NAME
udev - Dynamic device managementDESCRIPTION
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 /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 /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.
":="
Assign a value to a key finally; disallow any
later changes.
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[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. ACTIONMatch 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.
ENV{key}
Match against a device property value.
CONST{key}
Match against a system-wide constant.
Supported keys are:
"arch"
Unknown keys will never match.
TAG
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.
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*".
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}
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"
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
Execute an external program specified as the
assigned value. If no absolute path is given, the program is expected to live
in /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.
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"
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
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).
Rule and device options:
link_priority=value
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
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.
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"
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)NOTES
- 1.
- C-style escaped
systemd 252 |