Systemd timer running on every boot

4 min read

Disclaimer: This blog article has been generated with the assistance of AI. While the content is AI-generated, the software itself and the ideas behind it are the result of real development work and genuine user needs.

Problem Description

A systemd timer configured with OnCalendar=daily and Persistent=true was executing its associated service on every system boot, in addition to the scheduled execution time. The expected behavior was execution only at the scheduled time, with persistent timers catching up on missed runs only when the system was offline during the scheduled time.

Environment

  • System: User systemd instance (systemctl --user)
  • Timer: rednotebook-backup.timer
  • Service: rednotebook-backup.service
  • Systemd version: Standard systemd installation

Observed Behavior

Aug 13 20:59:24 - Service executed after boot
-- Boot f6bac27bcb034b92807c437ec2834916 --
Aug 13 21:10:50 - Service executed after boot
-- Boot 18a831a9d19b415da72eb836e78ad866 --
Aug 13 21:11:20 - Service executed after boot

The service executed immediately after each boot, regardless of the last execution time or scheduled interval.

Configuration Analysis

Original Timer Configuration

[Unit]
Description=Run RedNotebook backup daily
Requires=rednotebook-backup.service

[Timer]
OnCalendar=daily
Persistent=true
RandomizedDelaySec=300

[Install]
WantedBy=timers.target

Service Configuration

[Unit]
Description=RedNotebook GitHub Backup Service
After=network-online.target 
Wants=network-online.target

[Service]
Type=oneshot
ExecStart=%h/bin/rednotebook-backup.sh
ExecStartPre=/bin/sleep 30
Environment=HOME=%h
StandardOutput=journal
StandardError=journal
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket

Root Cause Analysis

The issue stems from the Requires=rednotebook-backup.service directive in the timer unit’s [Unit] section.

Systemd Dependency Resolution

  1. Timer activation: The timer unit is started by timers.target during boot
  2. Dependency resolution: Due to Requires=rednotebook-backup.service, systemd immediately activates the service unit when the timer starts
  3. Unintended execution: The service executes on boot, independent of the timer schedule

Systemd Timer Mechanics

Systemd timers automatically establish relationships with their target services through:

  1. Implicit naming convention: foo.timer automatically targets foo.service
  2. Automatic dependencies: Timer units gain an implicit Before= dependency on their target service
  3. No explicit linking required: The Unit= directive in [Timer] section is only needed when targeting a service with a different name

From systemd.timer(5):

Timer units automatically gain a Before= dependency on the service they are supposed to activate.

Solution

Remove the Requires= directive from the timer unit:

[Unit]
Description=Run RedNotebook backup daily
# Removed: Requires=rednotebook-backup.service

[Timer]
OnCalendar=daily
Persistent=true
RandomizedDelaySec=300

[Install]
WantedBy=timers.target

Implementation

systemctl --user stop rednotebook-backup.timer
systemctl --user edit rednotebook-backup.timer
# Add override: [Unit]\nRequires=
systemctl --user daemon-reload
systemctl --user start rednotebook-backup.timer

Verification

Post-fix timer state:

NEXT                            LEFT     LAST PASSED UNIT                     ACTIVATES                 
Thu 2025-08-14 00:02:17 IST    2h 54min  -    -      rednotebook-backup.timer rednotebook-backup.service

Multiple reboot test results:

  • Service does not execute on boot
  • Timer maintains correct scheduling
  • Service executes only at scheduled intervals

Common Misconceptions

  1. Explicit linking requirement: Many assume timers require explicit service dependencies
  2. Cargo-cult configuration: Copy-pasting examples with unnecessary Requires= directives
  3. Documentation gaps: Some tutorials incorrectly show Requires= as standard practice

Alternative Diagnostic Approaches

If removing Requires= doesn’t resolve the issue:

  1. Check service enablement: 
    systemctl --user is-enabled rednotebook-backup.service

    Services triggered by timers should show static or disabled

  2. Verify service dependencies: 
    systemctl --user list-dependencies rednotebook-backup.service
  3. Check for WantedBy directives: Remove WantedBy= from service [Install] section

Timer State Management

Systemd maintains timer state in:

  • System timers: /var/lib/systemd/timers/
  • User timers: ~/.local/share/systemd/

State corruption can cause similar symptoms but is less common than configuration issues.

Technical References

  • systemd.timer(5) - Timer unit configuration
  • systemd.unit(5) - Unit configuration directives
  • systemd.special(7) - Special systemd units
  • systemd GitHub Issue #14642 - Timer runs on every boot

Conclusion

The Requires= directive in systemd timer units creates an unnecessary dependency that causes service execution on timer activation (boot). Systemd’s implicit timer-to-service binding mechanism eliminates the need for explicit dependencies in standard configurations. This issue affects both user and system timer units and represents a common configuration anti-pattern in systemd timer implementations.