Ubuntu Manpage: udev - Dynamic device management

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 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.

NOTES

        1. Escape sequences in C
           https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences

NAME

DESCRIPTION

RULES FILES

SEE ALSO

NOTES