httpdate.sh - Get date information from http.

httpdate.sh

#!/usr/bin/env bash
#
# httpdate.sh - Get date information from http
# Copyright (c) 2017 Koichi OKADA. All rights reserved.
# This script is distributed under the MIT license.
#

function help ()
{
  cat<<-EOD
	Usage: "$(basename "$0")" [options] [URL]
	Options:
	  -u, --url URL    Set url to get date. 
	
	EOD
}

ARGS=()
OPT_URL=https://google.com/

while (( 0 < $# )); do
  case "$1" in
    -h|--help)
      OPT_HELP="$1"
      shift
      ;;
    -u|--url)
      OPT_URL="$1"
      shift
      ;;
    --offset)
      OPT_OFFSET="$2"
      shift 2
      ;;
    --*)
      echo "ERROR: unknown option ${1}"
      exit 1
      ;;
    --)
      ARGS+=( "${@:2}" )
      shift $#
      ;;
    *)
      ARGS+=( "$1" )
      shift
      ;;
  esac
done

if [ -n "$OPT_HELP" ] || (( 1 < ${#ARGS[@]} )); then
  help
  exit 1
fi

if (( 1 == ${#ARGS[@]} )); then
  OPT_URL="${ARGS[0]}"
fi

HTTP="$(wget -qSO/dev/null "$OPT_URL" |& grep Date: | tail -n1 |sed 's/.*Date: *//g')"
LOCAL="$(LANG=C date)"

OFFSET="$(( $(date -d "$LOCAL" +%s) - $(date -d "$HTTP" +%s) + ${OPT_OFFSET:-0} ))"
OFFSET_SIGN="$( (( OFFSET < 0 )) && echo -n - || echo -n +)"
OFFSET_ABS="$(( ${OFFSET_SIGN}1 * OFFSET ))"
OFFSET_HR="$(\
  printf "%s%d d %02d:%02d:%02d" \
    "${OFFSET_SIGN}" \
    $(( OFFSET_ABS / 86400 )) \
    $(( OFFSET_ABS % 86400 / 3600 )) \
    $(( OFFSET_ABS % 3600 / 60 )) \
    $(( OFFSET_ABS % 60 )) )"

echo -n "http   : "; date -d "$HTTP"
echo -n "local  : "; date -d "$LOCAL"
echo    "offset : $OFFSET s ($OFFSET_HR)"