File in Ubuntu equivalent to /etc/inittab file in RedHat

Ubuntu uses Upstart, an alternative to sysvinit, which works completely differently. The Upstart CookBook discusses runlevels and how to work with them.

Generally, though, the RedHat concept of runlevels isn't used on Debian or derivatives, including Ubuntu; they have only "single user" or "everything running", without any of the "network, but not graphics" style distinctions that RedHat draws.

As mentioned Ubuntu does now use Upstart.

The /etc/inittab and the scripts from /etc/init.d or /etc/rc.d/* have been incorporated into several scripts in the /etc/init directory or /etc/event.d (for older versions of Upstart).

Look for the item you want to modify to have a script in that directory, (serviceX) /etc/init/serviceX.conf and modify it accordingly. You can find the syntax and additional information online.

However, if you need to run a program that depends on SysV Upstart has a compatibility mode. Programs that still use /etc/rc.d/* can put scripts in there and have them still run.

As I was looking at the scripts in /etc/init, there was one named rc-sysinit.conf - and it provides the ability to parse the /etc/inittab and determine the default runlevel from this file should you add one.