Perform arithmetic on shell variables.
Syntax let expression [expression]
Each expression is an arithmetic expression to be evaluated. If the last expression evaluates to 0,
returns 1; otherwise 0 is returned.
`let' is a bash builtin command.
"Arithmetic is being able to count up to twenty without taking off your shoes" ~ Mickey Mouse
expr - Evaluate expressions
eval - Evaluate several commands/arguments
test - Evaluate a conditional expression