A function that attempts to list the dns servers that are used by a host

dnslist

dnslist() {
  case $(uname) in
    (Darwin)
      printf -- '%s\n' "Attempting lookup test using 'scutil' command..." >&2
      scutil --dns |
        awk '/nameserver/{ a[$3]++} END { for (b in a) {print b } }'
      return 0
    ;;
    ([Ll]inux)
      # TODO: Update to test against IP addresses rather than 'Global'
      if command -v resolvectl >/dev/null 2>&1; then
        printf -- '%s\n' "Attempting lookup test using 'resolvectl' command..." >&2
        # shellcheck disable=SC2046
        set -- $(resolvectl dns | grep Global)
        printf -- '%s\n' "${@:2}"
        return 0
      fi
      if command -v systemd-resolv >/dev/null 2>&1; then
        printf -- '%s\n' "Attempting lookup test using 'systemd-resolve' command..." >&2
        # shellcheck disable=SC2046
        set -- $(
          systemd-resolve --status | 
            awk '/DNS Server:/{flag=1;next}/DNS Domain/{flag=0}flag' | 
            paste -sd ' ' - | 
            tr -s '[:space:]'
        )
        printf -- '%s\n' "${@:3}"
        return 0
      fi
      if command -v nmcli >/dev/null 2>&1; then
        # printf -- '%s\n' "Attempting lookup test using 'nmcli' command..." >&2
        :
      fi
    ;;
  esac
  if command -v host >/dev/null 2>&1; then
    printf -- '%s\n' "Attempting lookup test using 'host' command..." >&2
    host -v something.unknown | awk -F "[ #]" '/Received /{print$5}' | uniq
    return 0
  fi
  if [ -r /etc/resolv.conf ]; then
    printf -- '%s\n' "Parsing /etc/resolv.conf..." >&2
    awk '/nameserver/{print $2}' /etc/resolv.conf
    return 0
  fi
  # If we get to this point, we have failed.
  printf -- '%s\n' "Unable to determine any dns servers" >&2
  return 1
}