#!/bin/sh
suffix="${2:-bak}"
orig="${1%%.$suffix}"
smv () {
  eval source=\$$(($#-1))
  if test -w "$source"
  then mv -v "$@"
  else sudo mv -v "$@"
  fi
}
if test -e "$orig.$suffix"
then
  test -e "$orig" && smv "$orig" /tmp
  smv -n "$orig.$suffix" "$orig"
else
  smv -n "$1" "$1.$suffix"
fi