Ubuntu – How to adjust the screen brightness in i3wm on Ubuntu 16.04

There's something very odd about the syntax you're using to write to the files in /sys/class/backlight/. Please try this, instead:

echo 128 | sudo tee /sys/class/backlight/radeon_bl0/brightness

By the way, you were not able to modify "max_brightness" because that file is just there to let you know the maximum brightness you can use. You need to change the file named "brightness".

About your shell syntax: The shell redirection operator > only works if you're already running a shell as root (for example, using sudo -s), but I should let you know that doing so is generally considered a bad idea. Logging in to a root shell gives you great power; and with great power comes great whopping mistakes. It's easy to forget you're running as root and then accidentally mess up your whole system. That's why I recommend sudo, as in the example above. Sudo is a reminder to be careful: anything after the word sudo is normally off-limits and potentially dangerous.

However, if you are curious, it is possible to use the > file redirection operator the way you were asking, but you'd need to change the arguments around. The part that comes after the > is the filename to write to, the part before it is the command whose output you want to redirect. So, for example,

$ sudo -s
# cd /sys/class/backlight/*/
# echo 128
128
# echo 100 > brightness
# cat brightness
100
# echo 200 > brightness
# cat max_brightness
255
# cat max_brightness > brightness
# cat brightness
255
# exit
$ 

(You'll notice your prompt changes to a hash mark when you're running a root shell. That's a subtle warning sign. Typing exit will get you back to the safety of your normal user account.)

I created a temporally bash script to adjust brightness level. Then I create 2 sortcuts at keyboard mapping settings to set up CTRL + Brightness UP or CTRL + Brightness Down with this script:

#!/bin/bash

# Step brightness value
STEP=2
# Max brightness value (defult 100)
MAX=100
# Min brightness value (defult 0)
MIN=0

COMMAND=$(xrandr --verbose | grep -i brightness | cut -f2 -d':' | tr -d "[:space:]")
ACTUAL=$(bc -l <<< $COMMAND*100 | cut -f1 -d'.')

while getopts ": u d" input
do
    case $input in
    u)  if [ $ACTUAL -lt $MAX ];then
                TOTAL=$(bc -l <<< $(($ACTUAL+$STEP))/100)
                $(xrandr --output DP-0 --brightness $TOTAL)
        fi;;
    d)  if [ $ACTUAL -gt $MIN ];then
                TOTAL=$(bc -l <<< $(($ACTUAL-$STEP))/100)
                $(xrandr --output DP-0 --brightness $TOTAL)
        fi;;
    ?)  printf "Usage: brightness [OPTION]\n"
        printf "Increase or decrease birghtness on Ubuntu Xorg.\n" 
        printf "WAYLAND MUST BE DISABLED\n\n"
        printf "  -u\t Increases brightness\n"
        printf "  -d\t Decreases brightness\n"
        exit 2;;
    esac
done

And in set custom shortcut I added this