UNIX for Dummies Questions & Answers

I am writing a script that needs to accept numbers into a variable by prompting and using the code
read num
case $num in
000)
break
;;
*)

I am fairly new to unix and am loving the versatility of the language but need a little shove in the right direction. I know how to define variables in C++, but how do you make sure that a variable is a number? And if a character string is entered instead of a number is there any way that i could send an error message and start the loop again.
thank you in advance

Code:

#!/bin/ksh
# define num as integer and set to zero
typeset -i num=0

num is now an integer variable. Works in the Korn shell.