How Linux works : what every superuser should know, 2ed

394 Pages·2015·5.068 MB·English

by  Brian Ward

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Unlike some operating systems, Linux doesn’t try to hide the • How development tools work and relate to shared
important bits from you—it gives you full control of your libraries
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computer. But to truly master Linux, you need to understand
• How to write effective shell scripts
its internals, like how the system boots, how networking W H A T E V E R Y S U P E R U S E R S H O U L D K N O W
works, and what the kernel actually does. You’ll also explore the kernel and examine key system

tasks inside user space, including system calls, input LL
In this completely revised second edition of the perennial
and output, and filesystems. With its combination of
best seller How Linux Works, author Brian Ward makes background, theory, real-world examples, and patient II B R I A N W A R D
the concepts behind Linux internals accessible to anyone
explanations, How Linux Works will teach you what NN
curious about the inner workings of the operating system.
you need to know to solve pesky problems and take
Inside, you’ll find the kind of knowledge that normally
control of your operating system. UU
comes from years of experience doing things the hard
way. You’ll learn: ABOUT THE AUTHOR
XX
• How Linux boots, from boot loaders to init implementa- Brian Ward has been working with Linux since 1993.
tions (systemd, Upstart, and System V) He is the author of the Linux Kernel-HOWTO, The Book
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of VMware (No Starch Press), and The Linux Problem
• How the kernel manages devices, device drivers, and
Solver (No Starch Press).
processes
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• How networking, interfaces, firewalls, and servers work
RR

KK
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Praise for the first edition of how linux works
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H o w L i n u x
w or k s
2 n d E d i t i o n
what Every superuser
should know
by Brian Ward
San Francisco
How linux works, 2nd Edition. Copyright © 2015 by Brian Ward.
All rights reserved. No part of this work may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying, recording, or by any information storage or retrieval
system, without the prior written permission of the copyright owner and the publisher.
Printed in USA
First printing
18 17 16 15 14 1 2 3 4 5 6 7 8 9
ISBN-10: 1-59327-567-6
ISBN-13: 978-1-59327-567-9
Publisher: William Pollock
Production Editor: Laurel Chun
Cover and Interior Design: Octopod Studios
Cover Illustration: Tina Salameh
Developmental Editor: William Pollock
Technical Reviewer: Jordi Gutiérrez Hermoso
Copyeditor: Gillian McGarvey
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The Library of Congress has cataloged the first edition as follows:
Ward, Brian.
How Linux works : what every superuser should know / Brian Ward.
p. cm.
Includes index.
ISBN 1-59327-035-6
1. Linux. 2. Operating systems (Computers). I. Title.
QA76.76.O63 W3654 2004
005.4'32--dc22
2004002692
No Starch Press and the No Starch Press logo are registered trademarks of No Starch Press, Inc. Other
product and company names mentioned herein may be the trademarks of their respective owners. Rather
than use a trademark symbol with every occurrence of a trademarked name, we are using the names only
in an editorial fashion and to the benefit of the trademark owner, with no intention of infringement of the
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liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or
indirectly by the information contained in it.
BriEf ContE nts
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xix
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiii
Chapter 1: The Big Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Chapter 2: Basic Commands and Directory Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Chapter 3: Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Chapter 4: Disks and Filesystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
Chapter 5: How the Linux Kernel Boots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93
Chapter 6: How User Space Starts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111
Chapter 7: System Configuration: Logging, System Time, Batch Jobs, and Users . . . . . . . . 149
Chapter 8: A Closer Look at Processes and Resource Utilization . . . . . . . . . . . . . . . . . . . 171
Chapter 9: Understanding Your Network and Its Configuration . . . . . . . . . . . . . . . . . . . .189
Chapter 10: Network Applications and Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
Chapter 11: Introduction to Shell Scripts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .249
Chapter 12: Moving Files Across the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271
Chapter 13: User Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
Chapter 14: A Brief Survey of the Linux Desktop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
Chapter 15: Development Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .309
Chapter 16: Introduction to Compiling Software from C Source Code . . . . . . . . . . . . . . . 329
Chapter 17: Building on the Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

ContEnts in dE taiL
PrEfacE xix
Who Should Read This Book? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx
Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx
How to Read This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx
A Hands-On Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx
How This Book Is Organized . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi
What’s New in the Second Edition? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi
A Note on Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii
acknowlEdgmEnts xxiii
1
tHE Big PicturE 1
1 .1 Levels and Layers of Abstraction in a Linux System . . . . . . . . . . . . . . . . . . . . . . . . 2
1 .2 Hardware: Understanding Main Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1 .3 The Kernel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1 .3 .1 Process Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1 .3 .2 Memory Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1 .3 .3 Device Drivers and Management . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1 .3 .4 System Calls and Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1 .4 User Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1 .5 Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1 .6 Looking Forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2
Basic commands and dirEctory HiErarcHy 11
2 .1 The Bourne Shell: /bin/sh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 .2 Using the Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 .2 .1 The Shell Window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 .2 .2 cat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 .2 .3 Standard Input and Standard Output . . . . . . . . . . . . . . . . . . . . . . . 13
2 .3 Basic Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 .3 .1 ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 .3 .2 cp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 .3 .3 mv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 .3 .4 touch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 .3 .5 rm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 .3 .6 echo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 .4 Navigating Directories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 .4 .1 cd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 .4 .2 mkdir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 .4 .3 rmdir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 .4 .4 Shell Globbing (Wildcards) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 .5 Intermediate Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 .5 .1 grep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 .5 .2 less . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 .5 .3 pwd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 .5 .4 diff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2 .5 .5 file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2 .5 .6 find and locate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2 .5 .7 head and tail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 .5 .8 sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 .6 Changing Your Password and Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 .7 Dot Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 .8 Environment and Shell Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 .9 The Command Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2 .10 Special Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2 .11 Command-Line Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2 .12 Text Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2 .13 Getting Online Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2 .14 Shell Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2 .14 .1 Standard Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2 .14 .2 Standard Input Redirection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2 .15 Understanding Error Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2 .15 .1 Anatomy of a UNIX Error Message . . . . . . . . . . . . . . . . . . . . . . . 28
2 .15 .2 Common Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2 .16 Listing and Manipulating Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2 .16 .1 Command Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2 .16 .2 Killing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2 .16 .3 Job Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2 .16 .4 Background Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2 .17 File Modes and Permissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2 .17 .1 Modifying Permissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2 .17 .2 Symbolic Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2 .17 .3 Creating Symbolic Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2 .18 Archiving and Compressing Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2 .18 .1 gzip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2 .18 .2 tar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2 .18 .3 Compressed Archives ( .tar .gz) . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2 .18 .4 zcat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2 .18 .5 Other Compression Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2 .19 Linux Directory Hierarchy Essentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2 .19 .1 Other Root Subdirectories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2 .19 .2 The /usr Directory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2 .19 .3 Kernel Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2 .20 Running Commands as the Superuser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2 .20 .1 sudo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2 .20 .2 /etc/sudoers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2 .21 Looking Forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3
dEvicEs 45
3 .1 Device Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3 .2 The sysfs Device Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
viii Contents in Detail
3 .3 dd and Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3 .4 Device Name Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3 .4 .1 Hard Disks: /dev/sd* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3 .4 .2 CD and DVD Drives: /dev/sr* . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3 .4 .3 PATA Hard Disks: /dev/hd* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3 .4 .4 Terminals: /dev/tty*, /dev/pts/*, and /dev/tty . . . . . . . . . . . . . . . 51
3 .4 .5 Serial Ports: /dev/ttyS* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3 .4 .6 Parallel Ports: /dev/lp0 and /dev/lp1 . . . . . . . . . . . . . . . . . . . . . . 52
3 .4 .7 Audio Devices: /dev/snd/*, /dev/dsp, /dev/audio, and More . . . . 53
3 .4 .8 Creating Device Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3 .5 udev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3 .5 .1 devtmpfs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3 .5 .2 udevd Operation and Configuration . . . . . . . . . . . . . . . . . . . . . . . 55
3 .5 .3 udevadm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3 .5 .4 Monitoring Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3 .6 In-Depth: SCSI and the Linux Kernel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3 .6 .1 USB Storage and SCSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3 .6 .2 SCSI and ATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3 .6 .3 Generic SCSI Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3 .6 .4 Multiple Access Methods for a Single Device . . . . . . . . . . . . . . . . . 63
4
disks and filEsystEms 65
4 .1 Partitioning Disk Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4 .1 .1 Viewing a Partition Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4 .1 .2 Changing Partition Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4 .1 .3 Disk and Partition Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4 .1 .4 Solid-State Disks (SSDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4 .2 Filesystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4 .2 .1 Filesystem Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4 .2 .2 Creating a Filesystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4 .2 .3 Mounting a Filesystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4 .2 .4 Filesystem UUID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4 .2 .5 Disk Buffering, Caching, and Filesystems . . . . . . . . . . . . . . . . . . . . 77
4 .2 .6 Filesystem Mount Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4 .2 .7 Remounting a Filesystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4 .2 .8 The /etc/fstab Filesystem Table . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4 .2 .9 Alternatives to /etc/fstab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4 .2 .10 Filesystem Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4 .2 .11 Checking and Repairing Filesystems . . . . . . . . . . . . . . . . . . . . . . . 82
4 .2 .12 Special-Purpose Filesystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4 .3 Swap Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4 .3 .1 Using a Disk Partition as Swap Space . . . . . . . . . . . . . . . . . . . . . . 85
4 .3 .2 Using a File as Swap Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4 .3 .3 How Much Swap Do You Need? . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4 .4 Looking Forward: Disks and User Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4 .5 Inside a Traditional Filesystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4 .5 .1 Viewing Inode Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4 .5 .2 Working with Filesystems in User Space . . . . . . . . . . . . . . . . . . . . . 90
4 .5 .3 The Evolution of Filesystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Contents in Detail ix