您不需要为回声... 和阅读这些反引号

echo "----------------"
echo "    menu"
echo "----------------"
echo "[1] hello"
echo "[2] bye"
echo "[3] exit"
echo "----------------"

read -p "Please Select A Number: " mc
if [[ "$mc" == "1" ]]; then
    echo "hello"
elif [[ "$mc" == "2" ]]; then
    echo "bye"
else
    echo "exit"
fi

修改

showMenu(){

echo "----------------"
echo "    menu"
echo "----------------"
echo "[1] hello"
echo "[2] bye"
echo "[3] exit"
echo "----------------"

read -p "Please Select A Number: " mc
return $mc
}


while [[ "$m" != "3" ]]
do
    if [[ "$m" == "1" ]]; then
        echo "hello"

    elif [[ "$m" == "2" ]]; then
        echo "bye"
    fi
    showMenu
    m=$?
done

exit 0;