| UDEV(7) | udev | UDEV(7) |
.SH "NAME" udev - Dynamic device management
.SH "DESCRIPTION"
.PP 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.
.PP 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.
.PP 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.
.SH "RULES FILES"
.PP 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 lexicographic 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.
.PP Every line in the rules file contains at least one key-value pair. A line
ending with a backslash (\) is joined with the following line, and the
result is processed as a single line. Lines beginning with # are ignored,
even within a continuation. An empty or whitespace-only line ends a pending
continuation and is otherwise 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.
.PP 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.
.PP 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.
.SS "Operators"
.PP ==
Compare for equality. (The specified key has the specified value.)
.RE
.PP !=
Compare for inequality. (The specified key does not have the specified value,
or the specified key is not present at all.)
.RE
.PP =
Assign a value to a key. Keys that represent a list are reset and only this
single value is assigned.
.RE
.PP +=
Add the value to a key that holds a list of entries.
.RE
.PP -=
Remove the value from a key that holds a list of entries.
Added in version 217.
.RE
.PP :=
Assign a value to a key finally; disallow any later changes.
Added in version 247.
.RE
.SS "Values"
.PP 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.
.PP 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.
.PP The string can be prefixed with a lowercase i (i"string") to
mark that the string or pattern will match case-insensitively. For example,
i"foo" will match foo, FOO, FoO and so on. The prefix can be used
only for match (==) or unmatch (!=) rules, e.g.
ATTR{foo}==i"abcd".
.PP Please note that NUL is not allowed in either string variant.
.SS "Keys"
.PP 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.
.PP ACTION
Match the name of the event action.
.RE
.PP DEVPATH
Match the devpath of the event device.
.RE
.PP KERNEL
Match the name of the event device.
.RE
.PP KERNELS
Search the devpath upwards for a matching device name.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP SUBSYSTEM
Match the subsystem of the event device.
.RE
.PP SUBSYSTEMS
Search the devpath upwards for a matching device subsystem name.
.RE
.PP 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.
.RE
.PP DRIVERS
Search the devpath upwards for a matching device driver name.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP SYSCTL{kernel parameter}
Match a kernel parameter value.
Added in version 240.
.RE
.PP ENV{key}
Match against a device property value.
.RE
.PP CONST{key}
Match against a system-wide constant. Supported keys are:
.PP arch
System's architecture. See ConditionArchitecture= in
systemd.unit(5) for possible values.
Added in version 244.
.RE
.PP virt
System's virtualization environment. See systemd-detect-virt(1) for
possible values.
Added in version 244.
.RE
.PP cvm
System's confidential virtualization technology. See
systemd-detect-virt(1) for possible values.
Added in version 254.
.RE
.sp
Unknown keys will never match.
Added in version 244.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP TEST{octal mode mask}
Test the existence of a file. An octal mode mask can be specified if
needed.
.RE
.PP 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 ==.
.RE
.PP 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.
.RE
.PP Most of the fields support shell glob pattern matching and alternate
patterns. The following special characters are supported:
.PP *
Matches zero or more characters.
.RE
.PP ?
Matches any single character.
.RE
.PP []
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.
.RE
.PP |
Separates alternative patterns. For example, the pattern string abc|x* would
match either abc or x*.
Added in version 217.
.RE
.PP The following keys can get values assigned:
.PP 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.
.RE
.PP 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.
.RE
.PP OWNER, GROUP, MODE
The permissions for the device node. Every specified value overrides the
compiled-in default value.
.RE
.PP SECLABEL{module}
Applies the specified Linux Security Module label to the device node.
Added in version 209.
.RE
.PP ATTR{key}
The value that should be written to a sysfs attribute of the event
device.
.RE
.PP SYSCTL{kernel parameter}
The value that should be written to kernel parameter.
Added in version 220.
.RE
.PP 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).
.RE
.PP 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.
.RE
.PP 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:
.PP 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.
.RE
.PP builtin
As program, but use one of the built-in programs rather than an
external one.
Added in version 199.
.RE
.sp
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.
.RE
.PP LABEL
A named label to which a GOTO may jump.
.RE
.PP GOTO
Jumps to the next LABEL with a matching name.
.RE
.PP IMPORT{type}
Import a set of variables as device properties, depending on type:
.PP 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.
.RE
.PP builtin
Similar to program, but use one of the built-in programs rather than an
external one.
Added in version 199.
.RE
.PP file
Import a text file specified as the assigned value, the content of which must
be in environment key format.
.RE
.PP 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.
.RE
.PP cmdline
Import a single property from the kernel command line. For simple flags the
value of the property is set to 1.
.RE
.PP 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).
.RE
.sp
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.
.RE
.PP OPTIONS
Rule and device options:
.PP link_priority=value
Specify the priority of the created symlinks. Devices with higher priorities
overwrite existing symlinks of other devices. The default is 0.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP watch
Watch the device node with inotify; when the node is closed after being
opened for writing, a change uevent is synthesized.
.RE
.PP nowatch
Disable the watching of a device node with inotify.
.RE
.PP 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.
.RE
.PP 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.
.RE
.PP dump
Dump the status of the event currently processing. It may be useful for
debugging udev rules by inserting this option.
Added in version 258.
.RE
.PP dump-json
Similar to dump, but dump the status of the event in JSON format.
Added in version 259.
.RE
.sp
.RE
.PP 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:
.PP $kernel, %k
The kernel name for this device.
.RE
.PP $number, %n
The kernel number for this device. For example, sda3 has kernel number 3.
.RE
.PP $devpath, %p
The devpath of the device.
.RE
.PP $id, %b
The name of the device matched while searching the devpath upwards for
SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
.RE
.PP $driver
The driver name of the device matched while searching the devpath upwards for
SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
.RE
.PP $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.
.RE
.PP $env{key},
%E{key}
A device property value.
.RE
.PP $major, %M
The kernel major number for the device.
.RE
.PP $minor, %m
The kernel minor number for the device.
.RE
.PP $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+}.
.RE
.PP $parent, %P
The node name of the parent device.
.RE
.PP $name
The current name of the device. If not changed by a rule, it is the name of
the kernel device.
.RE
.PP $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.
.RE
.PP $root, %r
The udev_root value.
.RE
.PP $sys, %S
The sysfs mount point.
.RE
.PP $devnode, %N
The name of the device node.
.RE
.PP %%
The % character itself.
.RE
.PP $$
The $ character itself.
.RE
.SH "SEE ALSO"
NOTES
- 1.
- Escape sequences in C
| systemd 262.2 |