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7 System Administration Collection
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8 System Administrator Collection
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| |
Crontab - Quick reference
Setting up cronjobs in Unix and Solaris
|
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
____________
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
To collect the cron execution execution log in a file :
30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log
This article covered
a significant aspect of system administration of setting up cronjobs .
Unix administration involves lots of different tasks and some of these tasks are
covered in this website but still there are many areas not covered here .
Following books available for online buying from Amazon.com . You
should have following two books in your bookshelf for ready
reference if you are involved in Unix system administration .
Essential
System Administration, Third Edition
by by Æleen Frisch
Solaris
Operating Environment Boot Camp
by David Rhodes, Dominic Butler
If you already own these books the amazon display panel below shows
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