Download File if Not Exists or Outdated

#!/bin/bash

# Function to display usage
usage() {
  echo "Usage: $0 <url> <destination_file>"
  echo "  <url>: The URL of the file to download."
  echo "  <destination_file>: The local path where the file should be saved."
  exit 1
}

# Check for required arguments
if [ -z "$1" ] || [ -z "$2" ]; then
  usage
fi

URL="$1"
DEST_FILE="$2"
DEST_DIR=$(dirname "$DEST_FILE")

# Create destination directory if it doesn't exist
mkdir -p "$DEST_DIR"

if [ -f "$DEST_FILE" ]; then
  echo "Local file '$DEST_FILE' exists. Checking if remote is newer..."
  # Use curl's -z (if-modified-since) to only download if remote is newer
  curl -sf -z "$DEST_FILE" -o "$DEST_FILE.tmp" "$URL"
  CURL_STATUS=$?

  if [ $CURL_STATUS -eq 0 ]; then
    # curl -z returns 0 even if no change if it doesn't actually download, but it also downloads to .tmp
    # Check if the .tmp file was actually created and has content
    if [ -s "$DEST_FILE.tmp" ]; then
      mv "$DEST_FILE.tmp" "$DEST_FILE"
      echo "File updated successfully."
    else
      # No new content downloaded (HTTP 304 or similar, or empty response)
      rm -f "$DEST_FILE.tmp" # Clean up empty temp file
      echo "Local file is already up to date or no newer version available."
    fi
  elif [ $CURL_STATUS -eq 33 ]; then # HTTP 304 Not Modified (curl specific error code for -z)
    echo "Local file is already up to date."
  else
    echo "ERROR: Failed to check/update file (curl exit code: $CURL_STATUS)."
    rm -f "$DEST_FILE.tmp" # Clean up potential partial temp file
    exit 1
  fi
else
  echo "Local file '$DEST_FILE' does not exist. Downloading..."
  # Directly download the file
  curl -sf -o "$DEST_FILE" "$URL"
  CURL_STATUS=$?

  if [ $CURL_STATUS -eq 0 ]; then
    echo "File downloaded successfully."
  else
    echo "ERROR: Failed to download file (curl exit code: $CURL_STATUS)."
    exit 1
  fi
fi