Cron & Scheduling

Schedule and manage recurring tasks. Covers cron syntax, crontab management, systemd timers, one-off scheduling, timezone handling, monitoring, and common failure patterns.

When to Use

• - Running scripts on a schedule (backups, reports, cleanup)
• Setting up systemd timers (modern cron alternative)
• Debugging why a scheduled job didn't run
• Handling timezones in scheduled tasks
• Monitoring and alerting on job failures
• Running one-off delayed commands

Cron Syntax

The five fields

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Common schedules

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Special strings (shorthand)

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Crontab Management

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Crontab best practices

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Systemd Timers

Create a timer (modern cron replacement)

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OnCalendar syntax

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Advantages over cron

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One-Off Scheduling

at (run once at a specific time)

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sleep-based (simplest)

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Timezone Handling

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DST pitfalls

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Monitoring and Debugging

Why didn't my cron job run?

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Job wrapper with logging and alerting

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Lock to prevent overlap

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Idempotent Job Patterns

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Tips

• - Always redirect output in cron jobs: >> /var/log/job.log 2>&1. Without this, output goes to mail (if configured) or is silently lost.
• Test cron jobs by running them with env -i to simulate cron's minimal environment. Most failures are caused by missing PATH or environment variables.
• Use flock to prevent overlapping runs when a job might take longer than its schedule interval.
• Make all scheduled jobs idempotent. If a job runs twice (DST, manual trigger, crash recovery), it should produce the same result.
• INLINECODE4 is invaluable for verifying timer schedules before deploying.
• Never schedule critical jobs between 1:00 AM and 3:00 AM if DST applies. Use UTC schedules instead.
• Log the start time, end time, and exit code of every cron job. Without this, debugging failures after the fact is guesswork.
• Prefer systemd timers over cron for production services: you get journald logging, missed-run catchup (Persistent=true), and resource limits for free.