Shell Programming and Scripting

#!/bin/bash
#Menu Script

clear
BLUE="\033[0;34;48m"
RED="\033[0;31;48m"
BLACK="\033[0;38;48m"

while true; do

echo "==Option Menu=="
echo "1 - Call a calculator"
echo "2 - Display files in user directory"
echo "3 - Display the contents of the file that user enters"
echo "4 - Sort the content of file that user enters"
echo "5 - Logout"
echo "Enter your option"
read option

case $option in

1)
echo ==Calculator Program==
echo "1 - Addition"
echo "2 - Subtraction"
echo "3 - Multiplication"
echo "4 - Division"
echo "5 - Exit"
echo "Please enter your option"
read option

case $option in

1)
echo ==Addition==;
echo Enter first number;
read a
echo Enter second number;
read b

op=`expr $a + $b`
if [ $op -ge 0 ]
then
echo -e Answer is: $BLUE"$op"$BLACK
else
echo -e Answer is: $RED"$op"$BLACK
fi
;;
2)
echo ==Subtraction==;
echo Enter first number;
read a
echo Enter second number;
read b

op=`expr $a - $b`
if [ $op -ge 0 ]
then
echo -e Answer is: $BLUE"$op"$BLACK
else
echo -e Answer is: $RED"$op"$BLACK
fi
;;
3)
echo ==Multiplication==
echo Enter first number;
read a
echo Enter second number;
read b

op=`expr $a \* $b`
if [ $op -ge 0 ]
then
echo -e Answer is: $BLUE"$op"$BLACK
else
echo -e Answer is: $RED"$op"$BLACK
fi
;;
4)
echo ==Division==
echo Enter first number;
read a
echo Enter second number;
read b

op=`expr $a / $b`
if [ $op -ge 0 ]
then
echo -e Answer is: $BLUE"$op"$BLACK
else
echo -e Answer is: $RED"$op"BLACK
fi
;;
5)
echo "Ok bye";
exit 0
;;
*)
echo "Enter valid option"
;;
esac
echo "Do you want to continue (Y/N)"
read opt;
echo $opt | grep -i 'y'
if [[ $? -eq 0 ]]
then
clear
else
exit 0
fi
;;
2)
echo Sort;
;;
3)
echo Delete;
;;
4)
echo Display;
;;
5)
echo "Ok bye";
exit 0
;;
esac
echo "Do you wnat to contine (Y/N)"
read opt;
echo $opt | grep -i 'y'
if [[ $? -eq 0 ]]
then
clear
else
exit 0
fi
done