因为偶尔遇到由于各种原因，我们在使用ntpdate的时候无法成功的跟服务器同步时间的现象，而一般情况下我们对时间的精度要求都不是太高，所以想到可以使用根据HTTP头里面的信息来校时的做法。
之前看到一个使用php编写的脚本，但是考虑到很多机器是没有php环境的，因此选用shell来进行代码的编写，尽量保证通用。

	#!/bin/bash
	#Set localtime based on server's http response time
	#By Orange
	host="www.google.com"
	#server host name
	timeout="5"
	#acceptable time out for curl
	proxyHost=""
	proxyPort=""
	proxyUser=""
	proxyPass=""
	#define proxy settings
	export TIMEFORMAT=$'Time cost: %lR'
	header=$((time curl --proxy "$proxyHost:$proxyPort" --proxy-user "$proxyUser:$proxyPass" --connect-timeout $timeout --silent --head $host) | tr -d '\r' 2>&1)
	#command to get server's http response header
	error_code=$?
	if [[ $error_code -ne 0 ]]; then
	echo "cUrl error on code $error_code"
	exit $error_code
	fi
	#Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
	regex1="Date: ((Mon|Tue|Wed|Thu|Fri|Sat|Sun), [0-9]{2} (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec) [0-9]{4} [0-9]{2}:[0-9]{2}:[0-9]{2} GMT)"
	#Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
	regex2="Date: ((Mon|Tues|Wednes|Thurs|Fri|Satur|Sun)day, [0-9]{2}-(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2} GMT)"
	#Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
	regex3="Date: ((Mon|Tue|Wed|Thu|Fri|Sat|Sun) (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec) *[0-9]{1,2} [0-9]{1,2}:[0-9]{2}:[0-9]{2} [0-9]{4})"
	#use regex to Match time in server's http response header
	if [[ $header =~ $regex1 ]]; then
	server_time=${BASH_REMATCH[1]}
	#get date time
	elif [[ $header =~ $regex2 ]]; then
	server_time=${BASH_REMATCH[1]}
	#get date time
	elif [[ $header =~ $regex3 ]]; then
	server_time=${BASH_REMATCH[1]}
	#get date time
	else
	error_code=$?
	echo "Get time from server failed! Error code $error_code"
	exit $error_code
	fi
	#date_str=$(date --date="$server_time")
	#convert date format
	set_result=$(date -s "$server_time")
	#set local time
	error_code=$?
	if [[ $error_code -ne 0 ]]; then
	echo "Set time with date -s error on code $error_code"
	exit $error_code
	else
	echo $set_result
	regex="(Time cost: [0-9]*m[0-9]*\.[0-9]*s)"
	if [[ $header =~ $regex ]]; then
	echo ${BASH_REMATCH[1]}
	fi
	fi

view raw http_time.sh hosted with by GitHub

PS：这篇文章正好测试一下Gist插件，看看效果如何。