MAWK(1)

NAME
mawk - pattern scanning and text processing language

SYNOPSIS
mawk [-W option] [-F value] [-v var=value] [--] 'program text' [file ...]
mawk [-W option] [-F value] [-v var=value] [-f program-file] [--] [file ...]

DESCRIPTION
mawk is an interpreter for the AWK Programming Language. The AWK language is useful for manipulation of data files, text retrieval and processing, 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 11.2) 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. Short programs are entered on the command line usually enclosed in ' ' to avoid shell interpretation. Longer programs can be read in from a file with the -f option. Data input is read from the list of files on the command line or from standard input when the list is empty. The 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.

OPTIONS

-F value
    sets the field separator, FS, to value.

-f file
    Program text is read from file instead of from the command line. Multiple -f options are allowed.

-v var=value
    assigns value to program variable var.

--  indicates the unambiguous end of options.

The above options will be available with any Posix compatible implementation of AWK, and implementation specific options are prefaced with -W. mawk provides three:

-W version
    mawk writes its version and copyright to stdout and compiled limits to stderr and exits 0.

-W dump
    writes an assembler like listing of the internal representation of the program to stderr.

-W sprintf=num
    adjusts the size of mawk's internal sprintf buffer to num bytes. More than rare use of this option indicates mawk should be recompiled.

-W posix_space
    forces mawk not to consider '\n' to be space.

THE AWK LANGUAGE

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

A pattern can be:
    BEGIN
    END
    expression
    expression , expression

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 semi-colon. 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.

    if ( expr ) statement

    if ( expr ) statement else statement

    while ( expr ) statement

    do statement while ( expr )

    for ( opt_expr ; opt_expr ; opt_expr ) statement

    for ( var in array ) statement

    continue

    break

  2. 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. (See section 4).

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 taken 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(3). 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(3), 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 exactly one operand is string and after trimming spaces and tabs from the front and back the remaining string is entirely numeric in form, then the string is converted to number and the comparison is numeric; otherwise, the numeric operand is converted to string and the comparison is string. The result of a comparison is numeric, 0 or 1.

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.

  3. 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(1). The regular expression metacharacters, i.e., those with special meaning in regular expressions are

	 ^ $ . [ ] | ( ) * + ?

Regular expressions are built up from characters as follows:

    c   matches any non-metacharacter c.

    \c  matches a character defined by the same escape sequences used in string constants or the literal character c if \c is not an escape sequence.

    .   matches any character (including newline).

    ^   matches the front of a string.

    $   matches the back of a string.

    [c1c2c3...]
        matches any character in the class c1c2c3... . An interval of characters is denoted c1-c2 inside a class [...].

    [^c1c2c3...]
        matches any character not in the class c1c2c3...

Regular expressions are built up from other regular expressions as follows:

    r1r2
        matches r1 followed immediately by r2 (concatenation).

    r1 | r2
        matches r1 or r2 (alternation).

    r*  matches r repeated zero or more times.

    r+  matches r repeated one or more times.

    r?  matches r zero or once.

    (r) matches r, providing grouping.

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

For example,

	/^[_a-zA-Z][_a-zA-Z0-9]*$/  and
	/^[-+]?([0-9]+\.?|\.[0-9])[0-9]*([eE][-+]?[0-9]+)?$/

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,

	BEGIN { identifier = "[_a-zA-Z][_a-zA-Z0-9]*" }
	$0 ~ "^" identifier

prints all lines that start with an AWK identifier.

mawk recognizes the empty regular expression, //, which matches the empty string and hence is matched by any string at the front, back and between every character. For example,

	echo  abc | mawk { gsub(//, "X") ; print }
	XaXbXcX

  4. 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,

	echo 24 24E |
	mawk '{ print($1>100, $1>"100", $2>100, $2>"100") }'
	0 0 1 1

$0 and $2 are string and $1 is number and string. The first and second comparisons are numeric and the last two are string. In the second "100" is converted to 100, and in the third 100 is converted to "100".

  5. Expressions and operators

The expression syntax is similar to C. Primary expressions are numeric constants, string constants, variables, fields, arrays and functions. 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.

  6. 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.

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]

  7. Builtin-variables

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

    ARGC
        number of command line arguments.

    ARGV
        array of command line arguments, 0..ARGC-1.

    CONVFMT
        format for internal conversion of numbers to string, initially = "%.6g".

    ENVIRON
        array indexed by environment variables. An environment string, var=value is stored as ENVIRON[var] = value.

    FILENAME
        name of the current input file.

    FNR current record number in FILENAME.

    FS  splits records into fields as a regular expression.

    NF  number of fields in the current record.

    NR  current record number in the total input stream.

    OFMT
        format for printing numbers; initially = "%.6g".

    OFS inserted between fields on output, initially = " ".

    ORS terminates each record on output, initially = "\n".

    RLENGTH
        length set by the last call to the built-in function, match().

    RS  input record separator, initially = "\n".

    RSTART
        index set by the last call to match().

    SUBSEP
        used to build multiple array subscripts, initially = "\034".

  8. Built-in functions
String functions

    gsub(r,s,t)  gsub(r,s)
        Global substitution, every match of regular expression r in variable t is replaced by string s. The number of replacements is returned. If t is omitted, $0 is used. An & in the replacement string s is replaced by the matched substring of t. \& puts a literal & in the replacement string.

    index(s,t)
        If t is a substring of s, then the position where t starts is returned, else 0 is returned. The first character of s is in position 1.

    length(s)  length()
        Returns the length of string s; without an argument, returns the length of $0.

    match(s,r)
        Returns the index of the first longest match of regular expression r in string s. Returns 0 if no match. As a side effect, RSTART is set to the return value. RLENGTH is set to the length of the match or -1 if no match. If the empty string is matched, RLENGTH is set to 0, and 1 is returned if the match is at the front, and length(s)+1 is returned if the match is at the back.

    split(s,A,r)  split(s,A)
        String s is split into fields by regular expression r and the fields are loaded into array A. The number of fields is returned. See section 11 below for more detail. If r is omitted, FS is used.

    sprintf(format,expr-list)
        Returns a string constructed from expr-list according to format. See the description of printf() below.

    sub(r,s,t)  sub(r,s)
        Single substitution, same as gsub() except at most one substitution.

    substr(s,i,n)  substr(s,i)
        Returns the substring of string s, starting at index i, of length n. If n is omitted, the suffix of s, starting at i is returned.

    tolower(s)
        Returns a copy of s with all upper case characters converted to lower case.

    toupper(s)
        Returns a copy of s with all lower case characters converted to upper case.

Arithmetic functions

    atan2(y,x)	Arctan of y/x between -π and π.

    cos(x)		Cosine function, x in radians.

    exp(x)		Exponential function.

    int(x)		Returns x truncated towards zero.

    log(x)		Natural logarithm.

    rand()		Returns a random number between zero and one.

    sin(x)		Sine function, x in radians.

    sqrt(x)		Returns square root of x.

    srand(expr)  srand()
        Seeds the random number generator, using the clock if expr is omitted, and returns the value of the previous seed. mawk seeds the random number generator from the clock at startup so there is no real need to call srand(). Srand(expr) is useful for repeating pseudo random sequences.

  9. Input and output
There are two output statements, print and printf.

    print
        writes $0 ORS to standard output.

    print expr1, expr2, ..., exprn
        writes expr1 OFS expr2 OFS ... exprn ORS to standard output. Numeric expressions are converted to string with OFMT.

    printf format, expr-list
        duplicates the printf C library function writing to standard output. The complete ANSI C format specifications are recognized with conversions %c, %d, %e, %E, %f, %g, %G, %i, %o, %s, %u, %x, %X and %%, and conversion qualifiers h and l.

The argument list to print or printf can optionally be enclosed in parentheses. Print formats numbers using OFMT or "%d" for exact integers. "%c" with a numeric argument prints the corresponding 8 bit character, with a string argument it prints the first character of the string. The output of print and printf can be redirected to a file or command by appending > file, >> file or | command to the end of the print statement. Redirection opens file or command only once, subsequent redirections append to the already open stream. By convention, mawk associates the filename "/dev/stderr" with stderr which allows print and printf to be redirected to stderr.

The input function getline has the following variations.

    getline
        reads into $0, updates the fields, NF, NR and FNR.

    getline < file
        reads into $0 from file, updates the fields and NF.

    getline var
        reads the next record into var, updates NR and FNR.

    getline var < file
        reads the next record of file into var.

    command | getline
        pipes a record from command into $0 and updates the fields and NF.

    command | getline var
        pipes a record from command into var.

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

Commands on the end of pipes are executed by /bin/sh.

The function close(expr) closes the file or pipe associated with expr. Close returns 0 if expr is an open file, the exit status if expr is a piped command, and -1 otherwise. Close() is used to reread a file or command, make sure the other end of an output pipe is finished or conserve file resources.

The function system(expr) uses /bin/sh to execute expr and returns the exit status of the command expr. Changes made to the ENVIRON array are not passed to commands executed with system or pipes.

  10. 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.

	function csplit(s, A,	n, i)
	{
	  n = length(s)
	  for( i = 1 ; i <= n ; i++ ) A[i] = substr(s, i, 1)
	  return n
	}

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.

  11. 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:

    (1) If sep is omitted, it is replaced by FS. Sep can be an expression or regular expression. If it is an expression of non-string type, it is converted to string.

    (2) If sep = " " (a single space), then <SPACE> is trimmed from the front and back of expr, and sep becomes <SPACE>. mawk defines <SPACE> as the regular expression /[ \t\n]+/. Otherwise sep is treated as a regular expression, except that meta-characters are ignored for a string of length 1, e.g., split(x, A, "*") and split(x, A, /\*/) are the same.

    (3) If expr is not string, it is converted to string. If expr is then the empty string "", split() returns 0 and A is unchanged. Otherwise, all non-overlapping, non-null and longest matches of sep in expr, separate expr into fields which are loaded into A. The fields are placed in A[1], A[2], ..., A[n] and split() returns n, the number of fields which is the number of matches plus one. Data placed in A that looks numeric is typed number and string.

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 "".

mawk splits files into records by the same algorithm, but with the slight difference that RS is really a terminator instead of a separator. (ORS is really a terminator too).

    E.g., if FS = ":+" and $0 = "a::b:" , then NF = 3 and $1 = "a", $2 = "b" and $3 = "", but if "a::b:" is the contents of an input file and RS = ":+", then there are two records "a" and "b".

RS = " " is not special.

  12. Multi-line records
Since mawk interprets RS as a regular expression, multi-line records are easy. Setting RS = "\n\n+", makes one or more blank lines separate records. If FS = " " (the default), then single newlines, by the rules for <SPACE> above, become space and single newlines are field separators.

    For example, if a file is "a b\nc\n\n", RS = "\n\n+" and FS = " ", then there is one record "a b\nc" with three fields "a", "b" and "c". Changing FS = "\n", gives two fields "a b" and "c"; changing FS = "", gives one field identical to the record.

If you want lines with spaces or tabs to be considered blank, set RS = "\n([ \t]*\n)+". For compatibility with other awks, setting RS = "" has the same effect as if blank lines are stripped from the front and back of files and then records are determined as if RS = "\n\n+". Posix requires that "\n" always separates records when RS = "" regardless of the value of FS. mawk does not support this convention, because defining "\n" as <SPACE> makes it unnecessary.

Most of the time when you change RS for multi-line records, you will also want to change ORS to "\n\n" so the record spacing is preserved on output.

  13. Program execution
This section describes the order of program execution. First ARGC is set to the total number of command line arguments passed to the execution phase of the program. ARGV[0] is set the name of the AWK interpreter and ARGV[1] ... ARGV[ARGC-1] holds the remaining command line arguments exclusive of options and program source. For example with

	mawk  -f  prog  v=1  A  t=hello  B

ARGC = 5 with ARGV[0] = "mawk", ARGV[1] = "v=1", ARGV[2] = "A", ARGV[3] = "t=hello" and ARGV[4] = "B". Next, each BEGIN block is executed in order. If the program consists entirely of BEGIN blocks, then execution terminates, else an input stream is opened and execution continues. If ARGC equals 1, the input stream is set to stdin, else the command line arguments ARGV[1] ... ARGV[ARGC-1] are examined for a file argument.

The command line arguments divide into three sets: file arguments, assignment arguments and empty strings "". An assignment has the form var=string. When an ARGV[i] is examined as a possible file argument, if it is empty it is skipped; if it is an assignment argument, the assignment to var takes place and i skips to the next argument; else ARGV[i] is opened for input. If it fails to open, execution terminates with exit code 1. If no command line argument is a file argument, then input comes from stdin. Getline in a BEGIN action opens input. "-" as a file argument denotes stdin.

Once an input stream is open, each input record is tested against each pattern, and if it matches, the associated action is executed. An expression pattern matches if it is boolean true (see the end of section 2). A BEGIN pattern matches before any input has been read, and an END pattern matches after all input has been read. A range pattern, expr1,expr2 , matches every record between the match of expr1 and the match expr2 inclusively.

When end of file occurs on the input stream, the remaining command line arguments are examined for a file argument, and if there is one it is opened, else the END pattern is considered matched and all END actions are executed.

In the example, the assignment v=1 takes place after the BEGIN actions are executed, and the data placed in v is typed number and string. Input is then read from file A. On end of file A, t is set to the string "hello", and B is opened for input. On end of file B, the END actions are executed.

Program flow at the pattern {action} level can be changed with the

	next   and
	exit  opt_expr

statements. A next statement causes the next input record to be read and pattern testing to restart with the first pattern {action} pair in the program. An exit statement causes immediate execution of the END actions or program termination if there are none or if the exit occurs in an END action. The opt_expr sets the exit value of the program unless overridden by a later exit or subsequent error.

EXAMPLES
1. emulate cat.
	{ print }
2. emulate wc.
	{ chars += length($0) + 1  # add one for the \n
	  words += NF
	}
	END{ print NR, words, chars }
3. count the number of unique "real words".
	BEGIN { FS = "[^A-Za-z]+" }
	{ for(i = 1 ; i <= NF ; i++)  word[$i] = "" }
	END { delete word[""]
	      for ( i in word )  cnt++
	      print cnt
	}
4. sum the second field of every record based on the first field.
	$1 ~ /credit|gain/ { sum += $2 }
	$1 ~ /debit|loss/  { sum -= $2 }
	END { print sum }
5. sort a file, comparing as string
	{ line[NR] = $0 "" }  # make sure of comparison type
			      # in case some lines look numeric
	END {  isort(line, NR)
	  for(i = 1 ; i <= NR ; i++) print line[i]
	}
	#insertion sort of A[1..n]
	function isort( A, n,	i, j, hold)
	{
	  for( i = 2 ; i <= n ; i++)
	  {
	    hold = A[j = i]
	    while ( A[j-1] > hold )
	    { j-- ; A[j+1] = A[j] }
	    A[j] = hold
	  }
	  # sentinel A[0] = "" will be created if needed
	}

COMPATIBILITY ISSUES
The Posix 1003.2(draft 11.2) definition of the AWK language is AWK as described in the AWK book with a few extensions that appeared in SystemVR4 nawk. The extensions are:

    New functions: toupper() and tolower(). New variables: ENVIRON[] and CONVFMT. ANSI C conversion specifications for printf() and sprintf(). New command options: -v var=value, multiple -f options and implementation options as arguments to -W.

Posix AWK is oriented to operate on files a line at a time. RS can be changed from "\n" to another single character, but it is hard to find any use for this — there are no examples in the AWK book. By convention, RS = "", makes one or more blank lines separate records, allowing multi-line records. When RS = "", "\n" is always a field separator regardless of the value in FS.

mawk, on the other hand, allows RS to be a regular expression. When "\n" appears in records, it is treated as space, and FS always determines fields.

Removing the line at a time paradigm can make some programs simpler and can often improve performance. For example, redoing example 3 from above,

	BEGIN { RS = "[^A-Za-z]+" }
	{ word[ $0 ] = "" }
	END { delete  word[ "" ]
	  for( i in word )  cnt++
	  print cnt
	}

counts the number of unique words by making each word a record. On moderate size files, mawk executes twice as fast, because of the simplified inner loop.

The following program replaces each comment by a single space in a C program file,

	BEGIN {
	  RS = "/\*([^*]|\*+[^/*])*\*+/"
		# comment is record separator
	  ORS = " "
	  getline  hold
       }
       { print hold ; hold = $0 }
       END { printf "%s" , hold }

Buffering one record is needed to avoid terminating the last record with a space.

With mawk, the following are all equivalent,

	x ~ /a\+b/    x ~ "a\+b"     x ~ "a\\+b"

The strings get scanned twice, once as string and once as regular expression. On the string scan, mawk ignores the escape on non-escape characters while the AWK book advocates \c be recognized as c which necessitates the double escaping of meta-characters in strings. Posix explicitly declines to define the behavior which passively forces programs that must run under a variety of awks to use the more portable but less readable, double escape.

Posix AWK does not recognize "/dev/stderr" or \x hex escape sequences in strings. Unlike ANSI C, mawk limits the number of digits that follows \x to two.

Finally, here is how mawk handles exceptional cases not discussed in the AWK book or the Posix draft. It is unsafe to assume consistency across awks and safe to skip to the next section.

    substr(s, i, n) returns the characters of s in the intersection of the closed interval [1, length(s)] and the half-open interval [i, i+n). When this intersection is empty, the empty string is returned; so substr("ABC", 1, 0) = "" and substr("ABC", -4, 6) = "A". Every string, including the empty string, matches the empty string at the front so, s ~ // and s ~ "", are always 1 as is match(s, //) and match(s, ""). The last two set RLENGTH to 0. index(s, t) is always the same as match(s, t1) where t1 is the same as t with metacharacters escaped. Hence consistency with match requires that index(s, "") always returns 1. Also the condition, index(s,t) != 0 if and only t is a substring of s, requires index("","") = 1. If getline encounters end of file, getline var, leaves var unchanged. Similarly, on entry to the END actions, $0, the fields and NF have their value unaltered from the last record.

SEE ALSO
egrep (1)

Aho, Kernighan and Weinberger, The AWK Programming Language, Addison-Wesley Publishing, 1988, (the AWK book), defines the language, opening with a tutorial and advancing to many interesting programs that delve into issues of software design and analysis relevant to programming in any language.

The GAWK Manual, The Free Software Foundation, 1991, is a tutorial and language reference that does not attempt the depth of the AWK book and assumes the reader may be a novice programmer. The section on AWK arrays is excellent. It also discusses Posix requirements for AWK.

BUGS
mawk cannot handle ascii NUL \0 in the source or data files. You can output NUL using printf with %c, and any other 8 bit character is acceptable input. mawk implements printf() and sprintf() using the C library functions, printf and sprintf, so full ANSI compatibility requires an ANSI C library. In practice this means the h conversion qualifier may not be available. Also mawk inherits any bugs or limitations of the library functions. Implementors of the AWK language have shown a consistent lack of imagination when naming their programs.

AUTHOR
Mike Brennan (brennan@boeing.com).