I have a shell script in my Ubuntu Server 14.04 LTS at ./ShellScript.sh. I setup /etc/rc.local to run the shell script after boot but before login using below code.

Run this: sudo nano /etc/rc.local then add following and save.

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.

#!/bin/bash

./ShellScript.sh

exit 0

Now I want to run/execute this shell script again and again having 15min of time interval between every run after boot but before login. So Can I do it?

Update 1:)

When I run crontab -e then I got the following. Now What to do?

no crontab for root - using an empty one
Select an editor.  To change later, run 'select-editor'.
1. /bin/ed
2. /bin/nano        <---- easiest
3. /usr/bin/vim.basic
4. /usr/bin/vim.tiny
Choose 1-4 [2]:

After selecting 2, I got crontab: "/usr/bin/sensible-editor" exited with status 2

UPDATE 2:)

Update ShellScript.sh like below...

#!/bin/bash
# Testing ShellScript...
while true
do
 echo "ShellScript Start Running..."
 **********************************
 All My Shell Script Codes/Script/Commands
 **********************************
 echo "ShellScript End Running..."
 exit 0
sleep 900
done

Then Run this: sudo nano /etc/rc.local then add following and save.

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.
sh ./ShellScript.sh &
exit 0