Dirty hacking to use the Function-Keys to change brightness with lubuntu and i3 on a thinkpad a275
i couldnt get the function keys working so i made a dirty hack to do it anyway, even if its highly unrecommended. i wrote scripts which take the value from /sys/class/.../brightness and add or decrese them by 5. (as its a value between 0 and 255 its a good portion and nice brightnesschange, in my oppinion) as normally the brightness-file is write protected for the user, i wrote a systemd service to change the filepermissions to 777 on each startup, so that anyone could do anything with it. (777 was the fastest because i didnt need to test or google when it would work) after that i bound the fn keys (233 and 232) to the scripts in my i3 config, so that they will be executed when pressing the keys. its dirty but it works...
i repeat that its really not recommended to do it this way - if you have a better way (that is working) please tell me, but after about 12 hours of trying and debugging i had enough. if you are there as well, feel free to try ;-)
put the scripts in your .config/ folder in home
i dont recommend to replace your config, but if you want, you can do that - i recommend to add the last 2 or 3 lines, the bindcodes, to your config. if you want to store the bright_dec.sh or bright_inc.sh in another location, dont forget to change it to that location here in your config.
put the brightnesshack.service file in the systemd directory: /etc/systemd/system/ and enable the service with systemctl enable brightnesshack.service as root (or with sudo)
just reboot the system and get the sweet feeling of sanity back, as the brightness finally changes when pressing the f*n-keys!