rewrite of this gist from this blog article (credit to @konradstrack) in C. Same functionality, except the program won't fail if the display is turned off, brightness can increase all the way to 100%, and the args are slightly different.
$ git clone https://github.com/hrfee/brightness.git
$ cd brightness
$ sed -i "s#BACKLIGHT_PATH#PATH_TO_BACKLIGHT_FILE#g" bright.c
$ gcc -o bright -lm bright.c
$ ./bright [-U/-B/-D]replace PATH_TO_BACKLIGHT_FILE with the path to the file which contains and controls your displays backlight. For me, this is /sys/class/backlight/intel_backlight/brightness.
where -U(up)/-B(big up) are small/large increase, and -D(down) is decrease.