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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.
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}
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.
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"
System's virtualization environment. See systemd-detect-virt(1) for possible
values.
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.
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.
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.
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, 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.
SEE ALSO
systemd-udevd.service(8), udevadm(8), systemd.link(5)
systemd 219 UDEV(7)
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