How can I start a Qt GUI application on boot without a display connected?

Question

I need to run a Qt GUI App on every system start and run it forever. I have created a system service for it.

Here is the service file content:

QtGUIAPP.service

[Unit]
Description=QtGUIAPP

[Service]
Environment="DISPLAY=:0"
Environment="XAUTHORITY=/home/user/.Xauthority"
ExecStart=/usr/bin/sudo /home/user/QtGUIAPP --no-sandbox
Restart=always
[Install]
WantedBy=multi-user.target

The above works fine without any problem.

But when I boot the PC without a monitor and access it remotely, the application seems not to be running. And I get an error in syslog like

 Invalid MIT-MAGIC-COOKIE-1 keyQStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'QXcbConnection: Could not connect to display :0Could not connect to any X display.
Oct 18 12:45:55 user systemd[1]: QtGUIAPP.service: Main process exited, code=exited, status=1/FAILURE
Oct 18 12:45:55 user systemd[1]: QtGUIAPP.service: Unit entered failed state.
Oct 18 12:45:55 user systemd[1]: QtGUIAPP.service: Failed with result 'exit-code'.
Oct 18 12:45:55 user avahi-daemon[799]: Host name conflict, retrying with user-2
Oct 18 12:45:55 user avahi-daemon[799]: Registering new address record for 10.0.7.15 on enp2s0.IPv4.
Oct 18 12:45:56 user systemd[1]: QtGUIAPP.service: Service hold-off time over, scheduling restart.

I think the problem is with the line

 Environment="DISPLAY=:0"

How can I change the above script to make it work both when a display is connected and not connected.

Answer 1

I don't think you can run Qt without X, so you need to run X. It won't start by default if a monitor is not detected.

I found this Ubuntu Forums thread (link to exact answer) that might help you get over the problem of not being able to connect a monitor:

I am not that great with Ubuntu (been using it for past 2 years), so there might be other options to do the same.

I was having the exact same problem - I want to run X on a machine that (for now) doesn't have a monitor attached.

Using the intel driver, X was finding no monitors attached, and giving up:

(II) intel(0): Output VGA disconnected
(WW) intel(0): No outputs definitely connected, trying again...
(II) intel(0): Output VGA disconnected
(WW) intel(0): Unable to find initial modes
(EE) intel(0): No valid modes.
(II) UnloadModule: "intel"

I stumbled across a solution pieced together from several different threads on these and other forums.

First I needed to make a basic xorg.conf file, in which I specified the VESA driver rather than the intel one (see bottom of post for my xorg.conf)

This led to me seeing a different error:

(EE) VESA: Kernel modesetting driver in use, refusing to load
(WW) Falling back to old probe method for vesa
(EE) No devices detected.

I then had to add nomodeset to my GRUB boot options (used to be in /boot/grub/menu.lst but now in /etc/default/grub)

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset"

...and run sudo update-grub.

With some basic settings for the (non-existent) monitor, the result is that I get X running with no monitor plugged in, using the VESA driver, and with a decent resolution - which is fine for what I wanted it for.

The xorg.conf file:

Section "Monitor"
  Identifier   "Monitor0"
  HorizSync    31-81
  VertRefresh  56-75
EndSection

Section "Device"
  Identifier  "Card0"
  Driver      "vesa"
EndSection

Section "Screen"
    Identifier "Screen0"
    Device     "Card0"
    Monitor    "Monitor0"
EndSection