#! /bin/bash
# Usage: yesno prompt...

if [ $# -eq 0 ]
then prompt="yes or no: "
else prompt="$*"
fi

while true
do
    read -p "$prompt" || exit 1
    case "$REPLY" in
    y*|Y*)	exit 0;;
    n*|n*)	exit 1;;
    *)		echo "Answer yes or no please";;
    esac
done