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GAWK(1) Utility Commands GAWK(1)
NAME
gawk - pattern scanning and processing language
SYNOPSIS
gawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
gawk [ POSIX or GNU style options ] [ -- ] program-text file ...
DESCRIPTION
Gawk is the GNU Project's implementation of the AWK programming lan‐
guage. It conforms to the definition of the language in the POSIX
1003.2 Command Language And Utilities Standard. This version in turn
is based on the description in The AWK Programming Language, by Aho,
Kernighan, and Weinberger, with the additional features found in the
System V Release 4 version of UNIX awk. Gawk also provides more recent
Bell Labs awk extensions, and some GNU-specific extensions.
The command line consists of options to gawk itself, 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 variables.
OPTION FORMAT
Gawk 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, gawk-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. Argu‐
ments 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 abbre‐
viation remains unique.
OPTIONS
Gawk accepts the following options.
-F fs
--field-separator fs
Use fs for the input field separator (the value of the FS prede‐
fined 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. The f flag sets the
maximum number of fields, and the r flag sets the maximum record
size. These two flags and the -m option are from the Bell Labs
research version of UNIX awk. They are ignored by gawk, since
gawk has no pre-defined limits.
-W traditional
-W compat
--traditional
--compat
Run in compatibility mode. In compatibility mode, gawk 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. (Per the GNU Coding Standards, these options
cause an immediate, successful exit.)
-W lint
--lint Provide warnings about constructs that are dubious or non-porta‐
ble 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 gawk 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 gawk on
the standard output. This is useful mainly for knowing if the
current copy of gawk on your system is up to date with respect
to whatever the Free Software Foundation is distributing. This
is also useful when reporting bugs. (Per the GNU Coding Stan‐
dards, these options cause an immediate, successful exit.)
-- 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 array for processing. This is particularly useful for running
AWK programs via the “#!” executable interpreter mechanism.
AWK PROGRAM EXECUTION
An AWK program consists of a sequence of pattern-action statements and
optional function definitions.
pattern { action statements }
function name(parameter list) { statements }
Gawk first reads the program source from the program-file(s) if speci‐
fied, from arguments to --source, or from the first non-option argument
on the command line. The -f and --source options may be used multiple
times on the command line. Gawk will read the program text as if all
the program-files and command line source texts had been concatenated
together. This is useful for building libraries of AWK functions,
without having to include them in each new AWK program that uses them.
It also provides the ability to mix library functions with command line
programs.
The environment variable AWKPATH specifies a search path to use when
finding source files named with the -f option. If this variable does
not exist, the default path is ".:/usr/local/share/awk". (The actual
directory may vary, depending upon how gawk was built and installed.)
If a file name given to the -f option contains a “/” character, no path
search is performed.
Gawk executes AWK programs in the following order. First, all variable
assignments specified via the -v option are performed. Next, gawk com‐
piles the program into an internal form. Then, gawk executes the code
in the BEGIN block(s) (if any), and then proceeds to read each file
named in the ARGV array. If there are no files named on the command
line, gawk reads the standard input.
If a filename on the command line has the form var=val it is treated as
a variable assignment. The variable var will be assigned the value
val. (This happens after any BEGIN block(s) have been run.) Command
line variable assignment is most useful for dynamically assigning val‐
ues to the variables AWK uses to control how input is broken into
fields and records. It is also useful for controlling state if multi‐
ple passes are needed over a single data file.
If the value of a particular element of ARGV is empty (""), gawk skips
over it.
For each record in the input, gawk tests to see if it matches any pat‐
tern in the AWK program. 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, gawk 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
arrays; arrays with multiple dimensions may be simulated. 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 compati‐
bility 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 new‐
line character always acts as a field separator, in addition to what‐
ever 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. Oth‐
erwise, 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 num‐
bers, 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:
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 recom‐
puted, 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., ENVI‐
RON["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 separa‐
tor. 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 speci‐
fied 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 expres‐
sion 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:
i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"
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 associa‐
tive, 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 con‐
text. 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 accom‐
plished 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 num‐
bers 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. How‐
ever, 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 AND ACTIONS
AWK is a line-oriented language. 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 single
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. 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 state‐
ments 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, other‐
wise 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 pat‐
tern1, 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 expres‐
sion. If there is one number in the braces, the preceding
regexp r is repeated n times. If there are two numbers sep‐
arated 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 charac‐
ters 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 charac‐
ters.)
[: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 charac‐
ters, 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 col‐
lating, 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 ele‐
ment, 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 cur‐
rently 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 spe‐
cial. (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). Char‐
acters 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 expres‐
sion 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 pat‐
tern 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 nonsignifi‐
cant 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 speci‐
fies 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 spe‐
cial 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 cur‐
rent process, in decimal, terminated with a newline.
/dev/pgrpid Reading this file returns the process group ID of the cur‐
rent 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 num‐
ber 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 begin‐
ning 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 sup‐
plied, $0 is used instead. Within the replace‐
ment 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 expres‐
sion 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 sys‐
tems).
strftime([format [, timestamp]])
formats timestamp according to the specification in format.
The timestamp should be of the same form as returned by sys‐
time(). 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 conver‐
sions 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 con‐
versions 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 recog‐
nized, 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 fol‐
lowing 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 expres‐
sions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).
In compatibility mode, the characters represented by octal and hexadec‐
imal escape sequences are treated literally when used in regexp con‐
stants. Thus, /a\52b/ is equivalent to /a\*b/.
FUNCTIONS
Functions in AWK are defined as follows:
function name(parameter list) { statements }
Functions are executed when they are called from within expressions in
either patterns or actions. Actual parameters supplied in the function
call are used to instantiate the formal parameters declared in the
function. Arrays are passed by reference, other variables are passed
by value.
Since functions were not originally part of the AWK language, the pro‐
vision 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 fol‐
low 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 be recursive. Function parame‐
ters used as local variables are initialized to the null string and the
number zero upon function invocation.
Use return expr to return a value from a function. The return value is
undefined if no value is provided, or if the function returns by “fall‐
ing off” the end.
If --lint has been provided, gawk 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.
Effective AWK Programming, Edition 1.0, published by the Free Software
Foundation, 1995.
POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX standard, as
well as with the latest version of UNIX awk. To this end, gawk incor‐
porates 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 hap‐
pens when awk would otherwise open the argument as a file, which is
after the BEGIN block is executed. However, in earlier implementa‐
tions, 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, gawk 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 argu‐
ments 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 gawk also returns its
current seed.
Other new features are: The use of multiple -f options (from MKS awk);
the ENVIRON array; the \a, and \v escape sequences (done originally in
gawk 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
Gawk has a number of extensions to POSIX awk. They are described in
this section. All the extensions described here can be disabled by
invoking gawk with the --traditional option.
The following features of gawk 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 avail‐
able.
· 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 array to delete the entire contents of an
array.
The AWK book does not define the return value of the close() function.
Gawk's close() returns the value from fclose(3), or pclose(3), when
closing a file or pipe, respectively.
When gawk 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 gawk -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: gawk -F'\t'
....
HISTORICAL FEATURES
There are two features of historical AWK implementations that gawk sup‐
ports. 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 gawk
will issue a warning about its use if --lint is specified on the com‐
mand line.
The other feature is the use of either the continue or the break state‐
ments outside the body of a while, for, or do loop. Traditional AWK
implementations have treated such usage as equivalent to the next
statement. Gawk will support this usage if --traditional has been
specified.
ENVIRONMENT VARIABLES
If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly
as if --posix had been specified on the command line. If --lint has
been specified, gawk will issue a warning message to this effect.
The AWKPATH environment variable can be used to provide a list of
directories that gawk 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 assign‐
ment 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 gawk than you would get on a system without those files.
When gawk 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 gawk, version 3.0.6.
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
gawk, 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 gawk com‐
patible with the new version of UNIX awk. Arnold Robbins is the cur‐
rent maintainer.
The initial DOS port was done by Conrad Kwok and Scott Garfinkle.
Scott Deifik is the current DOS maintainer. Pat Rankin did the port to
VMS, and Michal Jaegermann did the port to the Atari ST. The port to
OS/2 was done by Kai Uwe Rommel, with contributions and help from Dar‐
rel Hankerson. Fred Fish supplied support for the Amiga.
BUG REPORTS
If you find a bug in gawk, please send electronic mail to bug-
gawk@gnu.org. Please include your operating system and its revision,
the version of gawk, 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 gawk. 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
gawk 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 test‐
ing and debugging. We thank him.
COPYING PERMISSIONS
Copyright © 1996-2000 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 man‐
ual page into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a trans‐
lation approved by the Foundation.
Free Software Foundation May 17 2000 GAWK(1)
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