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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 program-
ming language. 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 sup-
plied via arguments to the -W option. Multiple -W options
may be supplied Each -W option has a corresponding long
option, as detailed below. Arguments to long options are
either joined with the option by an = sign, with no inter-
vening 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 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. Mul-
tiple -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 --trad-
itional is preferred over the other forms of this
option. See GNU EXTENSIONS, below, for more informa-
tion.
-W copyleft
-W copyright
--copyleft
--copyright
Print the short version of the GNU copyright informa-
tion message on the standard output, and exits success-
fully.
-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-portable to other AWK implementations.
-W lint-old
--lint-old
Provide warnings about constructs that are not portable
to the original version of Unix awk.
-W posix
--posix
This turns on compatibility mode, with the following
additional restrictions:
o \x escape sequences are not recognized.
o Only space and tab act as field separators when FS is
set to a single space, newline does not.
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 traditionally available
in the AWK language. The POSIX standard added them, to
make awk and egrep consistent with each other. How-
ever, 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 Standards, 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 process-
ing. 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 con-
catenated 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 compiles 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 values to the vari-
ables AWK uses to control how input is broken into fields
and records. It is also useful for controlling 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 executed.
The patterns are tested in the order they occur in the pro-
gram.
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 mul-
tiple 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 regu-
lar 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 newline character always acts
as a field separator, in addition to whatever value FS may
have.
Fields
As each input record is read, gawk splits the record into
fields, using the value of the FS variable as the field
separator. If FS is a single character, fields are
separated by that character. If FS is the null string, then
each individual character becomes a separate field. Other-
wise, FS is expected to be a full regular expression. In
the special case that FS is a single space, fields are
separated by runs of spaces and/or tabs and/or newlines.
(But see the discussion of --posix, below). Note that the
value of IGNORECASE (see below) will also affect how fields
are split when FS is a regular expression, and how records
are separated when RS is a regular expression.
If the FIELDWIDTHS variable is set to a space separated list
of numbers, each field is expected to have fixed width, and
gawk will split up the record using the specified widths.
The value of FS is ignored. Assigning a new value to FS
overrides the use of FIELDWIDTHS, and restores the default
behavior.
Each field in the input record may be referenced by its
position, $1, $2, and so on. $0 is the whole record. The
value of a field may be assigned to as well. Fields need
not be referenced by constants:
n = 5
print $n
prints the fifth field in the input record. The variable NF
is set to the total number of fields in the input record.
References to non-existent fields (i.e. fields after $NF)
produce the null-string. However, assigning to a non-
existent field (e.g., $(NF+2) = 5) will increase the value
of NF, create any intervening fields with the null string as
their value, and cause the value of $0 to be recomputed,
with the fields being separated by the value of OFS. Refer-
ences 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 pro-
cessed.
ARGV Array of command line arguments. The array is
indexed from 0 to ARGC - 1. Dynamically chang-
ing the contents of ARGV can control the files
used for data.
CONVFMT The conversion format for numbers, "%.6g", by
default.
ENVIRON An array containing the values of the current
environment. The array is indexed by the
environment variables, each element being the
value of that variable (e.g., ENVIRON["HOME"]
might be /home/arnold). Changing this array
does not affect the environment seen by programs
which gawk spawns via redirection or the sys-
tem() function. (This may change in a future
version of gawk.)
ERRNO If a system error occurs either doing a redirec-
tion for getline, during a read for getline, or
during a close(), then ERRNO will contain a
string describing the error.
FIELDWIDTHS A white-space separated list of fieldwidths.
When set, gawk parses the input into fields of
fixed width, instead of using the value of the
FS variable as the field separator. The fixed
field width facility is still experimental; the
semantics may change as gawk evolves over time.
FILENAME The name of the current input file. If no files
are specified on the command line, the value of
FILENAME is "-". However, FILENAME is undefined
inside the BEGIN block.
FNR The input record number in the current input
file.
FS The input field separator, a space by default.
See Fields, above.
IGNORECASE Controls the case-sensitivity of all regular
expression and string operations. If IGNORECASE
has a non-zero value, then string comparisons
and pattern matching in rules, field splitting
with FS, record separating with RS, regular
expression matching with ~ and !~, and the gen-
sub(), gsub(), index(), match(), split(), and
sub() pre-defined functions will all ignore case
when doing regular expression operations. Thus,
if IGNORECASE is not equal to zero, /aB/ matches
all of the strings "ab", "aB", "Ab", and "AB".
As with all AWK variables, the initial value of
IGNORECASE is zero, so all regular expression
and string operations are normally case-
sensitive. Under Unix, the full ISO 8859-1
Latin-1 character set is used when ignoring
case. NOTE: In versions of gawk prior to 3.0,
IGNORECASE only affected regular expression
operations. It now affects string comparisons
as well.
NF The number of fields in the current input
record.
NR The total number of input records seen so far.
OFMT The output format for numbers, "%.6g", by
default.
OFS The output field separator, a space by default.
ORS The output record separator, by default a 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 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 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 con-
sisting 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 associative, i.e. indexed by string
values.
The special operator in may be used in an if or while state-
ment to see if an array has an index consisting of a partic-
ular 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 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 conversion
is accomplished using atof(3). A number is converted to a
string by using the value of CONVFMT as a format string for
sprintf(3), with the numeric value of the variable as the
argument. However, even though all numbers in AWK are
floating-point, integral values are always converted as
integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a string value of "12" and not "12.00".
Gawk performs comparisons as follows: If two variables are
numeric, they are compared numerically. If one value is
numeric and the other has a string value that is a "numeric
string," then comparisons are also done numerically. Other-
wise, 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 com-
parison 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 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, how-
ever, 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 writ-
ten 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 expres-
sions. BEGIN and END patterns cannot have missing action
parts.
For /regular expression/ patterns, the associated statement
is executed for each input record that matches the regular
expression. Regular expressions are the same as those in
egrep(1), and are summarized below.
A relational expression may use any of the operators defined
below in the section on actions. These generally test
whether certain fields match certain regular expressions.
The &&, ||, and ! operators are logical AND, logical OR, and
logical NOT, respectively, as in C. They do short-circuit
evaluation, also as in C, and are used for combining more
primitive pattern expressions. As in most languages,
parentheses may be used to change the order of evaluation.
The ?: operator is like the same operator in C. If the
first pattern is true then the pattern used for testing is
the second pattern, otherwise it is the third. Only one of
the second and third patterns is evaluated.
The pattern1, pattern2 form of an expression is called a
range pattern. It matches all input records starting with a
record that matches pattern1, and continuing until a record
that matches pattern2, inclusive. It does not combine with
any other sort of pattern expression.
Regular Expressions
Regular expressions are the extended kind found in egrep.
They are composed of characters as follows:
c matches the non-metacharacter c.
\c matches the literal character c.
. matches any character including newline.
^ matches the beginning of a string.
$ matches the end of a string.
[abc...] character list, matches any of the characters
abc....
[^abc...] negated character list, matches any character
except abc....
r1|r2 alternation: matches either r1 or r2.
r1r2 concatenation: matches r1, and then r2.
r+ matches one or more r's.
r* matches zero or more r's.
r? matches zero or one r's.
(r) grouping: matches r.
r{n}
r{n,}
r{n,m} One or two numbers inside braces denote an inter-
val expression. If there is one number in the
braces, the preceding regexp r is repeated n
times. If there are two numbers separated by a
comma, r is repeated n to m times. If there is
one number followed by a comma, then r is
repeated at least n times.
Interval expressions are only available if either
--posix or --re-interval is specified on the com-
mand line.
\y matches the empty string at either the beginning
or the end of a word.
\B matches the empty string within a word.
\< matches the empty string at the beginning of a
word.
\> matches the empty string at the end of a word.
\w matches any word-constituent character (letter,
digit, or underscore).
\W matches any character that is not word-
constituent.
\` matches the empty string at the beginning of a
buffer (string).
\' matches the empty string at the end of a buffer.
The escape sequences that are valid in string constants (see
below) are also legal in regular expressions.
Character classes are a new feature introduced in the POSIX
standard. A character class is a special notation for
describing lists of characters that have a specific attri-
bute, but where the actual characters themselves can vary
from country to country and/or from character set to charac-
ter set. For example, the notion of what is an alphabetic
character differs in the USA and in France.
A character class is only valid in a regexp inside the
brackets of a character list. Character classes consist of
[:, a keyword denoting the class, and :]. Here are the
character classes defined by the POSIX standard.
[:alnum:]
Alphanumeric characters.
[:alpha:]
Alphabetic characters.
[:blank:]
Space or tab characters.
[:cntrl:]
Control characters.
[:digit:]
Numeric characters.
[:graph:]
Characters that are both printable and visible. (A
space is printable, but not visible, while an a is
both.)
[:lower:]
Lower-case alphabetic characters.
[:print:]
Printable characters (characters that are not control
characters.)
[:punct:]
Punctuation characters (characters that are not letter,
digits, control characters, or space characters).
[:space:]
Space characters (such as space, tab, and formfeed, to
name a few).
[:upper:]
Upper-case alphabetic characters.
[:xdigit:]
Characters that are hexadecimal digits.
For example, before the POSIX standard, to match
alphanumeric characters, you would have had to write
/[A-Za-z0-9]/. If your character set had other alphabetic
characters in it, this would not match them. With the POSIX
character classes, you can write /[[:alnum:]]/, and this
will match all the alphabetic and numeric characters in your
character set.
Two additional special sequences can appear in character
lists. These apply to non-ASCII character sets, which can
have single symbols (called collating elements) that are
represented with more than one character, as well as several
characters that are equivalent for collating, or sorting,
purposes. (E.g., in French, a plain "e" and a grave-
accented e` are equivalent.)
Collating Symbols
A collating symbols is a multi-character collating ele-
ment enclosed in [. and .]. For example, if ch is a
collating element, then [[.ch.]] is a regexp that
matches this collating element, while [ch] is a regexp
that matches either c or h.
Equivalence Classes
An equivalence class is a locale-specific name for a
list of characters that are equivalent. The name is
enclosed in [= and =]. For example, the name e might
be used to represent all of "e," "e`," and "e`." In
this case, [[=e]] is a regexp that matches any of
.BR e ,
.BR e' , or
.BR e` .
These features are very valuable in non-English speaking
locales. The library functions that gawk uses for regular
expression matching currently only recognize POSIX character
classes; they do not recognize collating symbols or
equivalence classes.
The \y, \B, \<, \>, \w, \W, \`, and \' operators are
specific to gawk; they are extensions based on facilities in
the GNU regexp libraries.
The various command line options control how gawk interprets
characters in regexps.
No options
In the default case, gawk provide all the facilities of
POSIX regexps and the GNU regexp operators described
above. However, interval expressions are not sup-
ported.
--posix
Only POSIX regexps are supported, the GNU operators are
not special. (E.g., \w matches a literal w). Interval
expressions are allowed.
--traditional
Traditional Unix awk regexps are matched. The GNU
operators are not special, interval expressions are not
available, and neither are the POSIX character classes
([[:alnum:]] and so on). Characters described by
octal and hexadecimal escape sequences are treated
literally, even if they represent regexp metacharac-
ters.
--re-interval
Allow interval expressions in regexps, even if --tradi-
tional 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 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, 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 exe-
cuted.
nextfile Stop processing the current input
file. The next input record read
comes from the next input file.
FILENAME and ARGIND are updated, FNR
is reset to 1, and processing starts
over with the first pattern in the AWK
program. If the end of the input data
is reached, the END block(s), if any,
are executed. NOTE: Earlier versions
of gawk used next file, as two words.
While this usage is still recognized,
it generates a warning message and
will eventually be removed.
print Prints the current record. The output
record is terminated with the value of
the ORS variable.
print expr-list Prints expressions. Each expression
is separated by the value of the OFS
variable. The output record is 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 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() func-
tion (see below) accept the following conversion specifica-
tion formats:
%c An ASCII character. If the argument used for %c is
numeric, it is treated as a character and printed.
Otherwise, the argument is assumed to be a string, and
the only first character of that string is printed.
%d
%i A decimal number (the integer part).
%e
%E A floating point number of the form [-]d.dddddde[+-]dd.
The %E format uses E instead of e.
%f A floating point number of the form [-]ddd.dddddd.
%g
%G Use %e or %f conversion, whichever is shorter, with
nonsignificant zeros suppressed. The %G format uses %E
instead of %e.
%o An unsigned octal number (also an integer).
%u An unsigned decimal number (again, an integer).
%s A character string.
%x
%X An unsigned hexadecimal number (an integer). The %X
format uses ABCDEF instead of abcdef.
%% A single % character; no argument is converted.
There are optional, additional parameters that may lie
between the % and the control letter:
- The expression should be left-justified within its
field.
space
For numeric conversions, prefix positive values with a
space, and negative values with a minus sign.
+ The plus sign, used before the width modifier (see
below), says to always supply a sign for numeric
conversions, even if the data to be formatted is posi-
tive. 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 out-
put 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 speci-
fies 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 pro-
cess (usually the shell). Other special filenames provide
access to information about the running gawk process. The
filenames are:
/dev/pid Reading this file returns the process ID of the
current process, in decimal, terminated with a
newline.
/dev/ppid Reading this file returns the parent process ID
of the current process, in decimal, terminated
with a newline.
/dev/pgrpid Reading this file returns the process group ID
of the current process, in decimal, terminated
with a newline.
/dev/user Reading this file returns a single record ter-
minated 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 descrip-
tor n.
These are particularly useful for error messages. For exam-
ple:
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 radi-
ans.
exp(expr) the exponential function.
int(expr) truncates to integer.
log(expr) the natural logarithm function.
rand() returns a random number between 0 and 1.
sin(expr) returns the sine of expr, which is in radians.
sqrt(expr) the square root function.
srand([expr]) uses expr as a new seed for the random number
generator. If no expr is provided, the time
of day will be used. The return value is the
previous seed for the random number generator.
String Functions
Gawk has the following pre-defined string functions:
gensub(r, s, h [, t]) search the target string t for
matches of the regular expression r.
If h is a string beginning with g or
G, then replace all matches of r
with s. Otherwise, h is a number
indicating which match of r to
replace. If no t is supplied, $0 is
used instead. Within the 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 tar-
get string is not changed.
gsub(r, s [, t]) for each substring matching the reg-
ular expression r in the string t,
substitute the string s, and return
the number of substitutions. If t
is not supplied, use $0. An & in
the replacement text is replaced
with the text that was actually
matched. Use \& to get a literal &.
See Effective AWK Programming for a
fuller discussion of the rules for
&'s and backslashes in the replace-
ment text of sub(), gsub(), and gen-
sub().
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 sup-
plied.
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 split-
ting, 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 sub-
string 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 counter-
parts. 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 counter-
parts. Non-alphabetic characters
are left unchanged.
Time Functions
Since one of the primary uses of AWK programs is processing
log files that contain time stamp information, gawk provides
the following two functions for obtaining time stamps and
formatting them.
systime() returns the current time of day as the number of
seconds since the Epoch (Midnight UTC, January 1,
1970 on POSIX systems).
strftime([format [, timestamp]])
formats timestamp according to the specification
in format. The timestamp should be of the same
form as returned by systime(). If timestamp is
missing, the current time of day is used. If for-
mat is missing, a default format equivalent to the
output of date(1) will be used. See the specifi-
cation for the strftime() function in ANSI C for
the format conversions that are guaranteed to be
available. A public-domain version of strftime(3)
and a man page for it come with gawk; if that ver-
sion was used to build gawk, then all of the
conversions described in that man page are avail-
able to gawk.
String Constants
String constants in AWK are sequences of characters enclosed
between double quotes ("). Within strings, certain escape
sequences are recognized, as in C. These are:
\\ A literal backslash.
\a The "alert" character; usually the ASCII BEL character.
\b backspace.
\f form-feed.
\n newline.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\xhex digits
The character represented by the string of hexadecimal
digits following the \x. As in ANSI C, all following
hexadecimal digits are considered part of the escape
sequence. (This feature should tell us something about
language design by committee.) E.g., "\x1B" is the
ASCII ESC (escape) character.
\ddd The character represented by the 1-, 2-, or 3-digit
sequence of octal digits. E.g., "\033" is the ASCII
ESC (escape) character.
\c The literal character c.
The escape sequences may also be used inside constant regu-
lar 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
language, the provision for local variables is rather
clumsy: They are declared as extra parameters in the parame-
ter 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 interven-
ing 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. Func-
tion parameters used as local variables are initialized to
the null string and the number zero upon function invoca-
tion.
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 "falling 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. Ker-
nighan, 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 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 execu-
tion 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 accommo-
date 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 --traditional
option.
The following features of gawk are not available in POSIX
awk.
o The \x escape sequence. (Disabled with --posix.)
o The fflush() function. (Disabled with --posix.)
o The systime(), strftime(), and gensub() functions.
o The special file names available for I/O redirection
are not recognized.
o The ARGIND, ERRNO, and RT variables are not special.
o The IGNORECASE variable and its side-effects are not
available.
o The FIELDWIDTHS variable and fixed-width field split-
ting.
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 vari-
able 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 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 supports. First, it is possible to call the
length() built-in function not only with no argument, but
even without parentheses! Thus,
a = length # Holy Algol 60, Batman!
is the same as either of
a = length()
a = length($0)
This feature is marked as "deprecated" in the POSIX stan-
dard, 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 com-
mand 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 vari-
able assignment feature; it remains only for backwards com-
patibility.
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 mes-
sage. 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 3.0.5.
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-
tion, wrote gawk, to be compatible with the original version
of awk distributed in Seventh Edition UNIX. John Woods con-
tributed a number of bug fixes. David Trueman, with contri-
butions from Arnold Robbins, made gawk compatible with the
new version of UNIX awk. Arnold Robbins is the current
maintainer.
The initial DOS port was done by Conrad Kwok and Scott Gar-
finkle. 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@gnu.org, with a carbon copy to arnold@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 testing and debugging. We thank him.
COPYING PERMISSIONS
Copyright O 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 ver-
sions of this manual page under the conditions for verbatim
copying, provided that the entire resulting derived work is
distributed under the terms of a permission notice identical
to this one.
Permission is granted to copy and distribute translations of
this manual page into another language, under the above con-
ditions for modified versions, except that this permission
notice may be stated in a translation approved by the Foun-
dation.
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