LPI Linux Certification in a Nutshell, 2nd Edition

Objective 1: Customizing System Startup and Boot Processes

In Chapter 14, we discussed boot procedures for a Linux system. Specifically, we covered boot-time kernel parameters, boot initialization scripts, and how to change system runlevels. The 201 exam takes this subject a step further and addresses how to customize your system's startup procedures.

When the kernel has mounted the root filesystem, it executes /sbin/init. This program's first task is to read /etc/inittab, which defines everything done thereafter.

/etc/inittab

The /etc/inittab file describes what processes are started at boot time and during normal operation. This file is well-documented. For the examples in this section, we'll use a typical inittab file from a Red Hat system. After some initial comments, the file might contain:

# 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, with no  networking)
#   3 - Full multiuser mode
#   4 - unused
#   5 - X11
#   6 - reboot (Do NOT set initdefault to this)
#
id:3:initdefault:

The comments explain when the various runlevels are used on this machine. The first noncomment line is the one tagged id:, which defines the default runlevel for this system. As explained in the listing, this is multiuser mode with no X Window System. On a Debian system, the default runlevel is 2. Debian specifies only that 2–5 are multiuser. In virtually all distributions, runlevel 0 is used to halt ...