Advanced Bash-Scripting Guide - Linux Documentation Project

Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Mendel Cooper

10

10 Mar 2014

Revision History Revision 6.5 'TUNGSTENBERRY' release Revision 6.6 'YTTERBIUMBERRY' release Revision 10 'PUBLICDOMAIN' release

05 Apr 2012 27 Nov 2012 10 Mar 2014

Revised by: mc Revised by: mc Revised by: mc

This tutorial assumes no previous knowledge of scripting or programming, yet progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little nuggets of UNIX? wisdom and lore. It serves as a textbook, a manual for self-study, and as a reference and source of knowledge on shell scripting techniques. The exercises and heavily-commented examples invite active reader participation, under the premise that the only way to really learn scripting is to write scripts.

This book is suitable for classroom use as a general introduction to programming concepts.

This document is herewith granted to the Public Domain. No copyright!

Dedication

For Anita, the source of all the magic

Advanced Bash-Scripting Guide

Table of Contents

Chapter 1. Shell Programming!.........................................................................................................................1

Chapter 2. Starting Off With a Sha-Bang........................................................................................................3 2.1. Invoking the script............................................................................................................................6 2.2. Preliminary Exercises.......................................................................................................................6

Part 2. Basics.......................................................................................................................................................7

Chapter 3. Special Characters...........................................................................................................................8

Chapter 4. Introduction to Variables and Parameters..................................................................................30 4.1. Variable Substitution......................................................................................................................30 4.2. Variable Assignment.......................................................................................................................33 4.3. Bash Variables Are Untyped..........................................................................................................34 4.4. Special Variable Types...................................................................................................................35

Chapter 5. Quoting...........................................................................................................................................41 5.1. Quoting Variables...........................................................................................................................41 5.2. Escaping..........................................................................................................................................43

Chapter 6. Exit and Exit Status.......................................................................................................................51

Chapter 7. Tests................................................................................................................................................54 7.1. Test Constructs...............................................................................................................................54 7.2. File test operators............................................................................................................................62 7.3. Other Comparison Operators..........................................................................................................65 7.4. Nested if/then Condition Tests.......................................................................................................70 7.5. Testing Your Knowledge of Tests..................................................................................................71

Chapter 8. Operations and Related Topics....................................................................................................72 8.1. Operators.........................................................................................................................................72 8.2. Numerical Constants.......................................................................................................................78 8.3. The Double-Parentheses Construct.................................................................................................80 8.4. Operator Precedence.......................................................................................................................81

Part 3. Beyond the Basics.................................................................................................................................84

Chapter 9. Another Look at Variables...........................................................................................................85 9.1. Internal Variables............................................................................................................................85 9.2. Typing variables: declare or typeset.............................................................................................104 9.2.1. Another use for declare.......................................................................................................107 9.3. $RANDOM: generate random integer..........................................................................................107

Chapter 10. Manipulating Variables.............................................................................................................119 10.1. Manipulating Strings...................................................................................................................119 10.1.1. Manipulating strings using awk........................................................................................127 10.1.2. Further Reference..............................................................................................................127 10.2. Parameter Substitution................................................................................................................128

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Advanced Bash-Scripting Guide

Table of Contents

Chapter 11. Loops and Branches..................................................................................................................138 11.1. Loops..........................................................................................................................................138 11.2. Nested Loops..............................................................................................................................152 11.3. Loop Control...............................................................................................................................153 11.4. Testing and Branching................................................................................................................156

Chapter 12. Command Substitution.............................................................................................................165

Chapter 13. Arithmetic Expansion................................................................................................................171

Chapter 14. Recess Time................................................................................................................................172

Part 4. Commands..........................................................................................................................................173

Chapter 15. Internal Commands and Builtins.............................................................................................181 15.1. Job Control Commands..............................................................................................................210

Chapter 16. External Filters, Programs and Commands...........................................................................215 16.1. Basic Commands........................................................................................................................215 16.2. Complex Commands...................................................................................................................221 16.3. Time / Date Commands..............................................................................................................231 16.4. Text Processing Commands........................................................................................................235 16.5. File and Archiving Commands...................................................................................................258 16.6. Communications Commands......................................................................................................276 16.7. Terminal Control Commands.....................................................................................................291 16.8. Math Commands.........................................................................................................................292 16.9. Miscellaneous Commands..........................................................................................................303

Chapter 17. System and Administrative Commands..................................................................................318 17.1. Analyzing a System Script..........................................................................................................349

Part 5. Advanced Topics.................................................................................................................................351

Chapter 18. Regular Expressions..................................................................................................................353 18.1. A Brief Introduction to Regular Expressions..............................................................................353 18.2. Globbing.....................................................................................................................................357

Chapter 19. Here Documents.........................................................................................................................359 19.1. Here Strings................................................................................................................................369

Chapter 20. I/O Redirection...........................................................................................................................373 20.1. Using exec...................................................................................................................................376 20.2. Redirecting Code Blocks............................................................................................................379 20.3. Applications................................................................................................................................384

Chapter 21. Subshells.....................................................................................................................................386

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Advanced Bash-Scripting Guide

Table of Contents

Chapter 22. Restricted Shells.........................................................................................................................391

Chapter 23. Process Substitution...................................................................................................................393

Chapter 24. Functions....................................................................................................................................398 24.1. Complex Functions and Function Complexities.........................................................................402 24.2. Local Variables...........................................................................................................................413 24.2.1. Local variables and recursion...........................................................................................414 24.3. Recursion Without Local Variables............................................................................................417

Chapter 25. Aliases.........................................................................................................................................420

Chapter 26. List Constructs...........................................................................................................................423

Chapter 27. Arrays.........................................................................................................................................427

Chapter 28. Indirect References....................................................................................................................456

Chapter 29. /dev and /proc.............................................................................................................................460 29.1. /dev..............................................................................................................................................460 29.2. /proc............................................................................................................................................463

Chapter 30. Network Programming.............................................................................................................469

Chapter 31. Of Zeros and Nulls.....................................................................................................................472

Chapter 32. Debugging...................................................................................................................................476

Chapter 33. Options........................................................................................................................................487

Chapter 34. Gotchas.......................................................................................................................................490

Chapter 35. Scripting With Style..................................................................................................................499 35.1. Unofficial Shell Scripting Stylesheet..........................................................................................499

Chapter 36. Miscellany...................................................................................................................................502 36.1. Interactive and non-interactive shells and scripts.......................................................................502 36.2. Shell Wrappers............................................................................................................................503 36.3. Tests and Comparisons: Alternatives..........................................................................................509 36.4. Recursion: a script calling itself..................................................................................................509 36.5. "Colorizing" Scripts....................................................................................................................512 36.6. Optimizations..............................................................................................................................525 36.7. Assorted Tips..............................................................................................................................528 36.7.1. Ideas for more powerful scripts.........................................................................................528 36.7.2. Widgets..............................................................................................................................539 36.8. Security Issues............................................................................................................................541 36.8.1. Infected Shell Scripts.........................................................................................................541 36.8.2. Hiding Shell Script Source................................................................................................541

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Advanced Bash-Scripting Guide

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Chapter 36. Miscellany 36.8.3. Writing Secure Shell Scripts.............................................................................................541

36.9. Portability Issues.........................................................................................................................541 36.9.1. A Test Suite.......................................................................................................................542

36.10. Shell Scripting Under Windows...............................................................................................543

Chapter 37. Bash, versions 2, 3, and 4..........................................................................................................544 37.1. Bash, version 2............................................................................................................................544 37.2. Bash, version 3............................................................................................................................548 37.2.1. Bash, version 3.1...............................................................................................................551 37.2.2. Bash, version 3.2...............................................................................................................552 37.3. Bash, version 4............................................................................................................................552 37.3.1. Bash, version 4.1...............................................................................................................559 37.3.2. Bash, version 4.2...............................................................................................................560

Chapter 38. Endnotes.....................................................................................................................................564 38.1. Author's Note..............................................................................................................................564 38.2. About the Author........................................................................................................................564 38.3. Where to Go For Help.................................................................................................................565 38.4. Tools Used to Produce This Book..............................................................................................565 38.4.1. Hardware...........................................................................................................................565 38.4.2. Software and Printware.....................................................................................................565 38.5. Credits.........................................................................................................................................566 38.6. Disclaimer...................................................................................................................................567

Bibliography....................................................................................................................................................569

Appendix A. Contributed Scripts..................................................................................................................577

Appendix B. Reference Cards........................................................................................................................787

Appendix C. A Sed and Awk Micro-Primer................................................................................................792 C.1. Sed................................................................................................................................................792 C.2. Awk..............................................................................................................................................795

Appendix D. Parsing and Managing Pathnames.........................................................................................798

Appendix E. Exit Codes With Special Meanings.........................................................................................802

Appendix F. A Detailed Introduction to I/O and I/O Redirection.............................................................803

Appendix G. Command-Line Options..........................................................................................................805 G.1. Standard Command-Line Options...............................................................................................805 G.2. Bash Command-Line Options......................................................................................................806

Appendix H. Important Files.........................................................................................................................808

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