shell script to add two numbers

add1.sh

#!/bin/bash  
# Above line to specify which shell to be used to execute the program

echo "Enter first Number"  # echo used to print the message to screen
read first_number # read and store the input from terminal
echo "enter second Number"
read second_number

sum=`expr $first_number + $second_number`  # things insed the backtick ` are evaluate or executed 
echo "$first_number + $second_number = $sum"  # $ to specify arguments

# give execution permission to file before running the program like bellow
# chmod +x add1.sh
# run the program by ./add1.sh