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Cron is a available tool on Linux allowing the implementation of planned tasks in order to automate processions & actions.

Introduction

Each user of a Linux system has a crontab allowing the planned tasks management.
The following command displays the crontab content :

It is also possible to display the crontab of another user if we have the right.

Or even for all the users :

It is finally possible to directly consult and edit the contab :


Configuration

The crontab is edited from the command :

From the crontab’s view, we can configure some tasks to plan and the repetition cycle of each one.
Each file’s line must follow the following formatting separating with a space at least :

Minutes, value between 0 and 59.

Hour, value between 0 and 23.

day of month, value between 1 and 31.

Month, value between 1 and 12.

Day of week, value between 1 and 7.

User’s login to whom cron applies (optional).

Command of the cron task to execute.

Once the at least one tasks are written, we leave and safeguard the crontab (ECHAP then :wq) as in an open file with the editor VIM.
A message of the changes’ consideration is sent back :

It can happen that cron crash and in this case, a restart is necessary :

The different meters can be maximized with the characters /, , (decimal point) and .
The month number mon can be replaced by jan, feb, mar, apr, may, jun, jul, aug, sep, oct, nov and/or dec.
The day of week number dow can be replaced by mon, tue, wed, thu, fri, sat and/or sun.

Dividing a period.

List periods.

Extend a period.

it is also possible to replace the * * * * * with a preset shortcut : @reboot, @yearly
@annually, @monthly, @weekly, @daily, @midnight, @hourly

Every hour (0 * * * *).

Everyday (0 0 * * *).

At each system start.


Examples

In order to become familiar with the five elements constituting of the cron tasks, here are several concrete examples of use :


Resume

In order to keep on having these information to hand, you can preserve them in a file :


Generator

In order to simply and quickly create cron tasks, you can use the following generator :


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