$ xrandr --output HDMI-0 --brightness 0.7

起動直後いちいちこれを打ち込むのしんどい、永続化できるか。

$ xrandr
Screen 0: minimum 320 x 200, current 1200 x 1600, maximum 8192 x 8192
HDMI-0 connected primary 1200x1600+0+0 right (normal left inverted right x axis y axis) 399mm x 299mm
1600x1200 60.00*+
VGA-0 disconnected (normal left inverted right x axis y axis)

$