How is the default runlevel specified in a SysV init system?

In a SysV init system, the default runlevel is specified in the file /etc/inittab. This file contains instructions for the init process, including the default runlevel that the system should enter during the boot process. When the system starts, init reads /etc/inittab and uses the specified runlevel to determine which services and processes to start.

Each runlevel corresponds to a specific operational state for the system, such as multi-user mode or single-user mode, and these runlevels dictate which services are started or stopped. By defining the default runlevel in /etc/inittab, system administrators can control the initial state of the system after booting.

Other options are not applicable for specifying the default runlevel in a SysV init environment. For instance, /etc/runlevel does not exist as a standard configuration file in these systems. Similarly, /etc/systemd is associated with the systemd initialization system, which is different from SysV init, while /etc/init.d contains scripts that manage services but does not define the default runlevel.