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Cron is a command-line job scheduler on Unix-like systems. It allows you to run automated tasks in the background and it's especially useful for repetitive jobs.
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Those tasks are called cron jobs. Each job consists of three parts. The time when the job should be executed, a user who will run the task, and valid shell command or script that will be executed.
Cron jobs are defined in files called crontabs. Each user can have its own crontab file and there is also a system-wide crontab.
In this quick and easy-to-follow tutorial, you will learn how to list and view all current cronjobs.
To list all active cron jobs for the current user (ser that is currently logged in) run the following command:
This will display your crontab. If you haven't created a crontab yet, it will print the message stating that the current user doesn't have any crontab.
System cron jobs are defined in the system crontab which is located in
/etc/crontab. To display this crontab, simply run the following command:
If you want to display cron jobs for a specific user, you can use the following
command and replace [username] with the user name of the select user:
Hourly, daily, weekly, monthly cron jobs are stored in /etc/cron.hourly,
/etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly folders respectively.
To display one of those files use the following command and replace the
[anacron] with the select crontab.
For example, the following command will display the daily cron:
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.
Every minute cron goes through all the crontabs and looks for the jobs that should be executed. The exact time of the execution can be specified using the cron syntax. Whenever the parameter matches the current date and time cron job is executed.
In this quick tutorial, we will take a look at how to set up a cron job to run at a specific time.
Cron can generate logs, which are very useful in troubleshooting your cron jobs. In this quick tutorial, we will take a look at cron logs – how to find them and how to read them.
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