When plugging or unplugging external monitor via hdmi cable, sometimes the change in external display won't register. This is an applescript to automate the process of using the display menu "detect displays" button.

detect-displays.command

#!/usr/bin/env osascript

# adapted from https://stackoverflow.com/a/12641263/3044358
# don't forget to 
# chmod 744 detect-displays.command

tell application "System Preferences"
    activate
    reveal pane "com.apple.preference.displays"
end tell

delay 0.5

tell application "System Events"
    tell process "System Preferences"
        try --don't even consider not using a try block!
            key down option
            delay 0.2
            click button "Detect Displays" of window 1
            delay 0.2
            key up option
        on error --logging out is the only other way to clear these
            key up option
        end try
    end tell
end tell