bash - How to compute a relative date from a given date?

Question

The GNU date command can output date computed from relative items such as -1 hour or 1 month ago. It can also output date from different input format (for instance calendar date, seconds since epoch, etc...).

Is it possible to combine both a date format and a relative time to compute a new date from a given date using the GNU§ date utility?

Something like:

date_epoch=`date "+%s"`
date --date="@$date_epoch -1 month"

The second command gives an "incorrect date" error...

Answer 1

你可以做到，当然：

$ date -d"$(date -d@$date_epoch) -1 month" "+%Y-%m-%d"
2013-09-16

来自：

$ date_epoch=$(date "+%s")
$ date -d@$date_epoch
Wed Oct 16 23:46:50 CEST 2013

你可以使用它“硬编码”：

$ date -d"Wed Oct 16 23:46:50 CEST 2013 -1 month"
Mon Sep 16 23:46:50 CEST 2013

或使用变量：

$ date -d"$(date -d@$date_epoch) -1 month"
Mon Sep 16 23:46:50 CEST 2013