Title: Using anacron to run periodic tasks

	Title: Using anacron to run periodic tasks
	Author: Solène
	Date: 28 June 2023
	Tags: openbsd anacron
	Description: In this article, you will learn how to use anacron to run
	periodic tasks on workstations that are not on 24/7

	# Introduction

	When you need to regularly run a program on your workstation that isn't
	powered 24/7 or even not every day, you can't rely on cronjob for that
	task.

	Fortunately, there is a good old tool for this job (first release June
	2000), it's called anacron and it will track when was the last time
	each configured tasks have been running.

	I'll use OpenBSD as an example for the setup, but it's easily adaptable
	to any other Unix-like system.

	Anacron official website

	# Installation

	The first step is to install the package `anacron`, this will provide
	the program `/usr/local/sbin/anacron` we will use later. You can also
	read OpenBSD specific setup instructions in
	`/usr/local/share/doc/pkg-readmes/anacron`.

	Configure root's crontab to run anacron at system boot, we will use the
	flag `-d` to not run anacron as a daemon, and `-s` to run each task in
	a sequence instead of in parallel.

	The crontab entry would look like this:

	```
	@reboot /usr/local/sbin/anacron -ds
	```

	If your computer is occasionally on for a few days, anacron won't run
	at all after the boot, so it would make sense to run it daily too just
	in case:

	```
	# at each boot
	@reboot /usr/local/sbin/anacron -ds

	# at 01h00 if the system is up
	0 1 * * * /usr/local/sbin/anacron -ds
	```

	# Anacron file format

	Now, you will configure the tasks you want to run, and at which
	frequency. This is configured in the file `/etc/anacrontab` using a
	specific format, different from crontab.

	There is a man page named `anacrontab` for official reference.

	The format consists of the following ordered fields:

	* the frequency in days at which the task should be started
	* the delay in minutes after which the task should be started
	* a readable name (used as an internal identifier)
	* the command to run

	I said it before but it's really important to understand, the purpose
	of anacron is to run daily/weekly/monthly scripts on a system that
	isn't always on, where cron wouldn't be reliable.
	Usually, anacron is started at the system boot and run each task from
	its anacrontab file, this is why a delay field is useful, you may not
	want your backup to start immediately upon reboot, while the system is
	still waiting to have a working network connection.

	Some variables can be used like in crontab, the most important are
	`PATH` and `MAILTO`.

	Anacron keeps the last run date of each task in the directory
	`/var/spool/anacron/` using the identifier field as a filename, it will
	contain the last run date in the format YYYYMMDD.

	# Example for OpenBSD periodic maintenance

	I really like the example provided in the OpenBSD package. By default,
	OpenBSD has some periodic tasks to run every day, week and month at
	night, we can use anacron to run those maintenance scripts on our
	workstations.

	Edit `/etc/anacrontab` with the following content:

	```
	SHELL=/bin/sh
	PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/bin
	MAILTO=""

	1 5 daily_maintenance /bin/sh /etc/daily
	7 5 weekly_maintenance /bin/sh /etc/weekly
	30 5 monthly_maintenance /bin/sh /etc/monthly
	```

	You can manually run anacron if you want to check it's working instead
	of waiting for a reboot, just type `doas anacron -ds`.

	What does the example mean?

	* every day, after 5 minutes (after anacron invokation) run `/bin/sh
	/etc/daily`
	* every 7 days, after 5 minutes, run `/bin/sh /etc/weekly`
	* every 30 days, after 5 minutes, run `/bin/sh /etc/monthly`

	# Useful examples

	Here is a list of tasks I think useful to run regularly on a
	workstation, that couldn't be handled by a cron job.

	* Backups: you may want to have a backup every day, or every few days
	* OpenBSD snapshot upgrade: use `sysupgrade -ns` every n days to
	download the sets, they will be installed at the next boot
	* OpenBSD packages update: use `pkg_add -u` every day
	* OpenBSD system update: use `syspatch` every day
	* Repositories update: keep your cloned git / fossil / cvs / svn
	repository up to date without doing it aggressively

	# Conclusion

	Anacron is a simple and effective way to keep your periodic tasks done
	even if you don't use your computer very often.