sh - function

#! /bin/bash

read -p "fisrt:  " x
read -p "second:  " y

plus ( )
{ expr $x + $y
}

minus ( )
{ expr $x - $y
}

read -p "choose the operation; + -  " out

case $out in
+)
echo "total is: `plus $x $y`" ;;
-)
echo "total is: `minus $x $y`" ;;
esac