In this lesson, you will learn how to enforce a precise startup sequence so that your system's critical prerequisites are fully ready before your scheduled tasks attempt to run.
The Problem of Premature Execution
When you decouple execution logic from timing logic using systemd timers, you often create a scenario where a timer triggers a service that requires a specific environment—such as a mounted network drive, a database connection, or a specific hardware interface. If your service fires the moment the timer hits its mark, but your required resource is still initializing, the job will fail.
Using Dependency Directives
systemd provides dependency directives that allow you to dictate the order of operations between units. While a timer unit acts as the "trigger," you should define your requirements within the [Unit] section of the service unit being triggered, rather than the timer itself.
To ensure your task only runs after necessary prerequisites are satisfied, use the After= directive. If the job absolutely cannot proceed without the prerequisite, use Requires= to bind the lifecycles together.
Consider a backup script that requires a mounted data partition:
[Unit]
Description=Daily Backup Service
# Ensure the mount is fully operational first
After=mnt-data.mount
Requires=mnt-data.mount
[Service]
ExecStart=/usr/local/bin/backup-data.sh
Why this matters
By placing the dependency inside the service unit, you create a "wait-and-verify" state. When the timer triggers, systemd inspects the service’s dependency chain. If mnt-data.mount is not yet active, systemd will delay the execution of your backup script until the mount is successfully achieved.
This approach prevents "race conditions" where scripts fail because they attempted to access filesystems or sockets before the system kernel or background daemons had finished setting them up. It turns an unstable, time-sensitive process into a robust, event-aware sequence that respects the state of your infrastructure.
Key Takeaway: Use the After= and Requires= directives within the service unit to ensure all system prerequisites are met before the timer executes the payload.