The GNU Awk User's Guide

General Introduction

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

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1. “A GNU Manual”
2. “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”

• Foreword: Some nice words about this Web page.
• Preface: What this Web page is about; brief history and acknowledgments.
• Getting Started: A basic introduction to using awk. How to run an awk program. Command-line syntax.
• Invoking Gawk: How to run gawk.
• Regexp: All about matching things using regular expressions.
• Reading Files: How to read files and manipulate fields.
• Printing: How to print using awk. Describes the print and printf statements. Also describes redirection of output.
• Expressions: Expressions are the basic building blocks of statements.
• Patterns and Actions: Overviews of patterns and actions.
• Arrays: The description and use of arrays. Also includes array-oriented control statements.
• Functions: Built-in and user-defined functions.
• Internationalization: Getting gawk to speak your language.
• Advanced Features: Stuff for advanced users, specific to gawk.
• Library Functions: A Library of awk Functions.
• Sample Programs: Many awk programs with complete explanations.
• Debugger: The dgawk debugger.
• Language History: The evolution of the awk language.
• Installation: Installing gawk under various operating systems.
• Notes: Notes about gawk extensions and possible future work.
• Basic Concepts: A very quick introduction to programming concepts.
• Glossary: An explanation of some unfamiliar terms.
• Copying: Your right to copy and distribute gawk.
• GNU Free Documentation License: The license for this Web page.
• Index: Concept and Variable Index.
• History: The history of gawk and awk.
• Names: What name to use to find awk.
• This Manual: Using this Web page. Includes sample input files that you can use.
• Conventions: Typographical Conventions.
• Manual History: Brief history of the GNU project and this Web page.
• How To Contribute: Helping to save the world.
• Acknowledgments: Acknowledgments.
• Running gawk: How to run gawk programs; includes command-line syntax.
• One-shot: Running a short throwaway awk program.
• Read Terminal: Using no input files (input from terminal instead).
• Long: Putting permanent awk programs in files.
• Executable Scripts: Making self-contained awk programs.
• Comments: Adding documentation to gawk programs.
• Quoting: More discussion of shell quoting issues.
• DOS Quoting: Quoting in Windows Batch Files.
• Sample Data Files: Sample data files for use in the awk programs illustrated in this Web page.
• Very Simple: A very simple example.
• Two Rules: A less simple one-line example using two rules.
• More Complex: A more complex example.
• Statements/Lines: Subdividing or combining statements into lines.
• Other Features: Other Features of awk.
• When: When to use gawk and when to use other things.
• Command Line: How to run awk.
• Options: Command-line options and their meanings.
• Other Arguments: Input file names and variable assignments.
• Naming Standard Input: How to specify standard input with other files.
• Environment Variables: The environment variables gawk uses.
• AWKPATH Variable: Searching directories for awk programs.
• Other Environment Variables: The environment variables.
• Exit Status: gawk's exit status.
• Include Files: Including other files into your program.
• Obsolete: Obsolete Options and/or features.
• Undocumented: Undocumented Options and Features.
• Regexp Usage: How to Use Regular Expressions.
• Escape Sequences: How to write nonprinting characters.
• Regexp Operators: Regular Expression Operators.
• Bracket Expressions: What can go between ‘[...]’.
• GNU Regexp Operators: Operators specific to GNU software.
• Case-sensitivity: How to do case-insensitive matching.
• Leftmost Longest: How much text matches.
• Computed Regexps: Using Dynamic Regexps.
• Records: Controlling how data is split into records.
• Fields: An introduction to fields.
• Nonconstant Fields: Nonconstant Field Numbers.
• Changing Fields: Changing the Contents of a Field.
• Field Separators: The field separator and how to change it.
• Default Field Splitting: How fields are normally separated.
• Regexp Field Splitting: Using regexps as the field separator.
• Single Character Fields: Making each character a separate field.
• Command Line Field Separator: Setting FS from the command-line.
• Field Splitting Summary: Some final points and a summary table.
• Constant Size: Reading constant width data.
• Splitting By Content: Defining Fields By Content
• Multiple Line: Reading multi-line records.
• Getline: Reading files under explicit program control using the getline function.
• Plain Getline: Using getline with no arguments.
• Getline/Variable: Using getline into a variable.
• Getline/File: Using getline from a file.
• Getline/Variable/File: Using getline into a variable from a file.
• Getline/Pipe: Using getline from a pipe.
• Getline/Variable/Pipe: Using getline into a variable from a pipe.
• Getline/Coprocess: Using getline from a coprocess.
• Getline/Variable/Coprocess: Using getline into a variable from a coprocess.
• Getline Notes: Important things to know about getline.
• Getline Summary: Summary of getline Variants.
• Command line directories: What happens if you put a directory on the command line.
• Print: The print statement.
• Print Examples: Simple examples of print statements.
• Output Separators: The output separators and how to change them.
• OFMT: Controlling Numeric Output With print.
• Printf: The printf statement.
• Basic Printf: Syntax of the printf statement.
• Control Letters: Format-control letters.
• Format Modifiers: Format-specification modifiers.
• Printf Examples: Several examples.
• Redirection: How to redirect output to multiple files and pipes.
• Special Files: File name interpretation in gawk. gawk allows access to inherited file descriptors.
• Special FD: Special files for I/O.
• Special Network: Special files for network communications.
• Special Caveats: Things to watch out for.
• Close Files And Pipes: Closing Input and Output Files and Pipes.
• Values: Constants, Variables, and Regular Expressions.
• Constants: String, numeric and regexp constants.
• Scalar Constants: Numeric and string constants.
• Nondecimal-numbers: What are octal and hex numbers.
• Regexp Constants: Regular Expression constants.
• Using Constant Regexps: When and how to use a regexp constant.
• Variables: Variables give names to values for later use.
• Using Variables: Using variables in your programs.
• Assignment Options: Setting variables on the command-line and a summary of command-line syntax. This is an advanced method of input.
• Conversion: The conversion of strings to numbers and vice versa.
• All Operators: gawk's operators.
• Arithmetic Ops: Arithmetic operations (‘+’, ‘-’, etc.)
• Concatenation: Concatenating strings.
• Assignment Ops: Changing the value of a variable or a field.
• Increment Ops: Incrementing the numeric value of a variable.
• Truth Values and Conditions: Testing for true and false.
• Truth Values: What is ``true'' and what is ``false''.
• Typing and Comparison: How variables acquire types and how this affects comparison of numbers and strings with ‘<’, etc.
• Variable Typing: String type versus numeric type.
• Comparison Operators: The comparison operators.
• POSIX String Comparison: String comparison with POSIX rules.
• Boolean Ops: Combining comparison expressions using boolean operators ‘||’ (``or''), ‘&&’ (``and'') and ‘!’ (``not'').
• Conditional Exp: Conditional expressions select between two subexpressions under control of a third subexpression.
• Function Calls: A function call is an expression.
• Precedence: How various operators nest.
• Locales: How the locale affects things.
• Pattern Overview: What goes into a pattern.
• Regexp Patterns: Using regexps as patterns.
• Expression Patterns: Any expression can be used as a pattern.
• Ranges: Pairs of patterns specify record ranges.
• BEGIN/END: Specifying initialization and cleanup rules.
• Using BEGIN/END: How and why to use BEGIN/END rules.
• I/O And BEGIN/END: I/O issues in BEGIN/END rules.
• BEGINFILE/ENDFILE: Two special patterns for advanced control.
• Empty: The empty pattern, which matches every record.
• Using Shell Variables: How to use shell variables with awk.
• Action Overview: What goes into an action.
• Statements: Describes the various control statements in detail.
• If Statement: Conditionally execute some awk statements.
• While Statement: Loop until some condition is satisfied.
• Do Statement: Do specified action while looping until some condition is satisfied.
• For Statement: Another looping statement, that provides initialization and increment clauses.
• Switch Statement: Switch/case evaluation for conditional execution of statements based on a value.
• Break Statement: Immediately exit the innermost enclosing loop.
• Continue Statement: Skip to the end of the innermost enclosing loop.
• Next Statement: Stop processing the current input record.
• Nextfile Statement: Stop processing the current file.
• Exit Statement: Stop execution of awk.
• Built-in Variables: Summarizes the built-in variables.
• User-modified: Built-in variables that you change to control awk.
• Auto-set: Built-in variables where awk gives you information.
• ARGC and ARGV: Ways to use ARGC and ARGV.
• Array Basics: The basics of arrays.
• Array Intro: Introduction to Arrays
• Reference to Elements: How to examine one element of an array.
• Assigning Elements: How to change an element of an array.
• Array Example: Basic Example of an Array
• Scanning an Array: A variation of the for statement. It loops through the indices of an array's existing elements.
• Controlling Scanning: Controlling the order in which arrays are scanned.
• Delete: The delete statement removes an element from an array.
• Numeric Array Subscripts: How to use numbers as subscripts in awk.
• Uninitialized Subscripts: Using Uninitialized variables as subscripts.
• Multi-dimensional: Emulating multidimensional arrays in awk.
• Multi-scanning: Scanning multidimensional arrays.
• Arrays of Arrays: True multidimensional arrays.
• Built-in: Summarizes the built-in functions.
• Calling Built-in: How to call built-in functions.
• Numeric Functions: Functions that work with numbers, including int(), sin() and rand().
• String Functions: Functions for string manipulation, such as split(), match() and sprintf().
• Gory Details: More than you want to know about ‘\’ and ‘&’ with sub(), gsub(), and gensub().
• I/O Functions: Functions for files and shell commands.
• Time Functions: Functions for dealing with timestamps.
• Bitwise Functions: Functions for bitwise operations.
• Type Functions: Functions for type information.
• I18N Functions: Functions for string translation.
• User-defined: Describes User-defined functions in detail.
• Definition Syntax: How to write definitions and what they mean.
• Function Example: An example function definition and what it does.
• Function Caveats: Things to watch out for.
• Calling A Function: Don't use spaces.
• Variable Scope: Controlling variable scope.
• Pass By Value/Reference: Passing parameters.
• Return Statement: Specifying the value a function returns.
• Dynamic Typing: How variable types can change at runtime.
• Indirect Calls: Choosing the function to call at runtime.
• I18N and L10N: Internationalization and Localization.
• Explaining gettext: How GNU gettext works.
• Programmer i18n: Features for the programmer.
• Translator i18n: Features for the translator.
• String Extraction: Extracting marked strings.
• Printf Ordering: Rearranging printf arguments.
• I18N Portability: awk-level portability issues.
• I18N Example: A simple i18n example.
• Gawk I18N: gawk is also internationalized.
• Nondecimal Data: Allowing nondecimal input data.
• Array Sorting: Facilities for controlling array traversal and sorting arrays.
• Controlling Array Traversal: How to use PROCINFO["sorted_in"].
• Array Sorting Functions: How to use asort() and asorti().
• Two-way I/O: Two-way communications with another process.
• TCP/IP Networking: Using gawk for network programming.
• Profiling: Profiling your awk programs.
• Library Names: How to best name private global variables in library functions.
• General Functions: Functions that are of general use.
• Strtonum Function: A replacement for the built-in strtonum() function.
• Assert Function: A function for assertions in awk programs.
• Round Function: A function for rounding if sprintf() does not do it correctly.
• Cliff Random Function: The Cliff Random Number Generator.
• Ordinal Functions: Functions for using characters as numbers and vice versa.
• Join Function: A function to join an array into a string.
• Gettimeofday Function: A function to get formatted times.
• Data File Management: Functions for managing command-line data files.
• Filetrans Function: A function for handling data file transitions.
• Rewind Function: A function for rereading the current file.
• File Checking: Checking that data files are readable.
• Empty Files: Checking for zero-length files.
• Ignoring Assigns: Treating assignments as file names.
• Getopt Function: A function for processing command-line arguments.
• Passwd Functions: Functions for getting user information.
• Group Functions: Functions for getting group information.
• Walking Arrays: A function to walk arrays of arrays.
• Running Examples: How to run these examples.
• Clones: Clones of common utilities.
• Cut Program: The cut utility.
• Egrep Program: The egrep utility.
• Id Program: The id utility.
• Split Program: The split utility.
• Tee Program: The tee utility.
• Uniq Program: The uniq utility.
• Wc Program: The wc utility.
• Miscellaneous Programs: Some interesting awk programs.
• Dupword Program: Finding duplicated words in a document.
• Alarm Program: An alarm clock.
• Translate Program: A program similar to the tr utility.
• Labels Program: Printing mailing labels.
• Word Sorting: A program to produce a word usage count.
• History Sorting: Eliminating duplicate entries from a history file.
• Extract Program: Pulling out programs from Texinfo source files.
• Simple Sed: A Simple Stream Editor.
• Igawk Program: A wrapper for awk that includes files.
• Anagram Program: Finding anagrams from a dictionary.
• Signature Program: People do amazing things with too much time on their hands.
• Debugging: Introduction to dgawk.
• Debugging Concepts: Debugging In General.
• Debugging Terms: Additional Debugging Concepts.
• Awk Debugging: Awk Debugging.
• Sample dgawk session: Sample dgawk session.
• dgawk invocation: dgawk Invocation.
• Finding The Bug: Finding The Bug.
• List of Debugger Commands: Main dgawk Commands.
• Breakpoint Control: Control of breakpoints.
• Dgawk Execution Control: Control of execution.
• Viewing And Changing Data: Viewing and changing data.
• Dgawk Stack: Dealing with the stack.
• Dgawk Info: Obtaining information about the program and the debugger state.
• Miscellaneous Dgawk Commands: Miscellaneous Commands.
• Readline Support: Readline Support.
• Dgawk Limitations: Limitations and future plans.
• V7/SVR3.1: The major changes between V7 and System V Release 3.1.
• SVR4: Minor changes between System V Releases 3.1 and 4.
• POSIX: New features from the POSIX standard.
• BTL: New features from Brian Kernighan's version of awk.
• POSIX/GNU: The extensions in gawk not in POSIX awk.
• Common Extensions: Common Extensions Summary.
• Ranges and Locales: How locales used to affect regexp ranges.
• Contributors: The major contributors to gawk.
• Gawk Distribution: What is in the gawk distribution.
• Getting: How to get the distribution.
• Extracting: How to extract the distribution.
• Distribution contents: What is in the distribution.
• Unix Installation: Installing gawk under various versions of Unix.
• Quick Installation: Compiling gawk under Unix.
• Additional Configuration Options: Other compile-time options.
• Configuration Philosophy: How it's all supposed to work.
• Non-Unix Installation: Installation on Other Operating Systems.
• PC Installation: Installing and Compiling gawk on MS-DOS and OS/2.
• PC Binary Installation: Installing a prepared distribution.
• PC Compiling: Compiling gawk for MS-DOS, Windows32, and OS/2.
• PC Testing: Testing gawk on PC systems.
• PC Using: Running gawk on MS-DOS, Windows32 and OS/2.
• Cygwin: Building and running gawk for Cygwin.
• MSYS: Using gawk In The MSYS Environment.
• VMS Installation: Installing gawk on VMS.
• VMS Compilation: How to compile gawk under VMS.
• VMS Installation Details: How to install gawk under VMS.
• VMS Running: How to run gawk under VMS.
• VMS Old Gawk: An old version comes with some VMS systems.
• Bugs: Reporting Problems and Bugs.
• Other Versions: Other freely available awk implementations.
• Compatibility Mode: How to disable certain gawk extensions.
• Additions: Making Additions To gawk.
• Accessing The Source: Accessing the Git repository.
• Adding Code: Adding code to the main body of gawk.
• New Ports: Porting gawk to a new operating system.
• Dynamic Extensions: Adding new built-in functions to gawk.
• Internals: A brief look at some gawk internals.
• Plugin License: A note about licensing.
• Sample Library: A example of new functions.
• Internal File Description: What the new functions will do.
• Internal File Ops: The code for internal file operations.
• Using Internal File Ops: How to use an external extension.
• Future Extensions: New features that may be implemented one day.
• Basic High Level: The high level view.
• Basic Data Typing: A very quick intro to data types.
• Floating Point Issues: Stuff to know about floating-point numbers.
• String Conversion Precision: The String Value Can Lie.
• Unexpected Results: Floating Point Numbers Are Not Abstract Numbers.
• POSIX Floating Point Problems: Standards Versus Existing Practice.