#!/bin/bash

SIZE=${1:-12}
re='^[0-9]+$'
if ! [[ $SIZE =~ $re ]]; then
	echo "error: SIZE=$SIZE Not a number" >&2
	exit 1
fi
if [[ $SIZE -lt 4 || $SIZE -gt 20 ]]; then
	echo "expected SIZE=$SIZE not in range [4..20]" >&2
	exit 1
fi

tr -cd '[:alnum:]' </dev/urandom | fold -w "$SIZE" | head -n1