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CentOS System Administration Essentials
CentOS System Administration Essentials

CentOS System Administration Essentials: Become an efficient CentOS administrator by acquiring real-world knowledge of system setup and configuration

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CentOS System Administration Essentials

Chapter 2. Cold Starts

In the Northern Hemisphere, I think we can all relate to the analogy of the cold start; those bleak January mornings where you are frantically trying to start your car. When it does finally splutter into some form of life, we then have to contend with a steering wheel too cold to hold. Thankfully, starting up a Linux system is not so unpleasant; perhaps air-conditioned server rooms have something to do with this, I am not sure…

Working through this chapter, we are going to build upon what you have already mastered—helping you understand your Linux systems. You will learn about the following topics:

  • The GRUB and the MBR: In this section, you will learn about the relationship that the GRand Unified Bootloader (GRUB) enjoys with the Master Boot Record (MBR), being able to slip its slender 466 bytes easily inside the 512-byte limit.
  • When is the root filesystem not the root filesystem?: In this section, we will understand the term root when used as...

The GRUB and MBR

This is not just a competition to see how many acronyms we can fit into a chapter heading, although, out of four words, having used two already is not a bad start. The GRUB is the system-supplied bootloader that ships with CentOS and Red Hat Enterprise Linux 6. This tiny piece of bootstrap code is used to load the kernel and allows us to dual boot different Linux versions or even with Microsoft Windows operating systems. The GRUB has been the bootloader of choice for many years, although other bootloaders do exist. These include:

  • Lilo: This is the original Linux loader
  • EXTLinux: This is part of the SYSLinux family that includes the following:
    • EXTLinux to boot from fixed drives
    • ISOLinux to boot from CDs and DVDs
    • SYSLinux to boot from a USB device
    • PXELinux to boot from the network
  • GRUB2: More recently, this is making its appearance as a replacement to GRUB, or what is now referred to as the legacy GRUB. GRUB2 is likely to debut in CentOS 7 in 2014.

The GRUB bootloader is most commonly...

When is the root filesystem not the root filesystem?

We now need to break down the menu entries within the file, identifying the core components so that we can understand how they relate to the system and, most importantly, how we can correct errors.

Editing stanzas in GRUB

Each entry in the GRUB menu is known as a stanza, and each stanza will start with the title word, containing three directives as follows:

  • root
  • kernel
  • initd

The title of the stanza also becomes the displayed item in the menu. Let's consider a stanza that begins with the following title:

title CentOS 6.5 OS

The menu will display CentOS 6.5 OS as the selectable item, and it is important to note that we do not add quotes around the text as they will also be displayed to the user. This is unless, of course, you want or need to display these quotes; we are most certainly not quote unfriendly at Packt Publishing!

Adding a root entry to a stanza

Directly following the stanza title will be a line that starts with the root directive...

Working on the GRUB console

When presented with the GRUB menu, as well as selecting the entry we wish to boot, we can either edit existing entries or shell out to the GRUB console. Working on the GRUB console enables us to enter our own sets of commands. Remember the trilogy that should accompany each stanza:

  • root
  • kernel
  • initrd

We can enter these commands, but also reinstall GRUB if required. More simply, in the console, we can also edit or append to the exiting entries; using the e key, we can edit an entry, and a can be used to append an option to the kernel line. From the following screenshot, we can view these options:

Working on the GRUB console

Editing the kernel arguments allows you to specify the runlevel target to boot to; using this method, it is possible to reset the password of the root user.

To recover a forgotten root password, we can boot the system to runlevel 1; by default, this will log you in directly as root.

  1. Firstly, we must select the entry in the menu to boot to. If there is more than one, do not use...

Protecting the GRUB menu with passwords

Now I can imagine that all of this talk to gain root access from the physical server can be quite alarming; the truth is that it really shouldn't be, as securing physical access to the server is normally not difficult or onerous. However, where there is a desire or need to take the security further, it can easily be implemented through GRUB passwords. Any password settings will normally be added to the global section that precedes any stanza. Firstly, let's review some of the GRUB global options before setting some passwords.

On visiting the /boot/grub/menu.lst file on CentOS, we will see that the first lines are commented out and generated by the installer anaconda, and that the file is named as grub.conf.

The menu.lst file does exist in Red Hat and CentOS but is in the guise of a symbolic link to /boot/grub/grub.conf. From the legacy GRUB documentation, the file should be menu.lst; CentOS provides this with the link, but I feel that the...

Boot splashing with plymouth

As soon as we have begun the boot process and just prior to handing control to the kernel, a boot splash screen can be displayed. This, as the name suggests, controls the splash screen you may see during the boot process. In CentOS, this defaults to the plymouth theme: rings. Plymouth is the boot splash manager; we can use other themes should we wish. Some of these are installed as standard, while others are included in the standard repositories. Yet, more themes can be found in third-party repositories.

You can, of course, build your own theme. Essentially, a minimal theme is just a wallpaper.

Applying different themes

Most of the time during the boot process, you will not see the splash screen unless CentOS is your desktop machine. However, I would recommend still working with plymouth to change the default splash from rings to basic. With the basic theme, we can see the services loading during the boot process rather than the rings that merely show the boot...

The GRUB and MBR


This is not just a competition to see how many acronyms we can fit into a chapter heading, although, out of four words, having used two already is not a bad start. The GRUB is the system-supplied bootloader that ships with CentOS and Red Hat Enterprise Linux 6. This tiny piece of bootstrap code is used to load the kernel and allows us to dual boot different Linux versions or even with Microsoft Windows operating systems. The GRUB has been the bootloader of choice for many years, although other bootloaders do exist. These include:

  • Lilo: This is the original Linux loader

  • EXTLinux: This is part of the SYSLinux family that includes the following:

    • EXTLinux to boot from fixed drives

    • ISOLinux to boot from CDs and DVDs

    • SYSLinux to boot from a USB device

    • PXELinux to boot from the network

  • GRUB2: More recently, this is making its appearance as a replacement to GRUB, or what is now referred to as the legacy GRUB. GRUB2 is likely to debut in CentOS 7 in 2014.

The GRUB bootloader is most commonly...

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Description

If you are a Linux administrator who is looking to gain knowledge that differentiates yourself from the crowd, then this is the book for you. Beginners who have a keen interest to learn more about Linux administration will also progress quickly with this resourceful learning guide.

What you will learn

  • Conquer the command line using shortcuts in the shell and within the editor Vim
  • Analyze the booting of CentOS utilizing MBR, GRUB, and Plymouth
  • Gain an understanding of the filesystem structure of hard links, inodes, and data using stat and ls
  • Manage your software installations with YUM
  • Handle your services as they begin to migrate from System V scripts
  • Establish centralized account information using openLDAP directory services
  • Centralize the configuration management of CentOS using Puppet, enabling updates from Puppet Master to be distributed to the clients
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Publication date, Length, Edition, Language, ISBN-13
Publication date : Nov 25, 2014
Length: 174 pages
Edition : 1st
Language : English
ISBN-13 : 9781783985920
Vendor :
The CentOS Project
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Length: 174 pages
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Language : English
ISBN-13 : 9781783985920
Vendor :
The CentOS Project
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Table of Contents

12 Chapters
1. Taming vi Chevron down icon Chevron up icon
2. Cold Starts Chevron down icon Chevron up icon
3. CentOS Filesystems – A Deeper Look Chevron down icon Chevron up icon
4. YUM – Software Never Looked So Good Chevron down icon Chevron up icon
5. Herding Cats – Taking Control of Processes Chevron down icon Chevron up icon
6. Users – Do We Really Want Them? Chevron down icon Chevron up icon
7. LDAP – A Better Type of User Chevron down icon Chevron up icon
8. Nginx – Deploying a Performance-centric Web Server Chevron down icon Chevron up icon
9. Puppet – Now You Are the Puppet Master Chevron down icon Chevron up icon
10. Security Central Chevron down icon Chevron up icon
11. Graduation Day Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

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Bret W. Apr 12, 2018
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Excellent book and extremely well written, however these were not suitable for my web server purposes as very broad. Returned with no quibbles and wouldn't hesitate to buy again.
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modernistShambles Mar 14, 2020
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Clearly written.
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Mike O Oct 22, 2015
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I don't think you could find a more knowledgeable guide to Linux than Andrew Mallett. Seriously... he's awesome, and so are his books and videos.
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John Jul 03, 2016
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I've watched a couple of Andrew's videos and he is very good at explaining things in an easy to grasp manner. He is clearly a person who not only has great technical skills but also has the ability to pass that knowledge on to others which is very rare indeed! I have to say the book was every bit as good. At times entertaining with sprinklings of Andrew's sense of humour but very easy to follow and a very good read. If you enjoy working with Linuc and CentOS in particular then you will love this book. I look forward to reading and watching more of Andrew's work.
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Luke_Vidler Jun 20, 2015
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This is definitely one of the better Linux books I've read. Covers the Essentials really really well, the goal of the author is comprehension of the essential concepts, with the occasional side of cool hacks. Succeeds in gifting the serious student with confidence that would have been missing otherwise. Great resource and just the right length for me.
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