xdisplay -l (--list) # list displays
xdisplay -b (--brightness) DP-1 up # increase brightness of DP-1 display
xdisplay -b (--brightness) DP-1 down # increase brightness of DP-1 display
xdisplay -b (--brightness) DP-1 0.5 # set brightness of DP-1 display (min = 0.1, max 1.0)
# if you want to save default display for ease-of-use
xdisplay -s (--save-default) DP-1 # save DP-1 as default display
xdisplay up # increase default display brightness
xdisplay down # decrease default display brightness
xdisplay 0.5 # set default display brightness
I am just too lazy to type a long-ass xrandr
command, besides I hate to type my external monitor display everything I want to change the brightness
so here I am writing some utility script under an hour.