UNIX Crontab

Cron is used to execute tasks as background job at the specific time in UNIX system. Crontab (Cron Table) is a file which contains the schedule of cron entries to be run at specified times.

1. Crontab Restriction
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
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval



* 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).

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 :



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 Mail
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

Using Multiple Yahoo Messenger Accounts at the Same Time

If you have several Yahoo Messenger Accounts, and want to use them at the same time on the same machine, you can still use Yahoo Messenger GUI. You do not have to use your account in the YM, and then use the other one in the web-based messenger or Yahoo Messenger substitute like GAIM. The substitute will not allow you use some features like buzz, avatar, audible, and background. Even the Webcam can not be used. The default configuration will not let you to perform this action. But with a little modification, it can happen.

1. Click Start, and select Run. You can also press windows+R
2. Type regedit
3. Select HKEY_CURRENT_USER --> Software --> Yahoo --> pager --> test
4. Right click on the test and create a new DWORD value (32 bit)
5. Rename the value as Plural and set its value to 1

And voila! you can use your accounts on the same machine and at the same time.