Setting up CRON jobs on shell scripts in Linux (Ubutu)

Open the CRON job editor

crontab -e

After that you can format the job as below

*     *     *     *     *  YOURCOMMAND
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- Day in Week (0 to 7) (Sunday is 0 and 7)
|     |     |     +------- Month (1 to 12)
|     |     +--------- Day in Month (1 to 31)
|     +----------- Hour (0 to 23)
+------------- Minute (0 to 59)

There are some shorts, too (if you don´t want the *):

@reboot --> only once at startup
@daily ---> once a day
@midnight --> once a day at midnight
@hourly --> once a hour
@weekly --> once a week
@monthly --> once a month
@annually --> once a year
@yearly --> once a year

If you want to use the shorts as cron (because they don´t work or so):

@daily --> 0 0 * * *
@midnight --> 0 0 * * *
@hourly --> 0 * * * *
@weekly --> 0 0 * * 0
@monthly --> 0 0 1 * *
@annually --> 0 0 1 1 *
@yearly --> 0 0 1 1 *

# For example, you can run a backup of all your user accounts

# at 5 a.m every week with:

# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/

//Every 4 hours

This will run the job every 4 hours, on the hours of 00:00, 04:00, 08:00 12:00, 16:00, 20:00.

0 0,4,8,12,16,20 * * * /scripts/test.sh

More info @http://serverfault.com/questions/39004/what-is-the-correct-syntax-to-run-cron-every-4-hours

Start/Stop cron service

sudo service cron restart