Download Files from URL with Progress and Checksum Verification

#!/bin/bash

# Configuration
DOWNLOAD_URL="https://example.com/assets/latest-version.zip"
EXPECTED_SHA256="a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b2"
OUTPUT_DIR="./downloads"
OUTPUT_FILENAME="latest-version.zip"

# Full path for the downloaded file
FULL_OUTPUT_PATH="$OUTPUT_DIR/$OUTPUT_FILENAME"

# Ensure output directory exists
if [ ! -d "$OUTPUT_DIR" ]; then
  echo "Creating download directory: $OUTPUT_DIR"
  mkdir -p "$OUTPUT_DIR" || {
    echo "Error: Failed to create directory '$OUTPUT_DIR'." >&2
    exit 1
  }
fi

echo "Downloading '$DOWNLOAD_URL' to '$FULL_OUTPUT_PATH'..."

# Download the file using curl with progress bar
# -L: follow redirects
# -o: write output to specified file
# -s: silent (don't show progress by default, we use --progress-bar instead)
# --progress-bar: show a simple progress bar
curl -L -o "$FULL_OUTPUT_PATH" -s --progress-bar "$DOWNLOAD_URL" || {
  echo "Error: Download failed." >&2
  exit 1
}

echo "Download complete."

# Verify checksum if provided
if [ -n "$EXPECTED_SHA256" ]; then
  echo "Verifying file integrity with SHA256 checksum..."
  if command -v sha256sum &> /dev/null; then
    ACTUAL_SHA256=$(sha256sum "$FULL_OUTPUT_PATH" | awk '{print $1}')
    
    if [ "$ACTUAL_SHA256" = "$EXPECTED_SHA256" ]; then
      echo "Checksum matches! File integrity verified."
    else
      echo "Error: Checksum mismatch! Expected '$EXPECTED_SHA256', but got '$ACTUAL_SHA256'." >&2
      rm -f "$FULL_OUTPUT_PATH" # Remove potentially corrupted file
      exit 1
    fi
  else
    echo "Warning: 'sha256sum' command not found. Cannot verify checksum." >&2
  fi
else
  echo "No expected SHA256 checksum provided. Skipping integrity verification."
fi

echo "File downloaded and verified successfully."