udev - ArchWiki

# udev

From ArchWiki
Jump to navigation Jump to search

Related articles

- udisks

From Wikipedia:udev:

  udev is a device manager for the Linux kernel. As the successor of devfsd and hotplug, udev primarily manages device nodes in the `/dev` directory. At the same time, udev also handles all user space events raised while hardware devices are added into the system or removed from it, including firmware loading as required by certain devices.

udev replaces the functionality of both hotplug and hwdetect.

udev loads kernel modules by utilizing coding parallelism to provide a potential performance advantage versus loading these modules serially. The modules are therefore loaded asynchronously. The inherent disadvantage of this method is that udev does not always load modules in the same order on each boot. If the machine has multiple block devices, this may manifest itself in the form of device nodes changing designations randomly. For example, if the machine has two hard drives, `/dev/sda` may randomly become `/dev/sdb`. See below for more info on this.

## Installation

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

A standalone fork is available as eudevAUR and eudev-gitAUR.

## About 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.

### udev rule 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 -a -p $(udevadm info -q path -n /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"`.

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 --environment --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 -a -n 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 -a -p /sys/class/backlight/acpi_video0

### Testing rules before loading

    # udevadm test $(udevadm info -q path -n 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

## Udisks

See Udisks.

## 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 exFAT (which is common to find on mp3 players or shareable thumb drives) and NTFS, "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. It was found to be fast and reliable. 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"

The factual accuracy of this article or section is disputed.

Reason: This statement should be backed up with references. (Discuss in Talk:Udev#)

As of this writing, however, it was found to be slow and unreliable.

- 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.

### Accessing firmware programmers and USB virtual comm devices

The following rule will allow users in the `users` group to access the USBtinyISP USB programmer for AVR microcontrollers.

    /etc/udev/rules.d/50-usbtinyisp.rules

    SUBSYSTEMS=="usb", ATTRS{idVendor}=="1781", ATTRS{idProduct}=="0c9f", GROUP="users", MODE="0660"
    SUBSYSTEMS=="usb", ATTRS{idVendor}=="16c0", ATTRS{idProduct}=="0479", GROUP="users", MODE="0660"

Use lsusb to get the vendor and product IDs for other devices.

### 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

### 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 a eSATA bay or an other 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 -q path -n /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 an udev rule as in the #udev rule 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 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 -n /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 wake up functionality of a USB device, like a mouse or a keyboard, so that it can be used to wake from sleep the machine.

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"

### 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 -v -t subsystems -c remove -s usb -a "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

The factual accuracy of this article or section is disputed.

Reason: This is a lengthy monologue on how to hardcode variables (Discuss in Talk:Udev#)

Invoking an external script containing calls to `notify-send` via udev can sometimes be challenging since the notification(s) never display on the Desktop. Here is an example of what commands and environmental variables need to be included in which files for `notify-send` to successfully be executed from a udev rule. NOTE: a number of variables are hardcoded in this example, thus consider making them portable (i.e., $USER rather than user's shortname) once you understand the example.

1) The following udev rule executes a script that plays a notification sound and sends a desktop notification when screen brightness is changed according to power state on a laptop. Create the file:

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

    Play a notification sound and send a desktop notification when screen brightness is changed according to power state on a laptop (a second ''udev'' rule actually changes the screen brightness)
    # Rule for when switching to battery
    ACTION=="change", SUBSYSTEM=="power_supply", ATTR{type}=="Mains", ATTR{online}=="0", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/USERNAME/.Xauthority" RUN+="/usr/bin/su USERNAME_TO_RUN_SCRIPT_AS -c /usr/local/bin/brightness_notification.sh"
    # Rule for when switching to AC
    ACTION=="change", SUBSYSTEM=="power_supply", ATTR{type}=="Mains", ATTR{online}=="1", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/USERNAME/.Xauthority" RUN+="/usr/bin/su USERNAME_TO_RUN_SCRIPT_AS -c /usr/local/bin/brightness_notification.sh"

Note: 1) `USERNAME_TO_RUN_SCRIPT_AS` and `USERNAME` need to be changed to that of the shortname for the user of the graphical session where the notification will be displayed and 2) the script needs to be executed with `/usr/bin/su`, which will place its ownership under the user of the graphical session (rather than root/the system) where the notification will be displayed.

2) Contents of the executable script to be run on trigger of the udev rule:

    /usr/local/bin/brightness_notification.sh

    #!/usr/bin/env bash

    export XAUTHORITY=/home/USERNAME_TO_RUN_SCRIPT_AS/.Xauthority
    export DISPLAY=:0
    export DBUS_SESSION_BUS_ADDRESS="unix:path=/run/user/UID_OF_USER_TO_RUN_SCRIPT_AS/bus"

    /usr/bin/sudo -u USERNAME_TO_RUN_SCRIPT_AS /usr/bin/paplay --server /run/user/UID_OF_USER_TO_RUN_SCRIPT_AS/pulse/native /home/USERNAME/.i3/sounds/Click1.wav > /dev/null 2>&1

    /usr/bin/notify-send -i /usr/share/icons/gnome/256x256/status/battery-full-charging.png 'Changing Power States' --expire-time=4000

Note: 1) `USERNAME_TO_RUN_SCRIPT_AS`, `UID_OF_USER_TO_RUN_SCRIPT_AS` and `USERNAME` needs to be changed to that of the shortname for the user and user's UID of the graphical session where the notification will be displayed; 2) `/usr/bin/sudo` is needed when playing audio via pulseaudio; and, 3) three environmental variables (i.e., `XAUTHORITY`, `DISPLAY` and `DBUS_SESSION_BUS_ADDRESS`) for the user of the graphical session where the notification will be displayed need to be defined and exported.

Note: The `XAUTHORITY`, `DISPLAY` and `DBUS_SESSION_BUS_ADDRESS` environment variables must be defined correctly.

3) Load/reload the new udev rule (see above) and test it by unplugging the power supply to the laptop.

Tip: See also xpubAUR as a method for getting the user's display environment variables and exporting the last into udev rules via `IMPORT` key.

### 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, you can use at (e.g. `your_command | at now`, or `batch`), or create a systemd unit which can be triggered directly from a udev rule.

## 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. See blacklisting. Not at boot-time or later on when a hotplug event is received (eg, 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 config 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:

    # fgrep -r GROUP /etc/udev/rules.d/ /usr/lib/udev/rules.d | perl -nle '/GROUP\s*=\s*"(.*?)"/ && print $1;' | sort | uniq > udev_groups
    # cut -f1 -d: /etc/gshadow /etc/group | sort | uniq > 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

### BusLogic devices can be broken and will cause a freeze during startup

This is a kernel bug and no fix has been provided yet.

### 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 /dev/sdb | grep ID_SERIAL

Then we create a rule in `/etc/udev/rules.d/` and set variables for either udisks or udisks2.

For udisks, set `UDISKS_SYSTEM_INTERNAL="0"`, which will mark the device as "removable" and thus "eligible for automounting". See udisks(7) for details.

    /etc/udev/rules.d/50-external-myhomedisk.rules

    ENV{ID_SERIAL_SHORT}=="serial_number", ENV{UDISKS_SYSTEM_INTERNAL}="0"

For udisks2, 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.

Note: Since `udev>=171`, the OSS emulation modules (`snd_seq_oss`, `snd_pcm_oss`, `snd_mixer_oss`) are not automatically loaded by default.

### IDE CD/DVD-drive support

Starting with version 170, udev does not support CD-ROM/DVD-ROM drives that are loaded as traditional IDE drives with the `ide_cd_mod` module and show up as `/dev/hd*`. The drive remains usable for tools which access the hardware directly, like cdparanoia, but is invisible for higher userspace programs, like KDE.

A cause for the loading of the ide_cd_mod module prior to others, like sr_mod, could be e.g. that you have for some reason the module piix loaded with your initramfs. In that case you can just replace it with ata_piix in your `/etc/mkinitcpio.conf`.

### 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 IDE CD devices
    SUBSYSTEMS=="ide", KERNEL=="hd[a-z]", ATTR{removable}=="1", ATTRS{media}=="cdrom*", GROUP="optical"

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

## See also

- udev home page
- 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

Category:

- Hardware detection and troubleshooting

Hidden categories:

- Pages or sections flagged with Template:Accuracy
- Pages or sections flagged with Template:Merge