Is there a way to turn off individual screens?

If you are using stock Ubuntu Desktop you should be able to use System > Preferences > Monitors. This should show all of your monitors, click on the one you wish to disable, and then use the 'on/off' radio buttons to disable that specific monitor.

If you want to do this from the command line you should be able to use xrandr --output <name> --off to disable the output which should trigger the monitor to power down.

Problem with existing answer

Although there was an answer seven years ago with three up-votes it wasn't accepted because it causes all open windows on the shut-off display to migrate to the remaining active display(s).

Software based brightness

On a laptop you can control brightness through the graphics card controlling the LCD panel. With external TV's and monitors you need a software solution to turn brightness down to 0. Turning it to zero is closest I've come to the illusion of turning it off whilst keeping the active windows on that external display.

Short version

From the command line (without scripts) use:

$ xrandr | grep " connected"
HDMI-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 1107mm x 623mm
eDP-1-1 connected primary 1920x1080+3840+2160 (normal left inverted right x axis y axis) 382mm x 215mm
DP-1-1 connected 3840x2160+1920+0 (normal left inverted right x axis y axis) 1600mm x 900mm

This shows you the list of monitor names assigned by xrandr. Now if you want to set DP-1-1 brightness off use:

xrandr --output DP-1-1 --brightness 0

Sony TV bash script

Here's one of three scripts I've written:

#!/bin/bash

# NAME: sony
# PATH: /mnt/e/bin
# DESC: Set brightness of Sony TV
# DATE: Dec 9, 2017.

if [[ $# -ne 1 ]]; then
    xrandr --verbose | grep -A5 "^HDMI-0" > /tmp/sony
    head -n1 /tmp/sony
    echo "$(tput setaf 1)" ; tail -n1 /tmp/sony ; echo "$(tput sgr0)"
    rm /tmp/sony
    echo 'One argument required for brightness level, e.g. "sony .63"'
    echo 'will set brightness level of Sony TV to level .63 using xrandr'
    exit 1
fi

xrandr --output HDMI-0  --brightness "$1"

To turn off the display you would use sony 0

Calling script with no parameters

When you call the script with no parameters it displays the xrandr screen name along with the current brightness in red. You can change the red color to another by modifying this line:

echo "$(tput setaf 1)" ; tail -n1 /tmp/sony ; echo "$(tput sgr0)"

After setaf change the 1 to:

• 2 for green
• 3 for orange
• 4 for blue, etc.

Modifying and installing script

Change the two occurrences of HDMI-0to match your screen name. You can get a list of all your screens typing xrandrin the terminal.

Place the script in a directory in your path such as:

/usr/local/bin/sony

Mark the script as executable using:

sudo chmod a+x /usr/local/bin/sony

Choosing which directory:/usr/local/bin vs ~/bin

When you create scripts in /usr/local/bin you must use sudo powers. A better alternative is to use the directory ~/bin which is shorthand for /home/YourUserID/bin:

• create the directory ~/bin
• close your terminal and reopen it.
• the newly created directory is now in your path.
• all the scripts you create in ~/bin can be created with your regular permissions.
• to enable execution of your scripts you just need to use chmod +x rather than chmod a+x (the a meaning all users)