BootProcess > init
Init is the first program to run after your system is booted, and continues to run as process number 1 until your system halts. Init's job is to start other programs that are essential to the operation of your system. All other processes are descended from init.
The system initialization process is handled by the init daemon. In jessie and later releases, /sbin/init is a symbolic link to one of the various init daemons: /lib/systemd/systemd (see systemd) or /lib/sysvinit/init (see below), or some other init system.
Only systemd is fully supported; sysvinit is mostly supported, but Debian packages are not required to provide sysvinit start scripts. runit is also packaged, but has not received the same level of testing and support as the others.
The remainder of this page describes the sysvinit init daemon, which was the default in wheezy and earlier releases, and which is a supported alternative in jessie and later releases.
id:2:initdefault: si::sysinit:/etc/init.d/rcS l2:2:wait:/etc/init.d/rc 2
Which causes the files /etc/init.d/rcS to run initialization scripts (in /etc/rcS.d/*), then scripts for requested runlevel (in /etc/rcS.[0-9]/*).
- update-rc.d, the equivalent to chkconfig