Using cron to run programs on a schedule

Using cron to run programs on a schedule is very useful feature :-

Cron is a Linux system process that will execute a program at a preset time, To use cron you must prepare a text file that describes the program that you want executed & the times that cron should execute them, Then you use the crontab program to load the text file that describes the cron jobs into cron.

Here is the format of a cron job file:

[min] [hour] [day of month] [month] [day of week] [program to be run]

where each field is defined as
[min] Minutes that program should be executed on. 0-59. Do not set as * or the program will be run once a minute.
[hour] Hour that program should be executed on. 0-23. * for every hour.
[day of month] Day of the month that process should be executed on. 1-31. * for every day.
[month] Month that program whould be executed on. 1-12 * for every month.
[day of week] Day of the week. 0-6 where Sunday = 0, Monday = 1, ...., Saturday = 6. * for every day of the week.
[program] Program to be executed, Include full path information.

Here are some examples below:-

0,15,30,45 * * * * /usr/bin/foo

Will run /usr/bin/foo every 15 minutes on every hour, day-of-month, month, and day-of-week. In other words, it will run every 15 minutes for as long as the machine it running.

10 3 * * * /usr/bin/foo

Will run /usr/bin/foo at 3:10am on every day.

10 * 1 * * /usr/bin/foo

Will run /usr/bin/foo at 12:10am on the first day of the month.

10 * * 1 * /usr/bin/foo

Will run /usr/bin/foo at 12:10am on the first month of the year.

10 14 * * 1 /usr/bin/foo

Will run /usr/bin/foo at 2:10pm on every Monday.


You must use crontab to load cron jobs into cron. First create a text file that uses the above rule to describe the cron job that you want to load into cron. But before you load it, type crontab -l to list any jobs that are currently loaded in crontab.

If none are listed, then it is safe to load your job,Example. If you wanted to run /usr/local/bin/foo once a day at 3:10am, then create a text file

10 3 * * * /usr/bin/foo

Save it as foo.cron. Then type crontab foo.cron. Check to see if it was loaded by typing crontab -l. It should display something like this:

# DO NOT EDIT THIS FILE - edit the master & reinstall.
# (ipwatch.cron installed on Thu Nov 18 11:48:02 2009)
# (Cron version -- $Id: crontab.c,v 2.13 2004/01/17 03:20:37 vixie Exp $)
10 3 * * * /usr/bin/foo

If you want to edit the cron job, then edit foo.cron and then remove the existing cron job (crontab -r) and load it again (crontab foo.cron). You can have multiple jobs. Just put each different one on a seperate line in foo.cron.

contab jobs will run under the user that was in effect when you loaded the job in crontab.