Ahmer Zaidi

What is Cron?

Cron is UNIX/Linux service or daemon to execute scheduled commands.

Generally, crontab uses a daemon, crond, which runs constantly in the background and checks once a minute to see if any of the scheduled jobs need to be executed. If so, it executes them. These jobs are generally referred to as cron jobs.

What is Crontab?

Crontab maintain crontab files for individual users.

Crontab Commands

crontab -e

Edit your crontab file, or create one if it doesn’t already exist.

crontab -l

Display your crontab entries.

crontab -r

Remove your current crontab entry.

crontab -u username -e

This option allows you to modify or view the crontab file of a user, only super user can use this option.

crontab -u username -l

Display crontab entries of the particular user.

Crontab Options and Explanation

A crontab file has five fields for specifying time, day and date followed by the command to be run at that interval. Below is Crontab syntax,

*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of        month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

• * Is treated as a wild card. Meaning all possible values.
• */5 Is treated as ever 5 minutes, hours, days, or months. Replacing the 5 with another numerical value will change this option.
• 4,8,12 “,” Treated as an “OR”, so if placed in the hours, this could mean at 2, 8 or 12 o-clock. Or if put in days it would be on 2,8 and12. “,” is used to define more options [date, time, day]
• 2-12 Treats for any value between 2 and 12. So, if placed in day of month this would be days 2 through 12. Or if put in hours it would be between 2 and 12. “-” is used to define the range [date, time]