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# udev

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udev (user space `/dev`) is a user space system that enables the operating system administrator to register user space handlers for events. The events received by udev's daemon are mainly generated by the Linux kernel in response to physical events relating to peripheral devices.

As such, udev's main purpose is to act upon peripheral detection and hot-plugging, including actions that return control to the kernel, e.g. loading kernel modules or device firmware. Another component of this detection is adjusting the permissions of the device to be accessible to non-root users and groups. udev manages device nodes in the `/dev` directory by adding, symlinking and renaming them.

Note The kernel module loading order is not preserved across boots, because udev handles separate events concurrently (in parallel). For example, if a machine has multiple block devices, device nodes may change designations between reboots—i.e. `/dev/sda` may on next boot become `/dev/sdb`.
Tip Rely on `…/by-id/…`, `…/by-path/…`, `…/by-uuid/…` and similar persistent identifiers.

## Installation

udev is part of systemd and thus installed by default. See systemd-udevd.service(8) for more information.

## Introduction to udev rules

udev rules written by the administrator go in `/etc/udev/rules.d/`, their file name has to end with .rules. The udev rules shipped with various packages are found in `/usr/lib/udev/rules.d/`. If there are two files by the same name under `/usr/lib` and `/etc`, the ones in `/etc` take precedence.

To learn about udev rules, refer to the udev(7) manual. Also see Writing udev rules and some practical examples are provided within the guide: Writing udev rules - Examples.

### Example

Below is an example of a rule that creates a symlink `/dev/video-cam` when a webcamera is connected.

Let say this camera is currently connected and has loaded with the device name `/dev/video2`. The reason for writing this rule is that at the next boot, the device could show up under a different name, like `/dev/video0`.

    $ udevadm info --attribute-walk --path=$(udevadm info --query=path --name=/dev/video2)

    Udevadm info starts with the device specified by the devpath and then walks up the chain of parent devices.
    It prints for every device found, all possible attributes in the udev rules key format.
    A rule to match, can be composed by the attributes of the device and the attributes from one single parent device.

    looking at device '/devices/pci0000:00/0000:00:04.1/usb3/3-2/3-2:1.0/video4linux/video2':
      KERNEL=="video2"
      SUBSYSTEM=="video4linux"
       ...
    looking at parent device '/devices/pci0000:00/0000:00:04.1/usb3/3-2/3-2:1.0':
      KERNELS=="3-2:1.0"
      SUBSYSTEMS=="usb"
      ...
    looking at parent device '/devices/pci0000:00/0000:00:04.1/usb3/3-2':
      KERNELS=="3-2"
      SUBSYSTEMS=="usb"
      ATTRS{idVendor}=="05a9"
      ATTRS{manufacturer}=="OmniVision Technologies, Inc."
      ATTRS{removable}=="unknown"
      ATTRS{idProduct}=="4519"
      ATTRS{bDeviceClass}=="00"
      ATTRS{product}=="USB Camera"
      ...

To identify the webcamera, from the video4linux device we use `KERNEL=="video2"` and `SUBSYSTEM=="video4linux"`, then walking up two levels above, we match the webcamera using vendor and product ID's from the usb parent `SUBSYSTEMS=="usb"`, `ATTRS{idVendor}=="05a9"` and `ATTRS{idProduct}=="4519"`. Note that this matching is case sensitive, so `"05A9"` can not be used to match the `idVendor` in this example.

We are now able to create a rule match for this device as follows:

    /etc/udev/rules.d/83-webcam.rules

    KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", SUBSYSTEMS=="usb", ATTRS{idVendor}=="05a9", ATTRS{idProduct}=="4519", SYMLINK+="video-cam"

Here we create a symlink using `SYMLINK+="video-cam"` but we could easily set user `OWNER="john"` or group using `GROUP="video"` or set the permissions using `MODE="0660"`.

If you intend to write a rule to do something when a device is being removed, be aware that device attributes may not be accessible. In this case, you will have to work with preset device environment variables. To monitor those environment variables, execute the following command while unplugging your device:

    $ udevadm monitor --property --udev

In this command's output, you will see value pairs such as `ID_VENDOR_ID` and `ID_MODEL_ID`, which match the previously used attributes `idVendor` and `idProduct`. A rule that uses device environment variables instead of device attributes may look like this:

    /etc/udev/rules.d/83-webcam-removed.rules

    ACTION=="remove", SUBSYSTEM=="usb", ENV{ID_VENDOR_ID}=="05a9", ENV{ID_MODEL_ID}=="4519", RUN+="/path/to/your/script"

### List the attributes of a device

To get a list of all of the attributes of a device you can use to write rules, run this command:

    $ udevadm info --attribute-walk --name=device_name

Replace `device_name` with the device present in the system, such as `/dev/sda` or `/dev/ttyUSB0`.

If you do not know the device name you can also list all attributes of a specific system path:

    $ udevadm info --attribute-walk --path=/sys/class/backlight/acpi_video0

To narrow down the search for a device, figure out the class and run:

    $ ls /dev/class/by-id

You can use the symlink outright or what it points as the input to `--name`. For example:

    $ udevadm info --attribute-walk --name=/dev/input/by-id/usb-foostan_Corne-event-kbd

To get the path of a bare USB device which does not populate any subordinate device you have to use the full USB device path. Start monitor mode and then plug in the USB device to get it:

    $ udevadm monitor

    ...
    KERNEL[26652.638931] add      /devices/pci0000:00/0000:00:01.2/0000:02:00.0/0000:03:05.0/0000:05:00.0/usb1/1-3 (usb)
    KERNEL[26652.639153] add      /devices/pci0000:00/0000:00:01.2/0000:02:00.0/0000:03:05.0/0000:05:00.0/usb1/1-3/1-3:1.0 (usb)
    ...

You can just choose the deepest path and `--attribute-walk` will show all parent's attributes anyway:

    $ udevadm info --attribute-walk --path=/devices/pci0000:00/0000:00:01.2/0000:02:00.0/0000:03:05.0/0000:05:00.0/usb1/1-3/1-3:1.0

### Testing rules before loading

    # udevadm test $(udevadm info --query=path --name=device_name) 2>&1

This will not perform all actions in your new rules but it will however process symlink rules on existing devices which might come in handy if you are unable to load them otherwise. You can also directly provide the path to the device you want to test the udev rule for:

    # udevadm test /sys/class/backlight/acpi_video0/

### Loading new rules

udev automatically detects changes to rules files, so changes take effect immediately without requiring udev to be restarted. However, the rules are not re-triggered automatically on already existing devices. Hot-pluggable devices, such as USB devices, will probably have to be reconnected for the new rules to take effect, or at least unloading and reloading the ohci-hcd and ehci-hcd kernel modules and thereby reloading all USB drivers.

If rules fail to reload automatically:

    # udevadm control --reload

To manually force udev to trigger your rules:

    # udevadm trigger

## Components of udev rules

### Action matching

`ACTION==""` is used to only match against a device when something specific is happening, usually when it is appearing or disappearing. There are eight types of actions that can be raised for udev rules to match against:

add/remove
  When device nodes (in `/dev/`) are created or removed.
bind/unbind
  When a driver is attached or detached from a device.
change
  Only raised manually by drivers on a "device state change", has no standard meaning; see #The change action.
offline
  When a device (usually memory or CPUs) becomes locked for hot-unplugging.
online
  When a previously offline device is unlocked.
move
  When a device (usually a network interface) is renamed, hopefully never.

#### The change action

The change action is somewhat special, because not all drivers use it the same way, if at all. A change event is only emitted as a result of the driver manually raising a userspace event (or uevent) of type `KOBJ_CHANGE`. This normally signals that something has happened to that device, but it takes some additional information to determine what exactly.

Since that event is only raised under a limited number of situations, here is a (best-effort, non-exhaustive) list of subsystems that do raise change events, and under what conditions.

SUBSYSTEM | Event trigger | Event-specific property | Documentation
--- | --- | --- | ---
typec | Change in alt-mode, USB-PD role or data role | None |
usb_role | Role change (for a USB OTG or Type-C device) | `USB_ROLE_SWITCH=` valued at `none`, `host` or `device`, based on the new role |
block | Media changes for a disc drive or an SD card reader | `DISK_MEDIA_CHANGE=1` for a new disc/card or `DISK_EJECT_REQUEST=1` when the media is being unloaded |
drm | Device detected as "wedged" (hanged and cannot be recovered by the driver) | `WEDGED=` with a list of available reset methods, like `rebind` or `bus-reset` | Device wedging
drm | Monitor hot-plugged (plugged or unplugged) | `HOTPLUG=1`, sometimes also `CONNECTOR=` with the internal ID of the connector |
drm | Connector property change (such as HDCP status) | `CONNECTOR=` and `PROPERTY=` with their respective internal IDs |
backlight | Brightness changed | `SOURCE=` valued at `sysfs`, `hotkey` or `unknown` based on what triggered the change. |
power_supply | Power supply state changed (power supply plugged/unplugged, battery charging/discharging, etc.) | None |
rfkill | Radio turned on or off by killswitch, button or menu | None |
thunderbolt | Tunneling mode change | `TUNNEL_EVENT=` with the state of the tunnel, possibly `TUNNEL_DETAILS=` as well | Thunderbolt tunneling events
Any device with a `power/` attribute group | Gaining or losing the ability to do remote wakeup1 (i.e: when the entire `power/wakeup*` attribute set appears or disappears) | None | Device Power Management Basics, though the uevent itself is not documented
Any (synthetic uevent) | Writing `change $(uuidgen) FOO=BAR HELLO=WORLD` to the uevent file of any sysfs device2 | `SYNTH_UUID=` with the UUID written to the file, also `SYNTH_ARG_HELLO=WORLD` for any key-value pair after the UUID | sysfs-uevent

1. Gaining or losing the ability to do remote wakeup is interesting in cases such as USB, since unconfigured USB devices cannot issue wakeups in this state. See #Force USB device features to be re-added after suspend.
2. Both `udevadm trigger` and the `OPTIONS+="watch"` udev rule directive will cause synthetic change events to be raised through this mechanism, with `ENV{SYNTH_UUID}=="0"` in both cases.

### Device attributes

Device attributes, listed as `ATTR{my_attr}=="..."` (or `ATTRS{my_attr}=="..."` for parent devices) in udev rules and `udevadm info --attribute-walk`, correspond to all the device information that is made available through sysfs, which exposes information from the "kobject" classes used by the kernel to keep track of device state. This means that there is no need for udev-specific tools to examine these, and that a simple `ls /sys/class/thermal/thermal_zone0/` or `cat /sys/class/input/input0/name` is all that is needed to explore device attributes, and some can even be changed with something like `echo 1 > "/sys/class/leds/input2::capslock/brightness"`. It is, however, a lot more convenient to use `udevadm info --attribute-walk /sys/class/input/mouse0`, which has the advantage of showing all matchable attributes for that device all at once in the same format that udev rules expect (as well as the attributes for any parent devices, also in their readily usable format).

Most of these attributes have well-documented meanings, behaviors and accepted values based on what kernel submodule is handling the device, such as sysfs-bus-usb for USB devices and sysfs-class-typec for USB type-C ports, for instance. Other attributes are present as subtrees, such as sysfs-devices-physical_location or sysfs-devices-power, which use slashes to separate levels, just like directories, giving `ATTR{power/control}=="auto"`.

### Event environment

The "environment" of an event is a set of device properties and event properties (and rarely global properties), both written as `ENV{MY_PROPERTY}=="..."` in udev rules and both reported as `E: MY_PROPERTY=...` by `udevadm monitor --property` and `udevadm info` (without `--attribute-walk`). Despite being called "environment", they have nothing to do with environment variables (although they do get passed as environment variables to programs started by `RUN+="..."`). These contain context information added to an event by the kernel module or other udev rules to make that information available to downstream rules or components. The only difference between event properties and device properties is that devices properties are stored and can be examined with `udevadm info`, whereas event properties are transient and can only be seen if the event is caught by `udevadm monitor --property`. A lot of device properties are also available as device attributes with similar names.

Unlike with attributes, udev rules are allowed to set event properties arbitrarily, and there is also no concept of "parent properties" to inspect beyond the ones that are set on the event (which is why there is no `ENVS{...}==""` directive). Note that setting a property with a udev rule sets a device property, which will be stored until the device is removed and will thus appear in every event raised by that device (unless the property's name starts with a period, like `ENV{.PART_SUFFIX`}, which will be added to the event properties with the leading dot in the name and be usable by other rules, but not stored).

#### Useful device properties

There are a number of device properties that other programs listen to, and which can be used to affect how they recognize or handle a device that has them.

Property name | Usage | Software listening | Documentation
--- | --- | --- | ---
`KEYBOARD_KEY_<hex scan code>` | Can be used to remap any keyboard scancode to any logical key by name in input-event-codes.h (with the name in lowercase, only those that start with `KEY_` without the prefix or those with `BTN_` with the prefix). | `60-evdev.rules` and udev-builtin-keyboard | #Keyboard_devices and 60-keyboard.hwdb
`LIBINPUT_IGNORE_DEVICE` | Input device is disregarded if property is non-zero. | libinput | Ignoring specific devices

### Device tags

Tags (added using `TAG+="foo"`, removed with `TAG-="foo"`, matched with `TAG=="foo"`) are used by userspace software interacting with udev to list and identify all devices they should be acting upon. Those can also be arbitrary, but there are a few well-known ones.

Tag | Software listening | Meaning | Documentation
--- | --- | --- | ---
systemd | systemd | Systemd should create a device unit for this device | systemd.device(5)
seat | systemd-logind | Device is eligible to be assigned to a seat, and `ENV{ID_SEAT`} indicates which seat it is currently assigned to, if any | sd-login(3)
master-of-seat | systemd-logind | Identifies a primary video device: must be present for a seat to be considered existent. | sd-login(3)
uaccess | `70-uaccess.rules` and `73-seat-late.rules` | Users should have read and write permissions granted on this device (via Access Control Lists) while it is assigned to their seat | see #Allowing regular users to use devices
power-switch | systemd-logind | Device will be watched for lid and key events to trigger suspend, screen lock and other such things | logind.conf(5) (HandlePowerKey)

## Hardware database

udev's hardware database (hwdb) contains lists of device properties to be assigned to specific devices based solely on modalias patterns, similar to how kernel modules declare what devices they should be attached to. This is commonly used to configure keyboard scancodes, mice DPI, and USB classes/models, among other things. The database is compiled from files with .hwdb extension located in directories `/usr/lib/udev/hwdb.d/`, `/run/udev/hwdb.d/` and `/etc/udev/hwdb.d/`. See hwdb(7) for details.

Each .hwdb file can apply properties to one or more devices based on hardware ID glob patterns. You may obtain device identification info by running evemu-describe(1) as the root user. This command is provided by the evemu package.

### Event devices (evdev)

Event devices (evdev), which include input devices like keyboards, mice and gamepads (but also power buttons, laptop lid switches, remote controls, and anything that can raise "input" events) can be matched with the `evdev:` prefix in one of the following formats:

- Using the kernel modalias:

      evdev:input:b<bus_id>v<vendor_id>p<product_id>e<version_id>-<input_modalias>

  with the following 4-digit hex uppercase fields:
  - `<vendor_id>`, `<product_id>` and `<version_id>`: vendor, product and version IDs matching the output of the `lsusb` command.
  - `<bus_id>` is the 4-digit hex bus id and should be 0003 for usb devices. The possible `<bus_id>` values are defined in `/usr/include/linux/input.h` (you can run `awk '/BUS_/ {print $2, $3}' /usr/include/linux/input.h` to get a list).
  - `<input_modalias>` is an arbitrary length input-modalias describing the device capabilities. The other fields are sufficient to uniquely identify the device, so you can use a glob here.
  If you have the device currently plugged into your computer then you can get the whole modalias at once using sysfs, as demonstrated in #Remap specific device.

- Input driver device name and DMI data match:

      evdev:name:<input device name>:dmi:bvn*:bvr*:bd*:svn<vendor>:pn*

  where `<input_device_name>` is the name device specified by the driver and `<vendor>` is the firmware-provided string exported by the kernel DMI modalias.

#### Keyboard devices

The scancodes-to-keycodes mapping for keyboards is defined in `/usr/lib/udev/hwdb.d/60-keyboard.hwdb`. The format of each line in the block body is `KEYBOARD_KEY_<scancode>=<keycode>`, where:

- The value of `<scancode>` is hexadecimal, but without the leading `0x` (i.e. specify `a0` instead of `0xa0`).
- The value of `<keycode>` is the lower-case keycode name string as listed in `/usr/include/linux/input-event-codes.h` (see the `KEY_<KEYCODE>` variables), with a sorted list is available at [1]. It is not possible to specify decimal value in `<keycode>`.

### Examples

#### Remap specific device

Suppose you want to remap scancodes for a keyboard that you currently have plugged in. You should already have the evdev path (e.g. `/dev/input/event17`), and you can follow Keyboard input#Identifying scancodes to obtain the scancode (e.g. `70039` for caps lock). Now, using the event number, you can query sysfs for the modalias:

    cat /sys/class/input/event17/device/modalias

    input:b0003v32ACp0012e0111-e0,1,4,1...

This device could be matched with the following hwdb rule:

    /etc/udev/hwdb.d/90-remap.hwdb

    evdev:input:b0003v32ACp0012e0111*
     KEYBOARD_KEY_70039=rightctrl # This example maps the 70039 scancode to the "rightctrl" keycode.

#### Remap all devices

Suppose we want to remap a couple of common keys for all AT keyboards:

    /etc/udev/hwdb.d/90-custom-keyboard.hwdb

    evdev:atkbd:*
     KEYBOARD_KEY_10=suspend
     KEYBOARD_KEY_a0=search

#### Disable key

To block the `Sleep` key, bind it to the "reserved" keyword. Alternatively, you can use "unknown" to map it to the `NoSymbol` key. For example:

    /etc/udev/hwdb.d/90-block-sleep.hwdb

    evdev:input:b0003v03F0p020C* # hp 5308 keyboard controller
     KEYBOARD_KEY_10082=reserved

### Updating the Hardware Database Index

After changing the configuration files, the hardware database index, `hwdb.bin` needs to be rebuilt by running:

    # systemd-hwdb update

This creates `/etc/udev/hwdb.bin`, which takes precedence over the existing `/usr/lib/udev/hwdb.bin`. Upon system upgrade, both files will be automatically kept up-to-date with the same contents:

- `/usr/lib/udev/hwdb.bin` is rebuilt by the `30-systemd-hwdb.hook` pacman hook on each systemd upgrade, and immediately effected by `30-systemd-udev-reload.hook`.
- `/etc/udev/hwdb.bin` is rebuilt by `systemd-hwdb-update.service` on the next boot after the upgrade (see systemd-update-done.service(8)), since it detects we have manually generated it.

### Reloading the Hardware Database Index

The kernel loads `hwdb.bin` as part of the boot process, rebooting the system will load the updated `hwdb.bin`.

It is possible to load new properties from the updated `hwdb.bin` by running:

    # udevadm trigger

Be aware that only added or changed properties are loaded so if we delete a property from the configuration file, the deleted property is still kept by the kernel until a reboot.

### Querying the database

You can check that your configuration was loaded either by pressing keys, or by running `udevadm info`. For the USB keyboard in the above example, this outputs the mapping we configured as follows:

    # udevadm info /dev/input/by-path/*-usb-*-kbd | grep KEYBOARD_KEY
    E: KEYBOARD_KEY_70039=leftalt
    E: KEYBOARD_KEY_700e2=leftctrl

## Tips and tricks

### Mounting drives in rules

To mount removable drives, do not call `mount` from udev rules. This is ill-advised for two reasons:

1. systemd by default runs `systemd-udevd.service` with a separate "mount namespace" (see namespaces(7)), which means that mounts will not be visible to the rest of the system.
2. Even if you change the service parameters to fix this (commenting out the `PrivateMounts` and `MountFlags` lines), there is another problem which is that processes started from Udev are killed after a few seconds. In case of FUSE filesystems, such as NTFS-3G, mount starts a user-space process to handle the filesystem internals; when this is killed you will get `Transport endpoint not connected` errors if you try to access the filesystem.

There are some options that work:

- Start a custom systemd service from the Udev rule; the systemd service can invoke a script which can start any number of long-running processes (like FUSE). A concise example which automatically mounts USB disks under `/media` is udev-media-automount. A variant of the same idea is explained in this blog post.
- Use `systemd-mount` instead of `mount` in your Udev rule. This is recommended by systemd developers. For example this Udev rule should mount USB disks under `/media`:

      ACTION=="add", SUBSYSTEMS=="usb", SUBSYSTEM=="block", ENV{ID_FS_USAGE}=="filesystem", RUN{program}+="/usr/bin/systemd-mount --no-block --automount=yes --collect $devnode /media"

- Use a package like udisks or udiskie. These are very powerful, but difficult to set up. Also, they are meant to be used in single user sessions, since they make some filesystems available under the ownership of the unprivileged user whose session is currently active.

### Spawning long-running processes

Programs started by udev will block further events from that device, and any tasks spawned from a udev rule will be killed after event handling is completed. If you need to spawn a long-running process with udev, the intended way is to have a systemd unit which handles running the actual command, and a udev rule that merely signals that this unit should run. However, using `systemctl` in a udev rule is discouraged, since it is meant for user interaction and may block, among other things.

The correct way of doing this is to have the rule tag the device as needing a systemd device unit (see systemd.device(5)) using `TAG+="systemd"` and adding a device property of either `ENV{SYSTEMD_WANTS}+=` for services that would run with `systemctl --system` or `ENV{SYSTEMD_USER_WANTS}+=` for services that should run with `systemctl --user`. For example:

    SUBSYSTEM=="tty", ACTION=="add", ATTRS{manufacturer}=="Pulse-Eight", ATTRS{product}=="CEC Adapter", TAG+="systemd", ENV{SYSTEMD_WANTS}+="inputattach-cec@$devnode.service"

    /etc/systemd/system/inputattach-cec@.service

    [Unit]
    Description=Configure USB serial device at %I

    [Service]
    Type=simple
    ExecStart=/usr/bin/inputattach --pulse8-cec %I

`SYSTEMD_WANTS` is equivalent to the `Wants=` directive elsewhere in systemd, meaning the device will not be affected if the service fails, does not exist, or completes successfully at any point.

### Allowing regular users to use devices

Warning Do not set the `uaccess` tag on input or block devices; those have their own access control mechanisms and unprivileged users should not have raw access to those. For instance, unprivileged raw keyboard access would make keylogging trivial, and unprivileged raw disk access would completely negate filesystem security.

When a kernel driver initializes a device, the default state of the device node is to be owned by `root:root`, with permissions `600`. [2] This makes devices inaccessible to regular users unless the driver changes the default, or a udev rule in user space changes the permissions.

The `OWNER`, `GROUP`, and `MODE` udev values can be used to provide access, though one encounters the issue of how to make a device usable to all users without an overly permissive mode. Ubuntu's approach is to create a `plugdev` group that devices are added to, but this practice is not only discouraged by the systemd developers, [3] but considered a bug when shipped in udev rules on Arch (FS#35602) and broken starting from systemd 258 [4][5]. Another approach historically employed, as described in Users and groups#Pre-systemd groups, is to have different groups corresponding to categories of devices.

The modern recommended approach for systemd systems is to use a `MODE` of `660` to let the group use the device, and then attach a `TAG` named `uaccess` [6]. This special tag makes udev apply a dynamic user ACL to the device node, which coordinates with systemd-logind(8) to make the device usable to logged-in users. For an example of a udev rule implementing this:

    /etc/udev/rules.d/71-device-name.rules

    ACTION!="remove", SUBSYSTEMS=="usb", ATTRS{idVendor}=="vendor_id", ATTRS{idProduct}=="product_id", MODE="0660", TAG+="uaccess"

Note For any rule adding the `uaccess` tag to be effective, the name of the file it is defined in has to lexically precede `/usr/lib/udev/rules.d/73-seat-late.rules`.

### Execute when HDMI cable is plugged in or unplugged

Create the rule `/etc/udev/rules.d/95-hdmi-plug.rules` with the following content:

    ACTION=="change", SUBSYSTEM=="drm", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/username/.Xauthority", RUN+="/path/to/script.sh"

Note If the rule triggers before the X server starts, it may not work as intended. See #X programs in RUN rules hang when no X server is present.

### Execute on VGA cable plug in

Create the rule `/etc/udev/rules.d/95-monitor-hotplug.rules` with the following content to launch arandr on plug in of a VGA monitor cable:

    KERNEL=="card0", SUBSYSTEM=="drm", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/username/.Xauthority", RUN+="/usr/bin/arandr"

Some display managers store the `.Xauthority` outside the user home directory. You will need to update the `ENV{XAUTHORITY}` accordingly. As an example GNOME Display Manager looks as follows:

    $ printenv XAUTHORITY

    /run/user/1000/gdm/Xauthority

Note If the rule triggers before the X server starts, it may not work as intended. See #X programs in RUN rules hang when no X server is present.

### Detect new eSATA drives

If your eSATA drive is not detected when you plug it in, there are a few things you can try. You can reboot with the eSATA plugged in. Or you could try:

    # echo 0 0 0 | tee /sys/class/scsi_host/host*/scan

Or you could install scsiaddAUR (from the AUR) and try:

    # scsiadd -s

Hopefully, your drive is now in `/dev`. If it is not, you could try the above commands while running:

    # udevadm monitor

to see if anything is actually happening.

### Mark internal SATA ports as eSATA

If you connected an eSATA bay or another eSATA adapter, the system will still recognize this disk as an internal SATA drive. GNOME and KDE will ask you for your root password all the time. The following rule will mark the specified SATA-Port as an external eSATA-Port. With that, a normal GNOME user can connect their eSATA drives to that port like a USB drive, without any root password and so on.

    /etc/udev/rules.d/10-esata.rules

    DEVPATH=="/devices/pci0000:00/0000:00:1f.2/host4/*", ENV{UDISKS_SYSTEM}="0"

Note The `DEVPATH` can be found after connection the eSATA drive with the following commands (replace `sdb` accordingly):

    $ udevadm info --query=path /dev/sdb

    /devices/pci0000:00/0000:00:1f.2/host4/target4:0:0/4:0:0:0/block/sdb

    $ find /sys/devices/ -name sdb

    /sys/devices/pci0000:00/0000:00:1f.2/host4/target4:0:0/4:0:0:0/block/sdb

### Setting static device names

Because udev loads all modules asynchronously, they are initialized in a different order. This can result in devices randomly switching names. A udev rule can be added to use static device names. See also Persistent block device naming for block devices and Network configuration#Change interface name for network devices.

#### Video device

For setting up the webcam in the first place, refer to Webcam setup.

Using multiple webcams will assign video devices as `/dev/video*` randomly on boot. The recommended solution is to create symlinks using a udev rule as in the #Example:

    /etc/udev/rules.d/83-webcam.rules

    KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", SUBSYSTEMS=="usb", ATTRS{idVendor}=="05a9", ATTRS{idProduct}=="4519", SYMLINK+="video-cam1"
    KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", SUBSYSTEMS=="usb", ATTRS{idVendor}=="046d", ATTRS{idProduct}=="08f6", SYMLINK+="video-cam2"

Note Using names other than `/dev/video*` will break preloading of `v4l1compat.so` and perhaps `v4l2convert.so`

#### Printer

If you use multiple printers, `/dev/lp[0-9]` devices will be assigned randomly on boot, which will break e.g. CUPS configuration.

You can create the following rule, which will create symlinks under `/dev/lp/by-id` and `/dev/lp/by-path`, similar to Persistent block device naming scheme:

    /etc/udev/rules.d/60-persistent-printer.rules

    ACTION=="remove", GOTO="persistent_printer_end"
    # This should not be necessary
    #KERNEL!="lp*", GOTO="persistent_printer_end"

    SUBSYSTEMS=="usb", IMPORT{builtin}="usb_id"
    ENV{ID_TYPE}!="printer", GOTO="persistent_printer_end"

    ENV{ID_SERIAL}=="?*", SYMLINK+="lp/by-id/$env{ID_BUS}-$env{ID_SERIAL}"

    IMPORT{builtin}="path_id"
    ENV{ID_PATH}=="?*", SYMLINK+="lp/by-path/$env{ID_PATH}"

    LABEL="persistent_printer_end"

### Identifying a disk by its serial

To perform some action on a specific disk device `/dev/sdX` identified permanently by its unique serial `ID_SERIAL_SHORT` as displayed with `udevadm info /dev/sdX`, one can use the below rule. It is passing as a parameter the device name found if any to illustrate:

    /etc/udev/rules.d/69-disk.rules

    ACTION=="add", KERNEL=="sd[a-z]", ENV{ID_SERIAL_SHORT}=="X5ER1ALX", RUN+="/path/to/script /dev/%k"

### Waking from suspend with USB device

A udev rule can be useful to enable the wakeup triggers of a USB device, like a mouse or a keyboard, so that it can be used to wake the system from sleep.

Note By default, the USB host controllers are all enabled for wakeup. The status can be checked using `cat /proc/acpi/wakeup`. The rule below is in this case not necessary but can be used as a template to perform other actions, like disabling the wakeup functionality for example.

First, identify the vendor and product identifiers of the USB device. They will be used to recognize it in the udev rule. For example:

    $ lsusb | grep Logitech

    Bus 007 Device 002: ID 046d:c52b Logitech, Inc. Unifying Receiver

Then, find where the device is connected to using:

    $ grep c52b /sys/bus/usb/devices/*/idProduct

    /sys/bus/usb/devices/1-1.1.1.4/idProduct:c52b

Now create the rule to change the `power/wakeup` attribute of both the device and the USB controller it is connected to whenever it is added:

    /etc/udev/rules.d/50-wake-on-device.rules

    ACTION=="add", SUBSYSTEM=="usb", DRIVERS=="usb", ATTRS{idVendor}=="046d", ATTRS{idProduct}=="c52b", ATTR{power/wakeup}="enabled", ATTR{driver/1-1.1.1.4/power/wakeup}="enabled"

### Force USB device features to be re-added after suspend

USB devices need to be reset after exiting a suspended state (whether from the system resuming from sleep or from a port being turned off when the device was idle for power saving), which the Linux kernel handles mostly transparently in a process called reset-resume, so that USB drives do not look like they have been disconnected and reconnected every time. This is mostly desirable, but some devices, like USB TTY interfaces, do need to be manually reconfigured after being power cycled, which is not signalled by any uevent from the relevent drivers.

The one uevent that does get raised, however, is the one from losing and regaining `power/wakeup`, since USB devices are unusable while they are unconfigured, so they cannot issue wakeup triggers while in this state. That uevent has no distinctive properties and is issued once for losing `power/wakeup` (when the device state becomes `default`), and then a second time when regaining it (as the device state reaches `addressed`). The first event can still be easily identified because `DEVNUM` is set to zero (which is not a valid device number) immediately before the device is unconfigured and loses `power/wakeup`. When that happens, one can simply touch the `bConfigurationValue` sysfs attribute to force the system to reconfigure the device non-transparently, as if it had been disconnected during sleep, which unbinds all drivers and removes all children devices before adding them back when the device is ready again.

Note `DEVNUM` will not be set to zero if the attribute `power/persist` (which controls the USB-persist feature) is set to `1`. USB-persist is enabled by default on the default kernel of several Linux distributions (including Arch's linux kernel) using the `CONFIG_USB_DEFAULT_PERSIST` option at compilation. If so, the USB-persist feature needs to be disabled on that device in order to work properly.

    # Make sure that power/persist is and stays off,
    # in case CONFIG_USB_DEFAULT_PERSIST enables it by default
    SUBSYSTEM=="usb" ATTR{manufacturer}=="Foo" ATTR{product}=="Bar" ATTR{power/persist}="0"

    # The actual rule to be run
    ACTION=="add" SUBSYSTEM=="tty" SUBSYSTEMS=="usb" ATTRS{manufacturer}=="Foo" ATTRS{product}=="Bar" TAG+="systemd" ENV{SYSTEMD_WANTS}="setup-usb-tty@$devnode.service"

    # The rule that will get the above rule to re-run when the computer comes out of sleep
    # Can't use $attr{bConfigurationValue} in ATTR{}, so make sure to replace
    # both "1" by whatever configuration value is being used when
    # the device is functional.
    ACTION=="change" SUBSYSTEM=="usb" ENV{DEVNUM}=="000" ATTR{manufacturer}=="Foo" ATTR{product}=="Bar" ATTR{bConfigurationValue}=="1" ATTR{bConfigurationValue}="1"

### Triggering events

This article or section is a candidate for merging with #Testing rules before loading.

Notes: similar trick (Discuss in Talk:Udev)

It can be useful to trigger various udev events. For example, you might want to simulate a USB device disconnect on a remote machine. In such cases, use `udevadm trigger`:

    # udevadm trigger --verbose --type=subsystems --action=remove --subsystem-match=usb --attr-match="idVendor=abcd"

This command will trigger a USB remove event on all USB devices with vendor ID `abcd`.

### Triggering desktop notifications from a udev rule

To trigger a desktop notification from a udev rule, use systemd-run(1) as explained in Desktop notifications#Send notifications to another user:

Create the file:

    /etc/udev/rules.d/99-powersupply_notification.rules

    # Rule for when switching to battery
    ACTION=="change", SUBSYSTEM=="power_supply", ATTRS{type}=="Mains", ATTRS{online}=="0", RUN+="/usr/bin/systemd-run --machine=target_user@.host --user notify-send 'Changing Power States' 'Using battery power'"
    # Rule for when switching to AC
    ACTION=="change", SUBSYSTEM=="power_supply", ATTRS{type}=="Mains", ATTRS{online}=="1", RUN+="/usr/bin/systemd-run --machine=target_user@.host --user notify-send 'Changing Power States' 'Using AC power'"

To launch multiple or long commands, an executable script can be given to systemd-run:

    /usr/local/bin/from_battery.sh

    #!/bin/sh

    paplay /usr/share/sounds/freedesktop/stereo/power-unplug.oga
    notify-send --icon=/usr/share/icons/Adwaita/symbolic/legacy/battery-good-symbolic.svg 'Changing Power States' 'Using battery power' --expire-time=4000

Note If the rule triggers before the X server starts, it may not work as intended. See #X programs in RUN rules hang when no X server is present.

## Troubleshooting

### Blacklisting modules

In rare cases, udev can make mistakes and load the wrong modules. To prevent it from doing this, you can blacklist modules. Once blacklisted, udev will never load that module – not at boot-time and not even later on when a hot-plug event is received (e.g., you plug in your USB flash drive).

### Debug output

To get hardware debug info, use the kernel parameter `udev.log-priority=debug`. Alternatively you can set

    /etc/udev/udev.conf

    udev_log=debug

This option can also be compiled into your initramfs by adding the configuration file to your `FILES` array:

    /etc/mkinitcpio.conf

    FILES=(... /etc/udev/udev.conf)

and then regenerate the initramfs.

### udevd hangs at boot

After migrating to LDAP or updating an LDAP-backed system, udevd can hang at boot at the message "Starting UDev Daemon". This is usually caused by udevd trying to look up a name from LDAP but failing, because the network is not up yet. The solution is to ensure that all system group names are present locally.

Extract the group names referenced in udev rules and the group names actually present on the system:

    # grep -Fr GROUP /etc/udev/rules.d/ /usr/lib/udev/rules.d/ | sed 's:.*GROUP="\([-a-z_]\{1,\}\)".*:\1:' | sort -u >udev_groups
    # cut -d: -f1 /etc/gshadow /etc/group | sort -u >present_groups

To see the differences, do a side-by-side diff:

    # diff -y present_groups udev_groups
    ...
    network							      <
    nobody							      <
    ntp							      <
    optical								optical
    power							      |	pcscd
    rfkill							      <
    root								root
    scanner								scanner
    smmsp							      <
    storage								storage
    ...

In this case, the `pcscd` group is for some reason not present in the system. Add the missing groups. Also, make sure that local resources are looked up before resorting to LDAP. `/etc/nsswitch.conf` should contain the following line:

    group: files ldap

### Some devices, that should be treated as removable, are not

You need to create a custom udev rule for that particular device. To get definitive information of the device you can use either `ID_SERIAL` or `ID_SERIAL_SHORT` (remember to change `/dev/sdb` if needed):

    $ udevadm info --query=property --property=ID_SERIAL,ID_SERIAL_SHORT --name=/dev/sdb

Then we set `UDISKS_AUTO="1"` to mark the device for automounting and `UDISKS_SYSTEM="0"` to mark the device as "removable". See udisks(8) for details.

    /etc/udev/rules.d/99-removable.rules

    ENV{ID_SERIAL_SHORT}=="serial_number", ENV{UDISKS_AUTO}="1", ENV{UDISKS_SYSTEM}="0"

Remember to reload udev rules with `udevadm control --reload`. Next time you plug your device in, it will be treated as an external drive.

### Sound problems with some modules not loaded automatically

- Some users have traced this problem to old entries in `/etc/modprobe.d/sound.conf`. Try cleaning that file out and trying again.

- The OSS emulation modules `snd_pcm_oss` and `snd_seq_oss` are not automatically loaded by default.

### Optical drives have group ID set to "disk"

If the group ID of your optical drive is set to `disk` and you want to have it set to `optical`, you have to create a custom udev rule:

    /etc/udev/rules.d

    # permissions for SCSI CD devices
    SUBSYSTEMS=="scsi", KERNEL=="s[rg][0-9]*", ATTRS{type}=="5", GROUP="optical"

### X programs in RUN rules hang when no X server is present

When xrandr or another X-based program tries to connect to an X server, it falls back to a TCP connection on failure. However, due to `IPAddressDeny` in the systemd-udev service configuration, this hangs. Eventually the program will be killed and event processing will resume.

If the rule is for a drm device and the hang causes event processing to complete once the X server has started, this can cause 3D acceleration to stop working with a `failed to authenticate magic` error.

## See also

- udev(7)
- An Introduction to udev
- udev mailing list information
- Scripting with udev
- Writing udev rules
- Device and Module Handling on an LFS System
- Running GUI or accessing display variables from udev rules
- openSUSE udev documentation

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- Hardware detection and troubleshooting

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