📖 ~1 min read
Table of contents
Symptom & Impact
Critical scheduled tasks silently fail, resulting in missed backups, reports, and maintenance operations.
Environment & Reproduction
Introduced after PAM policy changes, shell restrictions, or account expiration rules on service users.
Root Cause Analysis
Cron invokes PAM stack where account constraints deny non-interactive session execution for target user.
Quick Triage
Check cron logs, user account state, and recent PAM include changes before modifying job definitions.
Step-by-Step Diagnosis
Trace PAM decision path, validate user shell and expiry metadata, and test minimal cron command.
Solution – Primary Fix
Adjust PAM account policy for service users and ensure cron environment requirements are explicitly defined.
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Solution – Alternative Approaches
Migrate jobs to systemd timers, run via dedicated automation account, or central scheduler.
Verification & Acceptance Criteria
Scheduled jobs execute on time and produce expected artifacts with no PAM denial entries.
Rollback Plan
Restore previous PAM profile if policy updates introduce authentication regressions elsewhere.
Prevention & Hardening
Test cron-critical users during policy rollouts and monitor failed-run metrics by job owner.
Related Errors & Cross-Refs
Relate to systemd timer failures and shell environment issues; include linked Ubuntu automation tutorial.
Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.
View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Read PAM configuration manuals, cron behavior docs, and Ubuntu scheduling best practices.
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