Cron

vim /etc/crontab

In this file you should see this:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/ 

# For details see man 4 crontabs 

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed

In /etc, there are five directories associated with cron: cron.d, cron.daily, cron.hourly, cron.monthly and cron.weekly

To enable the scripts in these directories to be run you need to add them to the crontab, so, /etc/crontab should look like this:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/ 

# For details see man 4 crontabs 

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly 

What this means is that:
cron.hourly will run every hour on the first minute.
cron.daily will run at 4:02 am everyday
cron.weekly will run every week at 4:22 am on Sunday
cron.monthly will run on the first of every month at 4:42 am 

I added a script to clean out the /tmp directory every night like this: In /etc/cron.daily/ I created a file called clean_tmp. I added the lines:

#!/bin/bash 

rm -rf /tmp/*

Saved and quit. Then I typed chmod a+x clean_tmp That’s it!