Question

This is my first bash script ever, essentially it just shuts off my second monitor. But I’ve been having problems with it, as it keeps giving me errors when I run it.

#!/bin/bash


read -p "Do you want the 2nd monitor on or off? " ON_OFF

if [$ON_OFF == on]; then
xrandr --output DVI-I-3 --auto --right-of DVI-I-0
echo "done"
fi

if [$ON_OFF == off]; then
xrandr --output DVI-I-3 --off   
echo "done"
fi

When I run it I get

monitor_control.sh: 11: [[off: not found
monitor_control.sh: 16: [[off: not found

Can anybody explain to me why it’s not working?

Answer 1

You need to add space around [ and ], as they are separate commands in bash.

Moreover, either quotes need to be used around parameter expansions, or [[ ]] needs to be used instead of [ ].

That is, you can either use:

if [[ $ON_OFF = on ]]

…or you can use:

if [ "$ON_OFF" = on ]

Otherwise you will get error if $ON_OFF is empty.

Finally, it’s better to use if ... then ... else ... fi, like:

if [[ $ON_OFF = on ]]; then
    xrandr --output DVI-I-3 --auto --right-of DVI-I-0
else
    xrandr --output DVI-I-3 --off   
fi
echo "done."