Automatic unattended execution of 'dump' ?

[Ken A. Glidden]

  We have a uVAX II running Ultrix 1.2.  I'd like very much to use
the crontab to start up a shell script late at night to perform 
a dump.   Specifically I'd like the shell script to "simulate"
single user mode by preventing any logins, killing any processes which
have the potential to alter the disk during the dump, and preventing 
any processes which may alter the disk during the dump from starting
while the dump is in progress.  Once finish the script should undo all
its protections and allow business as usual.

  I haven't had too much luck yet.

  Before I press on I'd like to ask, "ANYONE ALREADY DONE THIS?"
"DOES ANYONE KNOW OF A REASON I SHOULDN'T DO IT THIS WAY?"

--Ken