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Most Linux distributions are packaged with a cron daemon, crond. Every minute, crond will scan and look for scheduled tasks that it should be performing at that time. If any are found, it will perform them and return to sleep until the next minute.

Developers can exploit the power of this cron daemon to perform a variety of tasks – the most common probably being the backing up of a database at a regular interval. The cron daemon is able to execute scripts from several different programming languages – including PHP and Python.

To access crond in order to add/alter cron jobs, connect to the desired linux server via secure shell software and use the command crontab –e (note – though all users are normally able to make use of crond, if a superuser has altered its permissions you may have restricted access).

Within you will either be presented with an empty file (if no cron jobs currently exist), or a list of scheduled tasks. The syntax of each job is as follows:

An explanation of Cron Jobs syntax

The five stars control how frequently the job will run. Each star can be replaced with a number, or left with a star to imply ‘all’.

For example, a job that runs at 13:36 every Sunday would look like 36 13 * * 0.

Using the MAILTO command, output of each cron job can be emailed to a specified address, meaning the developer can instantly be made aware of any failures.

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