How subtract days from date in bash shell on Solaris 11? - bash

Question

I've been trying subtrac some days $days from a date $date with format yyyy-MM-dd, but nothing has worked on Solaris 11. Some solution is a 'trick' with the timezone, but it depends on the timezone and I think that is exactly that, a trick.
I would like a cheaper solution, because the only thing I can think is to convert the date to julian representation and then subtract one day and again obtain yyyy-MM-dd representation, for example:
date=2000-12-31
days=1
julian=$(toJulian $date)
resultJulian=$(subtractDays $julian $days)
resultGregorian=toGregorian $resultJulian
So, how can I do it without all this proccess? Thanks.

Answer 1

If you don't have GNU date or GNU awk, consider perl:
subtractDays() {
local date numDays
date=$1
numDays=$2
date=$date days=$numDays perl -e '
use Env qw(date days);
use Time::Piece;
use Time::Seconds;
my $start_time = Time::Piece->strptime($date, "%Y-%m-%d");
my $end_time = $start_time - (ONE_DAY * $days);
print $end_time->ymd . "\n";'
}
...thereafter:
subtractDays 2000-12-31 1
...emits...
2000-12-30

Answer 2

Here's the solution for Solaris 11:
#!/bin/bash
mydate="2016-08-01"
days=3
date_in_the_past=$(gdate -d "${mydate} - ${days} days")
You may wonder why I use gdate when you say it is not available?
Well, GNU Date is in fact available by default on a Solaris 11 install. It is accessed by the gdate command (full path is /usr/bin/gdate).
Some background: GNU Date is part of "GNU Coreutils" package and this package gets installed on any Solaris 11 server unless your customer has actively selected to exclude it. I doubt that is the case.
So gdate is there somewhere, but you say you can't find it? The reason is probably that you are in a local zone, not the global zone. This particular package doesn't get propagated by default into local zones when they are created. This is done in a (misguided, if you ask me) effort to save disk space. This difference between global zones and local zones is somewhat unknown to many Solaris admins.
When you explain to Solaris admins that in fact the binary already physically reside on the disk then it suddenly becomes more acceptable for them to execute the command required:
from the local zone:
pkg install file/gnu-coreutils
The above command doesn't actually go outside the server. It doesn't fetch the package from a remote package repository. The command will work even if you execute it when the server is detached from any network because the package is already there on the disk. Once you explain that to your Solaris admin he's typically okay with executing the command.