#!/usr/bin/env bash  

_buffer() {
  [[ -n "$1" ]] && return 1
  "${FUNCNAME[1]}" "$(</dev/stdin)"
}

rot13(){
  _buffer "$@" && return $?
  tr '[A-Za-z]' '[N-ZA-Mn-za-m]' <<< "$*"
}

rot13 "$*"