Prevent Duplicate Cron Jobs with Linux flock: A Step‑by‑Step Guide

Example

To simulate a conflict, a PHP script that loops 180 times, prints messages and sleeps one second (≈3 minutes) is created (test.php). A crontab entry runs it every minute: * * * * * php /root/test.php >> /root/test.log After a minute, the process appears; after another minute, another instance appears, showing duplicate execution.

Solution

Use Linux flock file locking to ensure only one instance runs.

Replace the crontab entry with:

* * * * * flock -xn /tmp/mytest.lock -c 'php /root/testflock.php >> /root/test.log'

Now only a single testflock.php process is observed.

flock Explanation

The flock command obtains an exclusive lock before running the task. If the lock cannot be acquired, the command exits without running, preventing overlap.

Key options used: -x, --exclusive : acquire an exclusive lock. -n, --nonblock : fail immediately if the lock is not available. -c, --command : execute the given command in a shell.