Programming :: 30 Minute Timer In Bash

I've recently started coding in bash and I'm making a diagnostic check CD for rackmounted computers to print out to an LCD screen. So far it works but what i want it to do is, when it checks what ethernet ports are unplugged, it stays there for 30 minutes or so and proceeds through the script. I've looked up cron and that seems to be useful for making timers on permanent systems but I just want the cd to boot, do the check and then eject it and let the real OS take over and I've also tried using:

while [ `sleep 30m` ];
do
<script>

with no luck. Is there any command I can use to do this? I've noticed a few examples where people have captured the time and then used a while less than/greater than statement but I'm not sure how to do this. Is it possible to do a concept like:

while [ %current time -lt %current time+30 min ];
do
<script>