Empty journal

Storing the output of cron jobs in systemd’s journal

The output from cronjob’s are often unceremoniously redirected to /dev/null just to keep them quiet. This is a bad practice as you don’t get any feedback when potentially important tasks go wrong. (If the task isn’t important, why are you running it on a schedule?)

systems provides a simple tool called systemd-cat for directing stdout and stderr to the journal. Below is a complete example of a simple monthly cron job that will run certbot once every month and save whatever it outputs to the journal under the arbitrary identifier “certbot-cron”. A good identifier will make it easier to lookup the output in the journal if you need it later.

@monthly systemd-cat -t "certbot-cron" /usr/bin/certbot --renew

I’ll use the automated TLS certificate renewal program certbot from Let’s Encrypt as the example command throughout this article. It serves nicely as an example of something you need to know about if it at some point will return an unsuccessful condition.

You can then quickly look at certbot output in the journal using journalctl with the same identifier that you gave the cron job:

journalctl -t "certbot-cron"

systemd-cat only accepts one executable with any number of arguments. You can’t run multiple commands at once or use pipes. It will capture stdout and stderr, and assign them appropriate properties in the journal.

You can pipe output into systemd-cat, but this will only capture stdout. Important and actionable information will often show up in stderr, and you’d have to redirect the flow of stderr to stdout to capture it. I’ll not demonstrate this here, as I believe this is the wrong approach.

If you need to run more complex cron jobs with multiple commands, you should save them to a script, make that executable, and then run that script through systemd-cat as a single command. This will capture stdout as well as stderr, and keep your crontab more readable and easier to maintain.


Feature image based on the photo “Black Pencil on White Paper” © 2015 Miguel Á. Padriñán Alba.

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