Cron scheduler and task management with crontab

Cron (Command Run ON) is a system for automatically running programs and scripts on a server at specified times.

Job syntax

Each cron job consists of six columns separated by spaces. The first five define the schedule, the sixth is the command to run:

* * * * * command to execute
| | | | |
| | | | └── Day of week (0–7, where both 0 and 7 = Sunday)
| | | └──── Month (1–12)
| | └────── Day (1–31)
| └──────── Hour (0–23)
└────────── Minute (0–59)

Each time field accepts:

• a specific value: 5
• a comma-separated list: 1,4,22
• a range: 4-9
• all possible values: *
• a step value: */3 — every three units

Examples

Run a script every hour:

0 */1 * * * /usr/local/bin/php /home/login/html/cron.php

Run a script every three hours:

0 */3 * * * /usr/local/bin/php /home/login/html/cron.php

Run a script every three hours on Tuesdays and Fridays:

0 */3 * * 2,5 /usr/local/bin/php /home/login/html/cron.php

Run a script every Monday at 1:15 AM:

15 1 * * 1 /usr/local/bin/php /home/login/html/cron.php

Run a script on April 5th every year at 0:01 AM:

1 0 5 4 * /usr/local/bin/php /home/login/html/cron.php

Run a script on Friday the 13th at 13:13:

13 13 13 * 5 /usr/local/bin/php /home/login/html/cron.php

If a job is resource-intensive and only needs to run once a day, schedule it for off-peak hours — between 2 AM and 8 AM, when server load is at its lowest.

Managing jobs with crontab

Open the current user's crontab for editing:

crontab -e

Open alice's crontab:

crontab -u alice -e

List the current user's jobs:

crontab -l

List alice's jobs:

crontab -u alice -l

User crontab files are stored in /var/spool/cron/.