Answer / kiranraj

1. Crontab Restrictions
____________
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
__________
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
___________
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
_______

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/*

Answer / ashok

crontab -e

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

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