Section 24.2. Boot, Initialization, Shutdown, and Runlevels (Topic 1.106)

24.2. Boot, Initialization, Shutdown, and Runlevels (Topic 1.106)

24.2.1. Objective 1: Boot the System

24.2.1.1. LILO, the Linux loader

• LILO is a utility designed to load a Linux kernel (or another operating system) into memory and launch it. It has two parts:

  
  

  The boot loader

  A two-stage program intended to find and load a kernel. The first stage resides in the disk boot sector and is started by the system BIOS. It locates and launches a second, larger stage residing elsewhere on disk.

  
  

  The lilo command

  The map installer, used to install and configure the LILO boot loader. It reads /etc/lilo.conf and writes a corresponding map file.

• The /etc/lilo.conf file contains options and kernel image information. Popular directives are:

  
  

  boot

  The name of the hard disk partition that contains the boot sector.

  
  

  image

  Refers to a specific kernel file.

  
  

  install

  The file installed as the new boot sector.

  
  

  label

  Provides a label, or name, for each image.

  
  

  map

  Directory where the map file is located.

  
  

  prompt

  Prompts the user for input (such as kernel parameters or runlevels) before booting and without a keystroke from the user.

  
  

  read-only

  The root filesystem should initially be mounted read-only.

  
  

  root

  Used following each image, this specifies the device that should be mounted as root.

  
  

  timeout

  The amount of time, in tenths of a second, the system waits for user input.

24.2.1.2. Kernel parameters and module configuration

• LILO can pass kernel parameters using name=value pairs.

• Linux kernels are modular, with portions of kernel functionality compiled as modules to be used as needed.

• Parameters to modules can be specified in /etc/conf.modules.

24.2.1.3. Boot-time messages

• The kernel gives detailed status information as it boots. This information can also be found in system logs such as /var/log/messages and from the dmesg command.

24.2.2. Objective 2: Change Runlevels and Shut Down or Reboot System

• Runlevels specify how a system is used by controlling which services are running.

• Runlevels are numbered through 6, as well as with a few single characters.

• Runlevel 0 implies system shutdown.

• Runlevel 6 implies system reboot.

• The intermediate runlevels differ in meaning among distributions.

• Runlevel 1 (also s or S) is usually single-user (maintenance) mode.

• Runlevels 2 through 5 usually define some kind of multiuser state, including an X login screen.

24.2.2.1. Single-user mode

• Runlevel 1 is a bare-bones operating environment intended for maintenance. Remote logins are disabled, networking is disabled, and most daemons are shut down.

• Single-user mode can be entered with the single, or simply 1, parameter at the LILO prompt.

• Switching to single-user mode is done using init 1.

24.2.2.2. The /etc/rc.d directory

• The /etc/rc.d file contains initialization scripts and links controlling the boot process for many Linux distributions:

  
  

  rc.sysinit

  The startup script launched by init at boot time

  
  

  rc.local

  A script for local startup customizations, started automatically after the system is running

  
  

  rc

  A script used to change runlevels

  
  

  init.d

  The directory containing scripts to start and stop system services

  
  

  rc0.d through rc6.d

  Links to scripts in init.d

• Names of the links are [K|S][nn][init.d_name]:

  • K and S prefixes mean kill and start, respectively.

  • nn is a sequence number controlling startup or shutdown order.

  • init.d_name is the name of the script being linked.

24.2.2.3. Default runlevel, determining runlevel, changing runlevels

• The default runlevel is located in /etc/inittab on the line containing initdefault:

  
  

  id:n:initdefault:

  n is a valid runlevel number such as 3.

• Runlevel is determined by the runlevel command, which displays the previous and current runlevels. An N for previous runlevel indicates that the runlevel has not changed since startup.

• Runlevels can be changed using init:

  
  

  init n

  Change to runlevel n.

• System shutdown can also be initiated using shutdown:

  
  

  shutdown time

  Bring the system down in a secure, organized fashion. time is mandatory, in the form of hh:mm, now, or +n for n minutes.