Options and positional argument contracts

The idea

getopts parses short options using the shell’s positional parameters and updates OPTIND as it proceeds. An option followed by a colon in the option string requires an argument. After parsing, shifting by OPTIND minus one exposes the remaining operands. A reusable function should manage OPTIND deliberately so previous parsing does not affect a later independent call.

The command interface should define required values, defaults, unknown-option behavior, and whether options may precede operands. A double dash marks the end of option parsing for conventional interfaces, allowing following data to begin with a hyphen. Validate option values after syntactic parsing: recognizing that -n has an argument does not establish that the argument is a supported nonnegative integer or within a safe range.

set -- -n 3 alpha
OPTIND=1
limit=1
while getopts ":n:" option; do
 case $option in n) limit=$OPTARG ;; *) exit 2 ;; esac
done
shift "$((OPTIND-1))"
printf '%s:%s\n' "$limit" "$1"

3:alpha

The option value becomes three, and shifting the consumed parameters leaves alpha as the first operand.