What is a cron in Linux?

Information

The cron utility is a Linux program that schedules a command or script on Linux server or system to run automatically at a specific date and time.

Cron jobs include three Elements:

The action or the output of the script. It’s depending on what the script).

The cron script to be executed or called

The command executes the script on a recurring basis. It’s usually defined in web host panel.

In VPS and Dedicated Server run a cron job as often as you wish but In Shared and Reseller DO NOT run a cron job more frequently than every 15 minutes(it’s limitation of cron).

Timing patent of cron

Five part of the cron job

1. Minute 2.Hour 3.Day of month 4.Month 5.Day of week

Examples:

This will run on Sunday, every hour (24 times in one day, but only on Sundays):

0 * * * 1 command

Every 5 minutes:

*/5 * * * * command

Cron job every time the server/system is rebooted:

@reboot command

Edit the Crontab

Edit the Crontab for insertion or removal job, type the following command

crontab -e

You can also check if the update is performed by entering the following command

crontab -l

Output

Outputting to a File

Use the closing bracket (>) fro store the cron output in a file

15 * * * * /usr/bin/php /var/www/html/script.php >  /var/log/output.log

Discarding Output

Output will be discarded when you pull "> /dev/null 2>&1" at the end command

15 * * * * /usr/bin/php /var/www/html/script.php  > /dev/null 2>&1