Ubuntu 20.04 LTS Cron Jobs Not Running Under systemd Timers

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Maintenance and backup jobs silently stop running, leading to missed SLAs, stale data, and compliance risk.

Environment & Reproduction

Common where cron and systemd timers coexist with unclear ownership, wrong unit names, or disabled timer targets.

Root Cause Analysis

Scheduling definitions exist but are not enabled correctly, or execution environment assumptions differ between cron and systemd.

Quick Triage

Verify whether job ownership is cron or timer-based, then inspect enablement status and last-run timestamps.

Step-by-Step Diagnosis

Check timer unit metadata, journal logs, calendar expressions, and service exit codes for missed or failed executions.

Solution – Primary Fix

Correct unit dependencies and schedule expressions, enable timers persistently, and validate runtime environment variables for scripts.

Solution – Alternative Approaches

Return to classic cron for simple jobs, use orchestrator-native schedulers, or implement health-check wrappers with retries.

Verification & Acceptance Criteria

Jobs must execute on schedule across reboot and report successful completion with expected output artifacts.

Rollback Plan

Reinstate prior scheduler mechanism if timer migration causes missed executions in business-critical workflows.

Prevention & Hardening

Monitor scheduler health, alert on missed runs, and codify scheduled task ownership to avoid split-management drift.

Related Errors & Cross-Refs

See related failures involving environment variables, permissions, and filesystem mount timing for startup-triggered jobs.

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References & Further Reading

Use systemd.timer and cron documentation plus Ubuntu operations guidance for reliable task scheduling design.