Cron

Schedule tasks using the system crontab via shell_exec.

List current cron jobs

crontab -l

Add a recurring job

(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:

echo "echo 'Meeting reminder' >> /tmp/opc-reminders.log" | at 14:30

Or schedule with a specific date:

echo "echo 'Deadline reminder' >> /tmp/opc-reminders.log" | at 10:00 2026-03-20

Remove a job

Edit the crontab directly:

crontab -e

Or filter out a specific job:

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).