How to Determine if LCD Monitor is Turned on From Linux Command Line

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How do you tell if a computer's monitor(s) are turned on/off from the command line in Linux? I've traditionally thought of monitors as output-only devices, but I've noticed the Gnome Monitor Preferences dialog has a "detect monitor" function. Can this be generalized to determine if a monitor is physically turned off?

shell linux command-line
8 Answers

First, find the name of the display you want to inspect:

xrandr -q

Then change 'THE-MONITOR' to the proper name:

#!/bin/sh    
is_on="`xrandr -q | grep -A 1 'THE-MONITOR' | tail -1 | sed 's/[^\*]//g';`";

Pipeline:

  • xrandr -q lists all monitors;
  • grep -A 1 'THE-MONITOR' filters to two lines, the one containing the name of your display, and the consecutive line, which will have an "*" next to it if the monitor is on;
  • tail -1 discards the first line;
  • sed 's/[^\*]//g' filter out everything but the "*";

Now "$is_on" is a boolean string as "*" or empty.

This will only work if your preferred mode is at the top of the mode list, which is very usual.

The whole script for turning on and off:

#!/bin/bash
is_on="`xrandr | grep -A 1 'DVI-I-1' | tail -1 | sed 's/[^\*]//g';`";
if [ "$is_on" ]
then
    xrandr --output DVI-I-1 --off
else
    xrandr --output DVI-I-1 --auto --left-of HDMI-0 
fi
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