'\" t
.TH "UDEV" "7" "" "systemd 204" "udev"
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.SH "NAME"
udev \- Linux 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,
\fBsystemd-udevd.service\fR(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 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 file names replace each other\&. Files in
/etc
have the highest priority, files in
/run
take precedence over files with the same name in
/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
/lib, pointing to
/dev/null, disables the rules file entirely\&.
.PP
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\&. Except for empty lines or lines beginning with \*(Aq#\*(Aq, 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\&.
.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\-value pairs\&. Each key has a distinct operation, depending on the used operator\&. Valid operators are:
.PP
==
.RS 4
Compare for equality\&.
.RE
.PP
!=
.RS 4
Compare for inequality\&.
.RE
.PP
=
.RS 4
Assign a value to a key\&. Keys that represent a list are reset and only this single value is assigned\&.
.RE
.PP
+=
.RS 4
Add the value to a key that holds a list of entries\&.
.RE
.PP
:=
.RS 4
Assign a value to a key finally; disallow any later changes\&.
.RE
.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
\fBACTION\fR
.RS 4
Match the name of the event action\&.
.RE
.PP
\fBDEVPATH\fR
.RS 4
Match the devpath of the event device\&.
.RE
.PP
\fBKERNEL\fR
.RS 4
Match the name of the event device\&.
.RE
.PP
\fBNAME\fR
.RS 4
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
\fBSYMLINK\fR
.RS 4
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\&.
.RE
.PP
\fBSUBSYSTEM\fR
.RS 4
Match the subsystem of the event device\&.
.RE
.PP
\fBDRIVER\fR
.RS 4
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
\fBATTR{\fR\fB\fIfilename\fR\fR\fB}\fR
.RS 4
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\&.
.RE
.PP
\fBKERNELS\fR
.RS 4
Search the devpath upwards for a matching device name\&.
.RE
.PP
\fBSUBSYSTEMS\fR
.RS 4
Search the devpath upwards for a matching device subsystem name\&.
.RE
.PP
\fBDRIVERS\fR
.RS 4
Search the devpath upwards for a matching device driver name\&.
.RE
.PP
\fBATTRS{\fR\fB\fIfilename\fR\fR\fB}\fR
.RS 4
Search the devpath upwards for a device with matching sysfs attribute values\&. If multiple
\fBATTRS\fR
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
\fBTAGS\fR
.RS 4
Search the devpath upwards for a device with matching tag\&.
.RE
.PP
\fBENV{\fR\fB\fIkey\fR\fR\fB}\fR
.RS 4
Match against a device property value\&.
.RE
.PP
\fBTAG\fR
.RS 4
Match against a device tag\&.
.RE
.PP
\fBTEST{\fR\fB\fIoctal mode mask\fR\fR\fB}\fR
.RS 4
Test the existence of a file\&. An octal mode mask can be specified if needed\&.
.RE
.PP
\fBPROGRAM\fR
.RS 4
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\*(Aqs stdout is available in the RESULT key\&.
.sp
This can only be used for very short\-running foreground tasks\&. For details see
\fBRUN\fR\&.
.RE
.PP
\fBRESULT\fR
.RS 4
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\-style pattern matching\&. The following pattern characters are supported:
.PP
*
.RS 4
Matches zero or more characters\&.
.RE
.PP
?
.RS 4
Matches any single character\&.
.RE
.PP
[]
.RS 4
Matches any single character specified within the brackets\&. For example, the pattern string \*(Aqtty[SR]\*(Aq would match either \*(AqttyS\*(Aq or \*(AqttyR\*(Aq\&. Ranges are also supported via the \*(Aq\-\*(Aq character\&. For example, to match on the range of all digits, the pattern [0\-9] could be used\&. If the first character following the \*(Aq[\*(Aq is a \*(Aq!\*(Aq, any characters not enclosed are matched\&.
.RE
.PP
The following keys can get values assigned:
.PP
\fBNAME\fR
.RS 4
The name to use for a network interface\&. The name of a device node cannot be changed by udev, only additional symlinks can be created\&.
.RE
.PP
\fBSYMLINK\fR
.RS 4
The name of a symlink targeting the node\&. Every matching rule adds this value to the list of symlinks to be created\&.
.sp
The set of characters to name a symlink is limited\&. Allowed characters are [0\-9A\-Za\-z#+\-\&.:=@_/], valid utf8 character sequences, and "\ex00" hex encoding\&. All other characters are replaced by a \*(Aq_\*(Aq character\&.
.sp
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\&.
.sp
Symlink names must never conflict with the kernel\*(Aqs default device node names, as that would result in unpredictable behavior\&.
.RE
.PP
\fBOWNER\fR, \fBGROUP\fR, \fBMODE\fR
.RS 4
The permissions for the device node\&. Every specified value overrides the compiled\-in default value\&.
.RE
.PP
\fBATTR{\fR\fB\fIkey\fR\fR\fB}\fR
.RS 4
The value that should be written to a sysfs attribute of the event device\&.
.RE
.PP
\fBENV{\fR\fB\fIkey\fR\fR\fB}\fR
.RS 4
Set a device property value\&. Property names with a leading \*(Aq\&.\*(Aq are neither stored in the database nor exported to events or external tools (run by, say, the PROGRAM match key)\&.
.RE
.PP
\fBTAG\fR
.RS 4
Attach a tag to a device\&. This is used to filter events for users of libudev\*(Aqs 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
\fBRUN{\fR\fB\fItype\fR\fR\fB}\fR
.RS 4
Add a program to the list of programs to be executed after processing all the rules for a specific event, depending on
type:
.PP
program
.RS 4
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\&.
.sp
This is the default if no
\fItype\fR
is specified\&.
.RE
.PP
builtin
.RS 4
As
\fBprogram\fR, but use one of the built\-in programs rather than an external one\&.
.RE
.sp
The program name and following arguments are separated by spaces\&. Single quotes can be used to specify arguments with spaces\&.
.sp
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\&.
.sp
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\&.
.RE
.PP
\fBLABEL\fR
.RS 4
A named label to which a GOTO may jump\&.
.RE
.PP
\fBGOTO\fR
.RS 4
Jumps to the next LABEL with a matching name\&.
.RE
.PP
\fBIMPORT{\fR\fB\fItype\fR\fR\fB}\fR
.RS 4
Import a set of variables as device properties, depending on
type:
.PP
program
.RS 4
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
\fBRUN\fR\&.
.RE
.PP
builtin
.RS 4
As
\fBprogram\fR, but use one of the built\-in programs rather than an external one\&.
.RE
.PP
file
.RS 4
Import a text file specified as the assigned value, the content of which must be in environment key format\&.
.RE
.PP
db
.RS 4
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
.RS 4
Import a single property from the kernel command line\&. For simple flags the value of the property is set to \*(Aq1\*(Aq\&.
.RE
.PP
parent
.RS 4
Import the stored keys from the parent device by reading the database entry of the parent device\&. The value assigned to
\fBIMPORT{parent}\fR
is used as a filter of key names to import (with the same shell\-style pattern matching used for comparisons)\&.
.RE
.sp
This can only be used for very short\-running foreground tasks\&. For details see
\fBRUN\fR\&.
.RE
.PP
\fBWAIT_FOR\fR
.RS 4
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\&.
.RE
.PP
\fBOPTIONS\fR
.RS 4
Rule and device options:
.PP
\fBlink_priority=\fR\fB\fIvalue\fR\fR
.RS 4
Specify the priority of the created symlinks\&. Devices with higher priorities overwrite existing symlinks of other devices\&. The default is 0\&.
.RE
.PP
\fBevent_timeout=\fR
.RS 4
Number of seconds an event waits for operations to finish before giving up and terminating itself\&.
.RE
.PP
\fBstring_escape=\fR\fB\fInone|replace\fR\fR
.RS 4
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\&.
.RE
.PP
\fBstatic_node=\fR
.RS 4
Apply the permissions specified in this rule to the static device node with the specified name\&. Static device node creation can be requested by kernel modules\&. These nodes might not have a corresponding kernel device at the time systemd\-udevd is started; they can trigger automatic kernel module loading\&.
.RE
.PP
\fBwatch\fR
.RS 4
Watch the device node with inotify; when the node is closed after being opened for writing, a change uevent is synthesized\&.
.RE
.PP
\fBnowatch\fR
.RS 4
Disable the watching of a device node with inotify\&.
.RE
.RE
.PP
The
\fBNAME\fR,
\fBSYMLINK\fR,
\fBPROGRAM\fR,
\fBOWNER\fR,
\fBGROUP\fR,
\fBMODE\fR
and
\fBRUN\fR
fields support simple string substitutions\&. The
\fBRUN\fR
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
\fB$kernel\fR, \fB%k\fR
.RS 4
The kernel name for this device\&.
.RE
.PP
\fB$number\fR, \fB%n\fR
.RS 4
The kernel number for this device\&. For example, \*(Aqsda3\*(Aq has kernel number of \*(Aq3\*(Aq
.RE
.PP
\fB$devpath\fR, \fB%p\fR
.RS 4
The devpath of the device\&.
.RE
.PP
\fB$id\fR, \fB%b\fR
.RS 4
The name of the device matched while searching the devpath upwards for
\fBSUBSYSTEMS\fR,
\fBKERNELS\fR,
\fBDRIVERS\fR
and
\fBATTRS\fR\&.
.RE
.PP
\fB$driver\fR
.RS 4
The driver name of the device matched while searching the devpath upwards for
\fBSUBSYSTEMS\fR,
\fBKERNELS\fR,
\fBDRIVERS\fR
and
\fBATTRS\fR\&.
.RE
.PP
\fB$attr{\fR\fB\fIfile\fR\fR\fB}\fR, \fB%s{\fR\fB\fIfile\fR\fR\fB}\fR
.RS 4
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\&.
.sp
If the attribute is a symlink, the last element of the symlink target is returned as the value\&.
.RE
.PP
\fB$env{\fR\fB\fIkey\fR\fR\fB}\fR, \fB%E{\fR\fB\fIkey\fR\fR\fB}\fR
.RS 4
A device property value\&.
.RE
.PP
\fB$major\fR, \fB%M\fR
.RS 4
The kernel major number for the device\&.
.RE
.PP
\fB$minor\fR, \fB%m\fR
.RS 4
The kernel minor number for the device\&.
.RE
.PP
\fB$result\fR, \fB%c\fR
.RS 4
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:
\fB%c{N}\fR\&. If the number is followed by the \*(Aq+\*(Aq character, this part plus all remaining parts of the result string are substituted:
\fB%c{N+}\fR
.RE
.PP
\fB$parent\fR, \fB%P\fR
.RS 4
The node name of the parent device\&.
.RE
.PP
\fB$name\fR
.RS 4
The current name of the device\&. If not changed by a rule, it is the name of the kernel device\&.
.RE
.PP
\fB$links\fR
.RS 4
A space\-separated list of the current symlinks\&. The value is only set during a remove event or if an earlier rule assigned a value\&.
.RE
.PP
\fB$root\fR, \fB%r\fR
.RS 4
The udev_root value\&.
.RE
.PP
\fB$sys\fR, \fB%S\fR
.RS 4
The sysfs mount point\&.
.RE
.PP
\fB$devnode\fR, \fB%N\fR
.RS 4
The name of the device node\&.
.RE
.PP
%%
.RS 4
The \*(Aq%\*(Aq character itself\&.
.RE
.PP
$$
.RS 4
The \*(Aq$\*(Aq character itself\&.
.RE
.SH "SEE ALSO"
.PP
\fBsystemd-udevd.service\fR(8),
\fBudevadm\fR(8)