PART 1: Standard Cheats for PC/MAC/LINUX/NEXT/QNX/SGI/SUN Systems

Cheat codes are activated by typing a sequence of keys at the
keyboard at any time during normal play, although some cheats won't
do anything unless you're in a particular situation, eg. viewing the
map or participating in a multiplayer game. An incorrectly or
incompletely typed cheat code will have no effect. For most cheats,
there is no need to hold down a key such as Alt, or Control (the
exception being the multiplater/map cheat), and there is no need to
press Return.

The only cheats that work in Nightmare mode are iddt and idclev (see
below for what they do). In any other skill level, all the cheats
work; however, the following is effectively the same as Nightmare
mode with cheats, where ## is the level number:

doom -fast -respawn -skill 4 -warp ##