What is a crond? What is its use?

crond is a daemon that executes commands that need to be run regularly according to some schedule. Theschedule and corresponding commands are stored in the file /etc/crontab.Each entry in the /etc/crontab file entry contains six fields separated by spaces or tabs in the following form:

 

minute           hour  day_of_month      m onth  weekday    command  

 

These fields accept the following values:minute 0 through 59 hour 0 through 23 day_of_month 1 through 31 month 1 through 12 weekday 0 (Sun)through 6 (Sat) command a shell commandYou must specify a value for each field. Except for the command field, these fields can contain the following:

•

A number in the specified range, e.g. to run a command in May, specify 5 in the

month

field.

Two numbers separated by a dash to indicate an inclusive range, e.g. to run a cron job on Tuesdaythrough Friday, place 2-5 in the

weekday

field.

•

A list of numbers separated by commas, e.g. to run a command on the first and last day of January, youwould specify 1,31 in the

day_of_month

field.

•

* (asterisk), meaning all allowed values, e.g. to run a job every hour, specify an asterisk in the

hour 

field.You can also specify some execution environment options at the top of the /etc/crontab file:SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=rootTo run the calendar command at 6:30am. every Mon, Wed, and Fri, a suitable /etc/crontab entry would be:30 6 * * 1,3,5 /usr/bin/calendar The output of the command will be mailed to the user specified in the MAILTO environment option.You don't need to restart the cron daemon crond after changing /etc/crontab - it automatically detects changes.