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I am trying to get a way to execute a script from a root session, but I am having some trouble with it. I'll lay out the issue:

I have a script that performs some tasks as the regular user 'kolterdyx', and some others as root with sudo. What this script does is rotate a screen and update the touchscreen calibration matrix automatically. I spent about a week troubleshooting that and the script I came up with is as follows:

/home/kolterdyx/.cargo/bin/gnome-randr modify HDMI-1 --rotate "$1"

rule='ATTRS{idVendor}=="27c0", ATTRS{idProduct}=="0818", ENV{LIBINPUT_CALIBRATION_MATRIX}='

right='"0 -1 1 1 0 0"'
left='"0 1 0 -1 0 1"'
inverted='"-1 0 1 0 -1 1"'
normal='"1 0 0 0 1 0"'

case "$1" in
  left )
    RULE="$rule$left"
    ;;
  right )
    RULE="$rule$right"
    ;;
  inverted )
    RULE="$rule$inverted"
    ;;
  normal )
    RULE="$rule$normal"
  * )
    echo "You need to pass an argument"
    exit
    ;;
esac

sudo bash -c "echo '$RULE' > /etc/udev/rules.d/99-calibration.rules"


sudo udevadm control --reload-rules
sudo udevadm trigger
sudo bash -c 'echo 1-1 > /sys/bus/usb/drivers/usb/unbind'
sudo bash -c 'echo 1-1 > /sys/bus/usb/drivers/usb/bind'

The system this is running on is Raspberry Pi OS, which I installed without a desktop, and then installed Gnome, which by default uses Wayland and Gnome-Shell. That opened the can of worms that is trying to perform X11 tasks on a non X11 environment, through SSH (because I can't hook up a keyboard to the raspberry pi).

I managed to get that script working, but it only works if I run it as kolterdyx, directly from the user shell. sudo -u kolterdyx or runuser -u kolterdyx as any other user won't work, and the error I get is as follows:

Unable to autolaunch a dbus-daemon without a $DISPLAY for X11

This happens because of the first line, which is a command from gnome-randr-rust. For some reason, if I run it by logging in as kolterdyx it works fine, but if I run it as root either with or without sudo -u kolterdyx, I get the previous error. Isn't sudo supposed to make the target user run the command? What's the difference and how can I get the command to run as my user from the root user?

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Programs find your D-Bus session bus in a few ways:

  • Explicitly set $DBUS_SESSION_BUS_ADDRESS. This is the main method if you don't use systemd or if your distro doesn't have the "user bus" model enabled; in those cases usually a random address is chosen every time you log in.

  • A well-known path under $XDG_RUNTIME_DIR/bus. This is the systemd-ish approach where each UID has only one session bus and always at a semi-fixed per-UID location.

  • Auto-launch based on the X11 $DISPLAY; a legacy mechanism from the early days of D-Bus that most distros avoid now (not least because it's X11-dependent). Being the last fallback is why it ends up in those misleading error messages, though.

So you need to set one of the two environment variables mentioned above. On a systemd environment, XDG_RUNTIME_DIR would be the easier option because it's pretty much always set to /run/user/$UID and you can safely hardcode that format in scripts – and it will cover not only D-Bus but also the Wayland socket address and various other components that have settled around that path. Otherwise, though, you need to have your desktop session write out its DBUS_SESSION_BUS_ADDRESS into a file on startup, which the script will then read.

(In fact, with systemd being present, it might be easiest to systemd-run --user -M [email protected] to automatically find the bus address and spawn the command directly inside your session, having all necessary environment.)

As a side note, doesn't GNOME already have built-in support for detecting screen orientation via iio-sensor-proxy?

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  • That works! As for the side note, even if gnome has support for that, I don't have an accelerometer set up yet, and even when manually rotating, the touchscreen would not update. I had a lot of problems trying to rotate the screen manually from the command line, and the same for rotating the touchscreen. As you can see I ended up having to hardcode calibration values, reloading udev, and programatically unplugging and plugging in the touchscreen Commented Feb 2, 2024 at 13:22

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