Download a file using intermediate .part file and automatically resume.

download_file.sh

#!/bin/bash

# Function to handle the download
download_file() {
  local delete_flag="${1:-false}"
  local url="$2"
  local filename="$3"

  # Extract filename from URL if not provided
  if [ -z "$filename" ]; then
    filename="${url##*/}"
    filename="${filename%%\?*}"
  fi

  if [ -f "$filename" ]; then
    # Retrieve remote file information
    remote_info=$(curl -sL -I "$url" | tr -d '\r')
    read -r _ remote_size <<< "$(grep -i '^Content-Length:' <<< "$remote_info")"
    remote_size="${remote_size##*: }"
    remote_size="${remote_size//[[:space:]]/}"
    read -r _ remote_modified <<< "$(grep -i '^last-modified:' <<< "$remote_info")"
    remote_modified="${remote_modified##*: }"
    remote_modified="${remote_modified//,/}"
    remote_timestamp=$(date -d "$remote_modified" +"%s")
    
    # Retrieve local file information
    read -r local_size _ <<< "$(stat -c '%s %n' "$filename")"
    read -r local_modified _ <<< "$(stat -c '%y %n' "$filename")"
    local_modified="${local_modified%% *}"
    local_timestamp=$(date -d "$local_modified" +"%s")
    
    # Compare file sizes and timestamps
    if [ "$remote_size" = "$local_size" ] && [ "$remote_timestamp" -le "$local_timestamp" ]; then
      echo "Message: File already downloaded."
      return
    elif [ "$remote_size" = "$local_size" ] && [ "$remote_timestamp" -gt "$local_timestamp" ]; then
      echo "Warning: The local file is different from the remote file."
      if [ "$delete_flag" = true ]; then
        echo "Deleting the local file..."
        rm "$filename"
      fi
    fi
  fi

  # Start or continue the download
  curl -C - -L -o "${filename}.part" --progress-bar "$url"

  # Check if the download was successful
  if [ "$?" -eq 0 ]; then
     mv "${filename}.part" "$filename"
     echo "Download completed!"
  fi
}

# Print the usage information
print_usage() {
  echo "Usage: $0 [-f] <url> [filename]"
  echo "Options:"
  echo "  -f    Delete the local file if it differs from the remote file"
}

# Process command line options
if [[ $1 == "-f" ]]; then
  delete_flag=true
  shift
fi

while getopts ":h" opt; do
  case $opt in
    h)
      print_usage
      exit 0
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
  esac
done

shift "$((OPTIND - 1))"

# Check if the required arguments are provided
if [ $# -lt 1 ]; then
  echo "Error: Missing URL argument."
  print_usage
  exit 1
fi

# Call the main logic function
download_file "$delete_flag" "$1" "$2"