mawk - Man Page

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.

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 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(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 one operand is string, the non-string operand 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.

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 the -E option of grep(1). The regular expression metacharacters, i.e., those with special meaning in regular expressions are

	\ ^ $ . [ ] | ( ) * + ? { }

If the command line option -W traditional is used, these are omitted:

	{ }

are also regular expression metacharacters, and in this mode, require escaping to be a literal character.

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. (repetition).

(r)

matches r (grouping).

r{n}

matches r exactly n times.

r{n,}

matches r repeated n or more times.

r{n,m}

matches r repeated n to m (inclusive) times.

r{,m}

matches r repeated 0 to m times (a non-standard option).

The increasing precedence of operators is:

alternation concatenation repetition grouping

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

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 1 1 1

$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.

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

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 the delete array feature, 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]

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".

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. \& and \\ put  literal & and \, respectively, 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)

Returns the length of string or array s.

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.

Time functions

These are available on systems which support the corresponding C mktime and strftime functions:

mktime(specification)

converts a date specification to a timestamp with the same units as systime. The date specification is a string containing the components of the date as decimal integers:

YYYY

the year, e.g., 2012

MM

the month of the year starting at 1

DD

the day of the month starting at 1

HH

hour (0-23)

MM

minute (0-59)

SS

seconds (0-59)

DST

tells how to treat timezone versus daylight savings time:

positive

DST is in effect

zero (default)

DST is not in effect

negative

mktime() should (use timezone information and system databases to) attempt  to determine whether DST is in effect at the specified time.

strftime([format [, timestamp [, utc ]]])

formats the given timestamp using the format (passed to the C strftime function):

• If the format parameter is missing, "%c" is used.
• If the timestamp parameter is missing, the current value from systime is used.
• If the utc parameter is present and nonzero, the result is in UTC. Otherwise local time is used.

systime()

returns the current time of day as the number of seconds since the Epoch (1970-01-01 00:00:00 UTC on POSIX systems).

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. Srand(expr) is useful for repeating pseudo random sequences.

Note: mawk is normally configured to seed the random number generator from the clock at startup, making it unnecessary to call srand(). This feature can be suppressed via conditional compile, or overridden using the -Wrandom option.

There are two output statements, print and printf.

print

writes $0  ORS to standard output.

print expr₁, expr₂, ..., expr_n

writes expr₁ OFS expr₂ OFS ... expr_n 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,
• "/dev/stdout" with stdout,
• "-" and "/dev/stdin" with stdin.

The association with stderr is especially useful because it allows print and printf to be redirected to stderr. These names can also be passed to functions.

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 fflush(expr) flushes the output file or pipe associated with expr. Fflush returns 0 if expr is an open output stream else -1. Fflush without an argument flushes stdout. Fflush with an empty argument ("") flushes all open output.

The function system(expr) uses the C runtime system call to execute expr and returns the corresponding wait status of the command as follows:

• if the system call failed, setting the status to -1, mawk returns that value.
• if the command exited normally, mawk returns its exit-status.
• if the command exited due to a signal such as SIGHUP, mawk returns the signal number plus 256.

Changes made to the ENVIRON array are not passed to commands executed with system or pipes.

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.

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.

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 set empty. 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).

RS = " " is not special.

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

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.

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.

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 to 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 2. 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
	nextfile
	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.
• A nextfile statement tells mawk to stop processing the current input file. It then updates FILENAME to the next file listed on the command line, and resets FNR to 1.
• 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.

The POSIX 1003.2(draft 11.3) 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.

• For systems (MS-DOS or Windows) which provide a setmode function, an environment variable MAWKBINMODE and a built-in variable BINMODE. The bits of the BINMODE value tell mawk  how to modify the RS and ORS variables:

  0

  set standard input to binary mode, and if BIT-2 is unset, set RS to "\r\n" (CR/LF) rather than "\n" (LF).

  1

  set standard output to binary mode, and if BIT-2 is unset, set ORS to "\r\n" (CR/LF) rather than "\n" (LF).

  2

  suppress the assignment to RS and ORS of CR/LF, making it possible to run scripts and generate output compatible with Unix line-endings.

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/std{in,out,err}". Some systems provide an actual device for this, allowing AWKs which do not implement the feature directly to support it.

POSIX AWK does not recognize \x hex escape sequences in strings. Unlike ANSI C, mawk limits the number of digits that follows \x to two as the current implementation only supports 8 bit characters.

POSIX explicitly leaves the behavior of FS = "" undefined, and mentions splitting the record into characters as a possible interpretation, but currently this use is not portable across implementations.

Some features were not part of the POSIX standard until long after their introduction in mawk and other implementations. These were published in IEEE 1003.1-2024 (The Open Group Base Specifications Issue 8):

• The built-in fflush first appeared in a 1993 AT&T awk released to netlib. It was approved for the POSIX standard in 2012.
• The built-in nextfile first appeared in gawk in 1988, was adopted by BWK in 1996, and by mawk in 2012. It was approved for the POSIX standard in 2012.
• Aggregate deletion with delete array was approved in 2018.

POSIX does not prescribe a method for initializing random numbers at startup.

In practice, most implementations do nothing special, which makes srand and rand follow the C runtime library, making the initial seed value 1. Some implementations (Solaris XPG4 and Tru64) return 0 from the first call to srand, although the results from rand behave as if the initial seed is 1. Other implementations return 1.

While mawk can call srand at startup with no parameter (initializing random numbers from the clock), this feature may be suppressed using conditional compilation.

Mktime, strftime and systime are gawk extensions.

The "/dev/stdin" feature was added to mawk after 1.3.4, for compatibility with gawk and BWK awk. The corresponding "-" (alias for /dev/stdin) was present in mawk 1.3.3.

Interval expressions, e.g., a range {m,n} in Extended Regular Expressions (EREs), were not supported in awk (or even the original “nawk”):

• Gawk provided this feature in 1991 (and later, in 1998, options for turning it off, for compatibility with “traditional awk”).
• Interval expressions, were introduced into awk regular expressions in IEEE 1003.1-2001 (also known as Unix 03), along with some internationalization features.

• Apple modified its copy of the original awk in April 2006, making this version of awk support interval expressions.

  The updated source provides for compatibility with older “legacy” versions using an environment variable, making this “Unix 2003” feature (perhaps meant as Unix 03) the default.

• NetBSD developers copied this change in January 2018, omitting the compatibility option, and then applied it to BWK awk.
• The interval expression implementation in mawk is based on changes proposed by James Parkinson in April 2016.

Mawk also recognizes a few gawk-specific command line options for script compatibility:

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.