GNU awk

DOC DATE

June 2000

NAME

awk, Gawk - Aho, Weinberger and Kernighan's pattern processing language

WHAT IT IS

Gawk is the GNU Project's implementation of the AWK programming language, which is the command named awk in cLIeNUX. awk is a scripting language or batch interpreter. It is not a line-wise interactive interpreter or shell like sh, Forth or BASIC. awk takes it's current program as a whole and converts it as a whole into an internal form before doing anything. awk provides strings, floating point numbers, and subscripted-by-strings arrays that I will hereinafter call lists. At the highest level program flow in awk is controlled by pattern-action constructs similar to ed, which ease causing an awk program to traverse a file, but which are somewhat confusing. Within an action construct, however, awk is a pleasingly generic procedural language.

Generic stuff in awk includes unix-style string literals and concatenation like "\n\n\n\t\tHuh?\n\n\n" "Oh.\n", a nice little set of floating point math functions, A BASIC-like set of string parsing functions, the % interger remainder operator, and subroutines similar to the mis-named sh "functions". Variables and lists have no required prefix, and may be handled as strings or numbers, depending on context. That is, all regular variables are stored as strings. There are no required "variable declarations".

Not quite as generic, there is implicit support for a "current line". Data read into awk is implicitly split into records and fields based on the FS and RS delimiter variables and each field is assigned a number, and $ is the field reference operator. The defaults are for lines of text as records. By default then, $1 represents the first "word" of the current input line. awk also provides the ~ operator for comparing a string to a regular expression and providing a result flag. File input-output can be controlled within an awk program much like in the unix shell. The in operator checks a list for an item name, or can loop over list items via the for (VAR in list) construct.

AWK, it appears to me, is the main predecessor of Perl, and is present in cLIeNUX Core while Perl is not. Awk is much smaller than Perl, and to my tastes, has a cleaner syntax. The massive facilities of Perl exist elsewhere in cLIeNUX, and can be accessed from an awk program, via such things as netcat, as one example (for sockets). Perl is, I believe, a bit faster than awk, but both are terribly slow compared to compiled C, or even a relatively fast Forth like GForth. This slowness comes at a benefit, however. An awk-adept person can get handy, unusual little programs working very rapidly with startlingly little code. As soon as Bash or sh starts to look confining, look into awk.

INVOCATION

synopsis

awk [ options ] -f program-file [ -- ] [file...]
awk [ options ] [ -- ] program-text [file...]

examples

The command line consists of options to awk itself, which are listed toward the end of this seedoc, the AWK program text (if not supplied via the -f or --file options), and values to be made available in the ARGC and ARGV pre-defined AWK variable and list.

If the awk program is to be taken from standard input, it is usually the case that it must be excluded from interpretation by the shell. Thus the prevalent syntax for an awk quickie is... (with awk's answer hi-lited)



		:;awk '
		:;:;BEGIN { print "Dare I say it? HELLO WORLD!" }'
		Dare I say it? HELLO WORLD!
		:;
The right-quotes protect the awk program from the shell. The above example works at the shell prompt, and that format will be used for subsequent examples. This technique will also allow an awk program to be included within a pipe sequence. The above example uses a BEGIN "pattern" and affiliated {...}-delimited action, the print keyword, and a string literal. A BEGIN pattern doesn't scan an input file, so nothing in this example looks for an input file, so this program quits without waiting for more input. Note that the shell waited for the second ', and that therefor awk didn't do anything until the program was input completely, did it's thing, and then exited.

An awk program that doesn't look for an input data file can thus easily take arbitrary commandline arguments, since the program will not attempt to interpret them as files.


	:;awk ' BEGIN { print ARGV[3] } ' /dev/null /GPL woof  /dev/null
	woof
	:;
Here we used the built-in ARGV list. When awk itself generates a list the names of the list items (the subscripts) will be integers. Note that the arguments in this case can be shell-expanded. Can you guess what the above would return if we print ARGV[0]?

Flow of Execution (abbreviated)

First, all variable assignments specified via the -v VAR=val option construct are performed.

Next, awk compiles the program into an internal form.

Then, awk executes the code in the BEGIN block(s) (if any).

Then we proceed to read each file named in the ARGV list IF there are pattern-actions other than BEGIN or END.

If there are no files named on the command line, and if there are pattern-actions other than BEGIN or END, awk reads the standard input.

Line-wise Patterns
For each record in the data input file(s), awk tests to see if it matches any pattern in the AWK program (besides BEGIN and END). For each pattern that the record matches, the associated action is executed. The patterns are tested in the order they occur in the program.

Finally, after all the input is exhausted, awk executes the code in the END block(s) (if any).

VARIABLES, RECORDS AND FIELDS

AWK variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings, or both, depending upon how they are used. AWK also has one dimensional lists; lists with multiple dimensions may be simulated with "N-dimensional subscripts". Several pre-defined variables are set as a program runs; these will be described as needed and summarized below.

Records

Normally, records are separated by newline characters. You can control how records are separated by assigning values to the built-in variable RS. If RS is any single character, that character separates records. Otherwise, RS is a regular expression. Text in the input that matches this regular expression will separate the record. However, in compatibility mode, only the first character of its string value is used for separating records. If RS is set to the null string, then records are separated by blank lines. When RS is set to the null string, the newline character always acts as a field separator, in addition to whatever value FS may have.

Fields

As each input record is read, awk splits the record into fields, using the value of the FS variable as the field separator. If FS is a single character, fields are separated by that character. If FS is the null string, then each individual character becomes a separate field. Otherwise, FS is expected to be a full regular expression. In the special case that FS is a single space, fields are separated by runs of spaces and/or tabs and/or newlines. (But see the discussion of --posix, below). Note that the value of IGNORECASE (see below) will also affect how fields are split when FS is a regular expression, and how records are separated when RS is a regular expression.

If the FIELDWIDTHS variable is set to a space separated list of numbers, each field is expected to have fixed width, and awk will split up the record using the specified widths. The value of FS is ignored. Assigning a new value to FS overrides the use of FIELDWIDTHS, and restores the default behavior.

Each field in the input record may be referenced by its position, $1, $2, and so on. $0 is the whole record. The value of a field may be assigned to as well. Fields need not be referenced by constants:

n = 5
print $n

prints the fifth field in the input record. The variable NF is set to the total number of fields in the input record.

References to non-existent fields (i.e. fields after $NF) produce the null-string. However, assigning to a non-existent field (e.g., $(NF+2) = 5) will increase the value of NF, create any intervening fields with the null string as their value, and cause the value of $0 to be recomputed, with the fields being separated by the value of OFS. References to negative numbered fields cause a fatal error. Decrementing NF causes the values of fields past the new value to be lost, and the value of $0 to be recomputed, with the fields being separated by the value of OFS.

Built-in Variables

awk's built-in variables are:

ARGC
The number of command line arguments (does not include options to awk, or the program source).
ARGIND
The index in ARGV of the current file being processed.
ARGV
list of command line arguments. The list items are named from 0 to the value of ARGC - 1. Dynamically changing the contents of ARGV can control the files used for data.
CONVFMT
The conversion format for numbers, "%.6g", by default.
ENVIRON
A list containing the values of the current environment. The list is indexed (subscripted) by the environment variables, each element being the value of that variable (e.g., ENVIRON["HOME"] might be /home/arnold). Changing this list does not affect the environment seen by programs which awk spawns via redirection or the system() function. (This may change in a future version of awk.)
ERRNO
If a system error occurs either doing a redirection for getline, during a read for getline, or during a close(), then ERRNO will contain a string describing the error.
FIELDWIDTHS
A white-space separated list of fieldwidths. When set, awk parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator. The fixed field width facility is still experimental; the semantics may change as awk evolves over time.
FILENAME
The name of the current input file. If no files are specified on the command line, the value of FILENAME is ``-''. However, FILENAME is undefined inside the BEGIN block.
FNR
The input record number in the current input file.
FS
The input field separator, a space by default. See Fields, above.
IGNORECASE
Controls the case-sensitivity of all regular expression and string operations. If IGNORECASE has a non-zero value, then string comparisons and pattern matching in rules, field splitting with FS, record separating with RS, regular expression matching with ~ and !~, and the gensub(), gsub(), index(), match(), split(), and sub() pre-defined functions will all ignore case when doing regular expression operations.
NF
The number of fields in the current input record.
NR
The total number of input records seen so far.
OFMT
The output format for numbers, "%.6g", by default.
OFS
The output field separator, a space by default.
ORS
The output record separator, by default a newline.
RS
The input record separator, by default a newline.
RT
The record terminator. awk sets RT to the input text that matched the character or regular expression specified by RS.
RSTART
The index of the first character matched by match(); 0 if no match.
RLENGTH
The length of the string matched by match(); -1 if no match.
SUBSEP
The character used to separate multiple subscripts in list elements, by default "\034".

LISTS

Lists are subscripted with an expression between square brackets ([ and ]). If the expression is an expression list (expr, expr ...) then the list subscript is a string consisting of the concatenation of the (string) value of each expression, separated by the value of the SUBSEP variable. This facility is used to simulate multiply dimensioned lists. For example:

i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"

assigns the string "hello, world\n" to the element of the list x which is indexed by the string "A\034B\034C".

The special operator in may be used in an if or while statement to see if a list has an item name consisting of a particular value.

if (val in list)
        print list[val]

If the list has multiple subscripts, use (i, j) in list.

The in construct may also be used in a for loop to iterate over all the items in a list.

An element may be deleted from a list using the delete statement. The delete statement may also be used to delete the entire contents of a list, just by specifying the list name without a subscript.

Variable Typing And Conversion

Variables and fields may be (floating point) numbers, or strings, or both. How the value of a variable is interpreted depends upon its context. If used in a numeric expression, it will be treated as a number, if used as a string it will be treated as a string.

To force a variable to be treated as a number, add 0 to it; to force it to be treated as a string, concatenate it with the null string.

When a string must be converted to a number, the conversion is accomplished using atof(3). A number is converted to a string by using the value of CONVFMT as a format string for sprintf(3), with the numeric value of the variable as the argument. However, even though all numbers in AWK are floating-point, integral values are always converted as integers. Thus, given


	CONVFMT = "%2.2f"
	a = 12
	b = a ""

the variable b has a string value of "12" and not "12.00".

awk performs comparisons as follows: If two variables are numeric, they are compared numerically. If one value is numeric and the other has a string value that is a ``numeric string,'' then comparisons are also done numerically. Otherwise, the numeric value is converted to a string and a string comparison is performed. Two strings are compared, of course, as strings. According to the POSIX standard, even if two strings are numeric strings, a numeric comparison is performed. However, this is clearly incorrect, and awk does not do this. (This is from the GNU manpage. I can't make any sense of it. It looks like a thinko that should read "...a string comparison is performed. Rick. )

Note that string constants, such as "57", are not numeric strings, they are string constants. The idea of ``numeric string'' only applies to fields, getline input, FILENAME, ARGV elements, ENVIRON elements and the elements of a list created by split() that are strings representing integers.

Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string).

PATTERNS AND ACTIONS

In every statement, the pattern comes first, and then the action. Action statements are enclosed in { and }. Either the pattern may be missing, or the action may be missing, but, of course, not both. If the pattern is missing, the action will be executed for every record of input. A missing action is equivalent to

{ print }

which prints the entire record.

Comments begin with the ``#'' character, and continue until the end of the line. Blank lines may be used to separate statements. A newline in awk is analagous to ; in other languages; it terminates a statement. Normally, a statement ends with a newline, however, this is not the case for lines ending in a ,, {, ?, :, &&, or ||. Lines ending in do or else also have their statements automatically continued on the following line. In other cases, a line can be continued by ending it with a ``\'', in which case the newline will be ignored.

Multiple statements may be put on one line by separating them with a ``;''. This applies to both the statements within the action part of a pattern-action pair (the usual case), and to the pattern-action statements themselves.

Patterns

AWK patterns may be one of the following:

	BEGIN
	END
	/regular expression/
	relational expression
	pattern && pattern
	pattern || pattern
	pattern ? pattern : pattern
	(pattern)
	! pattern
	pattern1, pattern2

BEGIN and END are two special kinds of patterns which are not tested against the input. The action parts of all BEGIN patterns are merged as if all the statements had been written in a single BEGIN block. They are executed before any of the input is read. Similarly, all the END blocks are merged, and executed when all the input is exhausted (or when an exit statement is executed). BEGIN and END patterns cannot be combined with other patterns in pattern expressions. BEGIN and END patterns cannot have missing action parts.

For /regular expression/ patterns, the associated statement is executed for each input record that matches the regular expression. Regular expressions are the same as those in egrep(1), and are summarized below.

A relational expression may use any of the operators defined below in the section on actions. These generally test whether certain fields match certain regular expressions.

The &&, ||, and ! operators are logical AND, logical OR, and logical NOT, respectively, as in C. They do short-circuit evaluation, also as in C, and are used for combining more primitive pattern expressions. As in most languages, parentheses may be used to change the order of evaluation.

The ?: operator is like the same operator in C. If the first pattern is true then the pattern used for testing is the second pattern, otherwise it is the third. Only one of the second and third patterns is evaluated.

The pattern1, pattern2 form of an expression is called a range pattern. It matches all input records starting with a record that matches pattern1, and continuing until a record that matches pattern2, inclusive. It does not combine with any other sort of pattern expression.

Regular Expressions

Regular expressions are the extended kind found in egrep. They are composed of characters as follows:
c
matches the non-metacharacter c.
\c
matches the literal character c.
.
matches any character including newline.
^
matches the beginning of a string.
$
matches the end of a string.
[abc...]
character list, matches any of the characters abc....
[^abc...]
negated character list, matches any character except abc....
r1|r2
alternation: matches either r1 or r2.
r1r2
concatenation: matches r1, and then r2.
r+
matches one or more r's.
r*
matches zero or more r's.
r?
matches zero or one r's.
(r)
grouping: matches r.
r{n}
r{n,}
r{n,m} One or two numbers inside braces denote an interval expression. If there is one number in the braces, the preceding regexp r is repeated n times. If there are two numbers separated by a comma, r is repeated n to m times. If there is one number followed by a comma, then r is repeated at least n times.

Interval expressions are only available if either --posix or --re-interval is specified on the command line.

\y
matches the empty string at either the beginning or the end of a word.
\B
matches the empty string within a word.
\<
matches the empty string at the beginning of a word.
\>
matches the empty string at the end of a word.
\w
matches any word-constituent character (letter, digit, or underscore).
\W
matches any character that is not word-constituent.
\`
matches the empty string at the beginning of a buffer (string).
\'
matches the empty string at the end of a buffer.

The escape sequences that are valid in string constants (see below) are also legal in regular expressions.

Character classes are a new feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but where the actual characters themselves can vary from country to country and/or from character set to character set. For example, the notion of what is an alphabetic character differs in the USA and in France.

A character class is only valid in a regexp inside the brackets of a character list. Character classes consist of [:, a keyword denoting the class, and :]. Here are the character classes defined by the POSIX standard.

[:alnum:]
Alphanumeric characters.
[:alpha:]
Alphabetic characters.
[:blank:]
Space or tab characters.
[:cntrl:]
Control characters.
[:digit:]
Numeric characters.
[:graph:]
Characters that are both printable and visible. (A space is printable, but not visible, while an a is both.)
[:lower:]
Lower-case alphabetic characters.
[:print:]
Printable characters (characters that are not control characters.)
[:punct:]
Punctuation characters (characters that are not letter, digits, control characters, or space characters).
[:space:]
Space characters (such as space, tab, and formfeed, to name a few).
[:upper:]
Upper-case alphabetic characters.
[:xdigit:]
Characters that are hexadecimal digits.

For example, before the POSIX standard, to match alphanumeric characters, you would have had to write /[A-Za-z0-9]/. If your character set had other alphabetic characters in it, this would not match them. With the POSIX character classes, you can write /[[:alnum:]]/, and this will match all the alphabetic and numeric characters in your character set.

Two additional special sequences can appear in character lists. These apply to non-ASCII character sets, which can have single symbols (called collating elements) that are represented with more than one character, as well as several characters that are equivalent for collating, or sorting, purposes. (E.g., in French, a plain ``e'' and a grave-accented e` are equivalent.)

Collating Symbols
A collating symbols is a multi-character collating element enclosed in [. and .]. For example, if ch is a collating element, then [[.ch.]] is a regexp that matches this collating element, while [ch] is a regexp that matches either c or h.
Equivalence Classes
An equivalence class is a locale-specific name for a list of characters that are equivalent. The name is enclosed in [= and =]. For example, the name e might be used to represent all of ``e,'' ``e`,'' and ``e`.'' In this case, [[=e]] is a regexp that matches any of
 .BR e ,
 .BR e' , or
 .BR e` .

These features are very valuable in non-English speaking locales. The library functions that awk uses for regular expression matching currently only recognize POSIX character classes; they do not recognize collating symbols or equivalence classes.

The \y, \B, \<, \>, \w, \W, \`, and \' operators are specific to awk; they are extensions based on facilities in the GNU regexp libraries.

The various command line options control how awk interprets characters in regexps.

No options
In the default case, awk provide all the facilities of POSIX regexps and the GNU regexp operators described above. However, interval expressions are not supported.
--posix
Only POSIX regexps are supported, the GNU operators are not special. (E.g., \w matches a literal w). Interval expressions are allowed.
--traditional
Traditional Unix awk regexps are matched. The GNU operators are not special, interval expressions are not available, and neither are the POSIX character classes ([[:alnum:]] and so on). Characters described by octal and hexadecimal escape sequences are treated literally, even if they represent regexp metacharacters.
--re-interval
Allow interval expressions in regexps, even if --traditional has been provided.

Actions

Action statements are enclosed in braces, { and }. Action statements consist of the usual assignment, conditional, and looping statements found in most languages. The operators, control statements, and input/output statements available are patterned after those in C.

Operators

The operators in AWK, in order of decreasing precedence, are

(...)
Grouping
$
Field reference.
++ --
Increment and decrement, both prefix and postfix.
^
Exponentiation (** may also be used, and **= for the assignment operator).
+ - !
Unary plus, unary minus, and logical negation.
* / %
Multiplication, division, and modulus (integer remainder).
+ -
Addition and subtraction.
space
String concatenation.
< >
<= >=
!= == The regular relational operators.
~ !~
Regular expression match, negated match. NOTE: Do not use a constant regular expression (/foo/) on the left-hand side of a ~ or !~. Only use one on the right-hand side. The expression /foo/ ~ exp has the same meaning as (($0 ~ /foo/) ~ exp). This is usually not what was intended.
in
list membership, by item name.
&&
Logical AND.
||
Logical OR.
?:
The C conditional expression. This has the form expr1 ? expr2 : expr3. If expr1 is true, the value of the expression is expr2, otherwise it is expr3. Only one of expr2 and expr3 is evaluated.
= += -=
*= /= %= ^= Assignment. Both absolute assignment (var = value) and operator-assignment (the other forms) are supported.

Control Statements

The control statements are as follows:

	if (condition) statement [ else statement ]
	while (condition) statement 
	do statement while (condition)
	for (expr1; expr2; expr3) statement
	for (var in list) statement
	break
	continue
	delete list[item name]
	delete list
	exit [ expression ]
	{ statements }

I/O Statements

The input/output statements are as follows:

close(file)
Close file (or pipe, see below).
getline
Set $0 from next input record; set NF, NR, FNR.
getline <file
Set $0 from next record of file; set NF.
getline var
Set var from next input record; set NR, FNR.
getline var <file
Set var from next record of file.
next
Stop processing the current input record. The next input record is read and processing starts over with the first pattern in the AWK program. If the end of the input data is reached, the END block(s), if any, are executed.
nextfile
Stop processing the current input file. The next input record read comes from the next input file. FILENAME and ARGIND are updated, FNR is reset to 1, and processing starts over with the first pattern in the AWK program. If the end of the input data is reached, the END block(s), if any, are executed. NOTE: Earlier versions of awk used next file, as two words. While this usage is still recognized, it generates a warning message and will eventually be removed.
print
Prints the current record. The output record is terminated with the value of the ORS variable.
print expr-list
Prints expressions. Each expression is separated by the value of the OFS variable. The output record is terminated with the value of the ORS variable.
print expr-list >file
Prints expressions on file. Each expression is separated by the value of the OFS variable. The output record is terminated with the value of the ORS variable.
printf fmt, expr-list
Format and print.
printf fmt, expr-list >file
Format and print on file.
system(cmd-line)
Execute the command cmd-line, and return the exit status. (This may not be available on non-POSIX systems.)
fflush([file])
Flush any buffers associated with the open output file or pipe file. If file is missing, then standard output is flushed. If file is the null string, then all open output files and pipes have their buffers flushed.

Other input/output redirections are also allowed. For print and printf, >>file appends output to the file, while | command writes on a pipe. In a similar fashion, command | getline pipes into getline. The getline command will return 0 on end of file, and -1 on an error.

The printf Statement

The AWK versions of the printf statement and sprintf() function (see below) accept the following conversion specification formats:

%c
An ASCII character. If the argument used for %c is numeric, it is treated as a character and printed. Otherwise, the argument is assumed to be a string, and the only first character of that string is printed.
%d
%i A decimal number (the integer part).
%e
%E A floating point number of the form [-]d.dddddde[+-]dd. The %E format uses E instead of e.
%f
A floating point number of the form [-]ddd.dddddd.
%g
%G Use %e or %f conversion, whichever is shorter, with nonsignificant zeros suppressed. The %G format uses %E instead of %e.
%o
An unsigned octal number (again, an integer).
%s
A character string.
%x
%X An unsigned hexadecimal number (an integer). %X format uses ABCDEF instead of abcdef.
%%
A single % character; no argument is converted.

There are optional, additional parameters that may lie between the % and the control letter:

-
The expression should be left-justified within its field.
space
For numeric conversions, prefix positive values with a space, and negative values with a minus sign.
+
The plus sign, used before the width modifier (see below), says to always supply a sign for numeric conversions, even if the data to be formatted is positive. The + overrides the space modifier.
#
Use an ``alternate form'' for certain control letters. For %o, supply a leading zero. For %x, and %X, supply a leading 0x or 0X for a nonzero result. For %e, %E, and %f, the result will always contain a decimal point. For %g, and %G, trailing zeros are not removed from the result.
0
A leading 0 (zero) acts as a flag, that indicates output should be padded with zeroes instead of spaces. This applies even to non-numeric output formats. This flag only has an effect when the field width is wider than the value to be printed.
width
The field should be padded to this width. The field is normally padded with spaces. If the 0 flag has been used, it is padded with zeroes.
.prec
A number that specifies the precision to use when printing. For the %e, %E, and %f formats, this specifies the number of digits you want printed to the right of the decimal point. For the %g, and %G formats, it specifies the maximum number of significant digits. For the %d, %o, %i, %u, %x, and %X formats, it specifies the minimum number of digits to print. For a string, it specifies the maximum number of characters from the string that should be printed.

The dynamic width and prec capabilities of the ANSI C printf() routines are supported. A * in place of either the width or prec specifications will cause their values to be taken from the argument list to printf or sprintf().

Special File Names

When doing I/O redirection from either print or printf into a file, or via getline from a file, awk recognizes certain special filenames internally. These filenames allow access to open file descriptors inherited from awk's parent process (usually the shell). Other special filenames provide access to information about the running awk process. The filenames are:

/dev/pid
Reading this file returns the process ID of the current process, in decimal, terminated with a newline.
/dev/ppid
Reading this file returns the parent process ID of the current process, in decimal, terminated with a newline.
/dev/pgrpid
Reading this file returns the process group ID of the current process, in decimal, terminated with a newline.
/dev/user
Reading this file returns a single record terminated with a newline. The fields are separated with spaces. $1 is the value of the getuid(2) system call, $2 is the value of the geteuid(2) system call, $3 is the value of the getgid(2) system call, and $4 is the value of the getegid(2) system call. If there are any additional fields, they are the group IDs returned by getgroups(2). Multiple groups may not be supported on all systems.
/dev/stdin
The standard input.
/dev/stdout
The standard output.
/dev/stderr
The standard error output.
/dev/fd/n
The file associated with the open file descriptor n.

These are particularly useful for error messages. For example:

print "You blew it!" > "/dev/stderr"

whereas you would otherwise have to use

print "You blew it!" | "cat 1>&2"

These file names may also be used on the command line to name data files.

Numeric Functions

AWK has the following pre-defined arithmetic functions:

atan2(y, x)
returns the arctangent of y/x in radians.
cos(expr)
returns the cosine of expr, which is in radians.
exp(expr)
the exponential function.
int(expr)
truncates to integer.
log(expr)
the natural logarithm function.
rand()
returns a random number between 0 and 1.
sin(expr)
returns the sine of expr, which is in radians.
sqrt(expr)
the square root function.
srand([expr])
uses expr as a new seed for the random number generator. If no expr is provided, the time of day will be used. The return value is the previous seed for the random number generator.

String Functions

awk has the following pre-defined string functions:

gensub(r, s, h [, t])
search the target string t for matches of the regular expression r. If h is a string beginning with g or G, then replace all matches of r with s. Otherwise, h is a number indicating which match of r to replace. If no t is supplied, $0 is used instead. Within the replacement text s, the sequence \n, where n is a digit from 1 to 9, may be used to indicate just the text that matched the n'th parenthesized subexpression. The sequence \0 represents the entire matched text, as does the character &. Unlike sub() and gsub(), the modified string is returned as the result of the function, and the original target string is not changed.
gsub(r, s [, t])
for each substring matching the regular expression r in the string t, substitute the string s, and return the number of substitutions. If t is not supplied, use $0. An & in the replacement text is replaced with the text that was actually matched. Use \& to get a literal &. See AWK Language Programming for a fuller discussion of the rules for &'s and backslashes in the replacement text of sub(), gsub(), and gensub().
index(s, t)
returns the index of the string t in the string s, or 0 if t is not present.
length([s])
returns the length of the string s, or the length of $0 if s is not supplied.
match(s, r)
returns the position in s where the regular expression r occurs, or 0 if r is not present, and sets the values of RSTART and RLENGTH.
split(s, a [, r])
splits the string s into the list a on the regular expression r, and returns the number of fields. If r is omitted, FS is used instead. The list a is cleared first. Splitting behaves identically to field splitting, described above.
sprintf(fmt, expr-list)
prints expr-list according to fmt, and returns the resulting string.
sub(r, s [, t])
just like gsub(), but only the first matching substring is replaced.
substr(s, i [, n])
returns the at most n-character substring of s starting at i. If n is omitted, the rest of s is used.
tolower(str)
returns a copy of the string str, with all the upper-case characters in str translated to their corresponding lower-case counterparts. Non-alphabetic characters are left unchanged.
toupper(str)
returns a copy of the string str, with all the lower-case characters in str translated to their corresponding upper-case counterparts. Non-alphabetic characters are left unchanged.

Time Functions

Since one of the primary uses of AWK programs is processing log files that contain time stamp information, awk provides the following two functions for obtaining time stamps and formatting them.

systime()
returns the current time of day as the number of seconds since the Epoch (Midnight UTC, January 1, 1970 on POSIX systems).
strftime([format [, timestamp]])
formats timestamp according to the specification in format. The timestamp should be of the same form as returned by systime(). If timestamp is missing, the current time of day is used. If format is missing, a default format equivalent to the output of date(1) will be used. See the specification for the strftime() function in ANSI C for the format conversions that are guaranteed to be available. A public-domain version of strftime(3) and a man page for it come with awk; if that version was used to build awk, then all of the conversions described in that man page are available to awk.

String Constants

String constants in AWK are sequences of characters enclosed between double quotes ("). Within strings, certain escape sequences are recognized, as in C. These are:

\\
A literal backslash.
\a
The ``alert'' character; usually the ASCII BEL character.
\b
backspace.
\f
form-feed.
\n
newline.
\r
carriage return.
\t
horizontal tab.
\v
vertical tab.
\xhex digits
The character represented by the string of hexadecimal digits following the \x. As in ANSI C, all following hexadecimal digits are considered part of the escape sequence. (This feature should tell us something about language design by committee.) E.g., "\x1B" is the ASCII ESC (escape) character.
\ddd
The character represented by the 1-, 2-, or 3-digit sequence of octal digits. E.g. "\033" is the ASCII ESC (escape) character.
\c
The literal character c.

The escape sequences may also be used inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).

In compatibility mode, the characters represented by octal and hexadecimal escape sequences are treated literally when used in regexp constants. Thus, /a\52b/ is equivalent to /a\*b/.

SUBROUTINES

Subroutines in AWK are defined as follows:

function name(parameter list) { statements }

"Function" is a misnomer for awk subroutines, and many other uses of the term in unix. It is a defined named segment of program that may be invoked from anywhere in the program by name. Once named/declared/defined, an awk subroutine may be used in patterns or actions. Defining a subroutine is semantically and syntactically independant of program flow, and subroutine declarations may be above the pattern/action code.

The values the subroutine operates on are named for use inside the code of the subroutine when it is defined. The items that a particular invocation of a subroutine operates on are given to it when it is invoked, in the argument list. The declaration defines the names used to refer to a subroutine's arguments within the code of a particular instance of using the subroutine. What values the subroutine gets for particular named arguments is subject to what are called scoping issues.

Lists named in the argument list are accessed by the function "by name". That is, the function can change the contents of the list itself. The subroutine can thus change the list in ways that are external to the subroutine, i.e. other parts of the program will be effected. This is "call by reference", or "global scope". This is also called "side-effects", and is why C and awk "functions" aren't functions in the formal sense.

Regular variables passed to a use of a subroutine in it's argument list are converted to a local copy of thier value. The subroutine doesn't change the value of that variable as seen by any other part of the program. This is also called "call by value" or "local scope".

Since functions were not originally part of the AWK language, the provision for local variables is rather clumsy: They are declared as extra parameters in the parameter list. The convention is to separate local variables from real parameters by extra spaces in the parameter list. For example:

	function  f(p, q,     a, b)     # a & b are local
	{
	        .....
	}

	/abc/   { ... ; f(1, 2) ; ... }

The left parenthesis in a function call is required to immediately follow the function name, without any intervening white space. This is to avoid a syntactic ambiguity with the concatenation operator. This restriction does not apply to the built-in functions listed above.

Functions may call each other and may call themselves, which is "recursion". Function parameters used as local variables are initialized to the null string and the number zero upon function invocation.

If --lint has been provided, awk will warn about calls to undefined functions at parse time, instead of at run time. Calling an undefined function at run time is a fatal error.

The word func may be used in place of function.

EXAMPLES

	Print and sort the login names of all users:

		        BEGIN   { FS = ":" }
		                { print $1 | "sort" }

	Count lines in a file:

		                { nlines++ }
	        	END     { print nlines }

	Precede each line by its number in the file:

		        { print FNR, $0 }

	Concatenate and line number (a variation on a theme):

	        { print NR, $0 }

SEE ALSO

egrep(1), getpid(2), getppid(2), getpgrp(2), getuid(2), geteuid(2), getgid(2), getegid(2), getgroups(2)

The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X.

AWK Language Programming, Edition 1.0, published by the Free Software Foundation, 1995.

POSIX COMPATIBILITY

A primary goal for awk is compatibility with the POSIX standard, as well as with the latest version of UNIX awk. To this end, awk incorporates the following user visible features which are not described in the AWK book, but are part of the Bell Labs version of awk, and are in the POSIX standard.

The -v option for assigning variables before program execution starts is new. The book indicates that command line variable assignment happens when awk would otherwise open the argument as a file, which is after the BEGIN block is executed. However, in earlier implementations, when such an assignment appeared before any file names, the assignment would happen before the BEGIN block was run. Applications came to depend on this ``feature.'' When awk was changed to match its documentation, this option was added to accommodate applications that depended upon the old behavior. (This feature was agreed upon by both the AT&T and GNU developers.)

The -W option for implementation specific features is from the POSIX standard.

When processing arguments, awk uses the special option ``--'' to signal the end of arguments. In compatibility mode, it will warn about, but otherwise ignore, undefined options. In normal operation, such arguments are passed on to the AWK program for it to process.

The AWK book does not define the return value of srand(). The POSIX standard has it return the seed it was using, to allow keeping track of random number sequences. Therefore srand() in awk also returns its current seed.

Other new features are: The use of multiple -f options (from MKS awk); the ENVIRON list; the \a, and \v escape sequences (done originally in awk and fed back into AT&T's); the tolower() and toupper() built-in functions (from AT&T); and the ANSI C conversion specifications in printf (done first in AT&T's version).

GNU EXTENSIONS

awk has a number of extensions to POSIX awk. They are described in this section. All the extensions described here can be disabled by invoking awk with the --traditional option.

The following features of awk are not available in POSIX awk.

*
The \x escape sequence. (Disabled with --posix.)
*
The fflush() function. (Disabled with --posix.)
*
The systime(), strftime(), and gensub() functions.
*
The special file names available for I/O redirection are not recognized.
*
The ARGIND, ERRNO, and RT variables are not special.
*
The IGNORECASE variable and its side-effects are not available.
*
The FIELDWIDTHS variable and fixed-width field splitting.
*
The use of RS as a regular expression.
*
The ability to split out individual characters using the null string as the value of FS, and as the third argument to split().
*
No path search is performed for files named via the -f option. Therefore the AWKPATH environment variable is not special.
*
The use of nextfile to abandon processing of the current input file.
*
The use of delete list to delete the entire contents of a list.

gawk's close() returns the value from fclose(3), or pclose(3), when closing a file or pipe, respectively.

When awk is invoked with the --traditional option, if the fs argument to the -F option is ``t'', then FS will be set to the tab character. Note that typing awk -F\t ... simply causes the shell to quote the ``t,'', and does not pass ``\t'' to the -F option. Since this is a rather ugly special case, it is not the default behavior. This behavior also does not occur if --posix has been specified. To really get a tab character as the field separator, it is best to use quotes: awk -F'\t' ....

HISTORICAL FEATURES

There are two features of historical AWK implementations that awk supports. First, it is possible to call the length() built-in function not only with no argument, but even without parentheses! Thus,

a = length    # Holy Algol 60, Batman!

is the same as either of

a = length()
a = length($0)

This feature is marked as ``deprecated'' in the POSIX standard, and awk will issue a warning about its use if --lint is specified on the command line.

The other feature is the use of either the continue or the break statements outside the body of a while, for, or do loop. Traditional AWK implementations have treated such usage as equivalent to the next statement. awk will support this usage if --traditional has been specified.

OPTION FORMAT

awk options may be either the traditional POSIX one letter options, or the GNU style long options. POSIX options start with a single ``-'', while long options start with ``--''. Long options are provided for both GNU-specific features and for POSIX mandated features.

Following the POSIX standard, awk-specific options are supplied via arguments to the -W option. Multiple -W options may be supplied Each -W option has a corresponding long option, as detailed below. Arguments to long options are either joined with the option by an = sign, with no intervening spaces, or they may be provided in the next command line argument. Long options may be abbreviated, as long as the abbreviation remains unique.

OPTIONS

awk accepts the following options.

-F fs
--field-separator fs Use fs for the input field separator (the value of the FS predefined variable).
-v var=val
--assign var=val Assign the value val, to the variable var, before execution of the program begins. Such variable values are available to the BEGIN block of an AWK program.
-f program-file
--file program-file Read the AWK program source from the file program-file, instead of from the first command line argument. Multiple -f (or --file) options may be used.
-mf NNN
-mr NNN Set various memory limits to the value NNN.
-W traditional
-W compat
--traditional
--compat Run in compatibility mode. In compatibility mode, awk behaves identically to UNIX awk; none of the GNU-specific extensions are recognized. The use of --traditional is preferred over the other forms of this option. See GNU EXTENSIONS, below, for more information.
-W copyleft
-W copyright
--copyleft
--copyright Print the short version of the GNU copyright information message on the standard output, and exits successfully.
-W help
-W usage
--help
--usage Print a relatively short summary of the available options on the standard output.
-W lint
--lint Provide warnings about constructs that are dubious or non-portable to other AWK implementations.
-W lint-old
--lint-old Provide warnings about constructs that are not portable to the original version of Unix awk.
-W posix
--posix This turns on compatibility mode, with the following additional restrictions:
*
\x escape sequences are not recognized.
*
Only space and tab act as field separators when FS is set to a single space, newline does not.
*
The synonym func for the keyword function is not recognized.
*
The operators ** and **= cannot be used in place of ^ and ^=.
*
The fflush() function is not available.
-W re-interval
--re-interval Enable the use of interval expressions in regular expression matching (see Regular Expressions, below). Interval expressions were not traditionally available in the AWK language. The POSIX standard added them, to make awk and egrep consistent with each other. However, their use is likely to break old AWK programs, so awk only provides them if they are requested with this option, or when --posix is specified.
-W source program-text
--source program-text Use program-text as AWK program source code. This option allows the easy intermixing of library functions (used via the -f and --file options) with source code entered on the command line. It is intended primarily for medium to large AWK programs used in shell scripts.
-W version
--version Print version information for this particular copy of awk on the standard output.
--
Signal the end of options. This is useful to allow further arguments to the AWK program itself to start with a ``-''. This is mainly for consistency with the argument parsing convention used by most other POSIX programs.

In compatibility mode, any other options are flagged as illegal, but are otherwise ignored. In normal operation, as long as program text has been supplied, unknown options are passed on to the AWK program in the ARGV list for processing.

ENVIRONMENT VARIABLES

If POSIXLY_CORRECT exists in the environment, then awk behaves exactly as if --posix had been specified on the command line. If --lint has been specified, awk will issue a warning message to this effect.

The AWKPATH environment variable can be used to provide a list of directories that awk will search when looking for files named via the -f and --file options.

BUGS

The -F option is not necessary given the command line variable assignment feature; it remains only for backwards compatibility.

If your system actually has support for /dev/fd and the associated /dev/stdin, /dev/stdout, and /dev/stderr files, you may get different output from awk than you would get on a system without those files. When awk interprets these files internally, it synchronizes output to the standard output with output to /dev/stdout, while on a system with those files, the output is actually to different open files. Caveat Emptor.

Syntactically invalid single character programs tend to overflow the parse stack, generating a rather unhelpful message. Such programs are surprisingly difficult to diagnose in the completely general case, and the effort to do so really is not worth it.

VERSION INFORMATION

This man page documents awk, version 3.0.2.

AUTHORS

The original version of UNIX awk was designed and implemented by Alfred Aho, Peter Weinberger, and Brian Kernighan of AT&T Bell Labs. Brian Kernighan continues to maintain and enhance it.

Paul Rubin and Jay Fenlason, of the Free Software Foundation, wrote GNU awk, to be compatible with the original version of awk distributed in Seventh Edition UNIX. John Woods contributed a number of bug fixes. David Trueman, with contributions from Arnold Robbins, made awk compatible with the new version of UNIX awk. Arnold Robbins is the current maintainer.

BUG REPORTS

If you find a bug in GNU awk, please send electronic mail to bug-gnu-utils@prep.ai.mit.edu, with a carbon copy to arnold@gnu.ai.mit.edu. Please include your operating system and its revision, the version of awk, what C compiler you used to compile it, and a test program and data that are as small as possible for reproducing the problem.

Before sending a bug report, please do two things. First, verify that you have the latest version of awk. Many bugs (usually subtle ones) are fixed at each release, and if yours is out of date, the problem may already have been solved. Second, please read this man page and the reference manual carefully to be sure that what you think is a bug really is, instead of just a quirk in the language.

Whatever you do, do NOT post a bug report in comp.lang.awk. While the awk developers occasionally read this newsgroup, posting bug reports there is an unreliable way to report bugs. Instead, please use the electronic mail addresses given above.

ACKNOWLEDGEMENTS

Brian Kernighan of Bell Labs provided valuable assistance during testing and debugging. We (GNU) thank him. Kernighan is the K in K&R. Weinberger invented the filesystem abstraction layer known as VFS in the Linux implementation of it. I'm sure Aho is a kook too. What can you say about those Bell Labs boys? They also invented MERT, which rtlinux is based on.

COPYING PERMISSIONS

Copyright ©) 1996 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual page under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual page into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.