Values can be read from the command line using the
read command. See the following code.
read a read b echo $a echo $b
Invoked from the command line:
echo '1\n2'|./read.sh 1 2
To perform an operation, surround the variables with
 and precede with a
read a read b echo $[a+b] echo $((a-b))
A relevant and interesting discussion on Stack-Exchange for those getting started with handling numbers and arithmetic operations in Bash.