GNU Info
(gawk.info)Glossary
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. (*Note Actions:
Action Overview.)
Amazing `awk' Assembler
Henry Spencer at the University of Toronto wrote a retargetable
assembler completely as `sed' and `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. You can get it from
`ftp://ftp.freefriends.org/arnold/Awkstuff/aaa.tgz'.
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'. It is available over
the Internet from
`ftp://ftp.freefriends.org/arnold/Awkstuff/awf.tgz'.
Anchor
The regexp metacharacters `^' and `$', which force the match to
the beginning or end of the string, respectively.
ANSI
The American National Standards Institute. This organization
produces many standards, among them the standards for the C and
C++ programming languages. These standards often become
international standards as well. See also "ISO."
Array
A grouping of multiple values under the same name. Most languages
just provide sequential arrays. `awk' provides associative arrays.
Assertion
A statement in a program that a condition is true at this point in
the program. Useful for reasoning about how a program is supposed
to behave.
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". Note:
Assignment Expressions.
Associative Array
Arrays in which the indices may be numbers or strings, not just
sequential integers in a fixed range.
`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 also "Bourne Shell."
BBS
See "Bulletin Board System."
Bit
Short for "Binary Digit." All values in computer memory
ultimately reduce to binary digits: values that are either zero or
one. Groups of bits may be interpreted differently--as integers,
floating-point numbers, character data, addresses of other memory
objects, or other data. `awk' lets you work with floating-point
numbers and strings. `gawk' lets you manipulate bit values with
the built-in functions described in Note: Using `gawk''s Bit
Manipulation Functions.
Computers are often defined by how many bits they use to represent
integer values. Typical systems are 32-bit systems, but 64-bit
systems are becoming increasingly popular, and 16-bit systems are
waning in popularity.
Boolean Expression
Named after the English mathematician Boole. See also "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, I/O-related, and string computations. Examples are
`sqrt' (for the square root of a number) and `substr' (for a
substring of a string). `gawk' provides functions for timestamp
management, bit manipulation, and runtime string translation.
(Note: Built-in Functions.)
Built-in Variable
`ARGC', `ARGV', `CONVFMT', `ENVIRON', `FILENAME', `FNR', `FS',
`NF', `NR', `OFMT', `OFS', `ORS', `RLENGTH', `RSTART', `RS', and
`SUBSEP' are the variables that have special meaning to `awk'. In
addition, `ARGIND', `BINMODE', `ERRNO', `FIELDWIDTHS',
`IGNORECASE', `LINT', `PROCINFO', `RT', and `TEXTDOMAIN' are the
variables that have special meaning to `gawk'. Changing some of
them affects `awk''s running environment. (Note: 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
Info file points out similarities between `awk' and C when
appropriate.
In general, `gawk' attempts to be as similar to the 1990 version
of ISO C as makes sense. Future versions of `gawk' may adopt
features from the newer 1999 standard, as appropriate.
C++
A popular object-oriented programming language derived from C.
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
`http://cm.bell-labs.com/netlib/typesetting/chem.gz'.
Coprocess
A subordinate program with which two-way communications is
possible.
Compiler
A program that translates human-readable source code into
machine-executable object code. The object code is then executed
directly by the computer. See also "Interpreter."
Compound Statement
A series of `awk' statements, enclosed in curly braces. Compound
statements may be nested. (*Note Control Statements in Actions:
Statements.)
Concatenation
Concatenating two strings means sticking them together, one after
another, producing a new string. For example, the string `foo'
concatenated with the string `bar' gives the string `foobar'.
(Note: 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. (*Note Conditional Expressions:
Conditional Exp.)
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. (*Note Variable Typing and Comparison Expressions:
Typing and Comparison.)
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 Info file with "(d.c.)" in the text
and are indexed under the heading "dark corner."
Data Driven
A description of `awk' programs, where you specify the data you
are interested in processing, and what to do when that data is
seen.
Data Objects
These are numbers and strings of characters. Numbers are
converted into strings and vice versa, as needed. (Note:
Conversion of Strings and Numbers.)
Deadlock
The situation in which two communicating processes are each waiting
for the other to perform an action.
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 sometimes
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.
(Note: 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."
Epoch
The date used as the "beginning of time" for timestamps. Time
values in Unix systems are represented as seconds since the epoch,
with library functions available for converting these values into
standard date and time formats.
The epoch on Unix and POSIX systems is 1970-01-01 00:00:00 UTC.
See also "GMT" and "UTC."
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. (Note: Escape Sequences.)
FDL
See "Free Documentation License."
Field
When `awk' reads an input record, it splits the record into pieces
separated by whitespace (or by a separator regexp that 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. (Note:
Specifying How Fields Are Separated, and Note:
Reading Fixed-Width Data.)
Flag
A variable whose truth value indicates the existence or
non-existence of some condition.
Floating-Point Number
Often referred to in mathematical terms as a "rational" or real
number, this is just a number that can have a fractional part.
See also "Double-Precision" and "Single-Precision."
Format
Format strings are used to control the appearance of output in the
`strftime' and `sprintf' functions, and are used in the `printf'
statement as well. Also, data conversions from numbers to strings
are controlled by the format string contained in the built-in
variable `CONVFMT'. (Note: Format-Control Letters.
)
Free Documentation License
This document describes the terms under which this Info file is
published and may be copied. (Note: GNU Free Documentation
License.)
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. (Note: 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. (*Note GNU General Public License:
Copying.)
GMT
"Greenwich Mean Time." This is the old term for UTC. It is the
time of day used as the epoch for Unix and POSIX systems. See
also "Epoch" and "UTC."
GNU
"GNU's not Unix". An on-going project of the Free Software
Foundation to create a complete, freely distributable,
POSIX-compliant computing environment.
GNU/Linux
A variant of the GNU system using the Linux kernel, instead of the
Free Software Foundation's Hurd kernel. Linux is a stable,
efficient, full-featured clone of Unix that has been ported to a
variety of architectures. It is most popular on PC-class systems,
but runs well on a variety of other systems too. The Linux kernel
source code is available under the terms of the GNU General Public
License, which is perhaps its most important aspect.
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 (1 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 that is read in by `awk'. Usually, an
`awk' input record consists of one line of text. (Note: How Input
Is Split into Records.)
Integer
A whole number, i.e., a number that does not have a fractional
part.
Internationalization
The process of writing or modifying a program so that it can use
multiple languages without requiring further source code changes.
Interpreter
A program that reads human-readable source code directly, and uses
the instructions in it to process data and produce results. `awk'
is typically (but not always) implemented as an interpreter. See
also "Compiler."
Interval Expression
A component of a regular expression that lets you specify repeated
matches of some part of the regexp. Interval expressions were not
traditionally available in `awk' programs.
ISO
The International Standards Organization. This organization
produces international standards for many things, including
programming languages, such as C and C++. In the computer arena,
important standards like those for C, C++, and POSIX become both
American national and ISO international standards simultaneously.
This Info file refers to Standard C as "ISO C" throughout.
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'.
Lesser General Public License
This document describes the terms under which binary library
archives or shared objects, and their source code may be
distributed.
Linux
See "GNU/Linux."
LGPL
See "Lesser General Public License."
Localization
The process of providing the data necessary for an
internationalized program to work in a particular language.
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.
Matching
The act of testing a string against a regular expression. If the
regexp describes the contents of the string, it is said to "match"
it.
Metacharacters
Characters used within a regexp that do not stand for themselves.
Instead, they denote regular expression operations, such as
repetition, grouping, or alternation.
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. Modern `awk' implementations use
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).
P1003.2
See "POSIX."
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. (Note: Pattern
Elements.)
POSIX
The name for a series of standards 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."
Precedence
The order in which operations are performed when operators are used
without explicit parentheses.
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. (Note:
Naming Library Function Global Variables.)
Range (of input lines)
A sequence of consecutive lines from the input file(s). A pattern
can specify ranges of input lines for `awk' to process or it can
specify single lines. (Note: 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 something other than the
standard input stream, or performing output to something 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. (Note: Redirecting Output of
`print' and `printf', and Note: 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. (Note: 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 during its
execution. (Note: 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.
Scalar
A single value, be it a number or a string. Regular variables are
scalars; arrays and functions are not.
Search Path
In `gawk', a list of directories to search for `awk' program
source files. In the shell, a list of directories to search for
executable programs.
Seed
The initial value, or starting point, for a sequence of random
numbers.
`sed'
See "Stream Editor."
Shell
The command interpreter for Unix and POSIX-compliant systems. The
shell works both interactively, and as a programming language for
batch files, or shell scripts.
Short-Circuit
The nature of the `awk' logical operators `&&' and `||'. If the
value of the entire expression is determinable from evaluating just
the lefthand side of these operators, the righthand side is not
evaluated. (Note: 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.
(Note: 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 sometimes
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'. (Note: 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, as well as 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. (Note: Escape
Sequences.)
Tab
The character generated by hitting the `TAB' key on the keyboard.
It usually expands to up to eight spaces upon output.
Text Domain
A unique name that identifies an application. Used for grouping
messages that are translated at runtime into the local language.
Timestamp
A value in the "seconds since the epoch" format used by Unix and
POSIX systems. Used for the `gawk' functions `mktime',
`strftime', and `systime'. See also "Epoch" and "UTC."
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
environments 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 GNU/Linux, NetBSD, FreeBSD, and OpenBSD).
UTC
The accepted abbreviation for "Universal Coordinated Time." This
is standard time in Greenwich, England, which is used as a
reference time for day and date calculations. See also "Epoch"
and "GMT."
Whitespace
A sequence of space, tab, or newline characters occurring inside
an input record or a string.
automatically generated by info2www version 1.2.2.9