Arithmetic Expressions

Arithmetic expressions can be evaluated in the following three ways (spaces are important!):

Using the expr utility

expr is a standard but somewhat deprecated program. The syntax is as follows:
expr 8 + 8
echo $(expr 8 + 8)

Using the $((...)) syntax

This is the built-in shell format. The syntax is as follows:
echo $((x+1))

Using the built-in shell command let.

The syntax is as follows: let x=( 1 + 2 ); echo $x

In modern shell scripts, the use of expr is better replaced with var=$((...)).