'\" t .TH "UDEV" "7" "" "systemd 255" "udev" .\" ----------------------------------------------------------------- .\" * Define some portability stuff .\" ----------------------------------------------------------------- .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .\" http://bugs.debian.org/507673 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .ie \n(.g .ds Aq \(aq .el .ds Aq ' .\" ----------------------------------------------------------------- .\" * set default formatting .\" ----------------------------------------------------------------- .\" disable hyphenation .nh .\" disable justification (adjust text to left margin only) .ad l .\" ----------------------------------------------------------------- .\" * MAIN CONTENT STARTS HERE * .\" ----------------------------------------------------------------- .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, \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 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\&. .PP 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\&. .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 "==" .RS 4 Compare for equality\&. (The specified key has the specified value\&.) .RE .PP "!=" .RS 4 Compare for inequality\&. (The specified key doesn\*(Aqt have the specified value, or the specified key is not present at all\&.) .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 Remove the value from a key that holds a list of entries\&. .sp Added in version 217\&. .RE .PP ":=" .RS 4 Assign a value to a key finally; disallow any later changes\&. .sp 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 (\e")\&. Any other occurrences of a backslash followed by a character are not unescaped\&. That is, "\et\en" is treated as four characters: backslash, lowercase t, backslash, lowercase n\&. .PP The string can be prefixed with a lowercase e (e"string\en") to mark the string as C\-style escaped, see \m[blue]\fBEscape sequences in C\fR\m[]\&\s-2\u[1]\d\s+2\&. For example, e"string\en" 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 Please note that \fBNUL\fR 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 \fIACTION\fR .RS 4 Match the name of the event action\&. .RE .PP \fIDEVPATH\fR .RS 4 Match the devpath of the event device\&. .RE .PP \fIKERNEL\fR .RS 4 Match the name of the event device\&. .RE .PP \fIKERNELS\fR .RS 4 Search the devpath upwards for a matching device name\&. .RE .PP \fINAME\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 \fISYMLINK\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\&. If the operator is "!=", the token returns true only if there is no symlink matched\&. .RE .PP \fISUBSYSTEM\fR .RS 4 Match the subsystem of the event device\&. .RE .PP \fISUBSYSTEMS\fR .RS 4 Search the devpath upwards for a matching device subsystem name\&. .RE .PP \fIDRIVER\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 \fIDRIVERS\fR .RS 4 Search the devpath upwards for a matching device driver name\&. .RE .PP \fIATTR{\fR\fI\fIfilename\fR\fR\fI}\fR .RS 4 Match sysfs attribute value of the event device\&. .sp Trailing whitespace in the attribute values is ignored unless the specified match value itself contains trailing whitespace\&. .RE .PP \fIATTRS{\fR\fI\fIfilename\fR\fR\fI}\fR .RS 4 Search the devpath upwards for a device with matching sysfs attribute values\&. If multiple \fIATTRS\fR matches are specified, all of them must match on the same device\&. .sp Trailing whitespace in the attribute values is ignored unless the specified match value itself contains trailing whitespace\&. .RE .PP \fISYSCTL{\fR\fI\fIkernel parameter\fR\fR\fI}\fR .RS 4 Match a kernel parameter value\&. .sp Added in version 240\&. .RE .PP \fIENV{\fR\fI\fIkey\fR\fR\fI}\fR .RS 4 Match against a device property value\&. .RE .PP \fICONST{\fR\fI\fIkey\fR\fR\fI}\fR .RS 4 Match against a system\-wide constant\&. Supported keys are: .PP "arch" .RS 4 System\*(Aqs architecture\&. See \fBConditionArchitecture=\fR in \fBsystemd.unit\fR(5) for possible values\&. .sp Added in version 244\&. .RE .PP "virt" .RS 4 System\*(Aqs virtualization environment\&. See \fBsystemd-detect-virt\fR(1) for possible values\&. .sp Added in version 244\&. .RE .PP "cvm" .RS 4 System\*(Aqs confidential virtualization technology\&. See \fBsystemd-detect-virt\fR(1) for possible values\&. .sp Added in version 254\&. .RE .sp Unknown keys will never match\&. .sp Added in version 244\&. .RE .PP \fITAG\fR .RS 4 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 \fITAGS\fR .RS 4 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 \fITEST{\fR\fI\fIoctal mode mask\fR\fR\fI}\fR .RS 4 Test the existence of a file\&. An octal mode mask can be specified if needed\&. .RE .PP \fIPROGRAM\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 standard output is available in the \fIRESULT\fR key\&. .sp This can only be used for very short\-running foreground tasks\&. For details, see \fIRUN\fR\&. .sp Note that multiple \fIPROGRAM\fR keys may be specified in one rule, and "=", ":=", and "+=" have the same effect as "=="\&. .RE .PP \fIRESULT\fR .RS 4 Match the returned string of the last \fIPROGRAM\fR call\&. This key can be used in the same or in any later rule after a \fIPROGRAM\fR call\&. .RE .PP Most of the fields support shell glob pattern matching and alternate patterns\&. The following special 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 "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 "|" .RS 4 Separates alternative patterns\&. For example, the pattern string "abc|x*" would match either "abc" or "x*"\&. .sp Added in version 217\&. .RE .PP The following keys can get values assigned: .PP \fINAME\fR .RS 4 The name to use for a network interface\&. See \fBsystemd.link\fR(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 \fISYMLINK\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 UTF\-8 character sequences, and "\ex00" hex encoding\&. All other characters are replaced by a "_" 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 \fIOWNER\fR, \fIGROUP\fR, \fIMODE\fR .RS 4 The permissions for the device node\&. Every specified value overrides the compiled\-in default value\&. .RE .PP \fISECLABEL{\fR\fI\fImodule\fR\fR\fI}\fR .RS 4 Applies the specified Linux Security Module label to the device node\&. .sp Added in version 209\&. .RE .PP \fIATTR{\fR\fI\fIkey\fR\fR\fI}\fR .RS 4 The value that should be written to a sysfs attribute of the event device\&. .RE .PP \fISYSCTL{\fR\fI\fIkernel parameter\fR\fR\fI}\fR .RS 4 The value that should be written to kernel parameter\&. .sp Added in version 220\&. .RE .PP \fIENV{\fR\fI\fIkey\fR\fR\fI}\fR .RS 4 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 \fIPROGRAM\fR match key)\&. .RE .PP \fITAG\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 \fIRUN{\fR\fI\fItype\fR\fR\fI}\fR .RS 4 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\&. .sp \fItype\fR may be: .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 \fIprogram\fR, but use one of the built\-in programs rather than an external one\&. .sp 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\&. .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 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\&. .sp 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 \fISYSTEMD_WANTS\fR device property\&. See \fBsystemd.device\fR(5) for details\&. .RE .PP \fILABEL\fR .RS 4 A named label to which a \fIGOTO\fR may jump\&. .RE .PP \fIGOTO\fR .RS 4 Jumps to the next \fILABEL\fR with a matching name\&. .RE .PP \fIIMPORT{\fR\fI\fItype\fR\fR\fI}\fR .RS 4 Import a set of variables as device properties, depending on \fItype\fR: .PP "program" .RS 4 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 \fIRUN\fR\&. .sp Added in version 199\&. .RE .PP "builtin" .RS 4 Similar to "program", but use one of the built\-in programs rather than an external one\&. .sp Added in version 199\&. .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 "1"\&. .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 glob pattern matching used for comparisons)\&. .RE .sp This can only be used for very short\-running foreground tasks\&. For details see \fBRUN\fR\&. .sp Note that multiple \fIIMPORT{}\fR 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 \fIOPTIONS\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 \fBstring_escape=\fR\fB\fInone|replace\fR\fR .RS 4 When "replace", possibly unsafe characters in strings assigned to \fINAME\fR, \fISYMLINK\fR, and \fIENV{\fR\fI\fIkey\fR\fR\fI}\fR are replaced\&. When "none", no replacement is performed\&. When unset, the replacement is performed for \fINAME\fR, \fISYMLINK\fR, but not for \fIENV{\fR\fI\fIkey\fR\fR\fI}\fR\&. Defaults to unset\&. .RE .PP \fBstatic_node=\fR .RS 4 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/\fItag\fR 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 \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 .PP \fBdb_persist\fR .RS 4 Set the flag (sticky bit) on the udev database entry of the event device\&. Device properties are then kept in the database even when \fBudevadm info \-\-cleanup\-db\fR is called\&. This option can be useful in certain cases (e\&.g\&. Device Mapper devices) for persisting device state on the transition from initrd\&. .sp Added in version 241\&. .RE .PP \fBlog_level=\fR\fB\fIlevel\fR\fR .RS 4 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 \fBsystemd\-udevd\fR\&. .sp 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\&. .sp Example for debugging uevent processing for network interfaces: .sp .if n \{\ .RS 4 .\} .nf # /etc/udev/rules\&.d/00\-debug\-net\&.rules SUBSYSTEM=="net", OPTIONS="log_level=debug" .fi .if n \{\ .RE .\} .sp Added in version 248\&. .RE .RE .PP The \fINAME\fR, \fISYMLINK\fR, \fIPROGRAM\fR, \fIOWNER\fR, \fIGROUP\fR, \fIMODE\fR, \fISECLABEL\fR, and \fIRUN\fR fields support simple string substitutions\&. The \fIRUN\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, "sda3" has kernel number 3\&. .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 \fBKERNELS\fR, \fBSUBSYSTEMS\fR, \fBDRIVERS\fR, or \fBATTRS\fR 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 \fIPROGRAM\fR\&. 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 \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 \fB%%\fR .RS 4 The "%" character itself\&. .RE .PP \fB$$\fR .RS 4 The "$" character itself\&. .RE .SH "SEE ALSO" .PP \fBsystemd-udevd.service\fR(8), \fBudevadm\fR(8), \fBsystemd.link\fR(5) .SH "NOTES" .IP " 1." 4 Escape sequences in C .RS 4 \%https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences .RE