Is at missing from Pi-OS?

[Bosse_B]

I have read a number of sites describing how to run a process at a specific time using the at command.
I have tried it on my Ubuntu Server to schedule things that need to happen during the night.

Now turning towards the RaspberryPi I found to my astonishment that at is not recognized!

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$ at 01:45
-bash: at: command not found

This is on an RPi3B with PiOS installed a week or so ago (GUI with limited number of applications), so it is really up-to-date.
It is difficult to google at since it is such a common word, so I ask here instead.

What am I missing?
By what I read all how-tos just assume that at exists as part of Linux...

[GlowInTheDark]

Yes, it is not there by default. Surprised me, too.

But it is easily installable. apt-get install at

I have a list of missing packages that need to be installed onto a new Raspbian system. "at" is one of them.

[Bosse_B]

Yep!

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sudo install at

did it!

Here is an at question:
I am trying to set a job at a time a few days ahead but it does not work properly..

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at 04:10

is supposed to set the job on the closest match in time, i.e. the following early morning.
And this works.
To set it on the next match all the tutorials I have seen say this is the command to add a day to the match:

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at 04:10 + 1 days

But in fact it is still set the following early morning.
If I do this I can set it on a date of my choosing:

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at 04:10 2020-08-31

and it works, but is overly clumsy.

Surprise:

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at 04:10 + 2 days

will actually move it to the night following next night.

How come there is an offset by 1?
It does NOT work as the descriptions online say it does...
But if I enter a command that will execute today if the offset is not added and I add one:

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at 23:49 + 1 days

THEN it moves the execution to the next day if I enter it before 23:49, i.e. now it obeys the rule....

How very confusing that the fact I am scheduling a job past midnight means I have to add 1 to the day offset in order to get it right.

Ideas?

[davidcoton]

I don't use 'at', but one understanding of '+ n days' is: "pass midnight n times from now before activating at the set time". I think that is consistent with your observations.

[GlowInTheDark]

Yeah, that's just the way "at" is. I like "at", and have used it for certain things, but the date syntax is weird.

You have to read the man page carefully and (still) do lots of experimentation/trial-and-error.

[Bosse_B]

I don't use 'at', but one understanding of '+ n days' is: "pass midnight n times from now before activating at the set time". I think that is consistent with your observations.

So the part that says +1 days really means +1 midnight.
If one is at 23:55 when doing

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at 00:10 +1 days

then the result is the same as at 00:10 without any days.
But if one is at 00:01 when issuing

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at 00:10 + 1 days

it will be set an extra day later...

The command should have been

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at 00:10 + 1 midnight

instead.

I assumed that + 1 days means + 24 hours from the closest match...

[GlowInTheDark]

As I said, the date syntax of "at" is weird, but, to quote fearless leader, it is what it is.
It is obviously not going to be changed.

A workaround that I've used is, that allows me to use the more flexible and familiar "GNU date format" for specifying a time, is to do something like:

$ at "$(date '+%whatever' -d 'your string here')"

where:

1) whatever is a format that is acceptable to "at" (you determine this by trial-and-error)
2) your string here is any format that is acceptable to GNU date - which is pretty darn good and intuitive.

Then of course you wrap the above in an alias or script or whatever and then everything is fine.