Manipulating dates in shell

After a long time, I had to automate some of my workflow using shell scripts. One of the functions was to lookup something based on relative dates. Having used to Python and C for quite some time, I totally forgot the options and syntax to do the same thing in bash. I can use datetime and timedelta from datetime module to look up dates relative to a certain date.

>>> from datetime import datetime, timedelta
>>> for i in range(7):
...     print(datetime.today() - timedelta(days=i))
...
2020-08-29 13:36:35.981576
2020-08-28 13:36:35.981656
2020-08-27 13:36:35.981694
2020-08-26 13:36:35.981730
2020-08-25 13:36:35.981766
2020-08-24 13:36:35.981801
2020-08-23 13:36:35.981837

>>> # Say I want to do something in the days between christmas and new year
>>> xmas = datetime.strptime("12/25/20", "%m/%d/%y")
>>> newyear = datetime.strptime("01/01/21", "%m/%d/%y")
>>> date = xmas
>>> while (date != newyear):
...     # do something
...     print(date)
...     date += timedelta(days=1)
...
2020-12-25 00:00:00
2020-12-26 00:00:00
2020-12-27 00:00:00
2020-12-28 00:00:00
2020-12-29 00:00:00
2020-12-30 00:00:00
2020-12-31 00:00:00

I didn’t know date command in bash supports relative calculation. The -d/--date option to date command takes input in variety of formats (including human readable ones like today, next month, last year) that support both absolute and relative values.

Here are some examples.

ubuntu@ubuntu-lts:~$ date
Sat Aug 29 13:48:13 EDT 2020
ubuntu@ubuntu-lts:~$ date --date "last week"
Sat Aug 22 13:48:23 EDT 2020
ubuntu@ubuntu-lts:~$ date --date "-7 days"
Sat Aug 22 13:48:36 EDT 2020
ubuntu@ubuntu-lts:~$ for i in {1..7};do date --date "-$i days" +%c; done
Fri Aug 28 13:51:21 2020
Thu Aug 27 13:51:21 2020
Wed Aug 26 13:51:21 2020
Tue Aug 25 13:51:21 2020
Mon Aug 24 13:51:21 2020
Sun Aug 23 13:51:21 2020
Sat Aug 22 13:51:21 2020

The man page for date didn’t have full details on the free format for the --date option. The info page for coreutil had that information (this can also be viewed from the command line with info coreutils command).

The date tool in macOS is quite different though. The option -d is used to set the datetime in the kernel unlike the gnu counterpart where -d is used to specify the string to display the time. date uses the option -f to parse date strings similar to strptime. The option -v is the one to use adjust date and display the adjustment that works similar to --date with the gnu version. However, it doesn’t support human readable input strings.

~ % date
Sat Aug 29 14:11:37 EDT 2020
~ % date -v +1d
Sun Aug 30 14:11:45 EDT 2020
~ % date -v +1d -v -1m # go back 1month, 1day
Thu Jul 30 14:11:58 EDT 2020

~ % for i in {1..7};do  date -v -1m -v -${i}d +%c;done
Tue Jul 28 14:15:32 2020
Mon Jul 27 14:15:32 2020
Sun Jul 26 14:15:32 2020
Sat Jul 25 14:15:32 2020
Fri Jul 24 14:15:32 2020
Thu Jul 23 14:15:32 2020
Wed Jul 22 14:15:32 2020