1. Program structure

A pattern can be:

BEGIN
END
expression
expression , expression

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.

2. Data types, conversion and comparison

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

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"

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.

3. Regular expressions

	expr ~ /r/

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

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

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

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

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

For example,

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

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

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

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.

5. Expressions and operators

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			$

6. Arrays

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

	for ( var in array ) statement

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]

7. Builtin-variables

8. Built-in functions

Time functions

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

Arithmetic functions

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

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.

9. Input and output

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

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 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.

10. User defined functions

	function name( args ) { statements }

	return opt_expr

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

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.

11. Splitting strings, records and files

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 "".

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.

12. Multi-line records

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

	mawk  -f  prog  v=1  A  t=hello  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

• 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.

MAWK 1.3.3 versus POSIX 1003.2 Draft 11.3

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
	}

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 }

With mawk, the following are all equivalent,

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

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. The built-in fflush first appeared in a recent (1993) AT&T awk released to netlib, and is not part of the POSIX standard. Aggregate deletion with delete array is not part of the POSIX standard.

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.

Random numbers

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.

Extensions added for compatibility for GAWK and BWK

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.

Subtle Differences not in POSIX or the AWK Book