shell date 詳細用法 如date --date='1 month ago' '+%Y%m'

1.To print the date of the day before yesterday:
          date --date='2 days ago' 
2. To print the date of the day three months and one day hence:
          date --date='3 months 1 day'
3. To print the day of year of Christmas in the current year:
          date --date='25 Dec' +%j
4. To print the current full month name and the day of the month:
          date '+%B %d'
But this may not be what you want because for the first nine days
of the month, the `%d' expands to a zero-padded two-digit field,
for example `date -d 1may '+%B %d'' will print `May 01'.


5. To print a date without the leading zero for one-digit days of the
     month, you can use the (GNU extension) `-' flag to suppress the
     padding altogether:
          date -d 1may '+%B %-d


6. To print the current date and time in the format required by many
     non-GNU versions of `date' when setting the system clock:
          date +%m%d%H%M%Y.%S
7. To set the system clock forward by two minutes:
          date --set='+2 minutes'


8. To print the date in RFC 2822 format, use `date --rfc-2822'.  Here
is some example output:
          Fri, 09 Sep 2005 13:51:39 -0700


9. To convert a date string to the number of seconds since the epoch
(which is 1970-01-01 00:00:00 UTC), use the `--date' option with
the `%s' format.  That can be useful in sorting and/or graphing
and/or comparing data by date.  The following command outputs the
number of the seconds since the epoch for the time two minutes
after the epoch:
          date --date='1970-01-01 00:02:00 +0000' +%s
          date --date='1970-01-01 00:02:00 +0000' +%s
          120
If you do not specify time zone information in the date string,
`date' uses your computer's idea of the time zone when
interpreting the string.  For example, if your computer's time
zone is that of Cambridge, Massachusetts, which was then 5 hours
(i.e., 18,000 seconds) behind UTC:
          # local time zone used
          date --date='1970-01-01 00:02:00' +%s
          18120
10. If you're sorting or graphing dated data, your raw date values may
be represented as seconds since the epoch.  But few people can
look at the date `946684800' and casually note "Oh, that's the
first second of the year 2000 in Greenwich, England."
          date --date='2000-01-01 UTC' +%s
          946684800
An alternative is to use the `--utc' (`-u') option.  Then you may
omit `UTC' from the date string.  Although this produces the same
result for `%s' and many other format sequences, with a time zone
offset different from zero, it would give a different result for
zone-dependent formats like `%z'.
          date -u --date=2000-01-01 +%s
          946684800
To convert such an unwieldy number of seconds back to a more
readable form, use a command like this:
11. local time zone used
  date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
  1999-12-31 19:00:00 -0500
Often it is better to output UTC-relative date and time:
  date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
  2000-01-01 00:00:00 +0000