Keyboard Application Shortcuts Not Allowing If

I use a Linux Xfce (Xubuntu).

I tried to configure the brightness shortcut so that it would turn the screen down if brightness was to be set to 0.

    if [ "$(cat /sys/class/backlight/acpi_video0/actual_brightness)" == '0' ];
    then sh -c "sleep 1 && xset dpms force off";
    fi

This code works perfectly in the terminal. But when I go to xfce4-keyboard-settings (Keyboard settings of Xfce) and set some shortcut to be the code above I get (when I press the shortcut) the following error report

Failed to launch shortcut "Return"

Failed to execute child process "if" (No such file or directory)

Then I tried to try with some other if-Statements so as to try whether they are generally not accepted:

    if [ "0" == '0' ];
    then spotify;
    fi

returns the same error report but

    spotify

on the other hand works perfectly.

Do you have any ideas?

Thank you very much in advance,

Pol