How to Cron Job in CentOS 7/8

 A cron job is scheduled operation which can execute a script or an API at any defined interval.

To create a cron job in a CentOS Server you’ll need to edit a file called CronTab.

CronTab is a configuration file that contained commands that are read by Cronie, a command scheduler that is supposedly already installed in your CentOS server.

To edit the crontab file,

EDITOR=nano crontab -e

To creadte a scheduled task in Cron,

* * * * ping http://www.example.com

The first 5 fields of a crontab command define

• minute (0-59)
• hour (0-23)
• day of the month (1-31)
• month (1-12)
• day of the week (0-6)

An asterisk, * in a time runs the command at every valid value.
After first five field that define the schedule, next field is a shell command that is supposed to be executed.

Example,
To ping a web address at every minute,

* * * * ping http://www.example.com

To run a php script at every 2 minutes

*/2 * * * * php /var/www/html/myscript.php

To hit an GET REST API every week

0 0 * * 0 curl https://example.com/api/some_api

You can test out the required fields values for creating a schedule on crontab.guru

Some of the examples for creating a schedule

• * * * * * – Runs the command every minute.
• 12 * * * * – Runs the command 12 minutes after every hour.
• 0,15,30,45 * * * * – Runs the command every 15 minutes.
• */15 * * * * – Runs the command every 15 minutes.
• 0 4 * * * – Runs the command every day at 4:00 AM.
• 0 4 * * 2-4 – Runs the command every Tuesday, Wednesday, and Thursday at 4:00 AM.
• 20,40 */8 * 7-12 * – Runs the command on the 20th and 40th minute of every 8th hour every day of the last 6 months of the year.