Linux Boot

When a GNU/Linux system starts up,first the system’s kernel is loaded, then the first process begins; this process is called init and it has to execute and activate the rest of the system, through the management of different runlevels.

A runlevel is basically a configuration of programs and services that will be executed in order to carry out determined tasks.
The different runlevels carry different meanings, for instance
1 – single-user mode
3 – multi-user mode
6 – reboot

When the init process begins, it uses a configuration file called /etc/inittab to decide on the execution mode it will enter. This file defines the runlevel by default (initdefault) at startup.

Afterwards, according to the selected runlevel, the system will consult the files contained in /etc/rcn.d where n is the selected runlevel which contains a list of services that must be started.

A series of system commands help us to handle the runlevels, we must mention:
1. The scripts in /etc/init.d/ allow us to start up, halt or reboot the individual services.
2. telinit allows us to change the runlevel.we simply have to indicate the number.

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