A simple backup script.

backer.sh

#!/bin/bash
# AUTHOR
#  2013/10/08  Nobu C. Shirai
#  2018/10/06  Akira Hasegawa
description='
Creating backup files in the arguments in each backup directory.
The name of each backup file with backup directory is 
"org_filename.dir/X_org_filename" where "X" is the index number
of backups which starts from 0.

In the case that there is a directory named "back" in the current
directory (or in the specified directory by "-d" option),
the backup file is created under back/org_filename.dir/.
'

backer(){
  check_options $@
  Ind=1
  for TargetFile in ${ArgFiles[@]}
  do
    echo "[${Ind}] $TargetFile"
    if [[ ! -f "$TargetFile" ]]; then
      echo "$TargetFile does not exist"
      break
    fi
    setBackDir "$TargetFile"

    # Set NewBackFile and OldBackFile
    prefix_no="$(( $( find "${BackDir}" -regex ".*/[0-9][0-9]*_${FileName}" | wc -l ) ))"
    NewBackFile="${prefix_no}_${FileName}"
    OldBackFile=""
    if (( $prefix_no != 0 )); then
      OldBackFile="$(( prefix_no - 1 ))_${FileName}"
    fi

    # Backup TargetFile as $BackDir/$NewBackFile
    diff -q "$TargetFile" "$BackDir/$OldBackFile" 1>/dev/null 2>&1
    if (( $? != 0 )); then
      cp -vp "$TargetFile" "$BackDir/$NewBackFile"
    else
      echo " This file had already backuped. ($OldBackFile)"
    fi
  ((Ind++))
  done
}

check_options(){
  usage_msg1="\
Usage: $0 [options] files
"
  
  usage_msg2="\

Options:
  -h    show this help
  -d dir  select PATH to backup (default is current directory)
  -c    make backups at current directory
"
  TargetDir=""
  while getopts hd:c option
  do
    case "$option" in
    h)
      echo "$usage_msg1""$description""$usage_msg2"
      exit 0
      ;;
    d)
      TargetDir="$OPTARG"
      if [[ -d "$OPTARG" ]]; then
        echo "TargetDir = $OPTARG" 
      else
        echo "$OPTARG does not exist."
        exit 1
      fi
      ;;
    c)
      TargetDir="."
      ;;
    \?)
      echo "Usage: $0 [-h] [-d dir] files" 1>&2
      exit 1;;
    esac
  done
  shift `expr "$OPTIND" - 1`
  ArgFiles=($@)
  if (( ${#ArgFiles} == 0 )); then
    echo "$usage_msg1""$usage_msg2"
    exit 1
  fi
  if [[ -d "$TargetDir/Back" ]]; then
    TargetDir="$TargetDir/Back"
  elif [[ -d "$TargetDir/back" ]]; then
    TargetDir="$TargetDir/back"
  elif [[ -d "$TargetDir/BACK" ]]; then
    TargetDir="$TargetDir/BACK"
  else
    TargetDir="$TargetDir"
  fi
}

setBackDir(){
  file="$1"
  slash_count=`echo $file | grep -c "/"`
  if (( $slash_count != 0 )); then
    FileDir="${file%/*}"
  else
    FileDir="."
  fi
  FileName="${file##*/}"
  BackDirName="${FileName}.dir"
  if [[ -z "$TargetDir" ]]; then
    if [[ -d "$FileDir/Back" ]]; then
      BackDir="$FileDir/Back/$BackDirName"
    elif [[ -d "$FileDir/back" ]]; then
      BackDir="$FileDir/back/$BackDirName"
    elif [[ -d "$FileDir/BACK" ]]; then
      BackDir="$FileDir/BACK/$BackDirName"
    else
      BackDir="$FileDir/$BackDirName"
    fi
  else
    BackDir="$TargetDir/$BackDirName"
  fi
  mkdir_idne $BackDir
}

mkdir_idne(){
  if [[ ! -d $1 ]]; then
    mkdir $1
    echo "---> $1 was created."
  fi
}

backer $@