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 pro-
gramming language. It conforms to the definition of the
language in the POSIX 1003.2 Command Language And Utili-
ties 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 let-
ter 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, or multiple arguments may be sup-
plied together if they are separated by commas, or
enclosed in quotes and separated by white space. Case is
ignored in arguments to the -W option. 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
Gawk 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 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 argu-
ment. 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 infor-
mation message on the error output.
-W help
-W usage
--help
--usage
Print a relatively short summary of the available
options on the error output. (Per the GNU Coding
Standards, these options cause an immediate, suc-
cessful exit.)
-W lint
--lint Provide warnings about constructs that are dubious
or non-portable to other AWK implementations.
-W 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 follow-
ing additional restrictions:
o \x escape sequences are not recognized.
o The synonym func for the keyword function is not
recognized.
o The operators ** and **= cannot be used in place
of ^ and ^=.
o 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 tradition-
ally available in the AWK language. The POSIX stan-
dard 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 func-
tions (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.
The -W source= form of this option uses the rest of
the command line argument for program-text; no
other options to -W will be recognized in the same
argument.
-W version
--version
Print version information for this particular copy
of gawk on the error 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 Cod-
ing Standards, these options cause an immediate,
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 specified, 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 ``/'' charac-
ter, 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 compiles the program into an inter-
nal 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 values to the
state if multiple 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 pattern in the AWK program. For each pattern
that the record matches, the associated action is exe-
cuted. 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 charac-
ter, 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 sepa-
rated 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. 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
fied 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.
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 pro-
grams which gawk spawns via redirection or the
system() function. (This may change in a
rection for getline, during a read for get-
line, 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 IGNORE-
CASE has a non-zero value, then string compar-
isons 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 vari-
ables, 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 ver-
sions 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.
ORS The output record separator, by default a new-
line.
RS The input record separator, by default a new-
line.
RT The record terminator. Gawk sets RT to the
input text that matched the character or regu-
lar 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 sub-
scripts 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 multi-
ply 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 associative, i.e. indexed by string val-
ues.
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
Variables and fields may be (floating point) numbers, or
strings, or both. How the value of a variable is inter-
preted 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 conver-
sion 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 vari-
able as the argument. However, even though all numbers in
AWK are floating-point, integral values are always con-
verted 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 numeri-
cally. 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 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
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 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 state-
ment 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 expres-
sions.
and logical NOT, respectively, as in C. They do short-
circuit evaluation, also as in C, and are used for combin-
ing more primitive pattern expressions. As in most lan-
guages, 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 com-
bine 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... and newline.
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
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 begin-
ning 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.
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 con-
trol characters.)
[:punct:]
Punctuation characters (characters that are not
letter, digits, control characters, or space char-
acters).
[: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 alphanu-
meric 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 sev-
eral 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
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 list of equivalent char-
acters enclosed in [= and =]. Thus, [[=ee`=]] is
regexp that matches either e or 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 charac-
ter classes; they do not recognize collating symbols or
equivalence classes.
The \y, \B, \<, \>, \w, \W, \`, and \' operators are spe-
cific to gawk; they are extensions based on facilities in
the GNU regexp libraries.
The various command line options control how gawk inter-
prets characters in regexps.
No options
In the default case, gawk provide all the facili-
ties 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 reg-
exp 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 opera-
tors, control statements, and input/output statements
available are patterned after those in C.
The operators in AWK, in order of decreasing precedence,
are
(...) Grouping
$ Field reference.
++ -- Increment and decrement, both prefix and post-
fix.
^ 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, other-
wise 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
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
NF, 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 pro-
gram. 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 out-
print expr-list Prints expressions. Each expression
is separated by the value of the OFS
variable. The output record is ter-
minated 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 sys-
tems.)
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 spec-
ification 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
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 let-
ters. 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
the right of the decimal point. For the %g, and %G
formats, it specifies the maximum number of signif-
icant 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, termi-
nated 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 ter-
minated with a newline. The fields are sepa-
rated 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
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 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 in radians.
sqrt(expr) the square root function.
srand([expr]) uses expr as a new seed for the random num-
ber 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 sup-
plied, $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 indi-
sion. 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 substitu-
tions. 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 Pro-
gramming for a fuller discussion
of the rules for &'s and back-
slashes 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
tolower(str) returns a copy of the string str,
with all the upper-case characters
in str translated to their corre-
sponding 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 corre-
sponding upper-case counterparts.
Non-alphabetic characters are left
unchanged.
Time Functions
Since one of the primary uses of AWK programs is process-
ing 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 guar-
anteed 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, cer-
tain escape sequences are recognized, as in C. These are:
\\ A literal backslash.
\a The ``alert'' character; usually the ASCII BEL char-
acter.
\n newline.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\xhex digits
The character represented by the string of hexadeci-
mal digits following the \x. As in ANSI C, all fol-
lowing hexadecimal digits are considered part of the
escape sequence. (This feature should tell us some-
thing 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 reg-
ular 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/.
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 parame-
ters 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 lan-
guage, 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 inter-
vening white space. This is to avoid a syntactic ambigu-
ity 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. Func-
tion parameters used as local variables are initialized to
the null string and the number zero upon function invoca-
tion.
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.
AWK Language Programming, Edition 1.0, published by the
Free Software Foundation, 1995.
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 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 exe-
cution 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, 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 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 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 --tradi-
tional option.
The following features of gawk are not available in POSIX
awk.
o The \x escape sequence. (Disabled with --posix.)
tions.
o The special file names available for I/O redirec-
tion are not recognized.
o The ARGIND, ERRNO, and RT variables are not spe-
cial.
o The IGNORECASE variable and its side-effects are
not available.
o The FIELDWIDTHS variable and fixed-width field
splitting.
o The use of RS as a regular expression.
o The ability to split out individual characters
using the null string as the value of FS, and as
the third argument to split().
o No path search is performed for files named via
the -f option. Therefore the AWKPATH environment
variable is not special.
o The use of nextfile to abandon processing of the
current input file.
o The use of delete array to delete the entire con-
tents 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. 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.
HISTORICAL FEATURES
There are two features of historical AWK implementations
that gawk 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()
standard, and gawk 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. Gawk will sup-
port 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 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 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 differ-
ent 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.
The word ``GNU'' is incorrectly capitalized in at least
one file in the source code.
VERSION INFORMATION
This man page documents gawk, version 3.0.
AUTHORS
The original version of UNIX awk was designed and imple-
mented 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 Founda-
Woods contributed a number of bug fixes. David Trueman,
with contributions from Arnold Robbins, made gawk compati-
ble with the new version of UNIX awk. Arnold Robbins is
the current 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 Darrel
Hankerson. Fred Fish supplied support for the Amiga.
BUG REPORTS
If you find a bug in gawk, 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 sys-
tem 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 refer-
ence 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 unre-
liable way to report bugs. Instead, please use the elec-
tronic mail addresses given above.
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs provided valuable assistance
during testing and debugging. We thank him.
Release 1.1d7 of the Be OS
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