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After the kernel starts a program called init, the init process reads the file "/etc/inittab" and uses this file to determine how to create processes. It can boot OS to a certain state characterized by unique set of daemons, called runlevel.

The administrator can also dynamically change the current runlevel by using the init program.

The default runlevel into which the system is booted can be changed by editing the "/etc/inittab" file.

Which services are started in which runlevels can be managed with the chkconfig tool, which keeps its configuration settings in a set of link to scripts contained in /etc/rc.d/.

/sbin/chkconfig --list lists all the services controlled by chkconfig and whether they are on/off for each runlevel. Setting a service A controlled by chkconfig, for levels X, Y and Z is achieved using /etc/chkconfig --level XYZ A

See Runlevel - Wikipedia, the free encyclopedia

Default level is defined in /etc/inittab, for example id:5:initdefault

The inittab file

The "/etc/inittab" file tells init which runlevel to start the system at and describes the processes to be run at each runlevel. An entry in the inittab file has the following format:


Note the order of programs to run as specified above are:

  1. /etc/rc.d/rc.sysinit
  2. /etc/sbin/update
  3. /etc/rc.d/rc 3 - Note: we are running runlevel 3 here.

Therefore, the next thing that the system does is to run the rc.sysinit file, save buffers to the hard drive, then run system script files for the requested runlevel which will start up many system and network services as explained in the next section.

Determining the Default Boot runlevel

The default boot runlevel is set in the file /etc/inittab with the initdefault variable. When set to 3, the system boots up with the text interface on the VGA console; when set to 5, you get the GUI. Here is a snippet of the file (delete the initdefault line you don't need):

# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
id:3:initdefault:                         # Console Text Mode
id:5:initdefault:                         # Console GUI Mode

Note the following:

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The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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Last modified: March, 12, 2019