A Simple E-Mail Reminder System Based on cron and at

2023-02-10

Since I cannot keep everything I have to remember in my head, I need a reminder system. While I would prefer an analog system (ideally one that integrates with my system for keeping track of tasks), I have not found a satisfying analog solution so far.

I am not looking for a full-fledged calendar system. If I were, I would be using Remind or `calendar`^[1]. But they offer way more features than I need.

I basically have two requirements for my reminder system:

It has to remind me reliably by e-mail about things a certain time in advance.
It has to be simple and have as few external dependencies as possible.

The solution I have come up with is almost too trivial to share: It employs cron and at to schedule reminders, which are sent as e-mails via a remote SMTP server when they are due.

The Setup

The system depends on the following ingredients:

A machine which is up 24/7 – for scheduling reminders for arbitrary times.^[2] I am using a server hosted on OpenBSD Amsterdam.
smtpd – for relaying e-mails to a remote SMTP server.
cron – for scheduling recurring reminders (like birthdays).
at – for scheduling nonrecurring reminders (like one-time events, or events which recur in irregular intervals).
mail – for sending e-mails non-interactively.

On OpenBSD, all the required software is part of the base system; see smtpd(8), cron(8), at(1), mail(1).

Both cron and at basically take two kinds of data as input: A timestamp which specifies when something is to be done, and an action which specifies what is to be done at the specified time.

Regardless of whether a reminder is recurring or nonrecurring, I want an e-mail to be sent to me at the specified time. Therefore, the action in both cases is sending an e-mail to me reminding me of the given event. Since the e-mail needs to be sent non-interactively when the given time is reached, the mail utility is invoked, and the reminder text is passed to it as the mail subject (since they are usually short, I only specify a subject and leave the body empty; then I don’t even have to open the e-mail).

In order not to have to specify the non-changing details (like the e-mail address) for sending the mail each time, I employ a small script called mailrem which is basically a wrapper around mail which takes the reminder text as its argument:

#!/bin/sh

from="John Doe <john.doe@email.com>"
subject="$1"

echo | mail -r "$from" -s "$subject" "$from"

Finally, in order for mail to be able to send mails to a remote SMTP server, smtpd needs to be configured such that it relays outgoing mail to a remote server.

Usage

E-mail reminders are sent by calling mailrem with the specific reminder text as its argument, e.g.

$ mailrem "Prepare tax statement"

In order to schedule reminders for a specific time, mailrem is not invoked directly as above, but either by cron or by at, depending on whether the reminder is to be sent on a regular basis or just once.

Recurring reminders

Recurring reminders are scheduled by cron via corresponding crontab entries. Start editing the crontab file:

$ crontab -e

Then add an entry for each recurring reminder like this:

0 0 3 18 * $HOME/bin/mailrem "[Reminder] Paul's $(( $(/bin/date +\%Y) - 1976 )). birthday (1976)"

This entry would send an e-mail each year at midnight on March 18 with subject “[Reminder] Paul’s n. birthday (1976)”, where n is calculated each time the cron entry is triggered by subtracting Paul’s year of birth, 1976, from the current year obtained by the date utility^[3].

Nonrecurring reminders

Nonrecurring reminders are scheduled by at via corresponding atjobs. For instance, to schedule an atjob for midnight in three days:

$ at midnight + 3 days

When prompted, enter the action to be executed at the specified time like this:

> mailrem "[Reminder] Doctor's appointment at 9 am"

Then, press Ctrl + D to queue the atjob.

That’s it. A very simple system which does its job and is easy to setup and maintain.

Caveats

The system is certainly not suitable for time-critical reminders. This is obvious, since by nature the reminders are sent in an asynchroneous way (i.e. via e-mail).
There is no easy way to get a consolidated overview over all currently scheduled reminders. For recurring reminders, crontab -l could be used. Similarly, atq displays the queue of atjobs. However, the output of atq is pretty much unparseable (at least in the version which ships with OpenBSD). So one would have to manually extract the job ID from the output of atq and then do $ at -c <id> in order to see the content of the job.
Failures cannot easily be discovered in general, and there is no recovery mechanism. If, e.g., for some reason the cron daemon is not running at the time a reminder is due, it will not be sent and will subsequently be lost forever.

1. Man page of the OpenBSD version: calendar(1).

2. In principle, of course, it is also possible to run the system on a laptop. The severe drawback would be that reminders were lost if the laptop were not running at the scheduled time of a reminder. One could use tools like anacron instead of cron. This would mitigate the risk of losing reminders, but the time they are sent would then depend on when the laptop is running (which may differ greatly from the intended time of sending).

3. Man page of the OpenBSD version: date(1).