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Is there a command that will list displays connected to the computer?

Specifically, I'm looking to find out how my external monitor is being detected by the system when connected by HDMI cable and what it's (not sure of the correct term here) hardware address is (eg. "CRT-0" or "DFP-1").

This is just a general question, but for reference, I'm using:

  • Asus UL30JT laptop - running Ubuntu 12.04 beta and 11.10
  • Asus PA238 23" monitor
  • HDMI 1.4 cable
  • Optimus graphics (Intel + nVidia GeForce 310M) - running with Bumblebee
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Try this:

xrandr --query

This lists the display names and detected available resolutions. You can also reconfigure your displays using xrandr.

Note that this might not work if you're using the NVidia or ATI drivers; I'm not sure.

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I gave the xrandr ++query command and it says "Can't open display" What to do ? – user55334 Apr 18 '12 at 7:12
xrandr only works when you run it under X-windows. After all, it is X that handles all but the most basic video drivers. If you are in a terminal inside X and it still doesn't work, then you've most likely lost your $DISPLAY environment variable, somehow. – ams Apr 18 '12 at 8:44
xrandr --query not "xrandr ++query" – bmbaker May 1 '13 at 8:30
Don't forget to add -display :0.0 if trying this from remote log-in (putty?). – Nikola Malešević Mar 5 '15 at 0:31
xrandr --query | grep '\bconnected\b' if you just wanted to see the connected monitors and their resolutions. – mpen Dec 23 '15 at 16:40

For most machines with the proprietary driver loaded, /usr/lib/nvidia-current/bin/nvidia-xconfig --query-gpu-info --nvidia-cfg-path=/usr/lib/nvidia-current works. Note that I said "with the proprietary driver loaded". For instance, it does not work if the driver is unloaded or nouveau is loaded. Therefore, when using Bumblebee, run optirun /usr/lib/nvidia-current/bin/nvidia-xconfig --query-gpu-info. The --nvidia-cfg-path part is not needed here as optirun sets the correct library path.

See also the discussion on

/var/log/Xorg.0.log (where 0 is the display number) may also contain valuable information about available screens. For Optimus laptops, this log contains only details on the Intel screen, so replace 0 by 8 to find out the log from the X server started by Bumblebee.

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+1, This works when there are multiple graphics cards, whereas the xrandr answer does not. – Sanjay Manohar Sep 10 '14 at 9:39

xrandr --query not xrandr ++query I have just tried it in my Nvidia setup and it worked fine.

enter image description here

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