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The Evolution of the awk Language

This manual describes the GNU implementation of awk, which is patterned after the System V Release 4 version. 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. This chapter briefly describes the evolution of the awk language.

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 (see section awk Statements versus Lines).

• User-defined functions, and the return statement (see section User-defined Functions).

• The delete statement (see section The delete Statement).

• The do-while statement (see section 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 and system (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 (see section Operator Precedence: How Operators Nest).

• Regexps as the value of FS (see section Specifying How Fields Are Separated), or 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 (see section Explicit Input with getline).

• Multiple BEGIN and END rules (see section BEGIN and END Special Patterns).

• Simulation of multidimensional arrays (see section Multi-dimensional Arrays).

Minor Changes between S5R3.1 and S5R4

The System V Release 4 version of Unix awk added these features:

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

• Multiple `-f' options on the command line (see section Invocation of awk).

• The `-v' option for assigning variables before program execution begins (see section Invocation of awk).

• 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 (see section Using printf Statements 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/.

Extensions In gawk Not In S5R4

The GNU implementation, gawk, adds these features:

• The AWKPATH environment variable for specifying a path search for the `-f' command line option (see section Invocation of awk).

• The `-C' and `-V' command line options (see section Invocation of awk).

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

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

• The `-c' option to turn off these extensions (see section Invocation of awk).

• The `-a' and `-e' options to specify the syntax of regular expressions that gawk will accept (see section Invocation of awk).

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