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NAME | DESCRIPTION | RULES FILES | SEE ALSO | NOTES | COLOPHON |
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UDEV(7) udev UDEV(7)
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 program as part
of the event handling.
A rule consists of a comma-separated list of one or more
key-operator-value expressions. Each expression has a distinct
effect, depending on the key and operator used.
Operators
"=="
Compare for equality. (The specified key has the specified
value.)
"!="
Compare for inequality. (The specified key doesn't have the
specified value, or the specified key is not present at all.)
"="
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.
Added in version 217.
":="
Assign a value to a key finally; disallow any later changes.
Added in version 247.
Values
Values are written as double quoted strings, such as ("string").
To include a quotation mark (") in the value, precede it by a
backslash (\"). Any other occurrences of a backslash followed by
a character are not unescaped. That is, "\t\n" is treated as four
characters: backslash, lowercase t, backslash, lowercase n.
The string can be prefixed with a lowercase e (e"string\n") to
mark the string as C-style escaped, see Escape sequences in C[1].
For example, e"string\n" is parsed as 7 characters: 6 lowercase
letters and a newline. This can be useful for writing special
characters when a kernel driver requires them.
Please note that NUL is not allowed in either string variant.
Keys
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.
KERNELS
Search the devpath upwards for a matching device name.
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. If the operator is "!=", the token returns true only
if there is no symlink matched.
SUBSYSTEM
Match the subsystem of the event device.
SUBSYSTEMS
Search the devpath upwards for a matching device subsystem
name.
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.
DRIVERS
Search the devpath upwards for a matching device driver name.
ATTR{filename}
Match sysfs attribute value of the event device.
Trailing whitespace in the attribute values is ignored unless
the specified match value itself contains trailing
whitespace.
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.
SYSCTL{kernel parameter}
Match a kernel parameter value.
Added in version 240.
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.
Added in version 244.
"virt"
System's virtualization environment. See
systemd-detect-virt(1) for possible values.
Added in version 244.
"cvm"
System's confidential virtualization technology. See
systemd-detect-virt(1) for possible values.
Added in version 254.
Unknown keys will never match.
Added in version 244.
TAG
Match against one of device tags. It can be used once a TAG
key has been set in one of the preceding rules. There may be
multiple tags; only one needs to match. If the operator is
"!=", the token returns true only if there is no tag matched.
TAGS
Search the devpath upwards for a device with matching tag. If
the operator is "!=", the token returns true only if there is
no tag matched.
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*".
Added in version 217.
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.
Added in version 209.
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.
Added in version 220.
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 share a common 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.
Added in version 199.
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.
Added in version 199.
"builtin"
Similar to "program", but use one of the built-in
programs rather than an external one.
Added in version 199.
"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
When "replace", possibly unsafe characters in strings
assigned to NAME, SYMLINK, and ENV{key} are replaced.
When "none", no replacement is performed. When unset, the
replacement is performed for NAME, SYMLINK, but not for
ENV{key}. Defaults to unset.
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 initrd.
Added in version 241.
log_level=level
Takes a log level name like "debug" or "info", or a
special value "reset". When a log level name is
specified, the maximum log level is changed to that
level. When "reset" is set, then the previously specified
log level is revoked. Defaults to the log level of the
main process of systemd-udevd.
This may be useful when debugging events for certain
devices. Note that the log level is applied when the line
including this rule is processed. So, for debugging, it
is recommended that this is specified at earlier place,
e.g., the first line of 00-debug.rules.
Example for debugging uevent processing for network
interfaces:
# /etc/udev/rules.d/00-debug-net.rules
SUBSYSTEM=="net", OPTIONS="log_level=debug"
Added in version 248.
The ENV, GROUP, MODE, NAME, OWNER, PROGRAM, RUN, SECLABEL, and
SYMLINK 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.
systemd-udevd.service(8), udevadm(8), systemd.link(5)
1. Escape sequences in C
https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 257~devel UDEV(7)
Pages that refer to this page: udev_device_has_tag(3), nfsrahead(5), sysctl.d(5), sysfs(5), systemd.device(5), systemd.link(5), udev.conf(5), systemd.directives(7), systemd.index(7), systemd.net-naming-scheme(7), systemd.system-credentials(7), dmsetup(8), modprobe(8), systemd-udevd.service(8), systemd-udev-settle.service(8), udevadm(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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