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.opc/skills/cron/SKILL.md

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+---
+name: cron
+description: Schedule reminders, recurring tasks, and one-time jobs using system crontab.
+---
+
+# Cron
+
+Schedule tasks using the system crontab via `shell_exec`.
+
+## List current cron jobs
+
+```bash
+crontab -l
+```
+
+## Add a recurring job
+
+```bash
+(crontab -l 2>/dev/null; echo "*/20 * * * * echo 'Time to take a break!' >> /tmp/opc-reminders.log") | crontab -
+```
+
+## Common schedules
+
+| Schedule | Cron expression |
+|----------|----------------|
+| Every 20 minutes | `*/20 * * * *` |
+| Every hour | `0 * * * *` |
+| Every day at 8am | `0 8 * * *` |
+| Weekdays at 5pm | `0 17 * * 1-5` |
+| Every Monday at 9am | `0 9 * * 1` |
+
+## One-time scheduled task
+
+Use `at` for one-time jobs:
+```bash
+echo "echo 'Meeting reminder' >> /tmp/opc-reminders.log" | at 14:30
+```
+
+Or schedule with a specific date:
+```bash
+echo "echo 'Deadline reminder' >> /tmp/opc-reminders.log" | at 10:00 2026-03-20
+```
+
+## Remove a job
+
+Edit the crontab directly:
+```bash
+crontab -e
+```
+
+Or filter out a specific job:
+```bash
+crontab -l | grep -v 'pattern-to-remove' | crontab -
+```
+
+## Notes
+
+- Use full paths in cron commands (cron has a minimal `$PATH`).
+- Redirect output to a log file or `/dev/null` to avoid mail noise.
+- Check `at` availability: `which at` (install with `apt install at` if missing).