(7)
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