# udev

## 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 does not 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.
    
           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"_.
    
           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.
    
               **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.
    
           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
    


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