Table of Contents for
sed & awk, 2nd Edition

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition sed & awk, 2nd Edition by Arnold Robbins Published by O'Reilly Media, Inc., 1997
  1. sed & awk, 2nd Edition
  2. Cover
  3. sed & awk, 2nd Edition
  4. A Note Regarding Supplemental Files
  5. Dedication
  6. Preface
  7. Scope of This Handbook
  8. Availability of sed and awk
  9. Obtaining Example Source Code
  10. Conventions Used in This Handbook
  11. About the Second Edition
  12. Acknowledgments from the First Edition
  13. Comments and Questions
  14. 1. Power Tools for Editing
  15. 1.1. May You Solve Interesting Problems
  16. 1.2. A Stream Editor
  17. 1.3. A Pattern-Matching Programming Language
  18. 1.4. Four Hurdles to Mastering sed and awk
  19. 2. Understanding Basic Operations
  20. 2.1. Awk, by Sed and Grep, out of Ed
  21. 2.2. Command-Line Syntax
  22. 2.3. Using sed
  23. 2.4. Using awk
  24. 2.5. Using sed and awk Together
  25. 3. Understanding Regular Expression Syntax
  26. 3.1. That’s an Expression
  27. 3.2. A Line-Up of Characters
  28. 3.3. I Never Metacharacter I Didn’t Like
  29. 4. Writing sed Scripts
  30. 4.1. Applying Commands in a Script
  31. 4.2. A Global Perspective on Addressing
  32. 4.3. Testing and Saving Output
  33. 4.4. Four Types of sed Scripts
  34. 4.5. Getting to the PromiSed Land
  35. 5. Basic sed Commands
  36. 5.1. About the Syntax of sed Commands
  37. 5.2. Comment
  38. 5.3. Substitution
  39. 5.4. Delete
  40. 5.5. Append, Insert, and Change
  41. 5.6. List
  42. 5.7. Transform
  43. 5.8. Print
  44. 5.9. Print Line Number
  45. 5.10. Next
  46. 5.11. Reading and Writing Files
  47. 5.12. Quit
  48. 6. Advanced sed Commands
  49. 6.1. Multiline Pattern Space
  50. 6.2. A Case for Study
  51. 6.3. Hold That Line
  52. 6.4. Advanced Flow Control Commands
  53. 6.5. To Join a Phrase
  54. 7. Writing Scripts for awk
  55. 7.1. Playing the Game
  56. 7.2. Hello, World
  57. 7.3. Awk’s Programming Model
  58. 7.4. Pattern Matching
  59. 7.5. Records and Fields
  60. 7.6. Expressions
  61. 7.7. System Variables
  62. 7.8. Relational and Boolean Operators
  63. 7.9. Formatted Printing
  64. 7.10. Passing Parameters Into a Script
  65. 7.11. Information Retrieval
  66. 8. Conditionals, Loops, and Arrays
  67. 8.1. Conditional Statements
  68. 8.2. Looping
  69. 8.3. Other Statements That Affect Flow Control
  70. 8.4. Arrays
  71. 8.5. An Acronym Processor
  72. 8.6. System Variables That Are Arrays
  73. 9. Functions
  74. 9.1. Arithmetic Functions
  75. 9.2. String Functions
  76. 9.3. Writing Your Own Functions
  77. 10. The Bottom Drawer
  78. 10.1. The getline Function
  79. 10.2. The close( ) Function
  80. 10.3. The system( ) Function
  81. 10.4. A Menu-Based Command Generator
  82. 10.5. Directing Output to Files and Pipes
  83. 10.6. Generating Columnar Reports
  84. 10.7. Debugging
  85. 10.8. Limitations
  86. 10.9. Invoking awk Using the #! Syntax
  87. 11. A Flock of awks
  88. 11.1. Original awk
  89. 11.2. Freely Available awks
  90. 11.3. Commercial awks
  91. 11.4. Epilogue
  92. 12. Full-Featured Applications
  93. 12.1. An Interactive Spelling Checker
  94. 12.2. Generating a Formatted Index
  95. 12.3. Spare Details of the masterindex Program
  96. 13. A Miscellany of Scripts
  97. 13.1. uutot.awk—Report UUCP Statistics
  98. 13.2. phonebill—Track Phone Usage
  99. 13.3. combine—Extract Multipart uuencoded Binaries
  100. 13.4. mailavg—Check Size of Mailboxes
  101. 13.5. adj—Adjust Lines for Text Files
  102. 13.6. readsource—Format Program Source Files for troff
  103. 13.7. gent—Get a termcap Entry
  104. 13.8. plpr—lpr Preprocessor
  105. 13.9. transpose—Perform a Matrix Transposition
  106. 13.10. m1—Simple Macro Processor
  107. A. Quick Reference for sed
  108. A.1. Command-Line Syntax
  109. A.2. Syntax of sed Commands
  110. A.3. Command Summary for sed
  111. B. Quick Reference for awk
  112. B.1. Command-Line Syntax
  113. B.2. Language Summary for awk
  114. B.3. Command Summary for awk
  115. C. Supplement for Chapter 12
  116. C.1. Full Listing of spellcheck.awk
  117. C.2. Listing of masterindex Shell Script
  118. C.3. Documentation for masterindex
  119. masterindex
  120. C.3.1. Background Details
  121. C.3.2. Coding Index Entries
  122. C.3.3. Output Format
  123. C.3.4. Compiling a Master Index
  124. Index
  125. About the Authors
  126. Colophon
  127. Copyright

Applying Commands in a Script

Combining a series of edits in a script can have unexpected results. You might not think of the consequences one edit can have on another. New users typically think that sed applies an individual editing command to all lines of input before applying the next editing command. But the opposite is true. Sed applies the entire script to the first input line before reading the second input line and applying the editing script to it. Because sed is always working with the latest version of the original line, any edit that is made changes the line for subsequent commands. Sed doesn’t retain the original. This means that a pattern that might have matched the original input line may no longer match the line after an edit has been made.

Let’s look at an example that uses the substitute command. Suppose someone quickly wrote the following script to change “pig” to “cow” and “cow” to “horse”:

s/pig/cow/g
s/cow/horse/g

What do you think happened? Try it on a sample file. We’ll discuss what happened later, after we look at how sed works.

The Pattern Space

Sed maintains a pattern space, a workspace or temporary buffer where a single line of input is held while the editing commands are applied.[1] The transformation of the pattern space by a two-line script is shown in Figure 4.1. It changes “The Unix System” to “The UNIX Operating System.”

Initially, the pattern space contains a copy of a single input line. In Figure 4.1, that line is “The Unix System.” The normal flow through the script is to execute each command on that line until the end of the script is reached. The first command in the script is applied to that line, changing “Unix” to “UNIX.” Then the second command is applied, changing “UNIX System” to “UNIX Operating System.”[2] Note that the pattern for the second substitute command does not match the original input line; it matches the current line as it has changed in the pattern space.

When all the instructions have been applied, the current line is output and the next line of input is read into the pattern space. Then all the commands in the script are applied to that line.

The commands in the script change the contents of the pattern space.
Figure 4.1. The commands in the script change the contents of the pattern space.

As a consequence, any sed command might change the contents of the pattern space for the next command. The contents of the pattern space are dynamic and do not always match the original input line. That was the problem with the sample script at the beginning of this chapter. The first command would change “pig” to “cow” as expected. However, when the second command changed “cow” to “horse” on the same line, it also changed the “cow” that had been a “pig.” So, where the input file contained pigs and cows, the output file has only horses!

This mistake is simply a problem of the order of the commands in the script. Reversing the order of the commands—changing “cow” into “horse” before changing “pig” into “cow”—does the trick.

s/cow/horse/g
s/pig/cow/g

Some sed commands change the flow through the script, as we will see in subsequent chapters. For example, the N command reads another line into the pattern space without removing the current line, so you can test for patterns across multiple lines. Other commands tell sed to exit before reaching the bottom of the script or to go to a labeled command. Sed also maintains a second temporary buffer called the hold space. You can copy the contents of the pattern space to the hold space and retrieve them later. The commands that make use of the hold space are discussed in Chapter 6.


[1] One advantage of the one-line-at-a-time design is that sed can read very large files without any problems. Screen editors that have to read the entire file into memory, or some large portion of it, can run out of memory or be extremely slow to use in dealing with large files.

[2] Yes, we could have changed “Unix System” to “UNIX Operating System” in one step. However, the input file might have instances of “UNIX System” as well as “Unix System.” So by changing “Unix” to “UNIX” we make both instances consistent before changing them to “UNIX Operating System.”