Module 4: Automating Tasks with systemd

28/07/2025

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Beyond Cron: A Modern Guide to Automating Linux Tasks with Systemd Timers

For years, cron has been the go-to tool for scheduling tasks on Linux systems. It’s reliable, familiar, and gets the job done. But as Linux has evolved, so have its tools. Today, systemd, the default init system for most major Linux distributions, offers a more powerful, flexible, and integrated way to automate tasks: systemd timers.

If you’re a system administrator or developer looking for more control and better logging for your automated jobs, it’s time to look beyond cron. This guide will walk you through everything you need to know to master task automation with systemd timers.

Why Choose Systemd Timers Over Cron?

While cron is simple, systemd timers provide several distinct advantages that make them a superior choice for modern systems:

Integrated Logging: Each task run by a systemd timer is logged directly to the systemd journal. This means you can use the powerful journalctl command to view detailed output, execution status, and errors for each job without configuring custom log files.
Dependency Management: Timers can be configured to run only after specific services or system states are reached, like network connectivity. This is incredibly difficult to achieve reliably with cron.
Resource Control: You can use systemd’s built-in resource management features (cgroups) to limit the CPU, memory, or I/O usage of your scheduled tasks, preventing a rogue script from impacting system performance.
Granular Scheduling: Systemd offers more flexible time specifications, including options to run a task a certain amount of time after system boot, after a service was last run, or on complex calendar events.
Better Organization: Each automated task is defined by its own unit files, making it easier to manage, track, and version control your automations compared to a single, monolithic crontab file.

The Core Components: Service Units and Timer Units

Automating a task with systemd requires creating two simple text files, known as unit files. This separation of what to run from when to run it is a key part of systemd’s power.

The Service Unit (.service): This file defines the task you want to execute. It specifies the command or script to run and the environment in which it should operate. Think of this as the “what.”
The Timer Unit (.timer): This file defines the schedule for the task. It tells systemd when to trigger the corresponding service unit. This is the “when.”

These files are typically placed in the /etc/systemd/system/ directory to be available system-wide.

A Practical Example: Scheduling a Daily Backup Script

Let’s walk through the process of creating a systemd timer to run a backup script every day.

Step 1: Create the Backup Script

First, we need a script to run. For this example, we’ll create a simple backup script at /usr/local/bin/backup.sh.

#!/bin/bash
# A simple backup script
rsync -a --delete /home/user/documents/ /mnt/backups/documents/
echo "Backup completed successfully."

Make sure the script is executable:
chmod +x /usr/local/bin/backup.sh

Step 2: Create the Service Unit

Next, we define what to run. Create a file named daily-backup.service in /etc/systemd/system/:

[Unit]
Description=Run a daily backup of user documents

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

Let’s break this down:

[Unit]: The Description provides a human-readable summary of the service.
[Service]: This section defines the execution behavior.
- Type=oneshot: This indicates that the service is a short-lived process that does its job and then exits.
- ExecStart: This is the most important directive, specifying the full path to the command or script to execute.

Step 3: Create the Timer Unit

Now, we define when to run our service. Create a file named daily-backup.timer in /etc/systemd/system/.

[Unit]
Description=Run the daily backup script every day at 2 AM

[Timer]
OnCalendar=daily
Persistent=true
RandomizedDelaySec=1h

[Install]
WantedBy=timers.target

Here’s what these directives mean:

[Unit]: Again, a Description helps identify the timer’s purpose.
[Timer]: This section controls the schedule.
- OnCalendar=daily: This is the core scheduling directive. daily is a shorthand for *-*-* 00:00:00. You can be much more specific, for example: OnCalendar=*-*-* 02:00:00 runs the job at 2 AM every day.
- Persistent=true: This ensures that if the system was down during a scheduled run time, the job will be executed as soon as the system boots up.
- RandomizedDelaySec=1h: This is a great feature to prevent “thundering herd” problems where many jobs on many machines start at the exact same time. This directive will delay the job’s execution by a random amount of time up to one hour after its scheduled start.
[Install]: The WantedBy=timers.target line ensures that the timer is automatically started when the system boots.

Step 4: Enable and Start the Timer

After creating your unit files, you need to tell systemd to recognize them and activate the timer.

Reload the systemd daemon: This makes systemd aware of your new files.
```
sudo systemctl daemon-reload
```
Start the timer: This activates the schedule immediately.
```
sudo systemctl start daily-backup.timer
```
Enable the timer: This ensures the timer will start automatically on future system boots.
bash sudo systemctl enable daily-backup.timer

You can check the status of your timer and see when it’s scheduled to run next with:
sudo systemctl status daily-backup.timer

Monitoring and Security Best Practices

Checking Your Job’s Output
One of the best features of systemd automation is the centralized logging. To see the output and status of our backup job, you can use journalctl to query the logs for the service unit.

sudo journalctl -u daily-backup.service

This command will show you a detailed history of every time your service has run, including the “Backup completed successfully” message from our script or any errors that occurred.

Actionable Security Tip: Run as a Non-Root User
By default, systemd services run as the root user, which can be a security risk. It is a critical security best practice to run scheduled tasks with the minimum privileges necessary. You can easily specify a non-privileged user and group in your .service file.

Simply add the User and Group directives to the [Service] section:

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh
User=backupuser
Group=backupgroup

This ensures that your script runs with the permissions of backupuser, significantly reducing the potential damage if the script is ever compromised.

By embracing systemd timers, you gain a modern, robust, and transparent framework for automating tasks on your Linux systems. While cron still has its place, the advanced features of systemd timers make them the clear choice for reliable and secure system administration.

Source: https://linuxhandbook.com/courses/systemd/systemd-timers/