The GNU Awk User's Guide

This file documents awk, a program that you can use to select particular records in a file and perform operations upon them.

This is Edition 3 of GAWK: Effective AWK Programming: A User's Guide for GNU Awk, for the 3.1.0 version of the GNU implementation of AWK.

Foreword		Some nice words about this Web page.
Preface		What this Web page is about; brief history and acknowledgments.
2. Getting Started with awk		A basic introduction to using

                                   awk. How to run an awk
                                   program. Command-line syntax.

3. Regular Expressions		All about matching things using regular expressions.
4. Reading Input Files		How to read files and manipulate fields.
5. Printing Output		How to print using awk. Describes the print and printf statements. Also describes redirection of output.
6. Expressions		Expressions are the basic building blocks of statements.
7. Patterns, Actions, and Variables		Overviews of patterns and actions.
8. Arrays in awk		The description and use of arrays. Also includes array-oriented control statements.
9. Functions		Built-in and user-defined functions.
10. Internationalization with gawk		Getting gawk to speak your language.
11. Advanced Features of gawk		Stuff for advanced users, specific to

                                   gawk.

12. Running awk and gawk		How to run gawk.
13. A Library of awk Functions
14. Practical awk Programs		Many awk programs with complete explanations.
A. The Evolution of the awk Language		The evolution of the awk language.
B. Installing gawk		Installing gawk under various operating systems.
C. Implementation Notes		Notes about gawk extensions and possible future work.
D. Basic Programming Concepts		A very quick intoduction to programming concepts.
Glossary		An explanation of some unfamiliar terms.
GNU General Public License		Your right to copy and distribute

                                   gawk.

GNU Free Documentation License		The license for this Web page.
Index		Concept and Variable Index.

History of awk and gawk

The history of gawk and

                                   awk.

1.0 A Rose by Any Other Name		What name to use to find awk.
1.1 Using This Book		Using this Web page. Includes sample input files that you can use.
1.2 Typographical Conventions
The GNU Project and This Book		Brief history of the GNU project and this Web page.
How to Contribute		Helping to save the world.
Acknowledgments
2.1 How to Run awk Programs		How to run gawk programs; includes command-line syntax.
2.1.1 One-Shot Throw-Away awk Programs		Running a short throw-away awk program.
2.1.2 Running awk Without Input Files		Using no input files (input from terminal instead).
2.1.3 Running Long Programs		Putting permanent awk programs in files.
2.1.4 Executable awk Programs		Making self-contained awk programs.
2.1.5 Comments in awk Programs		Adding documentation to gawk programs.
2.1.6 Shell Quoting Issues		More discussion of shell quoting issues.
2.2 Data Files for the Examples		Sample data files for use in the

                                   awk programs illustrated in this
                                   Web page.

2.3 Some Simple Examples		A very simple example.
2.4 An Example with Two Rules		A less simple one-line example using two rules.
2.5 A More Complex Example		A more complex example.
2.6 awk Statements Versus Lines		Subdividing or combining statements into lines.
2.7 Other Features of awk
2.8 When to Use awk		When to use gawk and when to use other things.
3.1 How to Use Regular Expressions
3.2 Escape Sequences		How to write non-printing characters.
3.3 Regular Expression Operators
3.4 Using Character Lists		What can go between `[...]'.
3.5 gawk-Specific Regexp Operators		Operators specific to GNU software.
3.6 Case Sensitivity in Matching		How to do case-insensitive matching.
3.7 How Much Text Matches?		How much text matches.
3.8 Using Dynamic Regexps
4.1 How Input Is Split into Records		Controlling how data is split into records.
4.2 Examining Fields		An introduction to fields.
4.3 Non-Constant Field Numbers		Non-constant Field Numbers.
4.4 Changing the Contents of a Field
4.5 Specifying How Fields Are Separated		The field separator and how to change it.
4.5.1 Using Regular Expressions to Separate Fields		Using regexps as the field separator.
4.5.2 Making Each Character a Separate Field		Making each character a separate field.
4.5.3 Setting FS from the Command Line		Setting FS from the command-line.
4.5.4 Field Splitting Summary		Some final points and a summary table.
4.6 Reading Fixed-Width Data		Reading constant width data.
4.7 Multiple-Line Records		Reading multi-line records.
4.8 Explicit Input with getline		Reading files under explicit program control using the getline function.
4.8.1 Using getline with No Arguments		Using getline with no arguments.
4.8.2 Using getline into a Variable		Using getline into a variable.
4.8.3 Using getline from a File		Using getline from a file.
4.8.4 Using getline into a Variable from a File		Using getline into a variable from a file.
4.8.5 Using getline from a Pipe		Using getline from a pipe.
4.8.6 Using getline into a Variable from a Pipe		Using getline into a variable from a pipe.
4.8.7 Using getline from a Coprocess		Using getline from a coprocess.
4.8.8 Using getline into a Variable from a Coprocess		Using getline into a variable from a coprocess.
4.8.9 Points About getline to Remember		Important things to know about

                                   getline.

4.8.10 Summary of getline Variants
5.1 The print Statement		The print statement.
5.2 Examples of print Statements		Simple examples of print statements.
5.3 Output Separators		The output separators and how to change them.
5.4 Controlling Numeric Output with print		Controlling Numeric Output With

                                   print.

5.5 Using printf Statements for Fancier Printing		The printf statement.
5.5.1 Introduction to the printf Statement		Syntax of the printf statement.
5.5.2 Format-Control Letters		Format-control letters.
5.5.3 Modifiers for printf Formats		Format-specification modifiers.
5.5.4 Examples Using printf		Several examples.
5.6 Redirecting Output of print and printf		How to redirect output to multiple files and pipes.
5.7 Special File Names in gawk		File name interpretation in gawk.

                                   gawk allows access to inherited
                                   file descriptors.

5.7.1 Special Files for Standard Descriptors		Special files for I/O.
5.7.2 Special Files for Process-Related Information		Special files for process information.
5.7.3 Special Files for Network Communications		Special files for network communications.
5.7.4 Special File Name Caveats		Things to watch out for.
5.8 Closing Input and Output Redirections		Closing Input and Output Files and Pipes.
6.1 Constant Expressions		String, numeric and regexp constants.
6.1.1 Numeric and String Constants		Numeric and string constants.
6.1.2 Octal and Hexadecimal Numbers		What are octal and hex numbers.
6.1.3 Regular Expression Constants		Regular Expression constants.
6.2 Using Regular Expression Constants		When and how to use a regexp constant.
6.3 Variables		Variables give names to values for later use.
6.3.1 Using Variables in a Program		Using variables in your programs.
6.3.2 Assigning Variables on the Command Line		Setting variables on the command-line and a summary of command-line syntax. This is an advanced method of input.
6.4 Conversion of Strings and Numbers		The conversion of strings to numbers and vice versa.
6.5 Arithmetic Operators		Arithmetic operations (`+', `-', etc.)
6.6 String Concatenation		Concatenating strings.
6.7 Assignment Expressions		Changing the value of a variable or a field.
6.8 Increment and Decrement Operators		Incrementing the numeric value of a variable.
6.9 True and False in awk		What is "true" and what is "false".
6.10 Variable Typing and Comparison Expressions		How variables acquire types and how this affects comparison of numbers and strings with `<', etc.
6.11 Boolean Expressions		Combining comparison expressions using boolean operators `||' ("or"),

                                   `&&' ("and") and `!' ("not").

6.12 Conditional Expressions		Conditional expressions select between two subexpressions under control of a third subexpression.
6.13 Function Calls		A function call is an expression.
6.14 Operator Precedence (How Operators Nest)		How various operators nest.
7.1 Pattern Elements		What goes into a pattern.
7.1.1 Regular Expressions as Patterns		Using regexps as patterns.
7.1.2 Expressions as Patterns		Any expression can be used as a pattern.
7.1.3 Specifying Record Ranges with Patterns		Pairs of patterns specify record ranges.
7.1.4 The BEGIN and END Special Patterns		Specifying initialization and cleanup rules.
7.1.4.1 Startup and Cleanup Actions		How and why to use BEGIN/END rules.
7.1.4.2 Input/Output from BEGIN and END Rules		I/O issues in BEGIN/END rules.
7.1.5 The Empty Pattern		The empty pattern, which matches every record.
7.2 Using Shell Variables in Programs		How to use shell variables with

                                   awk.

7.3 Actions		What goes into an action.
7.4 Control Statements in Actions		Describes the various control statements in detail.
7.4.1 The if-else Statement		Conditionally execute some awk statements.
7.4.2 The while Statement		Loop until some condition is satisfied.
7.4.3 The do-while Statement		Do specified action while looping until some condition is satisfied.
7.4.4 The for Statement		Another looping statement, that provides initialization and increment clauses.
7.4.5 The break Statement		Immediately exit the innermost enclosing loop.
7.4.6 The continue Statement		Skip to the end of the innermost enclosing loop.
7.4.7 The next Statement		Stop processing the current input record.
7.4.8 Using gawk's nextfile Statement		Stop processing the current file.
7.4.9 The exit Statement		Stop execution of awk.
7.5 Built-in Variables		Summarizes the built-in variables.
7.5.1 Built-in Variables That Control awk		Built-in variables that you change to control awk.
7.5.2 Built-in Variables That Convey Information		Built-in variables where awk gives you information.
7.5.3 Using ARGC and ARGV		Ways to use ARGC and ARGV.
8.1 Introduction to Arrays
8.2 Referring to an Array Element		How to examine one element of an array.
8.3 Assigning Array Elements		How to change an element of an array.
8.4 Basic Array Example		Basic Example of an Array
8.5 Scanning All Elements of an Array		A variation of the for statement. It loops through the indices of an array's existing elements.
8.6 The delete Statement		The delete statement removes an element from an array.
8.7 Using Numbers to Subscript Arrays		How to use numbers as subscripts in

                                   awk.

8.8 Using Uninitialized Variables as Subscripts		Using Uninitialized variables as subscripts.
8.9 Multidimensional Arrays		Emulating multidimensional arrays in

                                   awk.

8.10 Scanning Multidimensional Arrays		Scanning multidimensional arrays.
8.11 Sorting Array Values and Indices with gawk		Sorting array values and indices.
9.1 Built-in Functions		Summarizes the built-in functions.
9.1.1 Calling Built-in Functions		How to call built-in functions.
9.1.2 Numeric Functions		Functions that work with numbers, including

                                   int, sin and rand.

9.1.3 String Manipulation Functions

Functions for string manipulation, such as

                                   split, match and
                                   sprintf.

9.1.3.1 More About `\' and `&' with sub, gsub, and gensub		More than you want to know about `\' and `&' with sub, gsub, and gensub.
9.1.4 Input/Output Functions		Functions for files and shell commands.
9.1.5 Using gawk's Timestamp Functions		Functions for dealing with timestamps.
9.1.6 Using gawk's Bit Manipulation Functions		Functions for bitwise operations.
9.1.7 Using gawk's String Translation Functions		Functions for string translation.
9.2 User-Defined Functions		Describes User-defined functions in detail.
9.2.1 Function Definition Syntax		How to write definitions and what they mean.
9.2.2 Function Definition Examples		An example function definition and what it does.
9.2.3 Calling User-Defined Functions		Things to watch out for.
9.2.4 The return Statement		Specifying the value a function returns.
9.2.5 Functions and Their Effect on Variable Typing		How variable types can change at runtime.
10.1 Internationalization and Localization
10.2 GNU gettext		How GNU gettext works.
10.3 Internationalizing awk Programs		Features for the programmer.
10.4 Translating awk Programs		Features for the translator.
10.4.1 Extracting Marked Strings		Extracting marked strings.
10.4.2 Rearranging printf Arguments		Rearranging printf arguments.
10.4.3 awk Portability Issues		awk-level portability issues.
10.5 A Simple Internationalization Example		A simple i18n example.
10.6 gawk Can Speak Your Language		gawk is also internationalized.
11.1 Allowing Non-Decimal Input Data		Allowing non-decimal input data.
11.2 Two-Way Communications with Another Process		Two-way communications with another process.
11.3 Using gawk for Network Programming		Using gawk for network programming.
11.4 Using gawk with BSD Portals		Using gawk with BSD portals.
11.5 Profiling Your awk Programs		Profiling your awk programs.
12.1 Invoking awk		How to run awk.
12.2 Command-Line Options		Command-line options and their meanings.
12.3 Other Command-Line Arguments		Input file names and variable assignments.
12.4 The AWKPATH Environment Variable		Searching directories for awk programs.
12.5 Obsolete Options and/or Features		Obsolete Options and/or features.
12.6 Undocumented Options and Features
12.7 Known Bugs in gawk
13.1 Naming Library Function Global Variables		How to best name private global variables in library functions.
13.2 General Programming		Functions that are of general use.
13.2.1 Implementing nextfile as a Function		Two implementations of a nextfile function.
13.2.2 Assertions		A function for assertions in awk programs.
13.2.3 Rounding Numbers		A function for rounding if sprintf does not do it correctly.
13.2.4 The Cliff Random Number Generator
13.2.5 Translating Between Characters and Numbers		Functions for using characters as numbers and vice versa.
13.2.6 Merging an Array into a String		A function to join an array into a string.
13.2.7 Managing the Time of Day		A function to get formatted times.
13.3 Data File Management		Functions for managing command-line data files.
13.3.1 Noting Data File Boundaries		A function for handling data file transitions.
13.3.2 Rereading the Current File		A function for rereading the current file.
13.3.3 Checking for Readable Data Files		Checking that data files are readable.
13.3.4 Treating Assignments as File Names		Treating assignments as file names.
13.4 Processing Command-Line Options		A function for processing command-line arguments.
13.5 Reading the User Database		Functions for getting user information.
13.6 Reading the Group Database		Functions for getting group information.
14.1 Running the Example Programs		How to run these examples.
14.2 Reinventing Wheels for Fun and Profit		Clones of common utilities.
14.2.1 Cutting out Fields and Columns		The cut utility.
14.2.2 Searching for Regular Expressions in Files		The egrep utility.
14.2.3 Printing out User Information		The id utility.
14.2.4 Splitting a Large File into Pieces		The split utility.
14.2.5 Duplicating Output into Multiple Files		The tee utility.
14.2.6 Printing Non-Duplicated Lines of Text		The uniq utility.
14.2.7 Counting Things		The wc utility.
14.3 A Grab Bag of awk Programs		Some interesting awk programs.
14.3.1 Finding Duplicated Words in a Document		Finding duplicated words in a document.
14.3.2 An Alarm Clock Program		An alarm clock.
14.3.3 Transliterating Characters		A program similar to the tr utility.
14.3.4 Printing Mailing Labels		Printing mailing labels.
14.3.5 Generating Word Usage Counts		A program to produce a word usage count.
14.3.6 Removing Duplicates from Unsorted Text		Eliminating duplicate entries from a history file.
14.3.7 Extracting Programs from Texinfo Source Files		Pulling out programs from Texinfo source files.
14.3.8 A Simple Stream Editor
14.3.9 An Easy Way to Use Library Functions		A wrapper for awk that includes files.
A.1 Major Changes Between V7 and SVR3.1		The major changes between V7 and System V Release 3.1.
A.2 Changes Between SVR3.1 and SVR4		Minor changes between System V Releases 3.1 and 4.
A.3 Changes Between SVR4 and POSIX awk		New features from the POSIX standard.
A.4 Extensions in the Bell Laboratories awk		New features from the Bell Laboratories version of awk.
A.5 Extensions in gawk Not in POSIX awk		The extensions in gawk not in POSIX awk.
A.6 Major Contributors to gawk		The major contributors to gawk.
B.1 The gawk Distribution		What is in the gawk distribution.
B.1.1 Getting the gawk Distribution		How to get the distribution.
B.1.2 Extracting the Distribution		How to extract the distribution.
B.1.3 Contents of the gawk Distribution		What is in the distribution.
B.2 Compiling and Installing gawk on Unix		Installing gawk under various versions of Unix.
B.2.1 Compiling gawk for Unix		Compiling gawk under Unix.
B.2.2 Additional Configuration Options		Other compile-time options.
B.2.3 The Configuration Process		How it's all supposed to work.
B.3 Installation on Other Operating Systems
B.3.1 Installing gawk on an Amiga
B.3.2 Installing gawk on BeOS
B.3.3 Installation on PC Operating Systems		Installing and Compiling gawk on MS-DOS and OS/2.
B.3.3.1 Installing a Prepared Distribution for PC Systems		Installing a prepared distribution.
B.3.3.2 Compiling gawk for PC Operating Systems		Compiling gawk for MS-DOS, Win32, and OS/2.
B.3.3.3 Using gawk on PC Operating Systems		Running gawk on MS-DOS, Win32 and OS/2.
B.3.4 How to Compile and Install gawk on VMS		Installing gawk on VMS.
B.3.4.1 Compiling gawk on VMS		How to compile gawk under VMS.
B.3.4.2 Installing gawk on VMS		How to install gawk under VMS.
B.3.4.3 Running gawk on VMS		How to run gawk under VMS.
B.3.4.4 Building and Using gawk on VMS POSIX		Alternate instructions for VMS POSIX.
B.4 Unsupported Operating System Ports		Systems whose ports are no longer supported.
B.4.1 Installing gawk on the Atari ST
B.4.1.1 Compiling gawk on the Atari ST		Compiling gawk on Atari.
B.4.1.2 Running gawk on the Atari ST		Running gawk on Atari.
B.4.2 Installing gawk on a Tandem
B.5 Reporting Problems and Bugs
B.6 Other Freely Available awk Implementations		Other freely available awk implementations.
C.1 Downward Compatibility and Debugging		How to disable certain gawk extensions.
C.2 Making Additions to gawk		Making Additions To gawk.
C.2.1 Adding New Features		Adding code to the main body of

                                   gawk.

C.2.2 Porting gawk to a New Operating System		Porting gawk to a new operating system.
C.3 Adding New Built-in Functions to gawk		Adding new built-in functions to

                                   gawk.

C.3.1 A Minimal Introduction to gawk Internals		A brief look at some gawk internals.
C.3.2 Directory and File Operation Built-ins		A example of new functions.
C.3.2.1 Using chdir and stat		What the new functions will do.
C.3.2.2 C Code for chdir and stat		The code for internal file operations.
C.3.2.3 Integrating the Extensions		How to use an external extension.
C.4 Probable Future Extensions		New features that may be implemented one day.
D.1 What a Program Does		The high level view.
D.2 Data Values in a Computer		A very quick intro to data types.
D.3 Floating-Point Number Caveats		Stuff to know about floating-point numbers.