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.
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},
SYSCTL{kernel parameter}
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. 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"
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.
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}
Add a program to the list of programs to be executed
after processing all the rules for a specific event, depending on
"type":
"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.
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.
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.
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.
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.
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.
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.