#!/bin/sh

offset=${1:-13}

if [ "$offset" -lt 1 ]; then
  echo "offset must be greater than zero" >&2
  exit 1
elif [ "$offset" -gt 25 ]; then
  echo "offset must be less than 26" >&2
  exit 1
fi

alphabet=aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ
sub1=$(echo "$alphabet" | cut -c $((offset * 2 + 1))-)
sub2=$(echo "$alphabet" | cut -c 1-$((offset * 2)))
sub=$sub1$sub2

tr "$alphabet" "$sub"