If you are a GNU/Linux user, their is a high chance that you have used live media images i.e. live CD, live DVD or ISO. I was curious about  how live images are created and recently got a chance to do some hands on.

There are multiple tools available for creating live media. However I am going to use livecd-tools to create live ISO. livecd tools need kickstart files to create the images and we will use CentOS upstream kickstart files [1].

As a first step install livecd-tools [2]. I using CentOS Vagrant box to create live ISOs.

$ sudo yum install livecd-tools git -y
$ git clone https://github.com/CentOS/sig-core-livemedia
$ cd sig-core-livemedia/kickstarts

#You can use any one of the .cfg files in /sig-core-livemedia/kickstarts
$ livecd-creator --config <Kickstart file>

If you are new to kickstart files and want to know more about it refer the documentation: https://github.com/rhinstaller/pykickstart/blob/master/docs/kickstart-docs.rst

[1] https://github.com/CentOS/sig-core-livemedia

[2] https://github.com/rhinstaller/livecd-tools

RHEL 7 and CentOS 7 moved to systemd from System V init facility.  So if we want to run a script at boot,  it is highly recommended that you should write a systemd unit file.  However to ease the transition to systemd, CentOS7 /RHEL7 offers a backward compatibility mode for init scripts.

In a System V init system you need to add an entry of the script to /etc/rc.local (or equivalent) and the script will be executed during boot at the end of multi user run level.

In CentOS7 /etc/rc.local is a symbolic link to /etc/rc.d/rc.local.

root@localhost ~]# ls -l /etc/rc.local 
lrwxrwxrwx. 1 root root 13 Apr  4  2016 /etc/rc.local -> rc.d/rc.local

To enable the init compatibility mode, you need to make  /etc/rc.d/rc.local executable and then add the script to /etc/rc.local.

Here are the steps you need to perform to to start a script e.g. hello.sh during boot.

$ chmod +x /etc/rc.d/rc.local

#make the script executable
$ chmod +x hello.sh

#Make a entry in /etc/rc.local
$ cat /etc/rc.local
!/bin/bash
# THIS FILE IS ADDED FOR COMPATIBILITY PURPOSES
#
# It is highly advisable to create own systemd services or udev rules
# to run scripts during boot instead of using this file.
#
# In contrast to previous versions due to parallel execution during boot
# this script will NOT be run after all other services.
#
# Please note that you must run 'chmod +x /etc/rc.d/rc.local' to ensure
# that this script will be executed during boot.

touch /var/lock/subsys/local

/root/hello.sh

Reference : https://www.centos.org/forums/viewtopic.php?t=48140