Before, we introduced you Redshift GUI to reduce your screen brightness. Reducing screen brightness may save your eyes and may prolong your laptop battery duration. But you need to install something if you want to do that. What if you want to reduce brightness without installing any program? Use built-in xrandr command.
Reducing into Half
xrandr --output LVDS1 --brightness 0.5
Reduce into Very Low
xrandr --output LVDS1 --brightness 0.1
Back To Normal
xrandr --output LVDS1 --brightness 1
Double Normal
xrandr --output LVDS1 --brightness 2
CAUTION: number above one is too bright. It may hurt your eyes.
Knowing Your Output
If the command above fails, probably your output screen LVDS1 is incorrect. LVDS1 is used in laptop. You should know your output screen whether it is LVDS1 or else. Use this command
xrandr -q
See the output. Look for the connected line. Ignore the disconnected lines. In this example, my correct output is LVDS1. In other hand, DP1, HDMI1, VGA1, and VIRTUAL1 are not my output. See picture above.