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The Evolution of the `awk` Language

This manual describes the GNU implementation of awk, which is patterned after the POSIX specification. Many awk users are only familiar with the original awk implementation in Version 7 Unix, which is also the basis for the version in Berkeley Unix (through 4.3--Reno). This chapter briefly describes the evolution of the awk language.

V7/S5R3.1: The major changes between V7 and System V Release 3.1.
S5R4: Minor changes between System V Releases 3.1 and 4.
POSIX: New features from the posix standard.
POSIX/GNU: The extensions in gawk not in posix awk.

Major Changes between V7 and S5R3.1

The awk language evolved considerably between the release of Version 7 Unix (1978) and the new version first made widely available in System V Release 3.1 (1987). This section summarizes the changes, with cross-references to further details.

The requirement for ; to separate rules on a line (note `awk' Statements versus Lines}).
User-defined functions, and the return statement (see section User-defined Functions).
The delete statement. The do-while statement (note: The do-while Statement).
The built-in functions atan2, cos, sin, rand and srand (see section Numeric Built-in Functions).
The built-in functions gsub, sub, and match (see section Built-in Functions for String Manipulation).
The built-in functions close, which closes an open file, and system, which allows the user to execute operating system commands (see section Built-in Functions for Input/Output).
The ARGC, ARGV, FNR, RLENGTH, RSTART, and SUBSEP built-in variables (see section Built-in Variables).
The conditional expression using the operators ? and : (see section Conditional Expressions).
The exponentiation operator ^ (see section Arithmetic Operators) and its assignment operator form ^= (see section Assignment Expressions).
C-compatible operator precedence, which breaks some old awk programs.
Regexps as the value of FS (see section Specifying how Fields are Separated), and as the third argument to the split function (see section Built-in Functions for String Manipulation).
Dynamic regexps as operands of the ~ and !~ operators (see section How to Use Regular Expressions).
Escape sequences (see section Constant Expressions) in regexps.
The escape sequences \b, \f, and \r (see section Constant Expressions).
Redirection of input for the getline function (note: Explicit Input with getline.
.Multiple BEGIN and END rules (note: BEGIN and END Special Patterns.
Simulated multi-dimensional arrays (see section Multi-dimensional Arrays).



Changes between S5R3.1 and S5R4

The System V Release 4 version of Unix awk added these features
(some of which originated in gawk):



The ENVIRON variable (see section Built-in Variables).


Multiple -f options on the command line
.The -v option for assigning variables before program execution begins
.The -- option for terminating command line options.


The \a, \v, and \x escape sequences
(see section Constant Expressions).

A defined return value for the srand built-in function
(see section Numeric Built-in Functions).


The toupper and tolower built-in string functions
for case translation
(see section Built-in Functions for String Manipulation).

A cleaner specification for the %c format-control letter in the
printf function
(note: Using printf Statements  for Fancier Printing.

The ability to dynamically pass the field width and precision ("%*.*d")
in the argument list of the printf function
(note: Using printf Stateme
nts  for Fancier Printing.

.The use of constant regexps such as /foo/ as expressions, where
they are equivalent to use of the matching operator, as in $0 ~
/foo/ (see section Constant Expressions).


Changes between S5R4 and POSIX awk

The POSIX Command Language and Utilities standard for awk
introduced the following changes into the language:



The use of -W for implementation-specific options.


The use of CONVFMT for controlling the conversion of numbers
to strings (see section Conversion of Strings and Numbers).


The concept of a numeric string, and tighter comparison rules to go
with it (see section Comparison Expressions).


More complete documentation of many of the previously undocumented
features of the language.


Extensions in gawk not in POSIX awk

The GNU implementation, gawk, adds these features:



The AWKPATH environment variable for specifying a path search for
the -f command line option

The various gawk specific features available via the -W
command line option.
The ARGIND variable, that tracks the movement of FILENAME
through ARGV.  (see section Built-in Variables).


The ERRNO variable, that contains the system error message when
getline returns -1, or when close fails.
(see section Built-in Variables).


The IGNORECASE variable and its effects
(see section Case-sensitivity in Matching).

The FIELDWIDTHS variable and its effects
(see section Reading Fixed-width Data).

The next file statement for skipping to the next data file
(note:The next file Statement).

The systime and strftime built-in functions for obtaining
and printing time stamps
(see section Functions for Dealing with Time Stamps).

The /dev/stdin, /dev/stdout, /dev/stderr, and
/dev/fd/n file name interpretation
(see section Standard I/O Streams).

The -W compat option to turn off these extensions
.

The -W posix option for full POSIX compliance
.

The Evolution of the `awk` Language

Major Changes between V7 and S5R3.1

Changes between S5R3.1 and S5R4

Changes between S5R4 and POSIX `awk`

Extensions in `gawk` not in POSIX `awk`