echo -n "Enter A, B, or C: "
read letter
case "$letter" in
    a|A)
        echo "You entered A"
        ;;
    b|B)
        echo "You entered B"
        ;;
    c|C)
        echo "You entered C"
        ;;
    *)
        echo "You did not enter A, B, or C"
        ;;
esac