udev

http://en.wikipedia.org/wiki/Udev

udev

udev
Developer(s)	Greg Kroah-Hartman and Kay Sievers
Initial release	November 2003
Stable release	184 (May 30, 2012; 18 months ago) [±]
Operating system	Linux kernel
Type	Device node
License	GPL v2
Website	www.freedesktop.org/software/systemd/libudev/

udev has been incorporated into systemd, though some Linux distributions, such as Debian ship it separately

udev is a device manager for the Linux kernel. Primarily, it manages device nodes in /dev. It is the successor of devfs and hotplug, which means that it handles the /dev directory and all user space actions when adding/removing devices, including firmware load.

[hide]

History[edit]

udev was introduced in Linux 2.5.

The Linux kernel version 2.6.13 introduced or updated a new version of the uevent interface. A system using a new version of udev will not boot with kernels older than 2.6.13 unless udev is disabled and a traditional /dev directory is used for device access.

In April 2012, udev's source tree was merged into systemd.^[1]^[2]

Overview[edit]

Unlike traditional Unix systems, where the device nodes in the /dev directory have been a static set of files, the Linux udev device manager dynamically provides only the nodes for the devices actually present on a system. Although devfs used to provide similar functionality, Greg Kroah-Hartman cited a number of reasons^[3] for preferring its implementation over devfs:

udev supports persistent device naming, which does not depend on, for example, the order in which the devices are plugged into the system. The default udev setup provides persistent names for storage devices. Any hard disk is recognized by its unique filesystem id, the name of the disk and the physical location on the hardware it is connected to.
udev executes entirely in user space, as opposed to devfs' kernel space. One consequence is that udev moved the naming policy out of the kernel and can run arbitrary programs to compose a name for the device from the device's properties, before the node is created; there, the whole process is also interruptible and it runs with a lower priority.

Operation[edit]

udev is a generic kernel device manager. It runs as a daemon on a Linux system and listens (via netlink socket) to uevents the kernel sends out if a new device is initialized or a device is removed from the system. The system provides a set of rules that match against exported values of the event and properties of the discovered device. A matching rule will possibly name and create a device node and run configured programs to set up and configure the device.

udev rules can match on properties like the kernel subsystem, the kernel device name, the physical location of the device, or properties like the device's serial number. Rules can also request information from external programs to name a device or specify a custom name that will always be the same, regardless of the order devices are discovered by the system.

In the past a common way to use udev on Linux systems was to let it send events through a socket to HAL, which would perform further device-specific actions. For example, HAL would notify other software running on the system that the new hardware had arrived by issuing a broadcast message on the D-Bus IPC system to all interested processes. In this way, desktops such as GNOME or K Desktop Environment 3 could start the file browser to browse the file systems of newly attached USB flash drives and SD cards.^[4]

By the middle of 2011 HAL had been deprecated by most Linux distributions as well as by the KDE, GNOME^[5] and XFCE^[6] desktop environments, among others. The functionality previously embodied in HAL has been integrated into udev itself, or moved to separate software such as udisks and upower.

Architecture[edit]

The system is divided into three parts:

The library libudev, that allows access to device information.
The daemon udevd, in user space, that manages the virtual /dev.
The administrative command udevadm for diagnostics.

The system gets calls from the kernel via netlink socket. Earlier versions used hotplug, adding a link to themselves in /etc/hotplug.d/default with this purpose.

Authors[edit]

udev was developed by Greg Kroah-Hartman and Kay Sievers, with much help from Dan Stekloff, among others.

udev(7) - Linux man page

Name

udev - dynamic device management

Description

udev provides a dynamic device directory containing only the files for actually present devices. It creates or removes device node files in the /dev directory, or it renames network interfaces.

Usually udev runs as udevd(8) and receives uevents directly from the kernel if a device is added or removed from the system.

If udev receives a device event, it matches its configured rules against the available device attributes provided in sysfs to identify the device. Rules that match may provide additional device information or specify a device node name and multiple symlink names and instruct udev to run additional programs as part of the device event handling.

Configuration

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

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:

udev_root

: Specifies where to place the device nodes in the filesystem. The default value is /dev.
udev_log: The logging priority. Valid values are the numerical syslog priorities or their textual representations: err, info and debug.

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 /run/udev/rules.d/. All rule files are sorted and processed in lexical order, regardless in which of these directories they live.

Rule files are required to have a unique name, duplicate file names are ignored. 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.

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 specify the name of the device node, add a symlink pointing to the node, or run a specified program as part of the event handling. If no matching rule is found, the default device node name is used.

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.

ACTION

: Match the name of the event action.
DEVPATH: Match the devpath of the event device.
KERNEL: Match the name of the event device.
NAME: 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.
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.
SUBSYSTEM: Match the subsystem of the event device.
DRIVER: 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.
ATTR{filename}: 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.
KERNELS: Search the devpath upwards for a matching device name.
SUBSYSTEMS: Search the devpath upwards for a matching device subsystem name.
DRIVERS: Search the devpath upwards for a matching device driver name.
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, if the specified match value does not contain trailing whitespace itself.
ENV{key}: Match against a device property value.
TEST{octal mode mask}: Test the existence of a file. An octal mode mask can be specified if needed.
PROGRAM: 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's output printed to stdout, is available in the RESULT key.
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 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 'tty[SR]' would match either 'ttyS' or 'ttyR'. Ranges are also supported within this match with the '-' character. For example, to match on the range of all digits, the pattern [0-9] would be used. If the first character following the '[' is a '!', any characters not enclosed are matched.
The following keys can get values assigned:

NAME

The name of the node to be created, or the name the network interface should be renamed to.

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 utf8 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 kernels default device node names, as that would result in unpredictable behavior.

OWNER, GROUP, MODE

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

ATTR{key}

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

ENV{key}

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

RUN

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 optionRUN{fail_event_on_error} is specified, and the executed program returns non-zero, the event will be marked as failed for a possible later handling.

If the specified string starts with socket:path, all current event values will be passed to the specified socket, as a message in the same format the kernel sends an uevent. If the first character of the specified path is an @ character, an abstract namespace socket is used, instead of an existing socket file.

LABEL

Named label where a GOTO can jump to.

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 import its output, which must be in environment key format.

file

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.

cmdline

Import a single property from the kernel commandline. For simple flags the value of the property will be 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-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

Wait for a file to become available.

OPTIONS

Rule and device options:

ignore_device

Ignore this event completely.

ignore_remove

Do not remove the device node when the device goes away. This may be useful as a workaround for broken device drivers.

link_priority=value

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

all_partitions

Create the device nodes for all available partitions of a block device. This may be useful for removable media devices where media changes are not detected.

event_timeout=

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

string_escape=none|replace

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.

watch

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

nowatch

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, 'sda3' has kernel number of '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, follow the chain of parent devices and use the value of the first attribute that matches. 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 '+' char 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 node. If not changed by a rule, it is the name of the kernel device.
$links: 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 '%' character itself.
$$: The '$' character itself.

Author

Written by Greg Kroah-Hartman greg@kroah.com and Kay Sievers kay.sievers@vrfy.org. With much help from Dan Stekloff and many others.

Referenced By

aoe-mkdevs(8), aoe-mkshelf(8), aoe-stat(8), ata_id(8), btkbdd(8), edd_id(8), hald(8), lvs(8), modprobe(8), nameif(8), pvs(8), scsi_id(8), udevinfo(8), udevmonitor(8),udevsettle(8), udevtest(8), udevtrigger(8), udisks(7), vgs(8), vol_id(8), wdaemon(1)

udevadm(8) - Linux man page

Name

udevadm - udev management tool

Synopsis

udevadm [--debug] [--version] [--help]

udevadm info options
udevadm trigger [options]
udevadm settle [options]
udevadm control command
udevadm monitor [options]
udevadm test [options] devpath

Description

udevadm expects a command and command specific options. It controls the runtime behavior of udev, requests kernel events, manages the event queue, and provides simple debugging mechanisms.

Options

--debug

Print debug messages to stderr.
--version: Print version number.
--help: Print help text.

udevadm info options

Queries the udev database for device information stored in the udev database. It can also query the properties of a device from its sysfs representation to help creating udev rules that match this device.

--query=type

: Query the database for specified type of device data. It needs the --path or --name to identify the specified device. Valid queries are: name, symlink, path, property,all.
--path=devpath: The devpath of the device to query.
--name=file: The name of the device node or a symlink to query
--root: The udev root directory: /dev. If used in conjunction with a name or symlink query, the query returns the absolute path including the root directory.
--attribute-walk: Print all sysfs properties of the specified device that can be used in udev rules to match the specified device. It prints all devices along the chain, up to the root of sysfs that can be used in udev rules.
--device-id-of-file=file: Print major/minor numbers of the underlying device, where the file lives on.
--export-db: Export the content of the udev database.
--version: Print version.
--help: Print help text.

udevadm trigger [options]

Request device events from the kernel. Usually used to replay events at system coldplug time.

--verbose

: Print the list of devices which will be triggered.
--dry-run: Do not actually trigger the event.
--type=type: Trigger a specific type of devices. Valid types are: devices, subsystems, failed. The default value is devices.
--action=action: Type of event to be triggered. The default value is add.
--subsystem-match=subsystem: Trigger events for devices which belong to a matching subsystem. This option can be specified multiple times and supports shell style pattern matching.
--subsystem-nomatch=subsystem: Do not trigger events for devices which belong to a matching subsystem. This option can be specified multiple times and supports shell style pattern matching.
--attr-match=attribute=value: Trigger events for devices with a matching sysfs attribute. If a value is specified along with the attribute name, the content of the attribute is matched against the given value using shell style pattern matching. If no value is specified, the existence of the sysfs attribute is checked. This option can be specified multiple times.
--attr-nomatch=attribute=value: Do not trigger events for devices with a matching sysfs attribute. If a value is specified along with the attribute name, the content of the attribute is matched against the given value using shell style pattern matching. If no value is specified, the existence of the sysfs attribute is checked. This option can be specified multiple times.
--property-match=property=value: Trigger events for devices with a matching property value. This option can be specified multiple times and supports shell style pattern matching.
--sysname-match=name: Trigger events for devices with a matching sys device name. This option can be specified multiple times and supports shell style pattern matching.

udevadm settle [options]

Watches the udev event queue, and exits if all current events are handled.

--timeout=seconds

: Maximum number of seconds to wait for the event queue to become empty. The default value is 180 seconds. A value of 0 will check if the queue is empty and always return immediately.
--seq-start=seqnum: Wait only for events after the given sequence number.
--seq-end=seqnum: Wait only for events before the given sequence number.
--exit-if-exists=file: Stop waiting if file exists.
--quiet: Do not print any output, like the remaining queue entries when reaching the timeout.
--help: Print help text.

udevadm control command

Modify the internal state of the running udev daemon.

--log-priority=value

: Set the internal log level of udevd. Valid values are the numerical syslog priorities or their textual representations: err, info and debug.
--stop-exec-queue: Signal udevd to stop executing new events. Incoming events will be queued.
--start-exec-queue: Signal udevd to enable the execution of events.
--reload-rules: Signal udevd to reload the rules files. The udev daemon detects changes automatically, this option is usually not needed. Reloading rules does not apply any changes to already existing devices.
--property=KEY=value: Set a global property for all events.
--max-childs=value: Set the maximum number of events, udevd will handle at the same time.
--help: Print help text.

udevadm monitor [options]

Listens to the kernel uevents and events sent out by a udev rule and prints the devpath of the event to the console. It can be used to analyze the event timing, by comparing the timestamps of the kernel uevent and the udev event.

--kernel

: Print the kernel uevents.
--udev: Print the udev event after the rule processing.
--property: Also print the properties of the event.
--help: Print help text.

udevadm test [options] devpath

Simulate a udev event run for the given device, and print debug output.

--action=string

: The action string.
--subsystem=string: The subsystem string.
--help: Print help text.

Author

Written by Kay Sievers kay.sievers@vrfy.org.

udev

udev

Contents

History[edit]

Overview[edit]

Operation[edit]

Architecture[edit]

Authors[edit]

See also[edit]

udev(7) - Linux man page

Name

Description

Configuration

Author

See Also

Referenced By

udevadm(8) - Linux man page

Name

Synopsis

Description

Options

Author

See Also