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, gawk 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 gawk 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:

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

Gawk's built-in variables are:

ARGC

The number of command line arguments (does not include options to gawk, or the program source).

ARGIND

The index in ARGV of the current file being processed.

ARGV

Array of command line arguments. The array is indexed from 0 to 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

An array containing the values of the current environment. The array is indexed by the environment variables, each element being the value of that variable (e.g., ENVIRON["HOME"] might be /home/arnold). Changing this array does not affect the environment seen by programs which gawk spawns via redirection or the system() function. (This may change in a future version of gawk.)

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, gawk 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 gawk 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. Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab", and "AB". As with all AWK variables, the initial value of IGNORECASE is zero, so all regular expression and string operations are normally case-sensitive. Under Unix, the full ISO 8859-1 Latin-1 character set is used when ignoring case. NOTE: In versions of gawk prior to 3.0, IGNORECASE only affected regular expression operations. It now affects string comparisons as well.

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. Gawk 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 array elements, by default "\034".

Arrays

Arrays are subscripted with an expression between square brackets ([ and ]). If the expression is an expression list (expr, expr ...) then the array 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 arrays. For example:

assigns the string "hello, world\n" to the element of the array x which is indexed by the string "A\034B\034C". All arrays in AWK are associative, i.e. indexed by string values.

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

if (val in array)
	print array[val]

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

The in construct may also be used in a for loop to iterate over all the elements of an array.

An element may be deleted from an array using the delete statement. The delete statement may also be used to delete the entire contents of an array, just by specifying the array 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".

Gawk 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 gawk does not do this.

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 an array created by split() that are numeric strings. The basic idea is that user input, and only user input, that looks numeric, should be treated that way.

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

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 gawk 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 gawk; they are extensions based on facilities in the GNU regexp libraries.

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

No options

In the default case, gawk 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.

+ -

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

Array membership.

&&

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 array) statement
break
continue
delete array[index]
delete array
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 gawk 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.

NOTE: If using a pipe to getline, or from print or printf within a loop, you must use close() to create new instances of the command. AWK does not automatically close pipes when they return EOF.

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 (also an integer).

%u

An unsigned decimal number (again, an integer).

%s

A character string.

%x

%X

An unsigned hexadecimal number (an integer). The %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, gawk recognizes certain special filenames internally. These filenames allow access to open file descriptors inherited from gawk's parent process (usually the shell). Other special filenames provide access to information about the running gawk 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:

whereas you would otherwise have to use

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

Gawk 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 Effective AWK 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 array a on the regular expression r, and returns the number of fields. If r is omitted, FS is used instead. The array 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, gawk 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 gawk; if that version was used to build gawk, then all of the conversions described in that man page are available to gawk.

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/.