Cron Jobs
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/*
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
___________
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
____________
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
________________
To collect the cron execution execution log in a file :
30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log
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