let - evaluate arithmetic expressions
let [expression ...]
This command is a Korn-shell built-in command.
Each expression is evaluated (see the section on arithmetic expressions in ksh(1)). If all expressions are successfully evaluated, the exit status is 0 (zero) if the last expression evaluated to non-zero (zero); and the exit status is 1 (one) if the last expression evaluated to 0. If an error occurs during the parsing or evaluation of an expression, the exit status is greater than 1. Since expressions might need to be quoted, (( expr )) is syntactic sugar for let "expr".