AWK

AWK is a programming language designed for text processing and typically used as a data extraction and reporting tool. It is a standard feature of most Unix-like operating systems.

The AWK language is a data-driven scripting language consisting of a set of actions to be taken against streams of textual data – either run directly on files or used as part of a pipeline – for purposes of extracting or transforming text, such as producing formatted reports. The language extensively uses the string datatype, associative arrays (that is, arrays indexed by key strings), and regular expressions. While AWK has a limited intended application domain and was especially designed to support one-liner programs, the language is Turing-complete, and even the early Bell Labs users of AWK often wrote well-structured large AWK programs.

AWK was created at Bell Labs in the 1970s, and its name is derived from the surnames of its authors—Alfred Aho, Peter Weinberger, and Brian Kernighan. The acronym is pronounced the same as the name of the bird auk (which acts as an emblem of the language such as on The AWK Programming Language book cover – the book is often referred to by the abbreviation TAPL). When written in all lowercase letters, as awk, it refers to the Unix or Plan 9 program that runs scripts written in the AWK programming language.

History

AWK was initially developed in 1977 by Alfred Aho, Peter Weinberger, and Brian Kernighan, from whose initials the language takes its name. As one of the early tools to appear in Version 7 Unix, AWK added computational features to a Unix pipeline besides the Bourne shell, the only scripting language available in a standard Unix environment. It is one of the mandatory utilities of the Single UNIX Specification, and is required by the Linux Standard Base specification.

AWK was significantly revised and expanded in 1985–88, resulting in the GNU AWK implementation written by Paul Rubin, Jay Fenlason, and Richard Stallman, released in 1988. GNU AWK is the most widely deployed version] because it is included with GNU-based Linux packages. GNU AWK has been maintained solely by Arnold Robbins since 1994. Brian Kernighan’s nawk (New AWK) source was first released in 1993 unpublicized, and publicly since the late 1990s; many BSD systems use it to avoid GPL license.

AWK was preceded by sed (1974). Both were designed for text processing. They share the line-oriented, data-driven paradigm, and are particularly suited to writing one-liner programs, due to the implicit main loop and current line variables. The power and terseness of early AWK programs – notably the powerful regular expression handling and conciseness due to implicit variables, which facilitate one-liners – together with the limitations of AWK at the time, were important inspirations for the Perl language (1987). In the 1990s, Perl became very popular, competing with AWK in the niche of Unix text-processing languages.

Structure of AWK programs

AWK is a language for processing text files. A file is treated as a sequence of records, and by default each line is a record. Each line is broken up into a sequence of fields, so we can think of the first word in a line as the first field, the second word as the second field, and so on. An AWK program is a sequence of pattern-action statements. AWK reads the input a line at a time. A line is scanned for each pattern in the program, and for each pattern that matches, the associated action is executed.” – Alfred V. Aho

An AWK program is a series of pattern action pairs, written as:

condition { action }

condition { action }

where condition is typically an expression and action is a series of commands. The input is split into records, where by default records are separated by newline characters so that the input is split into lines. The program tests each record against each of the conditions in turn, and executes the action for each expression that is true. Either the condition or the action may be omitted. The condition defaults to matching every record. The default action is to print the record. This is the same pattern-action structure as sed.

In addition to a simple AWK expression, such as foo == 1 or /^foo/, the condition can be BEGIN or END causing the action to be executed before or after all records have been read, or pattern1, pattern2 which matches the range of records starting with a record that matches pattern1 up to and including the record that matches pattern2 before again trying to match against pattern1 on future lines.

In addition to normal arithmetic and logical operators, AWK expressions include the tilde operator, ~, which matches a regular expression against a string. As handy syntactic sugar, /regexp/ without using the tilde operator matches against the current record; this syntax derives from sed, which in turn inherited it from the ed editor, where / is used for searching. This syntax of using slashes as delimiters for regular expressions was subsequently adopted by Perl and ECMAScript, and is now quite common. The tilde operator was also adopted by Perl, but has not seen as wide use.

Source: Wikipedia

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