crontab UNLEASHED

Your cron job looks like as follows:

1 2 3 4 5 /path/to/command arg1 arg2

Where,

• 1: Minute (0-59)
• 2: Hours (0-23)
• 3: Day (0-31)
• 4: Month (0-12 [12 == December])
• 5: Day of the week(0-7 [7 or 0 == sunday])
• /path/to/command - Script or command name to schedule

Same above five fields structure can be easily remembered with following diagram:

* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Use of operators

An operator allows you to specifying multiple values in a field. There are three operators:

1. The asterisk (*) : This operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.
2. The comma (,) : This operator specifies a list of values, for example: "1,5,10,15,20, 25".
3. The dash (-) : This operator specifies a range of values, for example: "5-15" days , which is equivalent to typing "5,6,7,8,9,....,13,14,15" using the comma operator.

Use special string to save time :

Instead of the first five fields, you can use any one of eight special strings. It will not just save your time but it will improve readability.

Special string	Meaning
@reboot	Run once, at startup.
@yearly	Run once a year, "0 0 1 1 *".
@annually	(same as @yearly)
@monthly	Run once a month, "0 0 1 * *".
@weekly	Run once a week, "0 0 * * 0".
@daily	Run once a day, "0 0 * * *".
@midnight	(same as @daily)
@hourly	Run once an hour, "0 * * * *".

Run ntpdate every hour:

@hourly /path/to/ntpdate

Make a backup everyday:

@daily /path/to/backup/script.sh

Understanding /etc/crontab file and /etc/cron.d/* directories

/etc/crontab is system crontabs file. Usually only used by root user or daemons to configure system wide jobs. All individual user must must use crontab command to install and edit their jobs as described above. /var/spool/cron/ or /var/cron/tabs/ is directory for personal user crontab files. It must be backup with users home directory.

Typical /etc/crontab file entries:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

Additionally, cron reads the files in /etc/cron.d/ directory. Usually system daemon such as sa-update or sysstat places their cronjob here. As a root user or superuser you can use following directories to configure cronjobs. You can directly drop your scripts here. run-parts command run scripts or programs in a directory via /etc/crontab

Directory	Description
/etc/cron.d/	Put all scripts here and call them from /etc/crontab file.
/etc/cron.daily/	Run all scripts once a day
/etc/cron.hourly/	Run all scripts once an hour
/etc/cron.monthly/	Run all scripts once a month
/etc/cron.weekly/	Run all scripts once a week