crontab -e

minuts 0-69
hours 0-24
days 0-30
month 0-12
years

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: 
month day and weekday. If both are specified in an entry, 
they are cumulative meaning both of the entries will get 
executed .

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/*