Can scripts modify Global Variables from other scripts?

Like the title says. Let's say I were to have a script that set a certain variable to something and it was set to be Global. (The variable, that is.) If I made another script that was to modify that variable, could I do it? If so, how would I call it so I can modify it? And would I need to use int/float/bool/whatever to define it, or does it become pre-defined when it's made Global? (Though, since the original variable would be in another script, I'd assume you'd need to define it again, or the compiler will say it's undefined, even though it isn't.)