you are still not being very clear on what it is you are talking about. i can assume that you mean:
you want to get a yes or no input from a user, and if the user dosnt enter a y Y n or N then the script will loop until they do or until it is killed. whenever you are looking for a certain kind of input and if that input is not given, you can use a while loop to ensure that you recieve the correct input.
ex: (watch your syntax, im typing this very loosely.)

Code:

while ($user_input != "")  # "" meaning they enter "nothing"
do
echo "please enter y for yes, or n for no"
if ($user_input == ("y" || "Y"))
do something
else if ($user_input == ("n" || "N"))
do something else
else
echo "please enter y for yes, or n for no"

etc etc.