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LINUX Web Hosting Commands and tools awkSYNOPSIS
gawk [ POSIX or GNU style options ] -f program-file [ -- ]
file ...
gawk [ POSIX or GNU style options ] [ -- ] program-text
file ...
pgawk [ POSIX or GNU style options ] -f program-file [ --
] file ...
pgawk [ 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 Laboratories awk exten-
sions, and a number of GNU-specific extensions.
Pgawk is the profiling version of gawk. It is identical
in every way to gawk, except that programs run more
slowly, and it automatically produces an execution profile
in the file awkprof.out when done. See the --profile
option, below.
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 traditional POSIX one letter
options, or GNU style long options. POSIX options start
with a single "-", while long options start with "--".
Long options are provided for both GNU-specific features
and for POSIX-mandated features.
Following the POSIX standard, gawk-specific options are
supplied via arguments to the -W option. Multiple -W
options may be supplied Each -W option has a corresponding
long option, as detailed below. 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, listed alphabetically.
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 Laboratories
research version of UNIX awk. They are ignored by
gawk, since gawk has no pre-defined limits.
-W compat
-W traditional
--compat
--traditional
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 standard output and exit suc-
cessfully.
-W dump-variables[=file]
--dump-variables[=file]
Print a sorted list of global variables, their
types and final values to file. If no file is pro-
vided, gawk uses a file named awkvars.out in the
current directory.
Having a list of all the global variables is a good
way to look for typographical errors in your pro-
grams. You would also use this option if you have
a large program with a lot of functions, and you
want to be sure that your functions don't inadver-
tently use global variables that you meant to be
local. (This is a particularly easy mistake to
make with simple variable names like i, j, and so
on.)
-W help
-W usage
--help
--usage
-W lint-old
--lint-old
Provide warnings about constructs that are not
portable to the original version of Unix awk.
-W gen-po
--gen-po
Scan and parse the AWK program, and generate a GNU
.po format file on standard output with entries for
all localizable strings in the program. The pro-
gram itself is not executed. See the GNU gettext
distribution for more information on .po files.
-W non-decimal-data
--non-decimal-data
Recognize octal and hexadecimal values in input
data. Use this option with great caution!
-W posix
--posix
This turns on compatibility mode, with the follow-
ing 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 You cannot continue lines after ? and :.
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 profile[=prof_file]
--profile[=prof_file]
Send profiling data to prof_file. The default is
awkprof.out. When run with gawk, the profile is
just a "pretty printed" version of the program.
When run with pgawk, the profile contains execution
counts of each statement in the program in the left
margin and function call counts for each user-
defined function.
-W re-interval
--re-interval
Enable the use of interval expressions in regular
expression matching (see Regular Expressions,
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
invalid, 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 reads 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 pro-
grams.
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 inter-
nal form. Then, gawk executes the code in the BEGIN
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
character, that character separates records. Otherwise,
RS is a regular expression. Text in the input that
matches this regular expression separates 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 new-
line character always acts as a field separator, in addi-
tion 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 and/or newlines.
(But see the discussion of --posix, below). NOTE: The
value of IGNORECASE (see below) also affects 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 splits 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.
Fields need not be referenced by constants:
n = 5
print $n
record to be rebuilt when $0 is referenced. Similarly,
assigning a value to $0 causes the record to be resplit,
creating new values for the fields.
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.
BINMODE On non-POSIX systems, specifies use of
"binary" mode for all file I/O. Numeric val-
ues of 1, 2, or 3, specify that input files,
output files, or all files, respectively,
should use binary I/O. String values of "r",
or "w" specify that input files, or output
files, respectively, should use binary I/O.
String values of "rw" or "wr" specify that all
files should use binary I/O. Any other string
value is treated as "rw", but generates a
warning message.
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.
ERRNO If a system error occurs either doing a redi-
rection for getline, during a read for get-
line, or during a close(), then ERRNO will
contain a string describing the error. The
value is subject to translation in non-English
locales.
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.
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 (unless
set by getline).
FNR The input record number in the current input
file.
matches all of the strings "ab", "aB", "Ab",
and "AB". As with all AWK variables, the ini-
tial value of IGNORECASE is zero, so all regu-
lar expression and string operations are nor-
mally case-sensitive. Under Unix, the full
ISO 8859-1 Latin-1 character set is used when
ignoring case.
LINT Provides dynamic control of the --lint option
from within an AWK program. When true, gawk
prints lint warnings. When false, it does not.
When assigned the string value "fatal", lint
warnings become fatal errors, exactly like
--lint=fatal. Any other true value just
prints warnings.
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.
PROCINFO The elements of this array provide access to
information about the running AWK program. On
some systems, there may be elements in the
array, "group1" through "groupn" for some n,
which is the number of supplementary groups
that the process has. Use the in operator to
test for these elements. The following ele-
ments are guaranteed to be available:
PROCINFO["egid"] the value of the getegid(2)
system call.
PROCINFO["euid"] the value of the geteuid(2)
system call.
PROCINFO["FS"] "FS" if field splitting
with FS is in effect, or
"FIELDWIDTHS" if field
splitting with FIELDWIDTHS
is in effect.
PROCINFO["gid"] the value of the getgid(2)
system call.
PROCINFO["pgrpid"] the process group ID of the
current process.
PROCINFO["pid"] the process ID of the cur-
rent process.
PROCINFO["ppid"] the parent process ID of
the current process.
PROCINFO["uid"] the value of the getuid(2)
system call.
RS The input record separator, by default a new-
line.
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
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 conver-
sion is accomplished using strtod(3). A number is con-
verted to a string by using the value of CONVFMT as a for-
mat string for sprintf(3), with the numeric value of the
variable as the argument. However, even though all num-
bers in AWK are floating-point, integral values are always
converted as integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a string value of "12" and not "12.00".
Gawk performs comparisons as follows: If two variables are
numeric, they are compared numerically. If one value is
numeric and the other has a string value that is a
"numeric string," then comparisons are also done 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. Note that the POSIX
standard applies the concept of "numeric string" every-
octal and hexadecimal constants in your AWK program source
code. For example, the octal value 011 is equal to deci-
mal 9, and the hexadecimal value 0x11 is equal to decimal
17.
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 charac-
ter.
\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 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 regular expression constants. Thus, /a\52b/
is equivalent to /a\*b/.
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 is executed for every single record
of input. A missing action is equivalent to
{ print }
which prints the entire record.
Comments begin with the "#" character, and continue until
the end of the line. Blank lines may be used to separate
statements. Normally, a statement ends with a newline,
however, this is not the case for lines ending in a ",",
{, ?, :, &&, or ||. Lines ending in do or else also have
their statements automatically continued on the following
line. In other cases, a line can be continued by ending
it with a "\", in which case the newline will be ignored.
! 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.
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 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....
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.
\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-con-
stituent.
\` 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 valid 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 regular expression
inside the brackets of a character list. Character
classes consist of [:, a keyword denoting the class, and
:]. The character classes defined by the POSIX standard
are:
[: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.)
[: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, and if your
character set collated differently from ASCII, this might
not even match the ASCII alphanumeric characters. With
the POSIX character classes, you can write /[[:alnum:]]/,
and this matches 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
A collating symbol is a multi-character collating
element enclosed in [. and .]. For example, if ch
is a collating element, then [[.ch.]] is a regular
expression that matches this collating element,
while [ch] is a regular expression 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," "`," and
"`." In this case, [[=e=]] is a regular expression
that matches any of e, ', or `.
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 regular expression libraries.
The various command line options control how gawk inter-
prets characters in regular expressions.
No options
In the default case, gawk provide all the facili-
on). Characters described by octal and hexadecimal
escape sequences are treated literally, even if
they represent regular expression metacharacters.
--re-interval
Allow interval expressions in regular expressions,
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 oper-
ators, 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.
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 [, how]) Close file, pipe or co-process. The
optional how should only be used
when closing one end of a two-way
pipe to a co-process. It must be a
string value, either "to" or "from".
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.
command | getline [var]
Run command piping the output either
into $0 or var, as above.
command |& getline [var]
Run command as a co-process piping
the output either into $0 or var, as
above. Co-processes are a gawk
extension.
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
put 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 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.
Additional output redirections are allowed for print and
printf.
print ... >> file
appends output to the file.
print ... | command
writes on a pipe.
print ... |& command
sends data to a co-process.
The getline command returns 0 on end of file and -1 on an
error. Upon an error, ERRNO contains a string describing
the problem.
NOTE: If using a pipe or co-process to getline, or from
print or printf within a loop, you must use close() to
create new instances of the command. AWK does not auto-
%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 for-
mat 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.
Optional, additional parameters may lie between the % and
the control letter:
count$ Use the count'th argument at this point in the for-
matting. This is called a positional specifier and
is intended primarily for use in translated ver-
sions of format strings, not in the original text
of an AWK program. It is a gawk extension.
- 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 always contains a
decimal point. For %g, and %G, trailing zeros are
specifies the number of digits you want printed to
the right of the decimal point. For the %g, and %G
formats, it specifies the maximum number of signif-
icant digits. For the %d, %o, %i, %u, %x, and %X
formats, it specifies the minimum number of digits
to print. For %s, 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 causes their values to be
taken from the argument list to printf or sprintf(). To
use a positional specifier with a dynamic width or preci-
sion, supply the count$ after the * in the format string.
For example, "%3$*2$.*1$s".
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). These file
names may also be used on the command line to name data
files. The filenames are:
/dev/stdin The standard input.
/dev/stdout The standard output.
/dev/stderr The standard error output.
/dev/fd/n The file associated with the open file
descriptor n.
These are particularly useful for error messages. For
example:
print "You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print "You blew it!" | "cat 1>&2"
The following special filenames may be used with the |&
co-process operator for creating TCP/IP network connec-
tions.
/inet/tcp/lport/rhost/rport File for TCP/IP connection on
local port lport to remote
host rhost on remote port
rport. Use a port of 0 to
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.
Numeric Functions
AWK has the following built-in arithmetic functions:
atan2(y, x) Returns the arctangent of y/x in radians.
cos(expr) Returns the cosine of expr, which is in
radians.
exp(expr) The exponential function.
int(expr) Truncates to integer.
log(expr) The natural logarithm function.
rand() Returns a random number between 0 and 1.
sin(expr) Returns the sine of expr, which is in radi-
ans.
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 is used. The return value is
the previous seed for the random number gen-
erator.
String Functions
Gawk has the following built-in 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 t is not 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-
cate just the text that matched
the n'th parenthesized subexpres-
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 &. (This must be typed as
"\\&"; see GAWK: Effective AWK
Programming for a fuller discus-
sion of the rules for &'s and
backslashes in the replacement
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
supplied.
match(s, r [, a]) 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.
Note that the argument order is
the same as for the ~ operator:
str ~ re. If array a is provided,
Splitting behaves identically to
field splitting, described above.
sprintf(fmt, expr-list) Prints expr-list according to fmt,
and returns the resulting string.
strtonum(str) Examines str, and returns its
numeric value. If str begins with
a leading 0, strtonum() assumes
that str is an octal number. If
str begins with a leading 0x or
0X, strtonum() assumes that str is
a hexadecimal number.
sub(r, s [, t]) Just like gsub(), but only the
first matching substring is
replaced.
substr(s, i [, n]) Returns the at most n-character
substring of s starting at i. If
n is omitted, the rest of s is
used.
tolower(str) Returns a copy of the string str,
with all the upper-case characters
in str translated to their 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 functions for obtaining time stamps
and formatting them.
mktime(datespec)
Rurns datespec into a time stamp of the same
form as returned by systime(). The datespec is
a string of the form YYYY MM DD HH MM SS[ DST].
The contents of the string are six or seven num-
bers representing respectively the full year
including century, the month from 1 to 12, the
day of the month from 1 to 31, the hour of the
fied time. If datespec does not contain enough
elements or if the resulting time is out of
range, mktime() returns -1.
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) is used. See the spec-
ification for the strftime() function in ANSI C
for the format conversions that are guaranteed
to be available. A public-domain version of
strftime(3) and a man page for it come with
gawk; if that version was used to build gawk,
then all of the conversions described in that
man page are available to gawk.
systime() Returns the current time of day as the number of
seconds since the Epoch (1970-01-01 00:00:00 UTC
on POSIX systems).
Bit Manipulations Functions
Starting with version 3.1 of gawk, the following bit
manipulation functions are available. They work by con-
verting double-precision floating point values to unsigned
long integers, doing the operation, and then converting
the result back to floating point. The functions are:
and(v1, v2) Return the bitwise AND of the values
provided by v1 and v2.
compl(val) Return the bitwise complement of val.
lshift(val, count) Return the value of val, shifted left
by count bits.
or(v1, v2) Return the bitwise OR of the values
provided by v1 and v2.
rshift(val, count) Return the value of val, shifted right
by count bits.
xor(v1, v2) Return the bitwise XOR of the values
provided by v1 and v2.
Internationalization Functions
Starting with version 3.1 of gawk, the following functions
may be used from within your AWK program for translating
strings at run-time. For full details, see GAWK: Effec-
Returns the translation of string in text domain
domain for locale category category. The default
value for domain is the current value of TEXTDO-
MAIN. The default value for category is "LC_MES-
SAGES".
If you supply a value for category, it must be a
string equal to one of the known locale categories
described in GAWK: Effective AWK Programming. You
must also supply a text domain. Use TEXTDOMAIN if
you want to use the current domain.
USER-DEFINED 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 and 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.
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.
extension(object, function)
Dynamically link the shared object file named by
object, and invoke function in that object, to
perform initialization. These should both be pro-
vided as strings. Returns the value returned by
function.
This function is provided and documented in GAWK: Effec-
tive AWK Programming, but everything about this feature is
likely to change in the next release. We STRONGLY recom-
mend that you do not use this feature for anything that
you aren't willing to redo.
SIGNALS
pgawk accepts two signals. SIGUSR1 causes it to dump a
profile and function call stack to the profile file, which
is either awkprof.out, or whatever file was named with the
--profile option. It then continues to run. SIGHUP
causes it to dump the profile and function call stack and
then exit.
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 }
INTERNATIONALIZATION
String constants are sequences of characters enclosed in
double quotes. In non-English speaking environments, it
is possible to mark strings in the AWK program as requir-
ing translation to the native natural language. Such
strings are marked in the AWK program with a leading
underscore ("_"). For example,
gawk 'BEGIN { print "hello, world" }'
always prints hello, world. But,
This allows gawk to find the .mo file associated with
your program. Without this step, gawk uses the mes-
sages text domain, which likely does not contain
translations for your program.
2. Mark all strings that should be translated with lead-
ing underscores.
3. If necessary, use the dcgettext() and/or bindtextdo-
main() functions in your program, as appropriate.
4. Run gawk --gen-po -f myprog.awk > myprog.po to gener-
ate a .po file for your program.
5. Provide appropriate translations, and build and
install a corresponding .mo file.
The internationalization features are described in full
detail in GAWK: Effective AWK Programming.
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 Laboratories version of awk, and are in
the POSIX standard.
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. How-
ever, in earlier implementations, when such an assignment
appeared before any file names, the assignment would hap-
pen before the BEGIN block was run. Applications came to
depend on this "feature." When awk was changed to match
its documentation, the -v option for assigning variables
before program execution was added to accommodate applica-
tions that depended upon the old behavior. (This feature
was agreed upon by both the Bell Laboratories and the 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 warns about but otherwise ignores 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
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 issues 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 supports
this usage if --traditional has been specified.
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 No path search is performed for files named via the -f
option. Therefore the AWKPATH environment variable is
not special.
o The \x escape sequence. (Disabled with --posix.)
o The fflush() function. (Disabled with --posix.)
o The ability to continue lines after ? and :. (Disabled
with --posix.)
o Octal and hexadecimal constants in AWK programs.
o The ARGIND, BINMODE, ERRNO, LINT, RT and TEXTDOMAIN
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 PROCINFO array is not available.
o The optional third argument to the match() function.
o The ability to use positional specifiers with printf and
sprintf().
o The use of delete array to delete the entire contents of
an array.
o The use of nextfile to abandon processing of the current
input file.
o The and(), asort(), bindtextdomain(), compl(), dcget-
text(), gensub(), lshift(), mktime(), or(), rshift(),
strftime(), strtonum(), systime() and xor() functions.
o Localizable strings.
o Adding new built-in functions dynamically with the
extension() function.
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 an output file or
pipe, respectively. It returns the process's exit status
when closing an input pipe. The return value is -1 if the
named file, pipe or co-process was not opened with a redi-
rection.
When gawk is invoked with the --traditional option, if the
fs argument to the -F option is "t", then FS is 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 single quotes: gawk -F'\t' ....
ENVIRONMENT VARIABLES
The AWKPATH environment variable can be used to provide a
list of directories that gawk searches when looking for
files named via the -f and --file options.
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 issues a
warning message to this effect.
SEE ALSO
egrep(1), getpid(2), getppid(2), getpgrp(2), getuid(2),
geteuid(2), getgid(2), getegid(2), getgroups(2)
message. Such programs are surprisingly difficult to
diagnose in the completely general case, and the effort to
do so really is not worth it.
AUTHORS
The original version of UNIX awk was designed and imple-
mented by Alfred Aho, Peter Weinberger, and Brian
Kernighan of Bell Laboratories. 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 ver-
sion of awk distributed in Seventh Edition UNIX. John
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,
Stephen Davies provided the Tandem port, and Martin Brown
provided the BeOS port.
VERSION INFORMATION
This man page documents gawk, version 3.1.0.
BUG REPORTS
If you find a bug in gawk, please send electronic mail to
bug-gawk@gnu.org. Please include your operating system
and its revision, the version of gawk (from gawk --ver-
sion), what C compiler you used to compile it, and a test
program and data that are as small as possible for repro-
ducing 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.
tim copying, provided that the entire resulting derived
work is distributed under the terms of a permission notice
identical to this one.
Permission is granted to copy and distribute translations
of this manual page into another language, under the above
conditions for modified versions, except that this permis-
sion notice may be stated in a translation approved by the
Foundation.
Free Software Foundation May 29 2001 GAWK(1)
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