udev - Linux dynamic device management


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 udevd(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 information 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 are provided by the library libudev.


udev configuration files are placed in /etc/udev/ and /lib/udev/. All empty lines, or lines beginning with \*(Aq#\*(Aq will be ignored.

3.1. Configuration file

udev expects its main configuration file at /etc/udev/udev.conf. It consists of a set of variables allowing the user to override default udev values. The following variables can be set:

    Specifies where to place the device nodes in the filesystem. The default value is /dev.

    The logging priority. Valid values are the numerical syslog priorities or their textual representations: err, info and debug.

3.2. Rules files

The udev rules are read from the files located in the default rules directory /lib/udev/rules.d/, the custom rules directory /etc/udev/rules.d/ and the temporary rules directory /dev/.udev/rules.d/. All rule files are sorted and processed in lexical order, regardless in which of these directories they live. Files in /etc/udev/rules.d/ have precedence over files with the same name in /lib/udev/rules.d/. This can be used to ignore a default rules file if needed.

Rule files must end in .rules, other extensions are ignored.

Every line in the rules file contains at least one key value pair. There are two kind of keys, match and assignment keys. If all match keys are matching against its value, the rule gets applied and the assign keys get the specified value 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 list of one or more key value pairs separated by a comma. Each key has a distinct operation, depending on the used operator. Valid operators are:

    Compare for equality.

    Compare for inequality.

    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.

    Assign a value to a key finally; disallow any later changes, which may be used to prevent changes by any later rules.

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.

    Match the name of the event action.

    Match the devpath of the event device.

    Match the name of the event device.

    Match the name of the node or network interface. It can be used once the NAME key has been set in one of the preceding rules.

    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.

    Match the subsystem of the event device.

    Match the driver name of the event device. Only set for devices which are bound to a driver at the time the event is generated.

    Match sysfs attribute values of the event device. Trailing whitespace in the attribute values is ignored, if the specified match value does not contain trailing whitespace itself.

    Search the devpath upwards for a matching device name.

    Search the devpath upwards for a matching device subsystem name.

    Search the devpath upwards for a matching device driver name.

    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, if the specified match value does not contain trailing whitespace itself.

    Match against a device property value.

    Match against a device tag.

TEST{\fBoctal mode mask\fR}
    Test the existence of a file. An octal mode mask can be specified if needed.

    Execute a program. 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 output printed to stdout, is available in the RESULT key.

    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 a shell style pattern matching. The following pattern characters are supported:

    Matches zero, or any number of characters.

    Matches any single character.

    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 within this match with the \*(Aq-\*(Aq character. For example, to match on the range of all digits, the pattern [0-9] would be used. If the first character following the \*(Aq[\*(Aq is a \*(Aq!\*(Aq, any characters not enclosed are matched.

The following keys can get values assigned:

    The name, a network interface should be renamed to. Or as a temporary workaround, the name a device node should be named. Usually the kernel provides the defined node name, or even creates and removes the node before udev even receives any event. Changing the node name from the kernel\*(Aqs default creates inconsistencies and is not supported. If the kernel and NAME specify different names, an error will be logged. Udev is only expected to handle device node permissions and to create additional symlinks, not to change kernel-provided device node names. Instead of renaming a device node, SYMLINK should be used. Symlink names must never conflict with device node names, it will result in unpredictable behavior.

    The name of a symlink targeting the node. Every matching rule will add this value to the list of symlinks to be created. Multiple symlinks may be specified by separating the names by the space character. In case multiple devices claim the same name, the link will always point to the device with the highest link_priority. If the current device goes away, the links will be re-evaluated and the device with the next highest link_priority will own the link. If no link_priority is specified, the order of the devices, and which one of them will own the link, is undefined. Claiming the same name for a symlink, which is or might be used for a device node, may result in unexpected behavior and is not supported.

    The permissions for the device node. Every specified value overwrites the compiled-in default value.

    The value that should be written to a sysfs attribute of the event device.

    Set a device property value. Property names with a leading \*(Aq.\*(Aq are not stored in the database or exported to external tool or events.

    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.

    Add a program to the list of programs to be executed for a specific device. This can only be used for very short running tasks. Running an event process for a long period of time may block all further events for this or a dependent device. Long running tasks need to be immediately detached from the event process itself. If the option RUN{\fBfail_event_on_error\fR} is specified, and the executed program returns non-zero, the event will be marked as failed for a possible later handling.

If no absolute path is given, the program is expected to live in /lib/udev, otherwise the absolute path must be specified. Program name and arguments are separated by spaces. Single quotes can be used to specify arguments with spaces.

    Named label where a GOTO can jump to.

    Jumps to the next LABEL with a matching name

    Import a set of variables as device properties, depending on type:

    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 RUN.

    Import a text file specified as the assigned value, which must be in environment key format.

    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.

    Import a single property from the kernel commandline. For simple flags the value of the property will be set to \*(Aq1\*(Aq.

    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-style pattern matching used for comparisons).

If no option is given, udev will choose between program and file based on the executable bit of the file permissions.

    Wait for a file to become available or until a 10 seconds timeout expires. The path is relative to the sysfs device, i. e. if no path is specified this waits for an attribute to appear.

    Rule and device options:

    Specify the priority of the created symlinks. Devices with higher priorities overwrite existing symlinks of other devices. The default is 0.

    Number of seconds an event will wait for operations to finish, before it will terminate itself.

    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.

    Apply the permissions specified in this rule to a static device node with the specified name. Static device nodes might be provided by kernel modules, or copied from /lib/udev/devices. These nodes might not have a corresponding kernel device at the time udevd is started, and allow to trigger automatic kernel module on-demand loading.

    Watch the device node with inotify, when closed after being opened for writing, a change uevent will be synthesised.

    Disable the watching of a device node with inotify.

The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE and RUN fields support simple printf-like string substitutions. The RUN format chars gets applied after all rules have been processed, right before the program is executed. It allows the use of device properties set by earlier matching rules. For all other fields, substitutions are applied 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, \*(Aqsda3\*(Aq has kernel number of \*(Aq3\*(Aq

$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.

    The driver name of the device matched while searching the devpath upwards for SUBSYSTEMS, KERNELS, DRIVERS and ATTRS.

$attr{\fBfile\fR}, %s{\fBfile\fR}
    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, use the attribute from that parent device. If the attribute is a symlink, the last element of the symlink target is returned as the value.

$env{\fBkey\fR}, %E{\fBkey\fR}
    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 \*(Aq+\*(Aq char this part plus all remaining parts of the result string are substituted: %c{N+}

$parent, %P
    The node name of the parent device.

    The current name of the device node. If not changed by a rule, it is the name of the kernel device.

    The current list of symlinks, separated by a space character. The value is only set if an earlier rule assigned a value, or during a remove events.

$root, %r
    The udev_root value.

$sys, %S
    The sysfs mount point.

$tempnode, %N
    The name of a created temporary device node to provide access to the device from a external program before the real node is created.

    The \*(Aq%\*(Aq character itself.

    The \*(Aq$\*(Aq character itself.


Written by Greg Kroah-Hartman and Kay Sievers . With much help from Dan Stekloff and many others.


udevd(8), udevadm(8)