AWK(1) Utility Commands AWK(1)NAMEawk - pattern scanning and processing language
SYNOPSISawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
awk [ POSIX or GNU style options ] [ -- ] program-text file ...
DESCRIPTION
Gawk is the GNU Project's implementation of the AWK programming lan‐
guage. In the 4.4BSD distribution, it is installed as awk. It con‐
forms 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 defined in the System V
Release 4 version of UNIX awk. Gawk also provides 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.
OPTIONS
Gawk options may be either the traditional POSIX one letter options, or
the GNU style long options. POSIX style options start with a single
``-'', while GNU long options start with ``--''. GNU style long
options are provided for both GNU-specific features and for POSIX man‐
dated features. Other implementations of the AWK language are likely
to only accept the traditional one letter options.
Following the POSIX standard, gawk-specific options are supplied via
arguments to the -W option. Multiple -W options may be supplied, or
multiple arguments may be supplied together if they are separated by
commas, or enclosed in quotes and separated by white space. Case is
ignored in arguments to the -W option. Each -W option has a corre‐
sponding GNU style long option, as detailed below.
Gawk accepts the following options.
-F fs
--field-separator=fs
Use fs for the input field separator (the value of the FS prede‐
fined variable).
-v var=val
--assign=var=val
Assign the value val, to the variable var, before execution of
the program begins. Such variable values are available to the
BEGIN block of an AWK program.
-f program-file
--file=program-file
Read the AWK program source from the file program-file, instead
of from the first command line argument. Multiple -f (or
--file) options may be used.
-W compat
--compat Run in compatibility mode. In compatibility mode, gawk
behaves identically to UNIX awk; none of the GNU-specific
extensions are recognized. See GNU EXTENSIONS, below, for
more information.
-W copyleft
-W copyright
--copyleft
--copyright Print the short version of the GNU copyright information
message on the standard error output.
-W help
-W usage
--help
--usage Print a relatively short summary of the available options
on the standard error output.
-W lint
--lint Provide warnings about constructs that are dubious or non-
portable to other AWK implementations.
-W posix
--posix This turns on compatibility mode, with the following addi‐
tional restrictions:
· \x escape sequences are not recognized.
· The synonym func for the keyword function is not recog‐
nized.
· The operators ** and **= cannot be used in place of ^ and
^=.
-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
size AWK programs used in shell scripts.
The -W source= form of this option uses the rest of the
command line argument for program-text; no other options to
-W will be recognized in the same argument.
-W version
--version Print version information for this particular copy of gawk
on the standard error output. This is useful mainly for
knowing if the current copy of gawk on your system is up to
date with respect to whatever the Free Software Foundation
is distributing.
-- 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.
Any other options are flagged as illegal, but are otherwise ignored.
AWK PROGRAM EXECUTION
An AWK program consists of a sequence of pattern-action statements and
optional function definitions.
pattern { action statements }
function name(parameter list) { statements }
Gawk first reads the program source from the program-file(s) if speci‐
fied, or from the first non-option argument on the command line. The
-f option may be used multiple times on the command line. Gawk will
read the program text as if all the program-files had been concatenated
together. This is useful for building libraries of AWK functions,
without having to include them in each new AWK program that uses them.
To use a library function in a file from a program typed in on the com‐
mand line, specify /dev/tty as one of the program-files, type your pro‐
gram, and end it with a ^D (control-d).
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/lib/awk:/usr/local/lib/awk". 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, gawk com‐
piles the program into an internal form. Next, all variable assign‐
ments specified via the -v option are performed. 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 com‐
mand 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 variables 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 line in the input, gawk tests to see if it matches any pattern
in the AWK program. For each pattern that the line matches, the asso‐
ciated action is executed. The patterns are tested in the order they
occur in the program.
Finally, after all the input is exhausted, gawk executes the code in
the END block(s) (if any).
VARIABLES 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; multiply dimensioned arrays may be simulated. Several pre-
defined variables are set as a program runs; these will be described as
needed and summarized below.
Fields
As each input line is read, gawk splits the line 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. Otherwise, FS is
expected to be a full regular expression. In the special case that FS
is a single blank, fields are separated by runs of blanks and/or tabs.
Note that the value of IGNORECASE (see below) will also affect how
fields are split when FS is a regular expression.
If the FIELDWIDTHS variable is set to a space separated list of num‐
bers, each field is expected to have fixed width, and gawk will split
up the record using the specified widths. The value of FS is ignored.
Assigning a new value to FS overrides the use of FIELDWIDTHS, and
restores the default behavior.
Each field in the input line may be referenced by its position, $1, $2,
and so on. $0 is the whole line. 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 line. The variable NF is set to
the total number of fields in the input line.
References to non-existent fields (i.e., fields after $NF) produce the
null-string. However, assigning to a non-existent field (e.g., $(NF+2)
= 5) will increase the value of NF, create any intervening fields with
the null string as their value, and cause the value of $0 to be recom‐
puted, with the fields being separated by the value of OFS.
Built-in Variables
AWK's built-in variables are:
ARGC The number of command line arguments (does not include
options to gawk, or the program source).
ARGIND The index in ARGV of the current file being processed.
ARGV Array of command line arguments. The array is indexed from
0 to ARGC - 1. Dynamically changing the contents of ARGV
can control the files used for data.
CONVFMT The conversion format for numbers, "%.6g", by default.
ENVIRON An array containing the values of the current environment.
The array is indexed by the environment variables, each
element being the value of that variable (e.g., ENVI‐
RON["HOME"] might be /u/arnold). Changing this array does
not affect the environment seen by programs which gawk
spawns via redirection or the system() function. (This may
change in a future version of gawk.)
ERRNO If a system error occurs either doing a redirection for
getline, during a read for getline, or during a close, then
ERRNO will contain a string describing the error.
FIELDWIDTHS A white-space separated list of fieldwidths. When set,
gawk parses the input into fields of fixed width, instead
of using the value of the FS variable as the field separa‐
tor. The fixed field width facility is still experimental;
expect the semantics to change as gawk evolves over time.
FILENAME The name of the current input file. If no files are speci‐
fied on the command line, the value of FILENAME is ``-''.
FNR The input record number in the current input file.
FS The input field separator, a blank by default.
IGNORECASE Controls the case-sensitivity of all regular expression
operations. If IGNORECASE has a non-zero value, then pat‐
tern matching in rules, field splitting with FS, regular
expression matching with ~ and !~, and the 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 operations are normally case-sensitive.
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 blank by default.
ORS The output record separator, a newline by default.
RS The input record separator, a newline by default. RS is
exceptional in that only the first character of its string
value is used for separating records. (This will probably
change in a future release of gawk.) If RS is set to the
null string, then records are separated by blank lines.
When RS is set to the null string, then the newline charac‐
ter always acts as a field separator, in addition to what‐
ever value FS may have.
RSTART The index of the first character matched by match(); 0 if
no match.
RLENGTH The length of the string matched by match(); -1 if no
match.
SUBSEP The character used to separate multiple subscripts in array
elements, "\034" by default.
Arrays
Arrays are subscripted with an expression between square brackets ([
and ]). If the expression is an expression list (expr, expr ...) then
the array subscript is a string consisting of the concatenation of the
(string) value of each expression, separated by the value of the SUBSEP
variable. This facility is used to simulate multiply dimensioned
arrays. For example:
i = "A" ; j = "B" ; k = "C"
x[i, j, k] = "hello, world\n"
assigns the string "hello, world\n" to the element of the array x which
is indexed by the string "A\034B\034C". All arrays in AWK are associa‐
tive, i.e., indexed by string values.
The special operator in may be used in an if or while statement to see
if an array has an index consisting of a particular value.
if (val in array)
print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct may also be used in a for loop to iterate over all the
elements of an array.
An element may be deleted from an array using the delete statement.
Variable Typing And Conversion
Variables and fields may be (floating point) numbers, or strings, or
both. How the value of a variable is interpreted depends upon its con‐
text. If used in a numeric expression, it will be treated as a number,
if used as a string it will be treated as a string.
To force a variable to be treated as a number, add 0 to it; to force it
to be treated as a string, concatenate it with the null string.
When a string must be converted to a number, the conversion is accom‐
plished using atof(3). A number is converted to a string by using the
value of CONVFMT as a format string for sprintf(3), with the numeric
value of the variable as the argument. However, even though all num‐
bers in AWK are floating-point, integral values are always converted as
integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a value of "12" and not "12.00".
Gawk performs comparisons as follows: If two variables are numeric,
they are compared numerically. If one value is numeric and the other
has a string value that is a ``numeric string,'' then comparisons are
also done numerically. Otherwise, the numeric value is converted to a
string and a string comparison is performed. Two strings are compared,
of course, as strings. According to the POSIX standard, even if two
strings are numeric strings, a numeric comparison is performed. How‐
ever, this is clearly incorrect, and gawk does not do this.
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
line of input. A missing action is equivalent to
{ print }
which prints the entire line.
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 end‐
ing in do or else also have their statements automatically continued on
the following line. In other cases, a line can be continued by ending
it with a ``\'', in which case the newline will be ignored.
Multiple statements may be put on one line by separating them with a
``;''. This applies to both the statements within the action part of a
pattern-action pair (the usual case), and to the pattern-action state‐
ments themselves.
Patterns
AWK patterns may be one of the following:
BEGIN
END
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN and END are two special kinds of patterns which are not tested
against the input. The action parts of all BEGIN patterns are merged
as if all the statements had been written in a single BEGIN block. They
are executed before any of the input is read. Similarly, all the END
blocks are merged, and executed when all the input is exhausted (or
when an exit statement is executed). BEGIN and END patterns cannot be
combined with other patterns in pattern expressions. BEGIN and END
patterns cannot have missing action parts.
For /regular expression/ patterns, the associated statement is executed
for each input line 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 eval‐
uation.
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 evalu‐
ated.
The pattern1, pattern2 form of an expression is called a range pattern.
It matches all input records starting with a line that matches pat‐
tern1, and continuing until a record that matches pattern2, inclusive.
It does not combine with any other sort of pattern expression.
Regular Expressions
Regular expressions are the extended kind found in egrep. They are
composed of characters as follows:
c matches the non-metacharacter c.
\c matches the literal character c.
. matches any character except newline.
^ matches the beginning of a line or a string.
$ matches the end of a line or a string.
[abc...] character class, matches any of the characters abc....
[^abc...] negated character class, matches any character except abc...
and newline.
r1|r2 alternation: matches either r1 or r2.
r1r2 concatenation: matches r1, and then r2.
r+ matches one or more r's.
r* matches zero or more r's.
r? matches zero or one r's.
(r) grouping: matches r.
The escape sequences that are valid in string constants (see below) are
also legal in regular expressions.
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 increasing precedence, are
= += -=
*= /= %= ^= Assignment. Both absolute assignment (var = value) and
operator-assignment (the other forms) are supported.
?: The C conditional expression. This has the form expr1 ?
expr2 : expr3. If expr1 is true, the value of the expres‐
sion is expr2, otherwise it is expr3. Only one of expr2
and expr3 is evaluated.
|| Logical OR.
&& Logical AND.
~ !~ 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.
< >
<= >=
!= == The regular relational operators.
blank String concatenation.
+ - Addition and subtraction.
* / % Multiplication, division, and modulus.
+ - ! Unary plus, unary minus, and logical negation.
^ Exponentiation (** may also be used, and **= for the
assignment operator).
++ -- Increment and decrement, both prefix and postfix.
$ Field reference.
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]
exit [ expression ]
{ statements }
I/O Statements
The input/output statements are as follows:
close(filename) Close file (or pipe, see below).
getline Set $0 from next input record; set NF, NR, FNR.
getline <file Set $0 from next record of file; set NF.
getline var Set var from next input record; set NF, FNR.
getline var <file Set var from next record of file.
next Stop processing the current input record. The
next input record is read and processing starts
over with the first pattern in the AWK program.
If the end of the input data is reached, the END
block(s), if any, are executed.
next file Stop processing the current input file. The next
input record read comes from the next input file.
FILENAME is updated, FNR is reset to 1, and pro‐
cessing 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.
print Prints the current record.
print expr-list Prints expressions.
print expr-list >file Prints expressions on file.
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.)
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. Getline
will return 0 on end of file, and -1 on an error.
The printf Statement
The AWK versions of the printf statement and sprintf() function (see
below) accept the following conversion specification formats:
%c An ASCII character. If the argument used for %c is numeric, it
is treated as a character and printed. Otherwise, the argument
is assumed to be a string, and the only first character of that
string is printed.
%d A decimal number (the integer part).
%i Just like %d.
%e A floating point number of the form [-]d.ddddddE[+-]dd.
%f A floating point number of the form [-]ddd.dddddd.
%g Use e or f conversion, whichever is shorter, with nonsignificant
zeros suppressed.
%o An unsigned octal number (again, an integer).
%s A character string.
%x An unsigned hexadecimal number (an integer).
%X Like %x, but using 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.
width The field should be padded to this width. If the number has a
leading zero, then the field will be padded with zeros. Other‐
wise it is padded with blanks.
.prec A number indicating the maximum width of strings or digits to
the right of the decimal point.
The dynamic width and prec capabilities of the ANSI C printf() routines
are supported. A * in place of either the width or prec specifications
will cause their values to be taken from the argument list to printf or
sprintf().
Special File Names
When doing I/O redirection from either print or printf into a file, or
via getline from a file, gawk recognizes certain special filenames
internally. These filenames allow access to open file descriptors
inherited from gawk's parent process (usually the shell). Other spe‐
cial filenames provide access information about the running gawk
process. The filenames are:
/dev/pid Reading this file returns the process ID of the current
process, in decimal, terminated with a newline.
/dev/ppid Reading this file returns the parent process ID of the cur‐
rent process, in decimal, terminated with a newline.
/dev/pgrpid Reading this file returns the process group ID of the cur‐
rent process, in decimal, terminated with a newline.
/dev/user Reading this file returns a single record terminated with a
newline. The fields are separated with blanks. $1 is the
value of the getuid(2) system call, $2 is the value of the
geteuid(2) system call, $3 is the value of the getgid(2)
system call, and $4 is the value of the getegid(2) system
call. If there are any additional fields, they are the
group IDs returned by getgroups(2). (Multiple groups may
not be supported on all systems.)
/dev/stdin The standard input.
/dev/stdout The standard output.
/dev/stderr The standard error output.
/dev/fd/n The file associated with the open file descriptor n.
These are particularly useful for error messages. For example:
print "You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print "You blew it!" | "cat 1>&2"
These file names may also be used on the command line to name data
files.
Numeric Functions
AWK has the following pre-defined arithmetic functions:
atan2(y, x) returns the arctangent of y/x in radians.
cos(expr) returns the cosine in radians.
exp(expr) the exponential function.
int(expr) truncates to integer.
log(expr) the natural logarithm function.
rand() returns a random number between 0 and 1.
sin(expr) returns the sine in radians.
sqrt(expr) the square root function.
srand(expr) use 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
AWK has the following pre-defined string functions:
gsub(r, s, t) for each substring matching the regular expres‐
sion r in the string t, substitute the string
s, and return the number of substitutions. If
t is not supplied, use $0.
index(s, t) returns the index of the string t in the string
s, or 0 if t is not present.
length(s) returns the length of the string s, or the
length of $0 if s is not supplied.
match(s, r) returns the position in s where the regular
expression r occurs, or 0 if r is not present,
and sets the values of RSTART and RLENGTH.
split(s, a, r) splits the string s into the array a on the
regular expression r, and returns the number of
fields. If r is omitted, FS is used.
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 n-character substring of s starting
at i. If n is omitted, the rest of s is used.
tolower(str) returns a copy of the string str, with all the
upper-case characters in str translated to
their corresponding lower-case counterparts.
Non-alphabetic characters are left unchanged.
toupper(str) returns a copy of the string str, with all the
lower-case characters in str translated to
their corresponding upper-case counterparts.
Non-alphabetic characters are left unchanged.
Time Functions
Since one of the primary uses of AWK programs is processing log files
that contain time stamp information, gawk provides the following two
functions for obtaining time stamps and formatting them.
systime() returns the current time of day as the number of seconds
since the Epoch (Midnight UTC, January 1, 1970 on POSIX sys‐
tems).
strftime(format, timestamp)
formats timestamp according to the specification in format.
The timestamp should be of the same form as returned by sys‐
time(). If timestamp is missing, the current time of day is
used. See the specification for the strftime() function in
ANSI C for the format conversions that are guaranteed to be
available. A public-domain version of strftime(3) and a man
page for it are shipped with gawk; if that version was used
to build gawk, then all of the conversions described in that
man page are available to gawk.
String Constants
String constants in AWK are sequences of characters enclosed between
double quotes ("). Within strings, certain escape sequences are recog‐
nized, as in C. These are:
\\ A literal backslash.
\a The ``alert'' character; usually the ASCII BEL character.
\b backspace.
\f form-feed.
\n newline.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\xhex digits
The character represented by the string of hexadecimal digits fol‐
lowing the \x. As in ANSI C, all following hexadecimal digits are
considered part of the escape sequence. (This feature should tell
us something about language design by committee.) E.g., "\x1B" is
the ASCII ESC (escape) character.
\ddd The character represented by the 1-, 2-, or 3-digit sequence of
octal digits. E.g. "\033" is the ASCII ESC (escape) character.
\c The literal character c.
The escape sequences may also be used inside constant regular expres‐
sions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).
FUNCTIONS
Functions in AWK are defined as follows:
function name(parameter list) { statements }
Functions are executed when called from within the action parts of reg‐
ular pattern-action statements. Actual parameters supplied in the func‐
tion call are used to instantiate the formal parameters declared in the
function. Arrays are passed by reference, other variables are passed
by value.
Since functions were not originally part of the AWK language, the pro‐
vision for local variables is rather clumsy: they are declared as extra
parameters in the parameter list. The convention is to separate local
variables from real parameters by extra spaces in the parameter list.
For example:
function f(p, q, a, b) { # a & b are local
..... }
/abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required to immediately fol‐
low the function name, without any intervening white space. This is to
avoid a syntactic ambiguity with the concatenation operator. This
restriction does not apply to the built-in functions listed above.
Functions may call each other and may be recursive. Function parame‐
ters used as local variables are initialized to the null string and the
number zero upon function invocation.
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 ALSOegrep(1)
The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter
J. Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X.
The GAWK Manual, Edition 0.15, published by the Free Software Founda‐
tion, 1993.
POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX standard, as
well as with the latest version of UNIX awk. To this end, gawk incor‐
porates the following user visible features which are not described in
the AWK book, but are part of awk in System V Release 4, and are in the
POSIX standard.
The -v option for assigning variables before program execution starts
is new. The book indicates that command line variable assignment hap‐
pens when awk would otherwise open the argument as a file, which is
after the BEGIN block is executed. However, in earlier implementa‐
tions, when such an assignment appeared before any file names, the
assignment would happen before the BEGIN block was run. Applications
came to depend on this ``feature.'' When awk was changed to match its
documentation, this option was added to accomodate 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 sig‐
nal the end of arguments, and warns about, but otherwise ignores, unde‐
fined options.
The AWK book does not define the return value of srand(). The System V
Release 4 version of UNIX awk (and 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 version); the tolower() and toupper()
built-in functions (from AT&T); and the ANSI C conversion specifica‐
tions in printf (done first in AT&T's version).
GNU EXTENSIONS
Gawk has some extensions to POSIX awk. They are described in this sec‐
tion. All the extensions described here can be disabled by invoking
gawk with the -W compat option.
The following features of gawk are not available in POSIX awk.
· The \x escape sequence.
· The systime() and strftime() functions.
· The special file names available for I/O redirection are not
recognized.
· The ARGIND and ERRNO variables are not special.
· The IGNORECASE variable and its side-effects are not avail‐
able.
· The FIELDWIDTHS variable and fixed width field splitting.
· No path search is performed for files named via the -f option.
Therefore the AWKPATH environment variable is not special.
· The use of next file to abandon processing of the current
input file.
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 -W compat option, if the fs argument to
the -F option is ``t'', then FS will be set to the tab character.
Since this is a rather ugly special case, it is not the default behav‐
ior. This behavior also does not occur if -W posix has been specified.
HISTORICAL FEATURES
There are two features of historical AWK implementations that gawk sup‐
ports. First, it is possible to call the length() built-in function
not only with no argument, but even without parentheses! Thus,
a = length
is the same as either of
a = length()
a = length($0)
This feature is marked as ``deprecated'' in the POSIX standard, and
gawk will issue a warning about its use if -W lint is specified on the
command line.
The other feature is the use of the continue statement 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 -W posix has not been specified.
BUGS
The -F option is not necessary given the command line variable assign‐
ment feature; it remains only for backwards compatibility.
If your system actually has support for /dev/fd and the associated
/dev/stdin, /dev/stdout, and /dev/stderr files, you may get different
output from gawk than you would get on a system without those files.
When gawk interprets these files internally, it synchronizes output to
the standard output with output to /dev/stdout, while on a system with
those files, the output is actually to different open files. Caveat
Emptor.
VERSION INFORMATION
This man page documents gawk, version 2.15.
Starting with the 2.15 version of gawk, the -c, -V, -C, -a, and -e
options of the 2.11 version are no longer recognized.
AUTHORS
The original version of UNIX awk was designed and implemented by Alfred
Aho, Peter Weinberger, and Brian Kernighan of AT&T Bell Labs. Brian
Kernighan continues to maintain and enhance it.
Paul Rubin and Jay Fenlason, of the Free Software Foundation, wrote
gawk, to be compatible with the original version of awk distributed in
Seventh Edition UNIX. John Woods contributed a number of bug fixes.
David Trueman, with contributions from Arnold Robbins, made gawk com‐
patible with the new version of UNIX awk.
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.
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs provided valuable assistance during test‐
ing and debugging. We thank him.
Free Software Foundation April 15 1993 AWK(1)