Cron Tab

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.

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)

Example:

0 6 * * 1 cd /data/test; ksh test.sh 1>>/logs/Test.log 2>>/logs/TestError.log

•  Fist 0 indicates the min
•  second "6" indicates the hour 6
•   last "1" indicates the Moday

Here cron job will run every Monday 6 AM

•  Test.log contians the normal SOP statements
• TestError.log Contains the error and stack statements