在linux下获取时间字符串
命令 date
# 以yyyymmdd格式输出23天之前现在这个时刻的时间
$ date +%Y%m%d –date=’23 days ago’

$ date -u
Thu Sep 28 09:32:04 UTC 2006

$ date -R
Thu, 28 Sep 2006 17:32:28 +0800

# 测试十亿分之一秒
$ date +’%Y%m%d %H:%M:%S.%N’;date +’%Y%m%d %H:%M:%S.%N’;date +’%Y%m%d %H:%M:%S.%N’;date +’%Y%m%d %H:%M:%S.%N’
20060928 17:44:20.906805000
20060928 17:44:20.909188000
20060928 17:44:20.911535000
20060928 17:44:20.913886000
date 参考
$ date –help
Usage: date [OPTION]… [+FORMAT]
or: date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
Display the current time in the given FORMAT, or set the system date.

-d, –date=STRING display time described by STRING, not `now’
# such as ‘n days ago |1 month ago|n years ago’
-f, –file=DATEFILE like –date once for each line of DATEFILE
-ITIMESPEC, –iso-8601[=TIMESPEC] output date/time in ISO 8601 format.
TIMESPEC=`date’ for date only,
`hours’, `minutes’, or `seconds’ for date and
time to the indicated precision.
–iso-8601 without TIMESPEC defaults to `date’.
-r, –reference=FILE display the last modification time of FILE
-R, –rfc-2822 output RFC-2822 compliant date string
-s, –set=STRING set time described by STRING
-u, –utc, –universal print or set Coordinated Universal Time
–help display this help and exit
–version output version information and exit

FORMAT controls the output. The only valid option for the second form
specifies Coordinated Universal Time. Interpreted sequences are:

%% 输出%符号 a literal %
%a 当前域的星期缩写 locale’s abbreviated weekday name (Sun..Sat)
%A 当前域的星期全写 locale’s full weekday name, variable length (Sunday..Saturday)
%b 当前域的月份缩写 locale’s abbreviated month name (Jan..Dec)
%B 当前域的月份全称 locale’s full month name, variable length (January..December)
%c 当前域的默认时间格式 locale’s date and time (Sat Nov 04 12:02:33 EST 1989)
%C n百年 century (year divided by 100 and truncated to an integer) [00-99]
%d 两位的天 day of month (01..31)
%D 短时间格式 date (mm/dd/yy)
%e 短格式天 day of month, blank padded ( 1..31)
%F 文件时间格式 same as %Y-%m-%d
%g the 2-digit year corresponding to the %V week number
%G the 4-digit year corresponding to the %V week number
%h same as %b
%H 24小时制的小时 hour (00..23)
%I 12小时制的小时 hour (01..12)
%j 一年中的第几天 day of year (001..366)
%k 短格式24小时制的小时 hour ( 0..23)
%l 短格式12小时制的小时 hour ( 1..12)
%m 双位月份 month (01..12)
%M 双位分钟 minute (00..59)
%n 换行 a newline
%N 十亿分之一秒 nanoseconds (000000000..999999999)
%p 大写的当前域的上下午指示 locale’s upper case AM or PM indicator (blank in many locales)
%P 小写的当前域的上下午指示 locale’s lower case am or pm indicator (blank in many locales)
%r 12小时制的时间表示（时:分:秒,双位） time, 12-hour (hh:mm:ss [AP]M)
%R 24小时制的时间表示 （时:分,双位）time, 24-hour (hh:mm)
%s 自基础时间 1970-01-01 00:00:00 到当前时刻的秒数 seconds since `00:00:00 1970-01-01 UTC’ (a GNU extension)
%S 双位秒 second (00..60); the 60 is necessary to accommodate a leap second
%t 横向制表位(tab) a horizontal tab
%T 24小时制时间表示 time, 24-hour (hh:mm:ss)
%u 数字表示的星期（从星期一开始 1-7）day of week (1..7); 1 represents Monday
%U 一年中的第几周星期天为开始 week number of year with Sunday as first day of week (00..53)
%V 一年中的第几周星期一为开始 week number of year with Monday as first day of week (01..53)
%w 一周中的第几天 星期天为开始 0-6 day of week (0..6); 0 represents Sunday
%W 一年中的第几周星期一为开始 week number of year with Monday as first day of week (00..53)
%x 本地日期格式 locale’s date representation (mm/dd/yy)
%X 本地时间格式 locale’s time representation (%H:%M:%S)
%y 两位的年 last two digits of year (00..99)
%Y 年 year (1970…)
%z RFC-2822 标准时间格式表示的域 RFC-2822 style numeric timezone (-0500) (a nonstandard extension)
%Z 时间域 time zone (e.g., EDT), or nothing if no time zone is determinable

By default, date pads numeric fields with zeroes. GNU date recognizes
the following modifiers between `%’ and a numeric directive.

`-’ (hyphen) do not pad the field
`_’ (underscore) pad the field with spaces