debug mode in mac pro?

how can I enter to debug mode when mac os starts up?

I am saying the entire screen should become a bunch of unix commands running drivers, startup items... stuff like that...

please help

i hold the shift key but no unix mode at all, still while screen then the login window shows "safe mode"

I guess I need to see all the commands running in the background before the login screen, and I thought it's called debug mode... any idea? thank you so mcuh!

works great now. Thank you sooooo much!