Linux init process

As we have seen before, the usual boot sequence of a Linux-based platform consists of several steps :

The init process :

• completes the boot procedure
• have the process ID '1'
• starts the system processes (as described in /etc/inittab)
• is not part of the kernel, it belongs to the user space ( /sbin/init )
• never dies : it handles the operating system's life cycle (restart, shutdown,..)

There are many alternatives for the init process, it is usually installed with one of the following startup programs :

• sysvinit : linux traditional init program
• upstart : used by most of the linux distributions
• systemd : Fedora
• busybox : for small embedded systems

When built, these projects generate an init process and a set of common tools like telinit or initctl.

Run level

At boot, init checks the inittab configuration file for the runlevel parameter. This value goes from 0 to 6 and a specific set of actions is executed for each of them.

Depending on the default init level setting, the system will execute the programs from one of the following folders :

• Run level 0 : /etc/rc.d/rc0.d
• Run level 1 : /etc/rc.d/rc1.d
• Run level 2 : /etc/rc.d/rc2.d
• Run level 3 : /etc/rc.d/rc3.d
• Run level 4 : /etc/rc.d/rc4.d
• Run level 5 : /etc/rc.d/rc5.d
• Run level 6 : /etc/rc.d/rc6.d

In these directories, the program names start with either S or K followed with a sequence number and the program name.

• The programs starting with an S are executed on system Start while the programs starting with a K are executed on system Kill. 
• The sequence number indicates when the program has the be executed.

For example, S12syslog has to be started before S80sendmail.

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