rithmetic expression parser enables simple arithmetic to be
performed at the CLI, and for that arithmetic to be tested during
logical tests such as those shown above. Arithmetic expressions are
enclosed within '((' and '))', unless the 'let' builtin is used
for evaluation.

In addition, one can use variables without prefixing them with a '$'
character within arithmetic expressions.

The result of an arithmetic expression is considered TRUE if it is
non-zero, and FALSE if it is 0.

As an example, one might want to loop a given number of times.

 router> x=0
 router> while (( x-- )); do something; done

For more information, please see the man pages for '((' and for 'let'.