Idle time - how is it measured?

[krivi]

Well, this isn't exactly a Gentoo-specific question, I believe, but anyhow...

How does the system determine how long it has been idle? I'm not thinking CPU idle or even HD idle, just how long it hasn't been in use...

I'm thinking of scripting something that'll shut down my PC after some inactivity time (electricity is rather expensive here ) - I was thinking of the output from ''w``, but:

$ w | gawk '{print $4}'

just says "?xdm?"...

Any thoughts?

-Jens
_________________
Regards/Hilsen
Jens Villadsen
Odense, Denmark