Glossary

Action

A series of awk statements attached to a rule. If the rule's pattern matches an input record, awk executes the rule's action. Actions are always enclosed in curly braces. See section Overview of Actions.

Amazing awk Assembler

Henry Spencer at the University of Toronto wrote a retargetable assembler completely as awk scripts. It is thousands of lines long, including machine descriptions for several eight-bit microcomputers. It is a good example of a program that would have been better written in another language.

Amazingly Workable Formatter (awf)

Henry Spencer at the University of Toronto wrote a formatter that accepts a large subset of the `nroff -ms' and `nroff -man' formatting commands, using awk and sh.

ANSI

The American National Standards Institute. This organization produces many standards, among them the standards for the C and C++ programming languages.

Assignment

An awk expression that changes the value of some awk variable or data object. An object that you can assign to is called an lvalue. The assigned values are called rvalues. See section Assignment Expressions.

awk Language

The language in which awk programs are written.

awk Program

An awk program consists of a series of patterns and actions, collectively known as rules. For each input record given to the program, the program's rules are all processed in turn. awk programs may also contain function definitions.

awk Script

Another name for an awk program.

Bash

The GNU version of the standard shell (the Bourne-Again shell). See "Bourne Shell."

BBS

See "Bulletin Board System."

Boolean Expression

Named after the English mathematician Boole. See "Logical Expression."

Bourne Shell

The standard shell (`/bin/sh') on Unix and Unix-like systems, originally written by Steven R. Bourne. Many shells (Bash, ksh, pdksh, zsh) are generally upwardly compatible with the Bourne shell.

Built-in Function

The awk language provides built-in functions that perform various numerical, time stamp related, and string computations. Examples are sqrt (for the square root of a number) and substr (for a substring of a string). See section Built-in Functions.

Built-in Variable

ARGC, ARGIND, ARGV, CONVFMT, ENVIRON, ERRNO, FIELDWIDTHS, FILENAME, FNR, FS, IGNORECASE, NF, NR, OFMT, OFS, ORS, RLENGTH, RSTART, RS, RT, and SUBSEP, are the variables that have special meaning to awk. Changing some of them affects awk's running environment. Several of these variables are specific to gawk. See section Built-in Variables.

Braces

See "Curly Braces."

Bulletin Board System

A computer system allowing users to log in and read and/or leave messages for other users of the system, much like leaving paper notes on a bulletin board.

C

The system programming language that most GNU software is written in. The awk programming language has C-like syntax, and this book points out similarities between awk and C when appropriate.

Character Set

The set of numeric codes used by a computer system to represent the characters (letters, numbers, punctuation, etc.) of a particular country or place. The most common character set in use today is ASCII (American Standard Code for Information Interchange). Many European countries use an extension of ASCII known as ISO-8859-1 (ISO Latin-1).

CHEM

A preprocessor for pic that reads descriptions of molecules and produces pic input for drawing them. It was written in awk by Brian Kernighan and Jon Bentley, and is available from netlib@research.att.com.

Compound Statement

A series of awk statements, enclosed in curly braces. Compound statements may be nested. See section Control Statements in Actions.

Concatenation

Concatenating two strings means sticking them together, one after another, giving a new string. For example, the string `foo' concatenated with the string `bar' gives the string `foobar'. See section String Concatenation.

Conditional Expression

An expression using the `?:' ternary operator, such as `expr1 ? expr2 : expr3'. The expression expr1 is evaluated; if the result is true, the value of the whole expression is the value of expr2, otherwise the value is expr3. In either case, only one of expr2 and expr3 is evaluated. See section Conditional Expressions.

Comparison Expression

A relation that is either true or false, such as `(a < b)'. Comparison expressions are used in if, while, do, and for statements, and in patterns to select which input records to process. See section Variable Typing and Comparison Expressions.

Curly Braces

The characters `{' and `}'. Curly braces are used in awk for delimiting actions, compound statements, and function bodies.

Dark Corner

An area in the language where specifications often were (or still are) not clear, leading to unexpected or undesirable behavior. Such areas are marked in this book with "(d.c.)" in the text, and are indexed under the heading "dark corner."

Data Objects

These are numbers and strings of characters. Numbers are converted into strings and vice versa, as needed. See section Conversion of Strings and Numbers.

Double Precision

An internal representation of numbers that can have fractional parts. Double precision numbers keep track of more digits than do single precision numbers, but operations on them are more expensive. This is the way awk stores numeric values. It is the C type double.

Dynamic Regular Expression

A dynamic regular expression is a regular expression written as an ordinary expression. It could be a string constant, such as "foo", but it may also be an expression whose value can vary. See section Using Dynamic Regexps.

Environment

A collection of strings, of the form name=val, that each program has available to it. Users generally place values into the environment in order to provide information to various programs. Typical examples are the environment variables HOME and PATH.

Empty String

See "Null String."

Escape Sequences

A special sequence of characters used for describing non-printing characters, such as `\n' for newline, or `\033' for the ASCII ESC (escape) character. See section Escape Sequences.

Field

When awk reads an input record, it splits the record into pieces separated by whitespace (or by a separator regexp which you can change by setting the built-in variable FS). Such pieces are called fields. If the pieces are of fixed length, you can use the built-in variable FIELDWIDTHS to describe their lengths. See section Specifying How Fields are Separated, and also see See section Reading Fixed-width Data.

Floating Point Number

Often referred to in mathematical terms as a "rational" number, this is just a number that can have a fractional part. See "Double Precision" and "Single Precision."

Format

Format strings are used to control the appearance of output in the printf statement. Also, data conversions from numbers to strings are controlled by the format string contained in the built-in variable CONVFMT. See section Format-Control Letters.

Function

A specialized group of statements used to encapsulate general or program-specific tasks. awk has a number of built-in functions, and also allows you to define your own. See section Built-in Functions, and section User-defined Functions.

FSF

See "Free Software Foundation."

Free Software Foundation

A non-profit organization dedicated to the production and distribution of freely distributable software. It was founded by Richard M. Stallman, the author of the original Emacs editor. GNU Emacs is the most widely used version of Emacs today.

gawk

The GNU implementation of awk.

General Public License

This document describes the terms under which gawk and its source code may be distributed. (see section GNU GENERAL PUBLIC LICENSE)

GNU

"GNU's not Unix". An on-going project of the Free Software Foundation to create a complete, freely distributable, POSIX-compliant computing environment.

GPL

See "General Public License."

Hexadecimal

Base 16 notation, where the digits are 0-9 and A-F, with `A' representing 10, `B' representing 11, and so on up to `F' for 15. Hexadecimal numbers are written in C using a leading `0x', to indicate their base. Thus, 0x12 is 18 (one times 16 plus 2).

I/O

Abbreviation for "Input/Output," the act of moving data into and/or out of a running program.

Input Record

A single chunk of data read in by awk. Usually, an awk input record consists of one line of text. See section How Input is Split into Records.

Integer

A whole number, i.e. a number that does not have a fractional part.

Keyword

In the awk language, a keyword is a word that has special meaning. Keywords are reserved and may not be used as variable names. gawk's keywords are: BEGIN, END, if, else, while, do...while, for, for...in, break, continue, delete, next, nextfile, function, func, and exit.

Logical Expression

An expression using the operators for logic, AND, OR, and NOT, written `&&', `||', and `!' in awk. Often called Boolean expressions, after the mathematician who pioneered this kind of mathematical logic.

Lvalue

An expression that can appear on the left side of an assignment operator. In most languages, lvalues can be variables or array elements. In awk, a field designator can also be used as an lvalue.

Null String

A string with no characters in it. It is represented explicitly in awk programs by placing two double-quote characters next to each other (""). It can appear in input data by having two successive occurrences of the field separator appear next to each other.

Number

A numeric valued data object. The gawk implementation uses double precision floating point to represent numbers. Very old awk implementations use single precision floating point.

Octal

Base-eight notation, where the digits are 0-7. Octal numbers are written in C using a leading `0', to indicate their base. Thus, 013 is 11 (one times 8 plus 3).

Pattern

Patterns tell awk which input records are interesting to which rules. A pattern is an arbitrary conditional expression against which input is tested. If the condition is satisfied, the pattern is said to match the input record. A typical pattern might compare the input record against a regular expression. See section Pattern Elements.

POSIX

The name for a series of standards being developed by the IEEE that specify a Portable Operating System interface. The "IX" denotes the Unix heritage of these standards. The main standard of interest for awk users is IEEE Standard for Information Technology, Standard 1003.2-1992, Portable Operating System Interface (POSIX) Part 2: Shell and Utilities. Informally, this standard is often referred to as simply "P1003.2."

Private

Variables and/or functions that are meant for use exclusively by library functions, and not for the main awk program. Special care must be taken when naming such variables and functions. See section Naming Library Function Global Variables.

Range (of input lines)

A sequence of consecutive lines from the input file. A pattern can specify ranges of input lines for awk to process, or it can specify single lines. See section Pattern Elements.

Recursion

When a function calls itself, either directly or indirectly. If this isn't clear, refer to the entry for "recursion."

Redirection

Redirection means performing input from other than the standard input stream, or output to other than the standard output stream. You can redirect the output of the print and printf statements to a file or a system command, using the `>', `>>', and `|' operators. You can redirect input to the getline statement using the `<' and `|' operators. See section Redirecting Output of print and printf, and section Explicit Input with getline.

Regexp

Short for regular expression. A regexp is a pattern that denotes a set of strings, possibly an infinite set. For example, the regexp `R.*xp' matches any string starting with the letter `R' and ending with the letters `xp'. In awk, regexps are used in patterns and in conditional expressions. Regexps may contain escape sequences. See section Regular Expressions.

Regular Expression

See "regexp."

Regular Expression Constant

A regular expression constant is a regular expression written within slashes, such as /foo/. This regular expression is chosen when you write the awk program, and cannot be changed doing its execution. See section How to Use Regular Expressions.

Rule

A segment of an awk program that specifies how to process single input records. A rule consists of a pattern and an action. awk reads an input record; then, for each rule, if the input record satisfies the rule's pattern, awk executes the rule's action. Otherwise, the rule does nothing for that input record.

Rvalue

A value that can appear on the right side of an assignment operator. In awk, essentially every expression has a value. These values are rvalues.

sed

See "Stream Editor."

Short-Circuit

The nature of the awk logical operators `&&' and `||'. If the value of the entire expression can be deduced from evaluating just the left-hand side of these operators, the right-hand side will not be evaluated (see section Boolean Expressions).

Side Effect

A side effect occurs when an expression has an effect aside from merely producing a value. Assignment expressions, increment and decrement expressions and function calls have side effects. See section Assignment Expressions.

Single Precision

An internal representation of numbers that can have fractional parts. Single precision numbers keep track of fewer digits than do double precision numbers, but operations on them are less expensive in terms of CPU time. This is the type used by some very old versions of awk to store numeric values. It is the C type float.

Space

The character generated by hitting the space bar on the keyboard.

Special File

A file name interpreted internally by gawk, instead of being handed directly to the underlying operating system. For example, `/dev/stderr'. See section Special File Names in gawk.

Stream Editor

A program that reads records from an input stream and processes them one or more at a time. This is in contrast with batch programs, which may expect to read their input files in entirety before starting to do anything, and with interactive programs, which require input from the user.

String

A datum consisting of a sequence of characters, such as `I am a string'. Constant strings are written with double-quotes in the awk language, and may contain escape sequences. See section Escape Sequences.

Tab

The character generated by hitting the TAB key on the keyboard. It usually expands to up to eight spaces upon output.

Unix

A computer operating system originally developed in the early 1970's at AT&T Bell Laboratories. It initially became popular in universities around the world, and later moved into commercial evnironments as a software development system and network server system. There are many commercial versions of Unix, as well as several work-alike systems whose source code is freely available (such as Linux, NetBSD, and FreeBSD).

Whitespace

A sequence of space or tab characters occurring inside an input record or a string.