This cannot be done without some programming.
First, test how to detect MIDI events. Go to a terminal, and run aseqdump -l to list the MIDI ports; this outputs something like this:
$ aseqdump -l Port Client name Port name 0:0 System Timer 0:1 System Announce 14:0 Midi Through Midi Through Port-0 24:0 Xonar D2 Xonar D2 MIDI 32:0 Yamaha DS-1E (YMF754) Yamaha DS-1E (YMF754) MIDI
Then run it with the client name to check whether events arrive:
$ aseqdump -p "Xonar D2" Waiting for data. Press Ctrl+C to end. Source Event Ch Data 24:0 Note on 0, note 64, velocity 86 24:0 Note on 0, note 48, velocity 80 24:0 Note off 0, note 48 24:0 Note on 0, note 68, velocity 84 24:0 Note on 0, note 52, velocity 88 24:0 Note off 0, note 64 24:0 Note off 0, note 52 24:0 Note off 0, note 68 ...
Second, to simulate key strokes, you need xdotool. If you do not yet have it installed, run sudo apt-get install xdotool.
You can use type to type text, or key to simulate special keys:
xdotool type Hello, World! xdotool key ctrl+p
Please note that not all special keys are handled correctly by xdotool.
And Ctrl+Alt+Del is handled very specially by the kernel and probably does not work when simulated; try running sudo reset instead of xdotool.
Finally, tie everything together with a script. Put this into a text file, for example, ~/bin/midi-to-keys:
#!/bin/bash
aseqdump -p "Xonar D2" | \
while IFS=" ," read src ev1 ev2 ch label1 data1 label2 data2 rest; do
case "$ev1 $ev2 $data1" in
"Note on 64" ) xdotool type hello ;;
"Note on 48" ) xdotool key ctrl+j ;;
esac
done
Make it executable (chmod +x ~/bin/midi-to-keys), and run it (~/bin/midi-to-keys).
Now, pressing E-5 or C-4 should have some effect.
Change or add lines of the form "Note on x" ) command ;; to do whatever you want.