Jun 29 2010

How To Getting Yesterdays or Tomorrows day with shell date command

Posted by Denie Category: BASH, Linux, Tutorials

When invoked without arguments, the date command displays the current date and time. Depending on the options specified, date will set the date and time or print it in a user defined way. I’ve seen many people writing a perl script for calculating yesterday or tomorrow. Computer loves numbers but we love relative terms like 2 days ago. Luckily GNU date command is designed to handle relative date calculation.

Why use relative date formats?

  • Ease of use
  • To write your own scripts
  • Automate task using cron (example run a job on last day of the month or Nth day of the month or 3rd Friday and so on)

First, print today’s date:
$ date
Sun Jun 17 12:17:24 CDT 2007

Now display Yesterday’s date:
$ date --date="1 days ago"
OR try:
$ date --date="yesterday"
Sat Jun 16 12:17:20 CDT 2007

Now display Tomorrow’s date:
$ date --date="-1 days ago"
Or better try:
$ date --date="next day"
Sat Jun 16 12:17:20 CDT 2007

Getting date in the future

To get tomorrow and day after tomorrow (tomorrow+N) use day word to get date in the future.

Getting date in the past

To get yesterday and earlier day in the past use string day ago:

Moving by whole years or months

You can add year and months keywords to get more accurate date:
$ date --date='2 year ago' # past
$ date --date='3 years' # go into future
$ date --date='2 days' # future
$ date --date='1 month ago' # past
$ date --date='2 months' # future

Moving date using more precise units

  • You can use fortnight for 14 day
  • Week for 7 days
  • hour for 60 minutes
  • minute for 60 seconds
  • second for one second
  • You can also use this / now / today keywords to stress the meaning

To print the date of this Friday:
$ date --date='this Friday'
To print the date of the day six months and 15 day
$ date --date='6 months 15 day'
To print the date of the day two months and 5 days ago:
$ date --date='2 months 5 day ago'

You can also use relative format to setup date and time. For example to set the system clock forward by 30 minutes, enter:
# date --set='+30 minutes'

To display date in epoch time:
$ date --date='1970-01-01 00:00:01 UTC +5 hours' +%s

SOURCE

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