Калькулятор

#!/bin/bash

while true
do
    first=""
    operator=""
    second=""

    read first operator second

    if [[ $first == "exit" ]]
        then
        echo "bye"
        exit
    fi

    if [[ $first == "" || $second == "" ]]
        then
        echo "error"
        exit
    fi

    if [ "$operator" != "+" ] && [ "$operator" != "-" ] && [ "$operator" != "*" ] && [ "$operator" != "/" ] && [ "$operator" != "%" ] && [ "$operator" != "**" ]
        then
        echo "error"
        exit
    fi

    let "result = $first $operator $second"

    echo $result

done