Shell Programming and Scripting

#!/bin/sh
bgcal() {
cal=""
echo "Enter the Option Number: \c"
read cal
if [ $cal = 1 ]
then
echo "You have selected addition"
elif [ $cal = 2 ]
then
echo "You have selected subtraction"
elif [ $cal = 3 ]
then
echo "You have selected multiplication"
elif [ $cal = 4 ]
then
echo "You have selected division"
else
echo "selection invalid. Please choose between 1-4"
fi
case $cal
in
1) echo "Enter two numbers for addition"
   read a b
   c=`expr $a + $b`
   echo " Addition of $a + $b is $c"
   echo " Do you want to continue"
   read ans
   if [ ans -eq 'yes' ]
   then
   bgcal
   else
   exit 0
   fi;;
2) echo "Enter two numbers for subtraction"
   read a b
   c=`expr $a - $b`
   echo " Subtraction of $a - $b is $c"
   echo " Do you want to continue"
   read ans
   if [ ans -eq 'yes' ]
   then
   bgcal
   else
   exit 0
   fi;;
3) echo "Enter two numbers for multiplication"
   read a b
   c=`expr $a \* $b`
   echo "Multiplication of $a * $b is $c"
   echo " Do you want to continue"
   read ans
   if [ ans -eq 'yes' ]
   then
   bgcal
   else
   exit 0 
   fi;;
4) echo "Enter two numbers for Division"
   read a b
   c=`expr $a / $b`
   echo "Division of $a / $b is $c"
   echo " Do you want to continue"
   read ans
   if [ ans -eq 'yes' ]
   then
   bgcal
   else
   exit 0
   fi;; 
*) echo "Do you want to continue?"
    read opt
     if [ opt -eq "yes" ]
     then
      bgcal
     else
     exit 0
     fi;;
esac
 }
echo "Choose the type of operation"
echo "============================"
echo "1-Addition"
echo "2-Subtraction"
echo "3-Multiplication"
echo "4-Division"
bgcal