GAWK: Effective AWK Programming

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iiGAWK: Effective AWK ProgrammingTable of ContentsForeword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3History of awk and gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3A Rose by Any Other Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Using This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Typographical Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6The GNU Project and This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7How to Contribute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Getting Started with awk . . . . . . . . . . . . . . . . . . . . . 111.1 How to Run awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.1.1 One-Shot Throwaway awk Programs . . . . . . . . . . . . . . . . . . . . . . 111.1.2 Running awk Without Input Files . . . . . . . . . . . . . . . . . . . . . . . . 121.1.3 Running Long Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.1.4 Executable awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.1.5 Comments in awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.1.6 Shell-Quoting Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.1.6.1 Quoting in MS-DOS Batch Files . . . . . . . . . . . . . . . . . . . . . 161.2 Data Files for the Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3 Some Simple Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.4 An Example with Two Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191.5 A More Complex Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.6 awk Statements Versus Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211.7 Other Features of awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221.8 When to Use awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 Regular Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . 242.1 How to Use Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.2 Escape Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.3 Regular Expression Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.4 Using Character Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.5 gawk-Specific Regexp Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.6 Case Sensitivity in Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.7 How Much Text Matches? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.8 Using Dynamic Regexps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.9 Where You Are Makes A Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3 Reading Input Files . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.1 How Input Is Split into Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.2 Examining Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3 Nonconstant Field Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.4 Changing the Contents of a Field. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.5 Specifying How Fields Are Separated . . . . . . . . . . . . . . . . . . . . . . . . . . 433.5.1 Using Regular Expressions to Separate Fields . . . . . . . . . . . . . 443.5.2 Making Each Character a Separate Field . . . . . . . . . . . . . . . . . 453.5.3 Setting FS from the Command Line . . . . . . . . . . . . . . . . . . . . . . 453.5.4 Field-Splitting Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.6 Reading Fixed-Width Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.7 Multiple-Line Records. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.8 Explicit Input with getline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.8.1 Using getline with No Arguments . . . . . . . . . . . . . . . . . . . . . . . 523.8.2 Using getline into a Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.8.3 Using getline from a File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.8.4 Using getline into a Variable from a File . . . . . . . . . . . . . . . . 543.8.5 Using getline from a Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.8.6 Using getline into a Variable from a Pipe . . . . . . . . . . . . . . . 563.8.7 Using getline from a Coprocess . . . . . . . . . . . . . . . . . . . . . . . . . 563.8.8 Using getline into a Variable from a Coprocess . . . . . . . . . . 563.8.9 Points to Remember About getline . . . . . . . . . . . . . . . . . . . . . 563.8.10 Summary of getline Variants. . . . . . . . . . . . . . . . . . . . . . . . . . . 57iii4 Printing Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.1 The print Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.2 Examples of print Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.3 Output Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 604.4 Controlling Numeric Output with print. . . . . . . . . . . . . . . . . . . . . . . 604.5 Using printf Statements for Fancier Printing . . . . . . . . . . . . . . . . . 614.5.1 Introduction to the printf Statement . . . . . . . . . . . . . . . . . . . . 614.5.2 Format-Control Letters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614.5.3 Modifiers for printf Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.5.4 Examples Using printf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654.6 Redirecting Output of print and printf . . . . . . . . . . . . . . . . . . . . . . 664.7 Special File Names in gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.7.1 Special Files for Standard Descriptors . . . . . . . . . . . . . . . . . . . . 694.7.2 Special Files for Process-Related Information . . . . . . . . . . . . . 704.7.3 Special Files for Network Communications . . . . . . . . . . . . . . . . 714.7.4 Special File Name Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.8 Closing Input and Output Redirections . . . . . . . . . . . . . . . . . . . . . . . . 71
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