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Using anacron to run periodic tasks

Written by Solène, on 28 June 2023.
Tags: #openbsd #anacron

Comments on Fediverse/Mastodon

1. 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

2. 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

3. 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.

4. 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:


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

5. 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

6. Conclusion §

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