Cron Jobs (Automated Processes)

Cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon.

Cron job is an automated process that operates at predefined time intervals. For example, reindexing your Web site using Excite at 3:00am every morning.

Cron daemon is a long-running process that executes commands at specific dates and times. You can use this to schedule activities, either as one-time events or as recurring tasks.

For commands that need to be executed repeatedly (e.g., hourly, daily, or weekly), you can use the crontab command. The crontab command creates a crontab file containing commands and instructions for the cron daemon to execute. You can use the crontab command with the following options:crontab -a filename Install filename as your crontab file. On many systems, this command is executed simply as crontab filename (i.e., without the -a option).

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 available on only a few systems.)
crontab -u  user   Used in conjunction with other options, this option allows you to modify or view the crontab file of user. When available, only administrators can use this option.

Each entry in a crontab file consists of six fields, specifying in the following order:

minute(s) hour(s) day(s) month(s) weekday(s) command(s)

The fields are separated by spaces or tabs. The first five are integer patterns and the sixth is the command to execute. The following table briefly describes each of the fields:Field Value Description

minute 0-59     The exact minute that the command sequence executes
hour 0-23         The hour of the day that the command sequence executes
day 1-31            The day of the month that the command sequence executes
month 1-12       The month of the year that the command sequence executes
weekday 0-6      The day of the week that the command sequence executes (Sunday = 0, Monday = 1, Tuesday = 2, and so forth)
command Special The complete sequence of commands to execute. The command string must conform to Bourne shell syntax. Commands, executables (such as scripts), or combinations are acceptable.

Each of the patterns from the first five fields may be either * (an asterisk), meaning all legal values, or a list of elements separated by commas. An element is either a number or an inclusive range, indicated by two numbers separated by a minus sign (e.g., 10-12). You can specify days with two fields: day of the month and day of the week. If you specify both of them as a list of elements, cron will observe both of them;

for example:

0 0 1,15 * 1 /mydir/myprogram

The cron daemon would run the program myprogram in the mydir directory on the first and fifteenth of each month, as well as on every Monday. To specify days by only one field, the other field should be set to * , for example:

0 0 * * 1 /mydir/myprogram

In the above example, the program would run only on Mondays.

If a cron job specified in your crontab entry produces any error messages when it runs, you will get a mail message reporting the errors.

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

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.