Crontab is a program used to schedule tasks to run automatically on a computer. It is commonly used to automate system maintenance or administration. With crontab, you can run a command or script at a specific time, day, week, or month. For example, you could use crontab to automatically run a backup script every night at midnight, or to send yourself an email reminder every Monday morning. To use crontab, you must have access to a Unix-like operating system, such as Linux or macOS.

To run a cron job every 3 days, you would use the following syntax in your crontab file:

0 0 */3 * * command_to_run

This cron job would run every day at midnight, and the */3 part of the syntax specifies that the job should run every 3 days.

Here’s a brief explanation of the other parts of the syntax:

• The first two numbers (0 and 0 in this case) specify the hour and minute when the job should run. In this case, the job will run at midnight (hour 0 and minute 0).
• The next two numbers (* and *) specify the day of the month and the month when the job should run. The * means that the job should run every day of the month and every month.
• The last number (*) specifies the day of the week when the job should run. The * means that the job should run every day of the week.

If you need more help with cron syntax or setting up cron jobs, you can type man crontab into a terminal to read the crontab man page. This will provide detailed information about the crontab file format and the options you can use in your cron jobs.