Difference between revisions of "Cronjobs in Unix and Solaris"

Revision as of 19:07, 27 June 2007

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 :

Crontab Restrictions, Crontab Commands, Crontab file - syntax, Crontab Example, Crontab Environment, Disable Email, and Generate log file for crontab activity

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.

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.)

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: monthday and weekday. If both are specified in an entry, they are cumulative .

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.

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.

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

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