Controlling monitor brightness from the command line

Start by loading the module allowing the kernel to communicate with your monitors:

# modprobe i2c-dev

Verify that your monitor(s) are detected:

# sudo ddcutil detect

Query your monitor(s) for the index of the brightness setting:

# sudo ddcutil -d 1 capabilities |grep -i brightness
   Feature: 05 (Restore factory brightness/contrast defaults)
   Feature: 10 (Brightness)

We see here that the Brightness index for monitor 1 (-d 1) has index #10. Let's query for the current setting:

# sudo ddcutil -d 1 getvcp 10
VCP code 0x10 (Brightness                    ): current value =    99, max value =   100

It's currently at 99, let's lower it slightly

# sudo ddcutil -d 1 setvcp 10 75