用unix模拟一个简单的计算器

用unix模拟一个简单的计算器，包括（+，-，x，/）和（MS, M+,MC, MR,X)功能。

while true
do
echo " Enter the operand1"
read operand1

echo " Enter the operator"
read operator
echo " Enter the operand2"
read operand2
if [ "operand1" == "$result" ]
then
echo "operand1 is: $result"
echo ""
fi

   if [ "$operator" == "+" ]
     then
       result=$(($operand1 + $operand2))

   elif [ "$operator" == "-" ]
    then
       result=$(($operand1 - $operand2))

   elif [ "$operator" == "x" ]
     then
       result=$(($operand1 x $operand2))

   elif [ "$operator" == "/" ]
     then
        if [ $operand2 -eq 0]
           then
             echo "Can't divide by zero. please re-enter divisor."
        else
            result=$(($operand1 / $operand2))
        fi

   elif [ "$operator" == "X" ]
       then
        exit
   elif [ "$operator" == "MS" ]
       then
         MS -> $result

   elif [ "$operator" == "MR" ]
       then
         operand2=$result
         MR -> $operand2

   elif [ "$operator" == "M+" ]
       then
         M+ -> $((result+MS))

   elif [ "$operator" == "MC" ]
       then
         $result=0
         MC -> 0
   else
       echo "Sorry, is not a valid operator. plrase re-enter operator. "
   fi

echo "$operand1 $operator $operand2 = $result"
done

只能计算（+，-, x , /),并且当数值不符合条件时不能做到重复，而且（MS, M+,MC, MR,X)功能并不能使用。

运用 while loop和if，只限一个小时内解决

可以模拟成正常我们日常生活中的计算器

$ ./calc
[ user instruction displayed ]
Enter operand: 3
Enter operator: +
Enter operand: 4
3 + 4 = 7
Enter operator: -
enter operand: 12
7 - 12 = -5
Enter operator: MS
-5 -> M
Enter operator: x
enter operand: 3
-5 x 3 = -15
Enter operator: C
results cleared(清除结果）
enter operand: MR
M -> -5
Enter operator: x
enter operand: MR
M -> -5
-5 x -5 = 25
Enter operator: M+
25 + -5 = 20 -> M
Enter operator: /
enter operand: 0
can't divide by zero. plesae re-enter divisor.(不能除以0，请重新输入）
enter operand: 0
can't divide by zero. plesae re-enter divisor.(不能除以0，请重新输入）
enter operand: -6
25 / -6 = -4
Enter operator: +
enter operand: C
results cleared(清除结果）
enter operand: -9
Enter operator: K
Sorry, k, is not a valid operator.please re-enter the operator. (k不是一个有效的operator，请重新输入）
Enter operator: Q
Sorry, Q, is not a valid operator.please re-enter the operator. (Q 不是一个有效的operator，请重新输入）
Enter operator: MC
0 -> M
Enter operator: +
enter operand:3
-9 + 3 = -6
Enter operator: X
Good bye.(再见）

在sh中添加echo和pause语句调试
……
虽然当时时间来不及了，我还是补上完美答案吧：

#!/bin/bash
echo "[ user instrution displayed ]"
echo "number + - x / C MS M+ MR MC supported, X exit"
result=0
memory=0
while true
do
    read -p "Enter the operand:" operand1
    if [ "$operand1" == "MR" ]
    then
        operand1=$memory
        echo "    M -> $operand1"
    fi
    while true
    do
        read -p "    Enter the operator:" operator
        if [ "$operator" == "+" ]
        then
            read -p "Enter the operand:" operand2
            if [ "$operand2" == "MR" ]
            then
                operand2=$memory
                echo "    M -> $operand2"
            elif [ "$operand2" == "C" ]
            then
                result=0
                echo "    results cleared."
                break
            fi
            result=$(($operand1 + $operand2))
            echo "    $operand1 + $operand2 = $result"
            operand1=$result
        elif [ "$operator" == "-" ]
        then
            read -p "Enter the operand:" operand2
            if [ "$operand2" == "MR" ]
            then
                operand2=$memory
                echo "    M -> $operand2"
                echo "$operand2"
            elif [ "$operand2" == "C" ]
            then
                result=0
                echo "    results cleared."
                break
            fi
            result=$(($operand1 - $operand2))
            echo "    $operand1 - $operand2 = $result"
            operand1=$result
        elif [ "$operator" == "x" ]
        then
            read -p "Enter the operand:" operand2
            if [ "$operand2" == "MR" ]
            then
                operand2=$memory
                echo "    M -> $operand2"
            elif [ "$operand2" == "C" ]
            then
                result=0
                echo "    results cleared."
                break
            fi
            result=$(($operand1 * $operand2))
            echo "    $operand1 x $operand2 = $result"
            operand1=$result
        elif [ "$operator" == "/" ]
        then
            while true
            do
                read -p "Enter the operand:" operand2
                if [ "$operand2" == "MR" ]
                then
                    operand2=$memory
                    echo "    M -> $operand2"
                elif [ "$operand2" == "C" ]
                then
                    result=0
                    echo "    results cleared."
                    break
                fi
                if [ $operand2 -eq 0 ]
                then
                    echo "Can't divide by zero. please re-enter divisor."
                else
                    result=$(($operand1 / $operand2))
                    echo "    $operand1 / $operand2 = $result"
                    operand1=$result
                    break
                fi
            done
            if [ "$operand2" == "C" ]
            then
                break
            fi
        elif [ "$operator" == "X" ]
        then
            echo "Good bye."
            exit
        elif [ "$operator" == "MS" ]
        then
            memory=$result
            echo "    $result -> M"
        elif [ "$operator" == "C" ]
        then
            result=0
            echo "    results cleared."
            break
        elif [ "$operator" == "MR" ]
        then
            operand1=$memory
            result=$operand1
            echo "    M -> $result"
        elif [ "$operator" == "M+" ]
        then
            oldmemory=$memory
            memory=$(($operand1 + $oldmemory))
            echo "    $operand1 + $oldmemory = $memory -> M"
        elif [ "$operator" == "MC" ]
        then
            memory=0
            echo "    0 -> M"
        else
            echo "Sorry, $operator is not a valid operator. please re-enter the operator."
        fi
    done
done

直接用bc命令呗