Resumen crontab

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 notappear 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 consistof 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 itdoesn'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 afew 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 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 rangeseparated 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 ofthe 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/*
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,...