How to map keyboard keys to joystick buttons using joy2key?

You can use antimicro, it is a fork of joy2key and has a GUI, use this PPA for 12.04 to 15.10

sudo add-apt-repository ppa:ryochan7/antimicro
sudo apt-get update
sudo apt-get install antimicro

For 16.04 to 17.10 use this PPA

sudo add-apt-repository ppa:mdeguzis/libregeek
sudo apt-get update
sudo apt-get install antimicro

Source

For those looking for a solution that's easily installable on Ubuntu 18.04+, I found qjoypad in the official Ubuntu repositories. Install it like this:

sudo apt install qjoypad

Then, it should be available in your launcher. It starts as a system tray icon, and you can launch the configurator by right clicking on it.

Personally, I really like the quick bind option that lets you press a key on a gamepad and then a key on your keyboard to update the mapping for that layout automatically. Note that it does indeed natively support multiple gamepads.

For me, I had some trouble with getting the right click to launch the configurator to work on the Unity 7.5 desktop - and the left-click menu doesn't appear to have an option for launching it. Thankfully, there's a workaround for this too. Simply launch qjoypad like so:

qjoypad --notray

....this will disable the system tray icon and instead launch a window with the icon in it instead - which works as expected.

This can be made to happen automatically by editing /usr/share/applications/qjoypad.desktop. Find the line that starts with Exec=, and append --notray to the end of it (make sure that there's a space after qjoypad at the end of the line and --notray).