I know that you can schedule a shutdown for a specific time via shutdown -h 21:45 and that you shouldn't use crontabs for such things because of their repetitive nature. How can I schedule a shutdown for a specific date like 31st of August at 20:00pm?

Recent Ubuntu versions use systemd and when the conventional atd and associated at scheduler for one-off commands is either not installed or not running a one-off command can be scheduled with systemd-run which is somewhat easier than manually generating a systemd timer:

 systemd-run --on-calendar="2019-08-31 20:00:00 CET" /sbin/shutdown now

The at command is for scheduling one off future executions.

e.g.

% at 8pm Aug 31
at> echo hello
at> <EOT>
job 161 at Sat Aug 31 20:00:00 2019

(the "<EOT>" was produced by pressing control-D)

% atq
161 Sat Aug 31 20:00:00 2019 a sweh

You can put your shutdown command here.

While at is the obvious way to do this, I think it will survive shutdowns, so if the machine reboots before the scheduled shutdown, it might shutdown again at the scheduled time. Therefore, using bash and GNU date we can use a delayed shutdown.

/sbin/shutdown +$(( ( $( date -d "30 may 2019 13:15" +%s ) - $( date +%s ) ) / 60 + 1 ))

NB: This is only accurate to one minute. It works by converting the scheduled time, and now to seconds since the UNIX epoch, calculating the difference, converting to minutes, and then using this as the delay to shutdown. It won't work correctly if the scheduled time is in the past.

You use a better shutdown command.

The van Smoorenburg, Upstart, and systemd shutdown commands do not allow a date specification. But the BSD shutdown command takes a date value in its specification of when to shut down, in the form yymmddhhmm. This has been the case ever since 4BSD, and remains so with FreeBSD and OpenBSD. NetBSD extended it to ccyymmddhhmm.

Because the nosh toolset is also usable on the BSDs, I gave my shutdown command the same capability, extending it to CCYYMMDDHHMM per NetBSD. And because the nosh toolset is also usable on Linux, that gives Linux a shutdown command that takes dates.

It of course works with the nosh system-manager. It also works with some other systems. It sends signals to process #1 to enact stuff and the same signals are understood by some other system managers, such as systemd, which can also be shut down using it.

You could use it, or a tool like it; or you could try to persuade the authors of other Linux shutdown programs to extend their tools to also be as capable as the BSD shutdown.

Further reading

• Jonathan de Boyne Pollard (2018). shutdown. nosh toolset manual pages. Softwares.


• Jonathan de Boyne Pollard (2018). system-manager. nosh toolset manual pages. Softwares.


• 
  shutdown. System Manager's Manual. NetBSD Manual pages. 2011-11-04.


• 
  shutdown. FreeBSD System Managers' Manual. 2018-01-01.


• 
  shutdown. OpenBSD Manual pages. 2015-01-21.


• https://unix.stackexchange.com/a/465337/5132

To run the job only this year:

0 20 31 8 * test $(/bin/date +%Y) = "2019" && /sbin/shutdown now

To run the job every year:

0 20 31 8 * /sbin/shutdown now

There may be an option to input the year to the shutdown command, but cron can be used to run commands only once at a specific point in time. For your example, try

0 0 31 8 ? 2019 shutdown

Beware: the above is not standard cron syntax.