How do you prevent duplicate cron jobs?

When working with cron jobs, it’s essential to ensure they don’t run concurrently, especially if their tasks could interfere with one another. To prevent duplicate executions without modifying the job or altering its schedule, you can use the flock utility to implement a locking mechanism.

What is flock?

In Linux, flock is a system call used to manage file locks, and the flock command-line utility leverages this functionality. By using flock, you can ensure that a cron job only runs if it successfully acquires a lock. While the job is running, the lock is held, preventing any other instance from starting. Once the job completes, the lock is released automatically.

Here’s how you can add flock to your cron jobs.

Example: Adding flock to a Cron Job

Before Using flock:

$ crontab -l
*/5 * * * * /home/crons/send-incremental-data.py

In this example, the cron job runs the script /home/crons/send-incremental-data.py every 5 minutes. However, if the script takes longer than 5 minutes to complete, another instance will start, causing duplicate executions.

After Adding flock:

crontab -l
*/5 * * * * flock -n /home/crons/send-incremental-data.lock /home/crons/send-incremental-data.py

Now, flock ensures that only one instance of the job runs at a time. Let’s break this down:

1. flock -n:

   • The -n option tells flock to fail immediately if the lock is already held by another process.

2. Lockfile (/home/crons/send-incremental-data.lock):

   • The lockfile serves as a marker to indicate whether the job is running.
   • If the lockfile doesn’t exist, flock creates it, acquires the lock, and allows the script to execute.
   • If the lockfile already exists, the job won’t run.

3. Automatic Lock Management:

   • When the script finishes, flock releases the lock, allowing future executions.

Manually Releasing a Lock

If a lock persists due to an unexpected issue (e.g., a crash), you can manually delete the lockfile:

rm /home/crons/send-incremental-data.lock

This frees the lock, enabling the job to run again.

Conclusion

Using flock is a simple yet effective way to prevent duplicate cron jobs, ensuring your scheduled tasks run smoothly without interference.