Backup files with sequential numbering and optional tag

bckfile

#!/bin/sh
# Backup files with sequential numbering and optional tag

# Display usage if given the incorrect number of parameters
if [ $# -ne 2 -a $# -ne 3 ]; then
  echo "${0##*/} [file] [dir] [*tag]- Backup files with sequential numbering, opt. tag"
  exit 1; fi

# Test if first and second parameters are a file and a directory
if [ ! -e "$1" ]; then
  echo "${0##*/}: non-file: "$1""
  error=1; fi
if [ ! -d "$2" ]; then
  echo "${0##*/}: non-dir.: "$2""
  error=1; fi
[ -n "$error" ] && exit 1

# Filename: path, dirname, name, basename (name w/o number, tag, ext.), ext.
fpth="$(realpath "$1")"
fdnm="${fpth%/*}"
fnme="${fpth##*/}"
fbse="${fnme%.*}"; fbse="${fbse%_[0-9][0-9]*}"
fext="${fnme##*.}"
if [ "$fext" = "$fnme" ]; then
  fext=""
else
  fext=".$fext";fi

# Set sequential number (parse out number previous, add to for number next)
fpre=$(ls -1 "$fdnm"/"$fbse"_[0-9][0-9]* 2> /dev/null) # File backups previous
if [ $? -eq 0 ]; then
  npre=$(echo "${fpre##*/}" | tail -n 1 | grep -o ^"$fbse"_[0-9][0-9] | grep \
         -o [0-9][0-9] )
  nnxt=$(printf "%02u" $((++npre)))
else
  nnxt=00
fi

# Define admin. program if write permission is needed
if [ ! -w "$2" ]; then
  [ ! hash sudo 2>&- ] && { echo "${0##*/}: program required: sudo"; exit 1; }
  sudo=sudo; fi

# Assign tag
[ -n "$3" ] && \
tag="-$3"   && \
tag="$(printf "%q" "$tag")"

# Backup file
$sudo cp -av "$fpth" "$fdnm"/"$fbse"_"$nnxt""$tag""$fext"