diffBackup.sh

#!/bin/bash

# path to tar file
# 1 input: path
# stdout: tar file
pathToTar() {
  path="$1"
  if [[ "$path" == /* ]]
  then
    tar -cf - --ignore-failed-read -C / "${path:1}"
  else
    tar -cf - --ignore-failed-read "$path"
  fi
}

# tar file to path
# 1 output: path
# stdin: tar file
tarToPath() {
  tar -C "$1" -xf -
}

# convert file to hex file
# stdin: file
# stdout: hex file
toHex() {
  xxd -ps -c 8
}

# convert hex file to file
# stdin: hex file
# stdout: file
fromHex() {
  xxd -r -ps
}

# file1 -> bin diff -> file2
# 1 input: file1
# 2 input: file2
# stdout: bin diff
binDiff() {
  diff <(cat "$1" | toHex) <(cat "$2" | toHex) | compress
}

# file1 -> bin diff -> file2
# stdin: bin diff
# 1 input: file1
# stdout: file2
applyBinDiff() {
  tmpfile="$(mktemp)"
  cat "$1" | toHex > "$tmpfile"
  decompress | patch -N -o - "$tmpfile" 2>/dev/null | fromHex
  rm "$tmpfile"
}

# file to compressed file
# stdin: file
# stdout: compressed file
compress() {
  gzip
}

# compressed file to file
# stdin: compressed file
# stdout: file
decompress() {
  gunzip
}

# 1 input: backup path
# stdout: full backup archive
pathToFullBackup() {
  pathToTar "$1" | tee >(wc -c >&2) | compress | tee >(wc -c >&2)
}

# stdin: full backup archive
# 1 input: backup path
# stdout: diff backup archive
pathToDiffBackup() {
  decompress | binDiff - <(pathToTar "$1" | tee >(wc -c >&2)) | tee >(wc -c >&2)
}

# stdin: diff backup archive
# 1 input: full backup archive
# stdout: full backup tar
diffToFullBackupTar() {
  applyBinDiff <(cat "$1" | decompress)
}

# Usage:
# backup                     [path] [archive]: create full backup
# backup    -d [full backup] [path] [archive]: create differential backup
# backup -r                  [path] [archive]: restore full backup
# backup -r -d [full backup] [path] [archive]: restore differential backup

# Cron example:
# 0 1 1 * * backup "/etc" "/var/backup-full-$(date '+%Y%m%d%H%M%S')"
# 0 4 * * * backup -d "/var/$(ls -1 "/var" | grep "backup-full-" | sort -rn | head -1)" "/etc" "/var/backup-$(date '+%Y%m%d%H%M%S')"

if [[ "$1" == "-r" ]]; then
  # restore
  shift
  if [[ "$1" == "-d" ]]; then
    # differential
    fullBackupPath="$2"
    shift 2

    cat "$2" | diffToFullBackupTar "$fullBackupPath" | tarToPath "$1"
  else
    # full
    cat "$2" | decompress | tarToPath "$1"
  fi
else
  # create
  log=$((
    if [[ "$1" == "-d" ]]; then
      # differential
      fullBackupPath="$2"
      shift 2

      cat "$fullBackupPath" | pathToDiffBackup "$1" > "$2"
    else
      # full
      pathToFullBackup "$1" > "$2"
    fi
  ) 3>&2 2>&1 1>&3) 2>&1

  sizePre=$(echo "$log" | cut -d $'\n' -f 1)
  sizePost=$(echo "$log" | cut -d $'\n' -f 2)

  echo "Size before compression: $sizePre"
  echo "Size after compression:  $sizePost"
  echo "Compression ratio: $(( $sizePre / $sizePost ))"
fi