On a Linux system, the cron daemon is responsible for running periodic tasks according to a schedule specified in configuration files called "crontabs". The cron daemon reads these crontabs and executes the tasks they contain at the specified times.
There are several predefined crontabs on a typical Linux system, including cron.daily, cron.weekly, and cron.monthly. These crontabs are located in the /etc/cron.d directory and contain tasks that are intended to be run on a daily, weekly, or monthly basis, respectively.
The exact time at which tasks in cron.daily, cron.weekly, and cron.monthly are run depends on the configuration of the system. On most systems, these tasks are run at a specific time every day, week, or month, as specified in the crontab.
For example, the cron.daily crontab might contain a line like this:
0 4 * * * root run-parts /etc/cron.dailySource:www.lautturi.com
This line specifies that the run-parts command should be run every day at 4:00 AM (using the 24-hour clock), and it will execute all the scripts in the /etc/cron.daily directory.
Similarly, the cron.weekly crontab might contain a line like this:
0 4 * * 0 root run-parts /etc/cron.weekly
This line specifies that the run-parts command should be run every week on Sunday at 4:00 AM, and it will execute all the scripts in the /etc/cron.weekly directory.
The cron.monthly crontab might contain a line like this:
0 4 1 * * root run-parts /etc/cron.monthly
This line specifies that the run-parts command should be run every month on the first day at 4:00 AM, and it will execute all the scripts in the /etc/cron.monthly directory.
You can use the crontab -e command to edit the crontabs on your system, including cron.daily, cron.weekly, and cron.monthly. You can also create your own custom crontabs if you need to schedule tasks that don't fit into the predefined categories.