# How to prevent duplicate cron jobs from running?

Sometimes you may find that duplicate cronjobs are running at the same time.
This may happen when the cronjob takes longer to complete than its execution
interval. Here is a simple way to prevent this from happening ever again.

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## Use flock

`Flock` is a very simple and easy-to-use utility found in most Linux systems.

Its functionality goes like this. It takes two parameters - a **lock file** and
a **command** (or shell script).

It locks the lock file at the start of the execution of the command and unlocks
it at the end. Then if the same cronjob is started it checks if the lock file is
still locked. If the lock file is locked, it means that the previous cronjob
isn't finished yet and the flock will prevent from starting a new one.

```bash
*   *  *   *   *  /usr/bin/flock -n /home/myfolder/file.lockfile /bin/backup.sh
```

This will create a cronjob that runs the `/bin/backup.sh` script and prevent
duplicate cronjobs from running using `/home/myfolder/file.lockfile` lock file.

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