Unix Commands - A quick reference to commonly used unix commands . Solaris Network Configuartion - Simplified : Quick reference to setting up network in Solaris system Performance Monitoring -  iostat , vmstat & netstat - Introduction to performance monitoring tools with example and command  syntax. crontab in Unix - A Quick Reference to setting up and using cron jobs in unix . Veritas Volume Manager-1:  Using vxdiskadm to add &  manage disks & disk groups . Veritas Volume Manager-2:  Using vxassist to create ,configure &  manage volumes . Admin's Guide to Solstice Disk Suite: Complete practical reference including root mirroring and trouble shooting. Securing Solaris :  A few suggestions to make your Solaris system secure from internal and external intrusion.   Solaris Installation Step by step guide to Solaris Installation. Jumpstart Server Network  installation of Solaris over one or multiple systems.  Booting Process Details of Solaris  booting process.  Booting Problems in Solaris Common booting related error messages and their possible solution DNS Server Setting up DNS server Trouble Shooting DNS  Some of the configuration and nslookup related errors explained. NIS+ Server , Clients setup and commands.  OpenBoot Parameters Reference table of important parameters and their values. Solaris Error Messages Alphabetical listing of common Solaris Error Messages and their explanation Checking & Repairing File systems with fsck   fsck operation ,syntax and explanation of some common error messages Vi Quick  Reference    vi basics for  reference explanations.   Open Directory    Unix , Solaris Administration  Sun.com    BigAdmin       FAQs unix , unix programming , VI  ,Sendmail  , Bind ssh ,BASH ,Security, Secure unix programming, Unix socket  SCSI Solaris 2 ,  Solaris x86  ,NIS+ FreeBSD ,RedHat , HPUX : Unix Unix History and timeline , Overview of the UNIX  Microsoft Windows NT Server 4.0 versus UNIX Overview of Unix Commands  Unix for advance Users , Unix Bourne Shell Guide with Ksh , Korn Shell BASH reference Guide,  : Solaris Documentation Solaris 7 System Administration Collection Solaris 8 System Administrator Collection Solaris 9 System Administartion Collection Creating Solaris Packages Multibooting Solaris  : Sun Sun System Handbook Sun Hardware Info. : Documentation Home Sun , BSD  ,Linux ,Hp,Veritas : Patches & Softwares Solaris Patches ,  Solaris Freeware ,  GNU Software,  big brother  ,  webmin .com ,  bugzilla.org : Certification Info  Sun  ,Red Hat , HP

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

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

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

 

8. Next Steps

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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