True, but my problem has been defined by others. Rightly or wrongly, this is my criterion. I have to identify which, if any, of the users is sitting at the machine logged in physically.

Security Cameras that you can watch via a web page to see if someone is sitting at the keyboard? There are no better solutions that what has been provided unfortunately.

I'm not really sure what has been provided. That I should give a "w" command and pick the tty? Or perhaps that I should look at the FROM and the WHAT and perform some test hinted at but not quite stated? What is the algorithm?

Find what works for you. Run it, look at the output, see which one you prefer and go with it. If it were me I would go with either the TTY or FROM column in w. Either way it will let you know who's sitting at the computer. The user who has a tty# instead of a pts# under the TTY column is the user sitting at the computer. Or the user who has a local display (:0, :0.0, :0.1, etc) instead of a pts# or remote IP/hostname under the FROM column is the user sitting at the computer.

See which one you prefer on your network with your machines and your users. The advantages and drawbacks to each method have been provided.

Okay, that kind of crystallizes it in my mind. Thanks, all.

You'll also probably want to use this; effectively a broadcast msg tool http://linux.die.net/man/1/wall