1. awk.js

mawk - pattern scanning and text processing language compiled to JavaScript

mawk is an interpreter for the AWK Programming Language. The AWK language is useful for manipulation of data files, text retrieval and pro cessing, and for prototyping and experimenting with algorithms. mawk is a new awk meaning it implements the AWK language as defined in Aho, Kernighan and Weinberger, The AWK Programming Language, Addison-Wesley Publishing, 1988 (hereafter referred to as the AWK book.) mawk conforms to the POSIX 1003.2 (draft 11.3) definition of the AWK language which contains a few features not described in the AWK book, and mawk provides a small number of extensions.

An AWK program is a sequence of pattern {action} pairs and function definitions. Data input is broken into records as determined by the record separator variable, RS. Initially, RS = "\n" and records are synonymous with lines. Each record is compared against each pattern and if it matches, the program text for {action} is executed.

May be invoked with the following command-line options:

mawk accepts abbreviations for any of these options, e.g., -W v and -Wv both tell mawk to show its version.

mawk allows multiple -W options to be combined by separating the options with commas, e.g., -Wsprint=2000,posix.

2. Program structure

An AWK program is a sequence of pattern {action} pairs and user function definitions.

A pattern can be:

One, but not both, of pattern {action} can be omitted. If {action} is omitted it is implicitly { print }. If pattern is omitted, then it is implicitly matched. BEGIN and END patterns require an action.

Statements are terminated by newlines, semi-colons or both. Groups of statements such as actions or loop bodies are blocked via { ... } as in C. The last statement in a block doesn't need a terminator. Blank lines have no meaning; an empty statement is terminated with a semicolon. Long statements can be continued with a backslash, \. A statement can be broken without a backslash after a comma, left brace, &&, ||, do, else, the right parenthesis of an if, while or for statement, and the right parenthesis of a function definition. A comment starts with # and extends to, but does not include the end of line.

The following statements control program flow inside blocks.

2.1. Data types, conversion and comparison

There are two basic data types, numeric and string. Numeric constants can be integer like -2, decimal like 1.08, or in scientific notation like -1.1e4 or .28E-3. All numbers are represented internally and all computations are done in floating point arithmetic. So for example, the expression 0.2e2 == 20 is true and true is represented as 1.0.

String constants are enclosed in double quotes: "This is a string with a newline at the end.\n"

Strings can be continued across a line by escaping (\) the newline. The following escape sequences are recognized.

\\ \
\" "
\a alert, ascii 7
\b backspace, ascii 8
\t tab, ascii 9
\n newline, ascii 10
\v vertical tab, ascii 11
\f formfeed, ascii 12
\r carriage return, ascii 13
\ddd 1, 2 or 3 octal digits for ascii ddd
\xhh 1 or 2 hex digits for ascii hh

If you escape any other character \c, you get \c, i.e., mawk ignores the escape.

There are really three basic data types; the third is number and string which has both a numeric value and a string value at the same time. User defined variables come into existence when first referenced and are initialized to null, a number and string value which has numeric value 0 and string value "". Non-trivial number and string typed data come from input and are typically stored in fields.

The type of an expression is determined by its context and automatic type conversion occurs if needed. For example, to evaluate the statements:

y = x + 2 ; z = x "hello"

the value stored in variable y will be typed numeric. If x is not numeric, the value read from x is converted to numeric before it is added to 2 and stored in y. The value stored in variable z will be typed string, and the value of x will be converted to string if necessary and concatenated with "hello". (Of course, the value and type stored in x is not changed by any conversions.) A string expression is converted to numeric using its longest numeric prefix as with atof. A numeric expression is converted to string by replacing expr with sprintf(CONVFMT, expr), unless expr can be represented on the host machine as an exact integer then it is converted to sprintf("%d", expr). Sprintf() is an AWK built-in that duplicates the functionality of sprintf, and CONVFMT is a built-in variable used for internal conversion from number to string and initialized to "%.6g". Explicit type conversions can be forced, expr "" is string and expr+0 is numeric.

To evaluate, expr1 rel-op expr2, if both operands are numeric or number and string then the comparison is numeric; if both operands are string the comparison is string; if one operand is string, the non-string op- erand is converted and the comparison is string. The result is numeric, 1 or 0.

In boolean contexts such as, if ( expr ) statement, a string expression evaluates true if and only if it is not the empty string ""; numeric values if and only if not numerically zero.

2.2. Regular Expressions

In the AWK language, records, fields and strings are often tested for matching a regular expression. Regular expressions are enclosed in slashes, and

expr ~ /r/

is an AWK expression that evaluates to 1 if expr "matches" r, which means a substring of expr is in the set of strings defined by r. With no match the expression evaluates to 0; replacing ~ with the "not match" operator, !~ , reverses the meaning. As pattern-action pairs,

/r/ { action } and $0 ~ /r/ { action }

are the same, and for each input record that matches r, action is executed. In fact, /r/ is an AWK expression that is equivalent to $0 ~ /r/ anywhere except when on the right side of a match operator or passed as an argument to a built-in function that expects a regular expression argument.

AWK uses extended regular expressions as with egrep. The regular expression metacharacters, i.e., those with special meaning in regular expressions are

^ $ . [ ] | ( ) * + ?

Regular expressions are built up from characters as follows:

The increasing precedence of operators is alternation, concatenation and unary (*, + or ?).

For example:




are matched by AWK identifiers and AWK numeric constants respectively. Note that "." has to be escaped to be recognized as a decimal point, and that metacharacters are not special inside character classes

Any expression can be used on the right hand side of the ~ or !~ operators or passed to a built-in that expects a regular expression. If needed, it is converted to string, and then interpreted as a regular expression. For example:

prints all lines that start with an AWK identifier

2.3. Records and fields

Records are read in one at a time, and stored in the field variable $0. The record is split into fields which are stored in $1, $2, ..., $NF. The built-in variable NF is set to the number of fields, and NR and FNR are incremented by 1. Fields above $NF are set to "".

Assignment to $0 causes the fields and NF to be recomputed. Assignment to NF or to a field causes $0 to be reconstructed by concatenating the $i's separated by OFS. Assignment to a field with index greater than NF, increases NF and causes $0 to be reconstructed.

Data input stored in fields is string, unless the entire field has numeric form and then the type is number and string. For example GIST:

$0 and $2 are string and $1 is number and string. The first comparison is numeric, the second is string, the third is string (100 is converted to "100"), and the last is string.

2.4. Expressions and operators

The expression syntax is similar to C. Primary expressions are numeric constants, string constants, variables, fields, arrays and function calls. The identifier for a variable, array or function can be a sequence of letters, digits and underscores, that does not start with a digit. Variables are not declared; they exist when first referenced and are initialized to null.

New expressions are composed with the following operators in order of increasing precedence.

assignment = += -= *= /= %= ^=
conditional ? :
logical or ||
logical and &&
array membership in
matching ~ !~
relational < > <= >= == !=
concatenation (no explicit operator)
add ops + -
mul ops * / %
unary + -
logical not !
exponentiation ^
inc and dec ++ -- (both post and pre)
field $

Assignment, conditional and exponentiation associate right to left; the other operators associate left to right. Any expression can be parenthesized.

2.5. Arrays

Awk provides one-dimensional arrays. Array elements are expressed as array[expr]. Expr is internally converted to string type, so, for example, A[1] and A["1"] are the same element and the actual index is "1". Arrays indexed by strings are called associative arrays. Initially an array is empty; elements exist when first accessed. An expression, expr in array evaluates to 1 if array[expr] exists, else to 0.

There is a form of the for statement that loops over each index of an array.

for ( var in array ) statement

sets var to each index of array and executes statement. The order that var transverses the indices of array is not defined.

The statement, delete array[expr], causes array[expr] not to exist. mawk supports an extension, delete array, which deletes all elements of array.

Multidimensional arrays are synthesized with concatenation using the built-in variable SUBSEP. array[expr1,expr2] is equivalent to array[expr1 SUBSEP expr2]. Testing for a multidimensional element uses a parenthesized index, such as:

if ( (i, j) in A ) print A[i, j]

2.6. Built-in variables

The following variables are built-in and initialized before program execution.

2.7. Built-in functions

String functions:

Time functions:

Arithmetic functions:

2.8. Input and output

There are two output statements, print and printf.

The input function getline has the following variations.

Getline returns 0 on end-of-file, -1 on error, otherwise 1.

2.9. User defined functions

The syntax for a user defined function is:

function name( args ) { statements }

The function body can contain a return statement:

return opt_expr

A return statement is not required. Function calls may be nested or recursive. Functions are passed expressions by value and arrays by reference. Extra arguments serve as local variables and are initialized to null. For example, csplit(s,A) puts each character of s into array A and returns the length of s:

Putting extra space between passed arguments and local variables is conventional. Functions can be referenced before they are defined, but the function name and the '(' of the arguments must touch to avoid confusion with concatenation.

A function parameter is normally a scalar value (number or string). If there is a forward reference to a function using an array as a parameter, the function's corresponding parameter will be treated as an array.

2.10. Splitting strings, records and files

Awk programs use the same algorithm to split strings into arrays with split(), and records into fields on FS. mawk uses essentially the same algorithm to split files into records on RS.

split(expr,A,sep)works as follows:

Splitting records into fields works the same except the pieces are loaded into $1, $2,..., $NF. If $0 is empty, NF is set to 0 and all $i to "".

If FS = "", then mawk breaks the record into individual characters, and, similarly, split(s,A,"") places the individual characters of s into A.

Simple Uses

GIST | In its simplest form, we can use awk like cat to simply print all lines of a text file out to the screen.

GIST | Let's try out awk's search filtering capabilities:

GIST | We can use the action section to specify which pieces of information we want to print. For instance, to print only the first column:

Built-in Variables

GIST | ARGC implies the number of arguments provided at the command line:

GIST | ARGV is an array that stores the command-line arguments. The array's valid index ranges from 0 to ARGC-1:

GIST | FS represents the (input) field separator and its default value is space. You can also change this by using -F command line option:

GIST | NF represents the number of fields in the current record:

GIST | NR represents the number of the current record. For instance, the following example prints only the first two records:


AWK has associative arrays and one of the best thing about it is – the indexes need not to be continuous set of number; you can use either string or number as an array index. Also, there is no need to declare the size of an array in advance – arrays can expand/shrink at runtime.

GIST | To gain more insight on array, let us create and access the elements of an array:

GIST | The following example deletes the element orange:

AWK only supports one-dimensional arrays. But you can easily simulate a multi-dimensional array using the one-dimensional array itself.

GIST | The following example simulates a 2-D array:

Control Flow

GIST | if else:


GIST | Initially, the for statement performs initialization action, then it checks the condition. If the condition is true, it executes actions, thereafter it performs increment or decrement operation. The loop execution continues as long as the condition is true. For instance, the following example prints 1 to 5 using for loop:

GIST | The following example prints 1 to 3 using while loop:

GIST | The following example prints 1 to 5 numbers using do-while loop:

GIST | Here is an example which ends the loop when the sum becomes greater than 50:

GIST | The following example uses continue statement to print the even numbers between 1 to 20:

User Defined Functions

GIST | Let us write two functions that calculate the minimum and the maximum number and call these functions from another function called main:

Further Reading