#!/bin/bash

# Script for performing incremental backups using rsync.

# Set the script to exit if any command returns a non-zero status.
set -o errexit
# Set the script to treat unset variables as errors.
set -o nounset
# Set the script to exit if a pipeline returns a non-zero status.
set -o pipefail

# Define the source directory you want to back up.
readonly SOURCE_DIR="<input folder>"

# Define the directory where the backups will be stored.
readonly BACKUP_DIR="<output folder>"

# Get the current date and time in the format 'YYYY-MM-DD_HH:MM:SS'.
readonly DATETIME="$(date '+%Y-%m-%d_%H:%M:%S')"

# Create a path for the current backup using the datetime.
readonly BACKUP_PATH="${BACKUP_DIR}/${DATETIME}"

# Create a symbolic link called 'latest' that points to the most recent backup.
readonly LATEST_LINK="${BACKUP_DIR}/latest"

# Ensure that the backup directory exists.
mkdir -p "${BACKUP_DIR}"

# Use rsync to perform the backup:
# - 'ahv' preserves permissions, owner, group, and provides verbose output.
# - '--progress' shows progress during transfer.
# - '--stats' shows statistics at the end of the backup.
# - '--delete' removes files from the backup that are no longer present in the source.
# - '--log-file' specifies a log file for recording backup details.
# - '--exclude' excludes the '.cache' directory from being backed up.
# - '--link-dest' specifies a directory with previous backups, enabling hard-linking of unchanged files.
# - "${SOURCE_DIR}/" is the source directory to be backed up.
# - "${BACKUP_PATH}" is the destination directory for the current backup.
rsync -ahv --progress --stats --delete \
  --log-file="${BACKUP_DIR}/${DATETIME}.log" \
  --exclude=".cache" \
  --link-dest "${LATEST_LINK}" \
  "${SOURCE_DIR}/" \
  "${BACKUP_PATH}"

# Remove the symbolic link 'latest' to the old backup.
rm -rfv "${LATEST_LINK}"

# Create a new symbolic link 'latest' pointing to the most recent backup.
ln -s "${BACKUP_PATH}" "${LATEST_LINK}"