Bash script to backup directory while keeping only X backups

directory_backup.sh

#!/bin/bash

# Check if the input directory, output directory, and filename prefix are provided
if [ "$#" -ne 4 ]; then
        echo "Makes backup of <input_directory> keeping last <retention> number of backups in <output directory>"
        echo "Usage: $0 <input_directory> <output_directory> <filename_prefix> <retention>"
  exit 1
fi

input_dir="$1"
output_dir="$2"
filename_prefix="$3"
max_backups="$4"

# Generate the date and time prefix
date_time_prefix=$(date +%Y%m%d_%H%M%S)

# File to store the last used backup ID
last_id_file="${output_dir}/last_id.txt"

# Check if the last ID file exists
if [ -f "$last_id_file" ]; then
  # Read the last used ID from the file
  last_id=$(cat "$last_id_file")
else
  # If the file doesn't exist, start with ID 1
  last_id=0
fi

# Increment the ID
backup_id=$((last_id + 1))

# Update the last ID file with the new ID
echo "$backup_id" > "$last_id_file"

# Create the backup directory if it doesn't exist
mkdir -p "$output_dir"

# Create the tar backup, ignoring failed reads
tar  --use-compress-program=zstdmt -cf "${output_dir}/${date_time_prefix}_${filename_prefix}_${backup_id}.tar.zst" --ignore-failed-read "$input_dir"

# Set the exit code to 0
exit_code=0

# Print the backup ID
echo "Backup created: ${date_time_prefix}_${filename_prefix}_${backup_id}"

# Delete old backups if there are more than the maximum number of backups
backup_files=("${output_dir}/"*.tar.zst)
num_backups=${#backup_files[@]}
if [ "$num_backups" -gt "$max_backups" ]; then
  num_to_delete=$((num_backups - max_backups))
  for (( i=0; i < num_to_delete; i++ )); do
    rm "${backup_files[$i]}"
    echo "Deleted backup: ${backup_files[$i]}"
  done
fi

# Exit with the tar command's exit code
exit "$exit_code"