Linux in a Nutshell, 6th Edition

Arithmetic Expressions

The let built-in command performs integer arithmetic. You can substitute arithmetic values (for use as command arguments or in variables); base conversion is also possible.

Expression	Meaning
$(( expr ))	Use the value of the enclosed arithmetic expression.
B# n	Interpret integer n in numeric base B. For example, 8#100 specifies the octal equivalent of decimal 64.