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The Linux Command Line

Second Internet Edition

William E. Shotts, Jr.

A LinuxCommand.org Book

Copyright ©2008-2013, William E. Shotts, Jr.

This work is licensed under the Creative Commons Attribution-Noncommercial-No De- rivative Works 3.0 United States License. To view a copy of this license, visit the link above or send a letter to Creative Commons, 171 Second Street, Suite 300, San Fran- cisco, California, 94105, USA. Linux® is the registered trademark of Linus Torvalds. All other trademarks belong to their respective owners. This book is part of the LinuxCommand.org project, a site for Linux education and advo- cacy devoted to helping users of legacy operating systems migrate into the future. You may contact the LinuxCommand.org project at http://linuxcommand.org. This book is also available in printed form, published by No Starch Press and may be purchased wherever fine books are sold. No Starch Press also offers this book in elec- tronic formats for most popular e-readers: http://nostarch.com/tlcl.htm

Release History

VersionDateDescription

13.07July 6, 2013Second Internet Edition.

09.12December 14, 2009First Internet Edition.

09.11November 19, 2009Fourth draft with almost all reviewer feedback

incorporated and edited through chapter 37.

09.10October 3, 2009Third draft with revised table formatting,

partial application of reviewers feedback and edited through chapter 18.

09.08August 12, 2009Second draft incorporating the first editing

pass.

09.07July 18, 2009Completed first draft.

Table of Contents

Why Use The Command Line?.....................................................................................xvi

What This Book Is About..............................................................................................xvii

Who Should Read This Book.......................................................................................xvii

What's In This Book.....................................................................................................xviii

How To Read This Book..............................................................................................xviii

Why I Don't Call It "GNU/Linux"...........................................................................xix

Your Feedback Is Needed!............................................................................................xx

What's New In The Second Internet Edition.................................................................xxi

Further Reading............................................................................................................xxi

Part 1 - Learning The Shell..............................................................1

1 - What Is The Shell?.....................................................................................2

Terminal Emulators..........................................................................................................2

Your First Keystrokes......................................................................................................2

Command History.......................................................................................................3

Cursor Movement.......................................................................................................3

A Few Words About Mice And Focus....................................................................3

Try Some Simple Commands..........................................................................................4

Ending A Terminal Session..............................................................................................5

The Console Behind The Curtain..........................................................................5

Summing Up....................................................................................................................5

Further Reading...............................................................................................................6

2 - Navigation...................................................................................................7

Understanding The File System Tree..............................................................................7

The Current Working Directory........................................................................................7

Listing The Contents Of A Directory................................................................................8

Changing The Current Working Directory.......................................................................9

Absolute Pathnames..................................................................................................9

Relative Pathnames...................................................................................................9

Some Helpful Shortcuts............................................................................................11

Important Facts About Filenames........................................................................11

i

Summing Up..................................................................................................................12

3 - Exploring The System.............................................................................13

More Fun With ls...........................................................................................................13

Options And Arguments............................................................................................14

A Longer Look At Long Format.................................................................................16

Determining A File's Type With file................................................................................17

Viewing File Contents With less....................................................................................17

What Is "Text"?.....................................................................................................17

Less Is More........................................................................................................19

A Guided Tour................................................................................................................19

Symbolic Links...............................................................................................................23

Hard Links.....................................................................................................................24

Summing Up..................................................................................................................24

Further Reading.............................................................................................................24

4 - Manipulating Files And Directories........................................................25

Character Ranges................................................................................................27

Wildcards Work In The GUI Too..........................................................................27

mkdir - Create Directories.............................................................................................28

cp - Copy Files And Directories....................................................................................28

Useful Options And Examples..................................................................................29

mv - Move And Rename Files......................................................................................30

Useful Options And Examples..................................................................................30

rm - Remove Files And Directories...............................................................................31

Useful Options And Examples..................................................................................31

Be Careful With rm!.............................................................................................32

ln - Create Links............................................................................................................33

Hard Links................................................................................................................33

Symbolic Links..........................................................................................................33

Let's Build A Playground................................................................................................34

Creating Directories..................................................................................................34

Copying Files............................................................................................................34

Moving And Renaming Files.....................................................................................35

Creating Hard Links..................................................................................................37

Creating Symbolic Links...........................................................................................38

Removing Files And Directories...............................................................................39

Creating Symlinks With The GUI.........................................................................40

Summing Up..................................................................................................................41

Further Reading.............................................................................................................41

5 - Working With Commands........................................................................42

What Exactly Are Commands?......................................................................................42

Identifying Commands...................................................................................................43

type - Display A Command's Type...........................................................................43

which - Display An Executable's Location...............................................................43

Getting A Command's Documentation..........................................................................44

help - Get Help For Shell Builtins............................................................................44

--help - Display Usage Information..........................................................................45

ii

man - Display A Program's Manual Page................................................................45

apropos - Display Appropriate Commands..............................................................47

whatis - Display A Very Brief Description Of A Command.......................................47 The Most Brutal Man Page Of Them All..............................................................48

info - Display A Program's Info Entry.......................................................................48

README And Other Program Documentation Files................................................49

Creating Your Own Commands With alias....................................................................50

Summing Up..................................................................................................................52

Further Reading.............................................................................................................52

6 - Redirection................................................................................................53

Standard Input, Output, And Error.................................................................................53

Redirecting Standard Output.........................................................................................54

Redirecting Standard Error............................................................................................55

Redirecting Standard Output And Standard Error To One File................................56

Disposing Of Unwanted Output................................................................................57

/dev/null In Unix Culture......................................................................................57

Redirecting Standard Input............................................................................................57

cat - Concatenate Files............................................................................................57

The Difference Between > and |..........................................................................60

uniq - Report Or Omit Repeated Lines.....................................................................61

wc - Print Line, Word, And Byte Counts..................................................................62

grep - Print Lines Matching A Pattern......................................................................62

head / tail - Print First / Last Part Of Files................................................................63

tee - Read From Stdin And Output To Stdout And Files..........................................64

Summing Up..................................................................................................................65

Linux Is About Imagination..................................................................................65

7 - Seeing The World As The Shell Sees It..................................................67

Pathname Expansion...............................................................................................68

Pathname Expansion Of Hidden Files.................................................................69

Tilde Expansion........................................................................................................69

Arithmetic Expansion................................................................................................70

Brace Expansion......................................................................................................71

Parameter Expansion...............................................................................................72

Command Substitution.............................................................................................73

Double Quotes..........................................................................................................75

Single Quotes...........................................................................................................76

Escaping Characters................................................................................................77

Backslash Escape Sequences............................................................................77

Summing Up..................................................................................................................78

Further Reading.............................................................................................................78

8 - Advanced Keyboard Tricks.....................................................................79

Command Line Editing..................................................................................................79

Cursor Movement.....................................................................................................79

iii

Modifying Text...........................................................................................................80

Cutting And Pasting (Killing And Yanking) Text........................................................80

The Meta Key......................................................................................................81

Programmable Completion..................................................................................83

Using History.................................................................................................................83

Searching History.....................................................................................................84

History Expansion.....................................................................................................86

Summing Up..................................................................................................................86

Further Reading.............................................................................................................87

9 - Permissions..............................................................................................88

Owners, Group Members, And Everybody Else............................................................89

Reading, Writing, And Executing...................................................................................90

chmod - Change File Mode.....................................................................................92

What The Heck Is Octal?.....................................................................................93

Setting File Mode With The GUI...............................................................................95

umask - Set Default Permissions............................................................................96

Some Special Permissions..................................................................................98

Changing Identities........................................................................................................99

su - Run A Shell With Substitute User And Group IDs............................................99 sudo - Execute A Command As Another User.......................................................101

Ubuntu And sudo...............................................................................................101

chown - Change File Owner And Group................................................................102

chgrp - Change Group Ownership.........................................................................103

Exercising Our Privileges............................................................................................103

Changing Your Password............................................................................................106

Summing Up................................................................................................................107

Further Reading..........................................................................................................107

10 - Processes.............................................................................................108

How A Process Works.................................................................................................108

Viewing Processes......................................................................................................109

Viewing Processes Dynamically With top..............................................................111

Controlling Processes.................................................................................................113

Interrupting A Process............................................................................................114

Putting A Process In The Background....................................................................114

Returning A Process To The Foreground...............................................................115

Stopping (Pausing) A Process................................................................................116

Sending Signals To Processes With kill.................................................................117

Sending Signals To Multiple Processes With killall................................................120

More Process Related Commands.............................................................................120

Summing Up................................................................................................................121

Part 2 - Configuration And The Environment.............................123

11 - The Environment..................................................................................124

iv

What Is Stored In The Environment?..........................................................................124

Examining The Environment..................................................................................124

Some Interesting Variables.....................................................................................126

How Is The Environment Established?.......................................................................127

What's In A Startup File?........................................................................................128

Modifying The Environment.........................................................................................130

Which Files Should We Modify?.............................................................................130

Text Editors.............................................................................................................130

Using A Text Editor.................................................................................................131

Why Comments Are Important..........................................................................134

Activating Our Changes..........................................................................................135

Summing Up................................................................................................................135

Further Reading..........................................................................................................135

12 - A Gentle Introduction To vi.................................................................136

Why We Should Learn vi.............................................................................................136

A Little Background ....................................................................................................137

Starting And Stopping vi..............................................................................................137

Compatibility Mode............................................................................................138

Editing Modes..............................................................................................................139

Entering Insert Mode..............................................................................................140

Saving Our Work....................................................................................................140

Moving The Cursor Around.........................................................................................141

Basic Editing................................................................................................................142

Appending Text.......................................................................................................142

Opening A Line.......................................................................................................143

Deleting Text...........................................................................................................144

Cutting, Copying, And Pasting Text........................................................................145

Joining Lines...........................................................................................................147

Searching Within A Line..........................................................................................147

Searching The Entire File.......................................................................................147

Global Search-And-Replace...................................................................................148

Editing Multiple Files...................................................................................................150

Switching Between Files.........................................................................................151

Opening Additional Files For Editing......................................................................151

Copying Content From One File Into Another........................................................152

Inserting An Entire File Into Another.......................................................................153

Saving Our Work.........................................................................................................154

Summing Up................................................................................................................155

Further Reading..........................................................................................................155

13 - Customizing The Prompt.....................................................................156

Anatomy Of A Prompt..................................................................................................156

Trying Some Alternative Prompt Designs....................................................................158

Adding Color................................................................................................................159

Terminal Confusion............................................................................................160

Moving The Cursor......................................................................................................162

Saving The Prompt......................................................................................................163

Summing Up................................................................................................................164

v

Further Reading..........................................................................................................164

Part 3 - Common Tasks And Essential Tools.............................165

14 - Package Management..........................................................................166

Packaging Systems.....................................................................................................166

How A Package System Works...................................................................................167

Package Files.........................................................................................................167

High And Low-level Package Tools........................................................................168

Common Package Management Tasks.......................................................................169

Finding A Package In A Repository........................................................................169

Installing A Package From A Repository.................................................................169

Installing A Package From A Package File.............................................................170

Removing A Package.............................................................................................170

Updating Packages From A Repository..................................................................171

Upgrading A Package From A Package File...........................................................171

Listing Installed Packages......................................................................................172

Determining If A Package Is Installed.....................................................................172

Displaying Info About An Installed Package...........................................................173

Finding Which Package Installed A File.................................................................173

Summing Up................................................................................................................173

The Linux Software Installation Myth.................................................................174

Further Reading..........................................................................................................175

15 - Storage Media.......................................................................................176

Mounting And Unmounting Storage Devices..............................................................176

Viewing A List Of Mounted File Systems................................................................178

Why Unmounting Is Important...........................................................................181

Determining Device Names....................................................................................182

Creating New File Systems.........................................................................................185

Manipulating Partitions With fdisk..........................................................................185

Creating A New File System With mkfs..................................................................188

Testing And Repairing File Systems............................................................................189

What The fsck?..................................................................................................189

Formatting Floppy Disks..............................................................................................189

Moving Data Directly To/From Devices.......................................................................190

Creating CD-ROM Images..........................................................................................191

Creating An Image Copy Of A CD-ROM.................................................................191

Creating An Image From A Collection Of Files.......................................................191

A Program By Any Other Name.........................................................................192

Writing CD-ROM Images.............................................................................................192

Mounting An ISO Image Directly............................................................................192

Blanking A Re-Writable CD-ROM...........................................................................193

Writing An Image....................................................................................................193

Summing Up................................................................................................................193

Further Reading..........................................................................................................193

Extra Credit..................................................................................................................193

vi

16 - Networking............................................................................................195

Examining And Monitoring A Network.........................................................................196

Transporting Files Over A Network..............................................................................199

lftp - A Better ftp.....................................................................................................202

Secure Communication With Remote Hosts...............................................................202

Tunneling With SSH..........................................................................................206

scp And sftp............................................................................................................207

An SSH Client For Windows?............................................................................208

Summing Up................................................................................................................208

Further Reading..........................................................................................................208

17 - Searching For Files..............................................................................209

locate - Find Files The Easy Way...............................................................................209

Where Does The locate Database Come From?..............................................211

find - Find Files The Hard Way...................................................................................211

Predefined Actions..................................................................................................217

User-Defined Actions..............................................................................................219

Improving Efficiency...............................................................................................220

Dealing With Funny Filenames..........................................................................221

A Return To The Playground..................................................................................221

Summing Up................................................................................................................225

Further Reading..........................................................................................................225

18 - Archiving And Backup.........................................................................226

Compressing Files.......................................................................................................226

Don't Be Compressive Compulsive...................................................................230

Archiving Files.............................................................................................................230

Synchronizing Files And Directories............................................................................238

Using rsync Over A Network...................................................................................240

Summing Up................................................................................................................241

Further Reading..........................................................................................................241

19 - Regular Expressions...........................................................................243

What Are Regular Expressions?............................................................................243

vii

Metacharacters And Literals........................................................................................245

The Any Character......................................................................................................246

A Crossword Puzzle Helper...............................................................................247

Bracket Expressions And Character Classes..............................................................248

Traditional Character Ranges.................................................................................249

POSIX Character Classes......................................................................................250

Reverting To Traditional Collation Order............................................................253

POSIX Basic Vs. Extended Regular Expressions.......................................................254

? - Match An Element Zero Or One Time...............................................................256

* - Match An Element Zero Or More Times............................................................257

+ - Match An Element One Or More Times............................................................258

{ } - Match An Element A Specific Number Of Times..............................................258

Putting Regular Expressions To Work.........................................................................259

Validating A Phone List With grep...........................................................................259

Finding Ugly Filenames With find...........................................................................260

Searching For Files With locate.............................................................................261

Searching For Text With less And vim....................................................................261

Summing Up................................................................................................................263

Further Reading..........................................................................................................263

20 - Text Processing....................................................................................264

Applications Of Text.....................................................................................................264

Web Pages.............................................................................................................265

Printer Output.........................................................................................................265

Program Source Code............................................................................................265

Revisiting Some Old Friends.......................................................................................265

MS-DOS Text Vs. Unix Text...............................................................................267

Slicing And Dicing........................................................................................................276

Expanding Tabs.................................................................................................279

Comparing Text...........................................................................................................283

Editing On The Fly.......................................................................................................288

ROT13: The Not-So-Secret Decoder Ring........................................................290 viii

People Who Like sed Also Like.........................................................................299

Summing Up................................................................................................................303

Further Reading..........................................................................................................303

Extra Credit..................................................................................................................304

21 - Formatting Output................................................................................305

Simple Formatting Tools..............................................................................................305

nl - Number Lines..................................................................................................305

fold - Wrap Each Line To A Specified Length........................................................309

fmt - A Simple Text Formatter................................................................................309

pr - Format Text For Printing..................................................................................313

printf - Format And Print Data................................................................................314

Document Formatting Systems...................................................................................317

Summing Up................................................................................................................324

Further Reading..........................................................................................................324

22 - Printing..................................................................................................326

A Brief History Of Printing............................................................................................326

Printing In The Dim Times......................................................................................326

Character-based Printers.......................................................................................327

Graphical Printers...................................................................................................328

Printing With Linux......................................................................................................329

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