How (and when) to use systemd timer instead of cronjob

Introduction

* * * * * bash demo.sh

Just a single line of code is sufficient to schedule a cron job. However, there are some scenarios where I find systemd timer more useful than cronjob.

How to use systemd timer

We need to create a service file(contains the script to be run) and a timer(contains the schedule).

# demo.service
[Unit]
Description=Demo service

[Service]
ExecStart=bash demo.sh


# demo.timer
[Unit]
Description=Run myscript.service every 1 minutes

[Timer]
OnBootSec=1min
OnUnitActiveSec=1min

[Install]
WantedBy=multi-user.target

We can copy these files to /etc/systemd/system/ and enable the timer.

$ sudo cp demo.service demo.timer /etc/systemd/system/

$ sudo systemctl daemon-reload

$ sudo systemctl enable --now demo.timer

We can use systemctl to see when the task is executed last and when it will be executed next.

$ sudo systemctl list-timers --all

Use Cases

• Singleton - In the above example, lets say demo.sh takes ~10 minutes to run. With cron job, in ten minutes we will have 10 instances of demo.sh running. This is not ideal. With systemd timer, it will ensure only one instance of demo.sh is running at a time.

• On demand runs - If we want to test out the script/job, systemd allows us to immediately run it with usual systemctl start demo without needing to run the script manually.

• Timer - With cron, we can run tasks upto a minute precision. Timer can run tasks till second level precision.
  

[Timer]
OnCalendar=*-*-* 15:30:15

In addition to that, we can run tasks based on system events. For example, we can run a script 15 minutes from reboot.

[Timer]
OnBootSec=15min

Conclusion

Systemd timer is a powerful tool that can replace cronjob in many scenarios. It provides more control and flexibility over cronjob. However, cronjob is still a good choice for simple scheduling tasks.