To modify /etc/inittab at the time of installation, using the sed class, you must complete the following tasks:
Add the sed class script to the prototype file.
The name of a script must be the name of the file that will be edited. In this case, the file to be edited is /etc/inittab and so the sed script is named /etc/inittab. There are no requirements for the mode, owner, and group of a sed script (represented in the sample prototype by question marks). The file type of the sed script must be e (indicating that it is editable).
Set the CLASSES parameter to include the sed class.
As shown in the example file, sed is the only class being installed. However, it could be one of any number of classes.
Create a sed class action script.
Your package cannot deliver a copy of /etc/inittab that looks the way you need it to, since /etc/inittab is a dynamic file and you have no way of knowing how it will look at the time of package installation. However, using a sed script allows you to modify the /etc/inittab file during package installation.
Create a postinstall script.
You need to execute the init q command to inform the system that /etc/inittab has been modified. The only place you can perform that action in this example is in a postinstall script. Looking at the example postinstall script, you will see that its only purpose is to execute the init q command.
This approach to editing /etc/inittab during installation has one drawback; you have to deliver a full script (the postinstall script) simply to perform the init q command.