Command help?

[Bradley]

Okay,

    I am starting to do some more advanced automated maintenance on my
system but can't find a nifty way to do something and was wondering if
anyone out there can help me with this.  I am configuring my system to
do an automated backup of all my data (2 to 4 hours per week) but need
it to do certain things to protect the process.

    For an unattended process, I need to know how to:

   1. Force all users currently logged on to be logged off (preferably
      with at least a 5 minute notice).
   2. Prevent anyone from logging on.
   3. Prevent the system from being shut down or rebooted.
   4. Shut down the X server (speeds up processing time considerably).
   5. After finished to restart X server and allow shutdown and logins.

    If anyone knows any commands to do at least some of these, I would
appreciate knowing how.  The part about shutting down the X server is
optional but would be nice but not allowing anyone to be logged in and
preventing system shut down is necessary.

Bradley