Ubuntu on Macbook Pro: set the function keys to act like F1-F12 by default (disable Fn default behavior)

put the following line in your ~/.bashrc or ~/.zshrc if you are using zsh

echo 2 | sudo tee /sys/module/hid_apple/parameters/fnmode

What? your sudo command asks your password? then 

sudo visudo
#add this line
your-user-name  ALL=(ALL:ALL) NOPASSWD:ALL

Don't you want to set this NOPASSWD option for security reasons? then put the following line into /etc/rc.local instead. It should work.

echo 2 > /sys/module/hid_apple/parameters/fnmode

Reference