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.

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.

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 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 .

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 :

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.

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 .

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