cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

Following points sum up the crontab functionality :

1. Crontab Restrictions
2. Crontab Commands
3. Crontab file - syntax
4. Crontab Example
5. Crontab Environment
6. Disable Email
7. Generate log file for crontab activity

8. Next Steps

1. Crontab Restrictions
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You can execute crontab if your name appears in the file
/usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file
/usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.

2. Crontab Commands
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export EDITOR=vi ;to specify a editor to open crontab file.

crontab -e     Edit your crontab file, or create one if it doesn't already exist.
crontab -l      Display your crontab file.
crontab -r      Remove your crontab file.
crontab -v      Display the last time you edited your crontab file. (This option is only available on a few systems.)
 

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

*     *   *   *    *  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)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

Note: The specification of days can be made in two fields: monthday and weekday. If both are specified in an entry, they are cumulative .

4. Crontab Example
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A line in crontab file like below  removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30     18     *     *     *         rm /home/someuser/tmp/*

 

Changing the parameter values as below will cause this command to run at different time schedule below :

min	hour	day/month	month	day/week	Execution time
30	0	1	1,6,12	*	-- 00:30 Hrs  on 1st of Jan, June & Dec.
:
0	20	*	10	1-5	--8.00 PM every weekday (Mon-Fri) only in Oct.
:
0	0	1,10,15	*	*	-- midnight on 1st ,10th & 15th of month
:
5,10	0	10	*	1	-- At 12.05,12.10 every Monday & on 10th of every month
:

Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.

5. Crontab Environment
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cron invokes the command from the user's HOME directory with the shell, (/usr/bin/sh).
cron supplies a default environment for every shell, defining:
HOME=user's-home-directory
LOGNAME=user's-login-id
PATH=/usr/bin:/usr/sbin:.
SHELL=/usr/bin/sh

Users who desire to have their .profile executed must explicitly do so
in the crontab entry or in a script called by the entry.

6. Disable Email
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By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

>/dev/null 2>&1

7. Generate log file
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To collect the cron execution execution log in a file :

30 18  *    *   *    rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log