DOWNLOAD_COMMAND=""
function set_download_command () {
  # Try curl.
  if command -v curl > /dev/null; then
    if curl --version | grep Protocols  | grep https > /dev/null; then
      DOWNLOAD_COMMAND="curl -fLSs --retry 5 --retry-delay 1 --retry-connrefused"
      return
    fi
    echo curl does not support https, will try wget for downloading files.
  else
    echo curl is not installed, will try wget for downloading files.
  fi