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Subject: Using crontab

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 - 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

2. Crontab Commands

3. Crontab file - syntax

4. Crontab Example

5. Crontab Environment

6. Disable Email

7. Generate log file for crontab activity

8. Next Steps

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/*

If you want to run a script called /home/scriptsample.sh every Thursday at 4 PM, use this syntax:

0 16 * * 4 rm /home/someuser/tmp/*

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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