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Chapter 37 System Startup and Shutdown - Notes

37.3 Learning Objectives:

  • Explain the boot process.
  • Identify several types of boot loaders.
  • Describe what the BIOS does.
  • Identify the relevant configuration files.
  • Describe how the system shuts down and reboots.

37.4 Boot Sequence

Basic steps in boot sequence:

  1. BIOS/UEFI locates and executes the boot program, or boot loader.
  2. Boot loader loads the kernel.
  3. Kernel starts the init process (pid=1).
  4. init manages system initialization, using systemd or the older Upstart and SysVinit startup scripts.

When power applied to computer, computer can only perform operations the BIOS (Basic Input Output System) orders it to do.

First, BIOS runs POST (Power On Self Test), which checks memory and hardware, then searches specific location or device for boot program. Typically, boot program found in device's MBR (Master Boot Record). Control of computer is then transferred to this boot program (usually GRUB).

Boot program then loads kernel into memory and executes it. On x86 platforms (and many others), kernel first has to decompress itself in place. It then performs hardware checks, gains access to important peripheral hardware, and eventually runs init process. This first process continues the system startup, managing either systemd or Upstart, or running appropriate init scripts is SysVinit is being used.

Newer computers are moving to UEFI, a replacement for BIOS, which performs many of the same functions.

boot_sequence_small The Boot Sequence

37.5 BIOS

On x86 architecture, BIOS contains all the code required to gain initial access to the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions. Once full system running, most of these devices will have enhanced capabilities when complete and specialized device drivers can be loaded and take over.

BIOS typically placed in ROM chip that comes with computer (often called ROM BIOS). This ensures that BIOS will always be available and will not be damaged by disk failures. This also makes it possible for computer to boot itself.

During boot process, BIOS loads boot loader from MBR.

37.6 Boot Loaders

Number of different boot loaders used in Linux:

  • GRUB
  • efibootmgr
  • LILO
  • Das U-Boot

Virtually, all (non-embedded) modern Linux distributions use GRUB (GRand Unified Boot Loader). GRUB's features include ability to boot multiple operating systems, both graphical and text-based interface allowing ease of use over serial cable, powerful command line interface for interactive configuration, network-based diskless booting, and other advanced features.

efibootmgr not actually a boot loader, but is a boot manager, used in conjunction with GRUB on multi-boot EFI systems.

The Linux Loader (LILO) is older and obsolete.

Das U-Boot is the most popular loader for embedded Linux systems. Some other boot loaders, including bareboot. However, not really considering embedded space in this course.

boot_loaders_small Boot Loaders

37.7 Configuration Files in /etc

Earlier, discussed about where Linux distributions cooperate, and hopefully follow agreed-upon standards to place certain kinds of files in standard places on system.

In particular, system-side configuration files generally placed in /etc and its subdirectories, while user-specific ones often placed in their individual home directories. Not completely true, though; eg, default configuration information might be stored in /usr/lib/systemd, but can be overridden by files in /etc/systemd.

FOr historical reasons, Linux distributions evolved their own rules about exactly where to place some information in /etc. Eg., all Red Hat-derived systems make extensive use of /etc/sysconfig, while Debian-based systems have used /etc/default. Interestingly, RHEL 7 and SUSE use both.

37.8 /etc/sysconfig

On RHEL 7 systems, files in /etc/sysconfig used when starting, stopping, configuring, or querying system services.

$ ls /etc/sysconfig

etcsysconfig

Can take a look at one file in screenshot here; this files reads and sets the selinux configuration at system startup.

$ cat /etc/sysconfig/selinux

etcselinux

37.9 /etc/default

Screenshot shows /etc/default directory on Ubuntu 17.04 system.

Use of this directory similar to that of Red Hat's /etc/sysconfig:

  • Files used to provide extra options when starting a service
  • Typically contain code to set environment variables.

Eg., file /etc/default/useradd sets defaults that are used when new user accounts are being created. As noted, RHEL 7 also has this directory as its use if becoming more universal.

ubuntuetcdefault

37.10 Shutting Down and Rebooting

shutdown used to bring system down in secure fashion, notifying all users that system is going down and then stopping it in a graceful and non-destructive way. After it is shutdown, system if either halted or rebooted. Can see some shutdown examples here:

$ sudo shutdown -h +1 "Power Failure imminent"
$ sudo shutdown -h now
$ sudo shutdown -r now
$ sudo shutdown now

Options can easily be listed by built-in help message.

shutdown

With no options (eg. shutdown now), default behavior is to power off the system completely. Some distributions, such as Ubuntu, violate this and go to single user mode instead.

One common mistake is failing to include a time argument (such as now or some actual time). This is required.

There are also lagacy commands reboot, halt, poweroff, which many veteran users use frequently.

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