In Linux, during boot up, the system looks for the boot sector on the hard disk. Once it finds the boot sector, it then searches for a boot loader. In turn, the boot loader loads the boot manager. In Linux, this will usually be either GRUB or GRUB2. After this stage, the user is presented with a boot menu. Finally, the user is given the opportunity to either select an operating system to load or edit an existing entry. The options available are usually going to be different versions of the Linux kernel. Sometimes, it might be an entirely different Linux distribution. However, in mixed environments, you may come into contact with another operating system, such as Microsoft Windows.

After the user selects a Linux kernel, depending on the Linux distribution release, a single process is started, known as init, which stands for initialization. init is often...

Now we'll switch gears and discuss the boot managers whose jobs are to present the boot menu, from which the user has the options to select which operating system/Linux kernel to load or edit. First, we will focus on GRUB and then move on to GRUB2.

GRUB stands for Grand Unified Bootloader. GRUB is primarily used for booting Linux distributions. However, GRUB can work with other boot loaders. A common use-case scenario is for dual booting with a Microsoft operating system: it does this by doing a hand-off to the Windows bootloader for Microsoft operating systems.

GRUB uses the /boot/grub/grub.conf file. Sometimes you will see /boot/grub/menu.lst, but this file is simply a symbolic link to /boot/grub...

Now we're going to interact with GRUB. We will add a custom boot entry. This will be presented upon reboot.

We will use the vi command, which will open /boot/grub/grub.conf in the visual editor:

[root@localhost ~]# cat /boot/grub/grub.conf
 # grub.conf generated by anaconda
 #
 # Note that you do not have to rerun grub after making changes to this file
 # NOTICE: You have a /boot partition. This means that
 # all kernel and initrd paths are relative to /boot/, eg.
 # root (hd0,0)
 # kernel /vmlinuz-version ro root=/dev/sda2
 # initrd /initrd-[generic-]version.img
 #boot=/dev/sda
 default=0
 timeout=5
 splashimage=(hd0,0)/grub/splash.xpm.gz
 hiddenmenu
 title CentOS (2.6.32-431.el6.x86_64)
 root (hd0,0)
 kernel /vmlinuz-2.6.32-431.el6.x86_64 ro root=UUID=05527d71-25b6-4931-a3bb-8fe505f3fa64...

We add a custom boot entry in GRUB2 in a slightly different way from GRUB. In GRUB2, instead of editing the actual /boot/grub/grub.cfg, we work with /etc/default/grub and /etc/grub.d. Let's do a listing of /etc/grub.d to see all of the available files:

philip@ubuntu:~$ ls -l /etc/grub.d/
 total 76
 -rwxr-xr-x 1 root root 9791 Apr 15 2016 00_header
 -rwxr-xr-x 1 root root 6258 Mar 15 2016 05_debian_theme
 -rwxr-xr-x 1 root root 12261 Apr 15 2016 10_linux
 -rwxr-xr-x 1 root root 11082 Apr 15 2016 20_linux_xen
 -rwxr-xr-x 1 root root 1992 Jan 28 2016 20_memtest86+
 -rwxr-xr-x 1 root root 11692 Apr 15 2016 30_os-prober
 -rwxr-xr-x 1 root root 1418 Apr 15 2016 30_uefi-firmware
 -rwxr-xr-x 1 root root 214 Apr 15 2016 40_custom
 -rwxr-xr-x 1 root root 216 Apr 15 2016 41_custom
 -rw-r--r-- 1 root root 483 Apr 15 2016 README
 philip@ubuntu:~$

In this chapter, we took a look at the boot process. We then discussed init and systemd. We used the pstree command and saw the first process that is loaded. We also used the ps command to identify the process number. We then looked at the boot messages that would normally scroll across the screen using the dmesg command. The messages displayed offer us hints as to what was loaded upon boot up. Furthermore, we can use the messages displayed to assist us whilst troubleshooting. Next, we covered GRUB and GRUB2, looking at the structure of GRUB, particularly /boot/grub/grub/conf. We looked at adding a custom menu entry in GRUB. We then explored interacting with GRUB in real time at the boot menu. After that, we looked at GRUB2, focusing on the structure of /boot/grub/grub.cfg. In addition, we looked into the other locations that play a role in the GRUB2 configuration: the...

1. Where is the boot loader located on the hard disk?

A. The boot sector
B. The secondary partition
C. The logical partition
D. None of the above

2. Which was the first commercial Unix operating system?

A. systemd
B. upstart
C. System X
D. System V

3. Which command displays the processes, starting with the parent process followed by the child processes?

A. dnf
B. systemctl
C. pstree
D. ps

4. Which is the first process that is started on a CentOS 5 system?

A. systemd
B. init
C. kickstart
D. upstart

5. What has replaced init in newer releases of the Linux kernel?

A. telinit
B. systemctl
C. systemb
D. systemd

6. Which command lists the processes that are running on a CentOS 7 distribution?

A. systemd list-unit-files
B. systemX list-unit-files
C. systemctl list-unit-files
D. service status unit-files

7. Which command lists the hardware drivers that are loaded during system boot...

• You can get more information about the CentOS distributions, such as installing, configuration best practices, and so on, at https://www.centos.org.
• The following website gives you a lot of useful tips and best practices from users in the Linux community, particularly for the Debian distributions, such as Ubuntu: https://askubuntu.com.
• This following link gives you information that relates to various commands that work on both CentOS and Ubuntu. You can post your questions there and other community members will respond: https://www.linuxquestions.org.