2.3.1. init

The single most important service in a UNIX system is provided by init init is started as the first process of every UNIX system, as the last thing the kernel does when it boots. When init starts, it continues the boot process by doing various startup chores (checking and mounting filesystems, starting daemons, etc).

The exact list of things that init does depends on which flavor it is; there are several to choose from. init usually provides the concept of single user mode, in which no one can log in and root uses a shell at the console; the usual mode is called multiuser mode. Some flavors generalize this as run levels; single and multiuser modes are considered to be two run levels, and there can be additional ones as well, for example, to run X on the console.

Linux allows for up to 10 runlevels, 0-9, but usually only some of these are defined by default. Runlevel 0 is defined as system halt''. Runlevel 1 is defined assingle user mode''. Runlevel 3 is defined as "multi user" because it is the runlevel that the system boot into under normal day to day conditions. Runlevel 5 is typically the same as 3 except that a GUI gets started also. Runlevel 6 is defined as ``system reboot''. Other runlevels are dependent on how your particular distribution has defined them, and they vary significantly between distributions. Looking at the contents of /etc/inittab usually will give some hint what the predefined runlevels are and what they have been defined as.

In normal operation, init makes sure getty is working (to allow users to log in) and to adopt orphan processes (processes whose parent has died; in UNIX all processes must be in a single tree, so orphans must be adopted).

When the system is shut down, it is init that is in charge of killing all other processes, unmounting all filesystems and stopping the processor, along with anything else it has been configured to do.