Cron Job Commands

How Does a Cron Job Work?

1. Cron Daemon (crond):
   • The cron daemon is a program that constantly runs in the background and checks every minute to see if there’s a scheduled task that needs to be executed.
2. Crontab (Cron Table):
   • The crontab is a file where you list all the tasks you want to schedule. Think of it as a “to-do list” for your computer.
3. Format of a Cron Job:
   • Each line in the crontab represents one task.
   • A line has two parts:
     1. Schedule (When the task should run).
     2. Command (What the task should do).

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Basic Format of a Cron Job

A cron job has 5 fields to define the schedule, followed by the command to run:

MINUTE HOUR DAY MONTH WEEKDAY COMMAND

• MINUTE: (0–59) What minute of the hour?
• HOUR: (0–23) What hour of the day?
• DAY: (1–31) What day of the month?
• MONTH: (1–12) Which month?
• WEEKDAY: (0–6) Which day of the week? (0 = Sunday, 1 = Monday, etc.)
• COMMAND: The program or script to run.

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Examples:

1. Reboot the system every day at 5:00 AM:

0 5 * * * /sbin/reboot

• 0: Minute = 0 (on the hour).
• 5: Hour = 5 AM.
• *: Any day of the month.
• *: Any month.
• *: Any weekday.

2. Delete temporary files at midnight on the 1st of every month:

0 0 1 * * rm -rf /tmp/*

• 0 0: At 12:00 AM (midnight).
• 1: On the 1st of the month.
• *: Any month.
• *: Any weekday.

3. Run a backup script every Friday at 6 PM:

0 18 * * 5 /path/to/backup.sh

• 0 18: At 6:00 PM.
• *: Any day.
• 5: Friday.

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How to Set a Cron Job

1. Open your crontab file for editing:

   crontab -e
   

2. Add your task in the correct format.
3. Save and exit the editor.

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Check Your Cron Jobs

• To view your scheduled jobs: ```bash crontab -l