Crontab Every 30 Seconds (Workaround)

Running a Job Every 30 Seconds with Cron

Standard cron has a minimum resolution of one minute — you cannot schedule a job more frequently than * * * * * (every minute). However, a common workaround uses the sleep command to achieve sub-minute intervals.

The Workaround

* * * * * /path/to/script.sh
* * * * * sleep 30 && /path/to/script.sh

This creates two crontab entries:

1. The first runs the script at the top of every minute (:00)
2. The second waits 30 seconds and then runs the script (:30)

Together, they execute the script every 30 seconds.

How It Works

At each minute tick, cron fires both entries simultaneously. The first entry runs the script immediately. The second entry executes sleep 30 (which blocks for 30 seconds), then runs the script. The net effect is execution at :00 and :30 of every minute.

Limitations

• Process overhead: Each cron invocation spawns a new shell process
• No overlap protection: If the script takes more than 30 seconds, runs will overlap
• Scaling: For intervals like every 10 or 15 seconds, you need more entries:

* * * * * /cmd
* * * * * sleep 15 && /cmd
* * * * * sleep 30 && /cmd
* * * * * sleep 45 && /cmd

Better Alternatives

For sub-minute scheduling in production, consider:

• systemd timers with OnUnitActiveSec=30s
• A loop-based daemon with sleep intervals
• Message queues for event-driven processing
• Watch/polling tools like inotifywait