Simple caesar cipher bash script

caesar

#!/bin/sh

if [ $# -ne 2 ] || [ $1 -lt -36 ] || [ $1 -gt 35 ]
then
echo 'Please specify how many random words would you like to generate !' 1>&2
echo 'example: caesar 5 "mjh5i9n 9pio 8jhpn"' 1>&2
echo 'This will do a caesar cipher of 5 places on the given string, returning romanes eunt domus' 1>&2
exit 0
fi

nr=$1

if [ $nr -lt 0 ]
then
  nr=$((nr+36))
fi

echo $2 | tr $(printf %${nr}s | tr ' ' '.')\0-9a-z 0-9a-z0-9a-z