Cheat mode:
Type 1982gonzo or gonzoopera while Duke is selected during game play to enable cheat mode:

result - Code:

Invisibility - [Ctrl] + I
Destroy everything - [Ctrl] + [Shift] + X
Mission editor - [Shift] + E or [Ctrl] + E
Select video mode - [Shift] + [F1] through [F4]
View debug output - [Ctrl] + [F9]
Invincibility - [Ctrl] + L
Place selected commandos under the pointer - [Shift] + X
Mission skip - [Ctrl] + [Shift] + N
Trace user - [Shift] + V
Display terrain information - [F9]
Window help - [Alt] + [F11]
Picture interface - [Alt] + I

Note:
If gonzoopera does not enable cheat mode in your version of the game, create a backup copy of the "commando.cfg" file in the game directory. Then, use a text editor to add the line ".DEVELOP 1" in that file.