System Initialization (old stuff)

System V init (sysvinit)

is based on a runlevel concept. The SysV init examines /etc/inittab for a line that sets a default runlevel:

id:3:initdefault:

If there is no such line, user is prompted to enter runlevel value manually. The initialization process is synchronous, i.e., the next task wouldn't start until the current one is finished. The init daemon runs scripts in /etc/init.d dir. The order of execution is defined by the symlinks in /etc/rc.d dir. The names of these symlinks are interpreted as follows: first letter S means that the script must be invoked during startup, first letter K - during shutdown, digits set the order of invocation. By modern standards sysvinit is slow and resource-consuming. It is also inconvenient for the desktop systems, that require fast and reliable interaction with USB devices, removable storage, etc.

Upstart

is an event-based system, that starts and stops services and tasks asynchronously, and supervises them while OS is running. The time and the order [of the specific service start] depend on a set of conditions (like the presence of processes that a service depends upon). The main Upstart programs are the init (daemon) and the initctl (it starts and stops init jobs, sends signals, and queries the status of jobs. The Upstart init daemon tries to perform tasks in parallel to finish the boot procedure as fast as possible. Upstart service config files (equivalent of Sys V init scripts) are in /etc/init dir. Here is an example of a simple service config file for a permanently running daemon (server application listening on some tcp port); this config file provides start/stop and auto restart in case of an abnormal exit:

# htpasd - this server app provides ... ...;

description "third-party daemon"

start on (net-device-up and local-filesystems)
stop on runlevel [!2345]

expect fork
respawn

exec htpasd

You can find details using  man 5 init  and multiple examples in /etc/init (if your system is Upstart-based, of course).

Classical /etc/inittab is not neccessary any more, though it can be present. To set the default runlevel, edit

/etc/init/rc-sysinit.conf

Upstart consists of 5 packages:

upstart

init daemon, initctl;

upstart-logd

logd daemon and job definition file for logd;

upstart-compat-sysv

SysV job definition files and utilities: reboot, runlevel, shutdown, telinit;

startup-tasks

job definition files for system startup tasks;

system-services

job definition files for tty services;

See usage examples in initctl.

Though Upstart is backward-compatible with sysvinit and runs the old-style scripts unmodified, you should not rely on the old tools like chkconfig, rc-sysv-conf, ntsysv - they control old-style services only. The new-style services are being started and stopped if/when the pre-defined conditions are met. To prevent the start of a new-style service during system start, change the extension of the corresponding config file, e.g.:

cd /etc/init

mv dovecot.conf dovecot.disabled

stop dovecot

You also have such options as moving the service .conf file to some dir, or totally deleting it (unwise), or removing the appropriate software pkg from the system (should be carefully considered). The last cmd (stop) is required if the service is currently running.

Runlevel

defines the certain system's state, characterized by the set of the running services (daemons). It is a System V paradigm. The last Ubuntu releases emulate runlevels for backward compatibility with the old-style scripts and for compatibility with the third-party applications intended for other distributions.

There could be up to 8 runlevels, but only 3 are mandatory:

The other runlevels are treated differently by different systems. It's supposed, that for each supported runlevel, /etc/inittab has a description of what should be done when the system switches from one runlevel to another.

In Ubuntu/Debian runlevels 2, 3, 5 are all the same (multi-user, GUI). To check the current runlevel:

runlevel

or

who -r