Caesar cipher in POSIX shell using cut and tr

caesar.sh

#!/bin/sh
#
# https://en.wikipedia.org/wiki/Caesar_cipher

KEY=${1:-3}
A=abcdefghijklmnopqrstuvwxyz
B=$(echo $A | cut -b${KEY}- | tr -d '\n'; echo $A | cut -b-$[KEY-1])
AU=$(echo $A | tr '[a-z]' '[A-Z]')
BU=$(echo $B | tr '[a-z]' '[A-Z]')

tr "[${A}${AU}]" "[${B}${BU}]"

$ sh caesar.sh 7
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