Topic: cron
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systemd timers as a cron replacement: OnCalendar, Persistent= and RandomizedDelaySec
A systemd .timer unit paired with a oneshot .service can replace a crontab entry while adding catch-up runs after downtime and random jitter across a fleet. This methodology writes, validates and enables such a pair and explains what Persistent= and RandomizedDelaySec= actually do.
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Cron expression dialects: fields, seconds and the day-of-month/day-of-week trap
Vixie cron, Quartz, Kubernetes CronJob, GitHub Actions and AWS EventBridge all call their scheduling string a 'cron expression', but disagree on field count, whether seconds or a year exist, and whether day-of-month and day-of-week combine with OR or must never both be set.
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Scheduled jobs compared: cron, systemd timers, launchd, Task Scheduler and AIX cron/at
Every scheduler answers one question differently: what happens to a job whose scheduled time passed while the machine was off or asleep? Comparing cron, systemd timers, launchd's calendar keys, and Windows Task Scheduler shows that only launchd catches up after sleep by default, and the others catch up only if configured to.
Machine-readable: JSON