Shell Programming and Scripting

#!/bin/sh 
clear 
x="y" 
echo "enter ur 1st no." 
read n1 
echo "enter ur 2nd no." 
read n2 
while [ $x = "y" ] 
do 
clear 
echo "1.sum" 
echo "2.subtraction" 
echo "3.product" 
echo "4.quotient" 
echo "5.exit" 
echo -n "Enetr ur choice between [1-5]" 
read ch; 

case $ch in 
1) sum = `expr $n1 + $n2` 
echo "the sum of two nos. provided by u is $sum";; 

2) subtraction = `expr $n1 - $n2` 
echo "the remainder after subtrating the smaller value from bigger one is $subtraction";; 

3) product = `expr $n1 \* $n2` 
echo "The product of 2 nos. is " $product;; 

4)quotient = `expr $n1 / $n2` 
echo "the quotient of two nos. is" $quotient;; 

5) exit 
echo "by-2" $USERNAME 
esac 
done

enter ur 1st no.
3
enter ur 2nd no.
4
1.sum
2.subtraction
3.product
4.quotient
5.exit
-n Enetr ur choice between [1-5]
1
sum: = No such file or directory
sum: 7 No such file or directory
the sum of two nos. provided by u is
1.sum
2.subtraction
3.product
4.quotient
5.exit
-n Enetr ur choice between [1-5]
5

case $ch in 
1) sum=`expr $n1 + $n2` 
echo "the sum of two nos. provided by u is $sum";; 
2) subtraction=`expr $n1 - $n2` 
echo "the remainder after subtrating the smaller value from bigger one is $subtraction";; 
3) product=`expr $n1 \* $n2` 
echo "The product of 2 nos. is " $product;; 
4)quotient=`expr $n1 / $n2` 
echo "the quotient of two nos. is" $quotient;; 
5) exit 
echo "by-2" $USERNAME 
esac 
done