THE LINU X T H E L IN U X COMM A ND L INE

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1 BA NISH YOUR MOUSE takes you from your very first terminal keystrokes to writing full programs in Bash, the most popular Linux shell. Along the way you ll learn the timeless skills handed down by generations of gray-bearded, mouse-shunning gurus: file navigation, environment configuration, command chaining, pattern matching with regular expressions, and more. In addition to that practical knowledge, author William Shotts reveals the philosophy behind these tools and the rich heritage that your desktop Linux machine has inherited from Unix supercomputers of yore. As you make your way through the book s short, easily digestible chapters, you ll learn how to: Create and delete files, directories, and symlinks Administer your system, including networking, package installation, and process management Use standard input and output, redirection, and pipelines Edit files with Vi, the world s most popular text editor Write shell scripts to automate common or boring tasks Slice and dice text files with cut, paste, grep, patch, and sed Once you overcome your initial shell shock, you ll find that the command line is a natural and expressive way to communicate with your computer. Just don t be surprised if your mouse starts to gather dust. ABOUT THE AUTHOR William E. Shotts, Jr., has been a software professional and avid Linux user for more than 15 years. He has an extensive background in software development, including technical support, quality assurance, and documentation. He is also the creator of LinuxCommand.org, a Linux education and advocacy site featuring news, reviews, and extensive support for using the Linux command line. T H E L IN U X COMM A ND L INE You ve experienced the shiny, point-and-click surface of your Linux computer now dive below and explore its depths with the power of the command line. T H E F I N E ST I N G E E K E N T E RTA I N M E N T I L I E F L AT. FSC LOGO $49.95 ($52.95 CDN) SHELVE IN: COMPUTERS/LINUX This book uses RepKover a durable binding that won t snap shut. SHOT TS w w w.nostarch.com THE LINU X CO M M A N D L I N E A COMPLETE IN T RODUC T ION WILLIAM E. SHOT TS, JR.

2 CONTENTS IN DETAIL ACKNOWLEDGMENTS xxiii INTRODUCTION xxv Why Use the Command Line?...xxvi What This Book Is About...xxvi Who Should Read This Book...xxvii What s in This Book...xxvii How to Read This Book...xxviii Prerequisites...xxviii PART 1 LEARNING THE SHELL 1 WHAT IS THE SHELL? 3 Terminal Emulators...3 Your First Keystrokes...4 Command History...4 Cursor Movement...4 Try Some Simple Commands...5 Ending a Terminal Session NAVIGATION 7 Understanding the Filesystem Tree...7 The Current Working Directory...8 Listing the Contents of a Directory...8 Changing the Current Working Directory...9 Absolute Pathnames...9 Relative Pathnames...9 Some Helpful Shortcuts...10

3 3 EXPLORING THE SYSTEM 13 More Fun with ls...13 Options and Arguments...14 A Longer Look at Long Format...15 Determining a File s Type with file...16 Viewing File Contents with less...17 A Guided Tour...19 Symbolic Links MANIPULATING FILES AND DIRECTORIES 25 Wildcards...26 mkdir Create Directories...28 cp Copy Files and Directories...28 mv Move and Rename Files...30 rm Remove Files and Directories...31 ln Create Links...32 Hard Links...32 Symbolic Links...32 Let s Build a Playground...33 Creating Directories...33 Copying Files...33 Moving and Renaming Files...34 Creating Hard Links...35 Creating Symbolic Links...36 Removing Files and Directories...37 Final Note WORKING WITH COMMANDS 39 What Exactly Are Commands?...40 Identifying Commands...40 type Display a Command s Type...40 which Display an Executable s Location...41 Getting a Command s Documentation...41 help Get Help for Shell Builtins help Display Usage Information...42 man Display a Program s Manual Page...42 apropos Display Appropriate Commands...43 whatis Display a Very Brief Description of a Command...44 info Display a Program s Info Entry...44 README and Other Program Documentation Files...45 Creating Your Own Commands with alias...46 Revisiting Old Friends...47 x

4 6 REDIRECTION 49 Standard Input, Output, and Error...50 Redirecting Standard Output...50 Redirecting Standard Error...51 Redirecting Standard Output and Standard Error to One File...52 Disposing of Unwanted Output...52 Redirecting Standard Input...53 Pipelines...54 Filters...55 uniq Report or Omit Repeated Lines...55 wc Print Line, Word, and Byte Counts...55 grep Print Lines Matching a Pattern...56 head/tail Print First/Last Part of Files...56 tee Read from Stdin and Output to Stdout and Files...57 Final Note SEEING THE WORLD AS THE SHELL SEES IT 59 Expansion...59 Pathname Expansion...60 Tilde Expansion...61 Arithmetic Expansion...62 Brace Expansion...63 Parameter Expansion...64 Command Substitution...64 Quoting...65 Double Quotes...65 Single Quotes...67 Escaping Characters...67 Final Note ADVANCED KEYBOARD TRICKS 69 Command Line Editing...70 Cursor Movement...70 Modifying Text...70 Cutting and Pasting (Killing and Yanking) Text...70 Completion...72 Using History...73 Searching History...74 History Expansion...75 Final Note...76 xi

5 9 PERMISSIONS 77 Owners, Group Members, and Everybody Else...78 Reading, Writing, and Executing...79 chmod Change File Mode...81 Setting File Mode with the GUI...84 umask Set Default Permissions...84 Changing Identities...87 su Run a Shell with Substitute User and Group IDs...87 sudo Execute a Command as Another User...88 chown Change File Owner and Group...90 chgrp Change Group Ownership...91 Exercising Your Privileges...91 Changing Your Password PROCESSES 95 How a Process Works...96 Viewing Processes with ps...96 Viewing Processes Dynamically with top...98 Controlling Processes Interrupting a Process Putting a Process in the Background Returning a Process to the Foreground Stopping (Pausing) a Process Signals Sending Signals to Processes with kill Sending Signals to Multiple Processes with killall More Process-Related Commands PART 2 CONFIGURATION AND THE ENVIRONMENT 11 THE ENVIRONMENT 109 What Is Stored in the Environment? Examining the Environment Some Interesting Variables How Is the Environment Established? Login and Non-login Shells What s in a Startup File? xii

6 Modifying the Environment Which Files Should We Modify? Text Editors Using a Text Editor Activating Our Changes Final Note A GENTLE INTRODUCTION TO VI 121 Why We Should Learn vi A Little Background Starting and Stopping vi Editing Modes Entering Insert Mode Saving Our Work Moving the Cursor Around Basic Editing Appending Text Opening a Line Deleting Text Cutting, Copying, and Pasting Text Joining Lines Search and Replace Searching Within a Line Searching the Entire File Global Search and Replace Editing Multiple Files Switching Between Files Opening Additional Files for Editing Copying Content from One File into Another Inserting an Entire File into Another Saving Our Work CUSTOMIZING THE PROMPT 139 Anatomy of a Prompt Trying Some Alternative Prompt Designs Adding Color Moving the Cursor Saving the Prompt Final Note xiii

7 PART 3 COMMON TASKS AND ESSENTIAL TOOLS 14 PACKAGE MANAGEMENT 149 Packaging Systems How a Package System Works Package Files Repositories Dependencies High- and Low-Level Package Tools Common Package Management Tasks Finding a Package in a Repository Installing a Package from a Repository Installing a Package from a Package File Removing a Package Updating Packages from a Repository Upgrading a Package from a Package File Listing Installed Packages Determining Whether a Package Is Installed Displaying Information About an Installed Package Finding Which Package Installed a File Final Note STORAGE MEDIA 159 Mounting and Unmounting Storage Devices Viewing a List of Mounted Filesystems Determining Device Names Creating New Filesystems Manipulating Partitions with fdisk Creating a New Filesystem with mkfs Testing and Repairing Filesystems Formatting Floppy Disks Moving Data Directly to and from Devices Creating CD-ROM Images Creating an Image Copy of a CD-ROM Creating an Image from a Collection of Files Writing CD-ROM Images Mounting an ISO Image Directly Blanking a Rewritable CD-ROM Writing an Image Extra Credit xiv

8 16 NETWORKING 175 Examining and Monitoring a Network ping Send a Special Packet to a Network Host traceroute Trace the Path of a Network Packet netstat Examine Network Settings and Statistics Transporting Files over a Network ftp Transfer Files with the File Transfer Protocol lftp A Better ftp wget Non-interactive Network Downloader Secure Communication with Remote Hosts ssh Securely Log in to Remote Computers scp and sftp Securely Transfer Files SEARCHING FOR FILES 187 locate Find Files the Easy Way find Find Files the Hard Way Tests Actions A Return to the Playground Options ARCHIVING AND BACKUP 201 Compressing Files gzip Compress or Expand Files bzip2 Higher Compression at the Cost of Speed Archiving Files tar Tape Archiving Utility zip Package and Compress Files Synchronizing Files and Directories rsync Remote File and Directory Synchronization Using rsync over a Network REGULAR EXPRESSIONS 215 What Are Regular Expressions? grep Search Through Text Metacharacters and Literals The Any Character Anchors xv

9 Bracket Expressions and Character Classes Negation Traditional Character Ranges POSIX Character Classes POSIX Basic vs. Extended Regular Expressions Alternation Quantifiers...226? Match an Element Zero Times or One Time * Match an Element Zero or More Times Match an Element One or More Times { } Match an Element a Specific Number of Times Putting Regular Expressions to Work Validating a Phone List with grep Finding Ugly Filenames with find Searching for Files with locate Searching for Text with less and vim Final Note TEXT PROCESSING 233 Applications of Text Documents Web Pages Printer Output Program Source Code Revisiting Some Old Friends cat Concatenate Files and Print on Standard Output sort Sort Lines of Text Files uniq Report or Omit Repeated Lines Slicing and Dicing cut Remove Sections from Each Line of Files paste Merge Lines of Files join Join Lines of Two Files on a Common Field Comparing Text comm Compare Two Sorted Files Line by Line diff Compare Files Line by Line patch Apply a diff to an Original Editing on the Fly tr Transliterate or Delete Characters sed Stream Editor for Filtering and Transforming Text aspell Interactive Spell Checker Final Note Extra Credit xvi

10 21 FORMATTING OUTPUT 267 Simple Formatting Tools nl Number Lines fold Wrap Each Line to a Specified Length fmt A Simple Text Formatter pr Format Text for Printing printf Format and Print Data Document Formatting Systems The roff Family and TEX groff A Document Formatting System Final Note PRINTING 285 A Brief History of Printing Printing in the Dim Times Character-Based Printers Graphical Printers Printing with Linux Preparing Files for Printing pr Convert Text Files for Printing Sending a Print Job to a Printer lpr Print Files (Berkeley Style) lp Print Files (System V Style) Another Option: a2ps Monitoring and Controlling Print Jobs lpstat Display Print System Status lpq Display Printer Queue Status lprm and cancel Cancel Print Jobs COMPILING PROGRAMS 297 What Is Compiling? Are All Programs Compiled? Compiling a C Program Obtaining the Source Code Examining the Source Tree Building the Program Installing the Program Final Note xvii

11 PART 4 WRITING SHELL SCRIPTS 24 WRITING YOUR FIRST SCRIPT 309 What Are Shell Scripts? How to Write a Shell Script Script File Format Executable Permissions Script File Location Good Locations for Scripts More Formatting Tricks Long Option Names Indentation and Line Continuation Final Note STARTING A PROJECT 315 First Stage: Minimal Document Second Stage: Adding a Little Data Variables and Constants Creating Variables and Constants Assigning Values to Variables and Constants Here Documents Final Note TOP-DOWN DESIGN 325 Shell Functions Local Variables Keep Scripts Running Final Note FLOW CONTROL: BRANCHING WITH IF 333 Using if Exit Status Using test File Expressions String Expressions Integer Expressions xviii

12 A More Modern Version of test (( )) Designed for Integers Combining Expressions Control Operators: Another Way to Branch Final Note READING KEYBOARD INPUT 347 read Read Values from Standard Input Options Separating Input Fields with IFS Validating Input Menus Final Note Extra Credit FLOW CONTROL: LOOPING WITH WHILE AND UNTIL 357 Looping while Breaking out of a Loop until Reading Files with Loops Final Note TROUBLESHOOTING 363 Syntactic Errors Missing Quotes Missing or Unexpected Tokens Unanticipated Expansions Logical Errors Defensive Programming Verifying Input Testing Stubs Test Cases Debugging Finding the Problem Area Tracing Examining Values During Execution Final Note xix

13 31 FLOW CONTROL: BRANCHING WITH CASE 375 case Patterns Combining Multiple Patterns Final Note POSITIONAL PARAMETERS 381 Accessing the Command Line Determining the Number of Arguments shift Getting Access to Many Arguments Simple Applications Using Positional Parameters with Shell Functions Handling Positional Parameters En Masse A More Complete Application Final Note FLOW CONTROL: LOOPING WITH FOR 393 for: Traditional Shell Form for: C Language Form Final Note STRINGS AND NUMBERS 399 Parameter Expansion Basic Parameters Expansions to Manage Empty Variables Expansions That Return Variable Names String Operations Arithmetic Evaluation and Expansion Number Bases Unary Operators Simple Arithmetic Assignment Bit Operations Logic bc An Arbitrary-Precision Calculator Language Using bc An Example Script Final Note Extra Credit xx

14 35 ARRAYS 415 What Are Arrays? Creating an Array Assigning Values to an Array Accessing Array Elements Array Operations Outputting the Entire Contents of an Array Determining the Number of Array Elements Finding the Subscripts Used by an Array Adding Elements to the End of an Array Sorting an Array Deleting an Array Final Note EXOTICA 423 Group Commands and Subshells Performing Redirections Process Substitution Traps Asynchronous Execution wait Named Pipes Setting Up a Named Pipe Using Named Pipes Final Note INDEX 433 xxi