On Ubuntu with Unity desktop environment, one can use
qdbus to issue some of the system commands via
dbus service, and there are two commands for increasing and decreasing brightness.
For increasing ,
qdbus org.gnome.SettingsDaemon.Power /org/gnome/SettingsDaemon/Power org.gnome.SettingsDaemon.Power.Screen.StepUp
and for decreasing
qdbus org.gnome.SettingsDaemon.Power /org/gnome/SettingsDaemon/Power org.gnome.SettingsDaemon.Power.Screen.StepDown
These commands can be bound to keyboard shortcuts for convenience. Go to System-Settings -> Keyboard -> Shortcuts -> Custom Shortcuts , click on + icon to add new shortcut.
A popup will appear, and add one of the commands above. In the screenshot you can see me adding the step-down command
Once you click apply, you will see a new entry added to the list. You can click on the
Disabled text, and it will wait for you to press the key combination you want to correspond to this command.
I've set mine to Alt+Shift+Arrow Down for decreasing brightness. Feel free to select your own combination.