below is bash script for a calculator other operations work fine, but I cannot make the multiply work...i tried to use \* instead * but to no avail, please help

#!/bin/sh input="yes" while [ $input = "yes" ] do echo Enter a Number read num1 echo Enter desired operation +,-,/,* read operation echo Enter another number read num2 result=`expr $num1 "$operation" $num2` if [ $operation = '+' ] then echo The sum of $num1 and $num2 is $result elif [ $operation = '-' ] then echo The difference of $num1 and $num2 is $result elif [ $operation = '*' ] then echo The product of $num1 and $num2 is $result elif [ $operation = '/' -a $num2 = '0' ] then echo The division by $num2 occured and it is invalid elif [ $operation = '/' ] then echo The quotient of $num1 and $num2 is $result fi echo "Do you want to calculate again(yes/no):" read input echo "Thank you for using this program" done