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GAWK(1) Utility Commands GAWK(1)
NAME
gawk - pattern scanning and processing language
SYNOPSIS
gawk [ -W _g_a_w_k-_o_p_t_i_o_n_s ] [ -F_f_s ] [ -v _v_a_r=_v_a_l ] -f
_p_r_o_g_r_a_m-_f_i_l_e [ -- ] file ...
gawk [ -W _g_a_w_k-_o_p_t_i_o_n_s ] [ -F_f_s ] [ -v _v_a_r=_v_a_l ] [ -- ]
_p_r_o_g_r_a_m-_t_e_x_t file ...
DESCRIPTION
_G_a_w_k 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 (draft 11). This version in turn is based on the
description in _T_h_e _A_W_K _P_r_o_g_r_a_m_m_i_n_g _L_a_n_g_u_a_g_e, by Aho, Ker-
nighan, and Weinberger, with the additional features defined
in the System V Release 4 version of UNIX _a_w_k. _G_a_w_k also
provides some GNU-specific extensions.
The command line consists of options to _g_a_w_k itself, the AWK
program text (if not supplied via the -f option), and values
to be made available in the ARGC and ARGV pre-defined AWK
variables.
OPTIONS
_G_a_w_k accepts the following options, which should be avail-
able on any implementation of the AWK language.
-F_f_s Use _f_s for the input field separator (the value of the
FS predefined variable).
-v _v_a_r=_v_a_l
Assign the value _v_a_l, to the variable _v_a_r, before exe-
cution of the program begins. Such variable values are
available to the BEGIN block of an AWK program.
-f _p_r_o_g_r_a_m-_f_i_l_e
Read the AWK program source from the file _p_r_o_g_r_a_m-_f_i_l_e,
instead of from the first command line argument. Mul-
tiple -f options may be used.
-- 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.
Following the POSIX standard, _g_a_w_k-specific options are sup-
plied 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
Free Software FoundationLast change: Jul 20 1992 1
GAWK(1) Utility Commands GAWK(1)
arguments to the -W option.
The -W option accepts the following arguments:
compat Run in _c_o_m_p_a_t_i_b_i_l_i_t_y mode. In compatibility mode,
_g_a_w_k behaves identically to UNIX _a_w_k; none of the
GNU-specific extensions are recognized. See GNU
EXTENSIONS, below, for more information.
copyleft
copyright Print the short version of the GNU copyright
information message on the error output.
lint Provide warnings about constructs that are dubious
or non-portable to other AWK implementations.
posix This turns on _c_o_m_p_a_t_i_b_i_l_i_t_y mode, with the follow-
ing additional restrictions:
o+ \x escape sequences are not recognized.
o+ The synonym func for the keyword function is not
recognized.
o+ The operators ** and **= cannot be used in place
of ^ and ^=.
version Print version information for this particular copy
of _g_a_w_k on the error output. This is useful
mainly for knowing if the current copy of _g_a_w_k on
your system is up to date with respect to whatever
the Free Software Foundation is distributing.
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.
_p_a_t_t_e_r_n { _a_c_t_i_o_n _s_t_a_t_e_m_e_n_t_s }
function _n_a_m_e(_p_a_r_a_m_e_t_e_r _l_i_s_t) { _s_t_a_t_e_m_e_n_t_s }
_G_a_w_k first reads the program source from the _p_r_o_g_r_a_m-_f_i_l_e(s)
if specified, or from the first non-option argument on the
command line. The -f option may be used multiple times on
the command line. _G_a_w_k will read the program text as if all
the _p_r_o_g_r_a_m-_f_i_l_es had been concatenated together. This is
useful for building libraries of AWK functions, without hav-
ing 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 command line, specify /dev/tty as one of the
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GAWK(1) Utility Commands GAWK(1)
_p_r_o_g_r_a_m-_f_i_l_es, type your program, 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.
_G_a_w_k executes AWK programs in the following order. First,
_g_a_w_k compiles the program into an internal form. Next, all
variable assignments specified via the -v option are per-
formed. Then, _g_a_w_k 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, _g_a_w_k reads the standard input.
If a filename on the command line has the form _v_a_r=_v_a_l it is
treated as a variable assignment. The variable _v_a_r will be
assigned the value _v_a_l. (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 mul-
tiple passes are needed over a single data file.
If the value of a particular element of ARGV is empty (""),
_g_a_w_k skips over it.
For each line in the input, _g_a_w_k tests to see if it matches
any _p_a_t_t_e_r_n in the AWK program. For each pattern that the
line matches, the associated _a_c_t_i_o_n is executed. The pat-
terns are tested in the order they occur in the program.
Finally, after all the input is exhausted, _g_a_w_k 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 dimension arrays; multiply dimen-
sioned arrays may be simulated. Several pre-defined vari-
ables are set as a program runs; these will be described as
needed and summarized below.
Fields
As each input line is read, _g_a_w_k splits the line into
_f_i_e_l_d_s, 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
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GAWK(1) Utility Commands GAWK(1)
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 numbers, each field is expected to have fixed width, and
_g_a_w_k 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 posi-
tion, $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 recomputed,
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 _g_a_w_k, or the program source).
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 /u/arnold). Changing this array does
not affect the environment seen by programs
which _g_a_w_k spawns via redirection or the
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GAWK(1) Utility Commands GAWK(1)
system() function. (This may change in a future
version of _g_a_w_k.)
FIELDWIDTHS A white-space separated list of fieldwidths.
When set, _g_a_w_k 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;
expect the semantics to change as _g_a_w_k 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 ``-''.
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 pattern matching in rules,
field splitting with FS, regular expression
matching with ~ and !~, and the gsub(), index(),
match(), split(), and sub() pre-defined func-
tions 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, by default a new-
line.
RS The input record separator, by default a new-
line. 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 _g_a_w_k.) If RS is set to
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GAWK(1) Utility Commands GAWK(1)
the null string, then records are separated by
blank lines. When RS is set to the null string,
then the newline character always acts as a
field separator, in addition to whatever 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 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
(_e_x_p_r, _e_x_p_r ...) 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.
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 context. If used in a numeric expression,
it will be treated as a number, if used as a string it will
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GAWK(1) Utility Commands GAWK(1)
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 _a_t_o_f(3). A number is converted to a
string by using the value of CONVFMT as a format string for
_s_p_r_i_n_t_f(3), with the numeric value of the variable as the
argument. However, even though all numbers in AWK are
floating-point, integral values are _a_l_w_a_y_s converted as
integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a value of "12" and not "12.00".
_G_a_w_k 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. Oth-
erwise, 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
(draft 11), even if two strings are numeric strings, a
numeric comparison is performed. However, this is clearly
incorrect, and _g_a_w_k 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 miss-
ing, 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, 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
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GAWK(1) Utility Commands GAWK(1)
on the following line. In other cases, a line can be con-
tinued 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
/_r_e_g_u_l_a_r _e_x_p_r_e_s_s_i_o_n/
_r_e_l_a_t_i_o_n_a_l _e_x_p_r_e_s_s_i_o_n
_p_a_t_t_e_r_n && _p_a_t_t_e_r_n
_p_a_t_t_e_r_n || _p_a_t_t_e_r_n
_p_a_t_t_e_r_n ? _p_a_t_t_e_r_n : _p_a_t_t_e_r_n
(_p_a_t_t_e_r_n)
! _p_a_t_t_e_r_n
_p_a_t_t_e_r_n_1, _p_a_t_t_e_r_n_2
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 expressions.
BEGIN and END patterns cannot have missing action parts.
For /_r_e_g_u_l_a_r _e_x_p_r_e_s_s_i_o_n/ patterns, the associated statement
is executed for each input line that matches the regular
expression. Regular expressions are the same as those in
_e_g_r_e_p(1), and are summarized below.
A _r_e_l_a_t_i_o_n_a_l _e_x_p_r_e_s_s_i_o_n 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.
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GAWK(1) Utility Commands GAWK(1)
The _p_a_t_t_e_r_n_1, _p_a_t_t_e_r_n_2 form of an expression is called a
range pattern. It matches all input records starting with a
line that matches _p_a_t_t_e_r_n_1, and continuing until a record
that matches _p_a_t_t_e_r_n_2, inclusive. It does not combine with
any other sort of pattern expression.
Regular Expressions
Regular expressions are the extended kind found in _e_g_r_e_p.
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.
[_a_b_c...] character class, matches any of the characters
_a_b_c....
[^_a_b_c...] negated character class, matches any character
except _a_b_c... and newline.
_r_1|_r_2 alternation: matches either _r_1 or _r_2.
_r_1_r_2 concatenation: matches _r_1, and then _r_2.
_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
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GAWK(1) Utility Commands GAWK(1)
= += -=
*= /= %= ^= Assignment. Both absolute assignment (_v_a_r =
_v_a_l_u_e) and operator-assignment (the other forms)
are supported.
?: The C conditional expression. This has the form
_e_x_p_r_1 ? _e_x_p_r_2 : _e_x_p_r_3. If _e_x_p_r_1 is true, the
value of the expression is _e_x_p_r_2, otherwise it
is _e_x_p_r_3. Only one of _e_x_p_r_2 and _e_x_p_r_3 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/ ~ _e_x_p has the same meaning as (($0 ~
/foo/) ~ _e_x_p). This is usually _n_o_t what was
intended.
< >
<= >=
!= == The regular relational operators.
_b_l_a_n_k 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 post-
fix.
$ Field reference.
Control Statements
The control statements are as follows:
if (_c_o_n_d_i_t_i_o_n) _s_t_a_t_e_m_e_n_t [ else _s_t_a_t_e_m_e_n_t ]
while (_c_o_n_d_i_t_i_o_n) _s_t_a_t_e_m_e_n_t
do _s_t_a_t_e_m_e_n_t while (_c_o_n_d_i_t_i_o_n)
for (_e_x_p_r_1; _e_x_p_r_2; _e_x_p_r_3) _s_t_a_t_e_m_e_n_t
for (_v_a_r in _a_r_r_a_y) _s_t_a_t_e_m_e_n_t
break
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GAWK(1) Utility Commands GAWK(1)
continue
delete _a_r_r_a_y[_i_n_d_e_x]
exit [ _e_x_p_r_e_s_s_i_o_n ]
{ _s_t_a_t_e_m_e_n_t_s }
I/O Statements
The input/output statements are as follows:
close(_f_i_l_e_n_a_m_e) Close file (or pipe, see below).
getline Set $0 from next input record; set NF,
NR, FNR.
getline <_f_i_l_e Set $0 from next record of _f_i_l_e; set
NF.
getline _v_a_r Set _v_a_r from next input record; set
NF, FNR.
getline _v_a_r <_f_i_l_e Set _v_a_r from next record of _f_i_l_e.
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.
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 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.
print Prints the current record.
print _e_x_p_r-_l_i_s_t Prints expressions.
print _e_x_p_r-_l_i_s_t >_f_i_l_e Prints expressions on _f_i_l_e.
printf _f_m_t, _e_x_p_r-_l_i_s_t Format and print.
printf _f_m_t, _e_x_p_r-_l_i_s_t >_f_i_l_e
Format and print on _f_i_l_e.
system(_c_m_d-_l_i_n_e) Execute the command _c_m_d-_l_i_n_e, and
return the exit status. (This may not
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GAWK(1) Utility Commands GAWK(1)
be available on non-POSIX systems.)
Other input/output redirections are also allowed. For print
and printf, >>_f_i_l_e appends output to the _f_i_l_e, while | _c_o_m_-
_m_a_n_d writes on a pipe. In a similar fashion, _c_o_m_m_a_n_d | get-
line pipes into getline. Getline will return 0 on end of
file, and -1 on an error.
The _p_r_i_n_t_f 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 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 non-
significant 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.
_w_i_d_t_h
The field should be padded to this width. If the number
has a leading zero, then the field will be padded with
zeros. Otherwise it is padded with blanks.
._p_r_e_c
A number indicating the maximum width of strings or
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GAWK(1) Utility Commands GAWK(1)
digits to the right of the decimal point.
The dynamic _w_i_d_t_h and _p_r_e_c 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, _g_a_w_k recognizes certain
special filenames internally. These filenames allow access
to open file descriptors inherited from _g_a_w_k's parent pro-
cess (usually the shell). 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 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(_e_x_p_r) returns the cosine in radians.
exp(_e_x_p_r) the exponential function.
int(_e_x_p_r) truncates to integer.
log(_e_x_p_r) the natural logarithm function.
rand() returns a random number between 0 and 1.
sin(_e_x_p_r) returns the sine in radians.
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sqrt(_e_x_p_r) the square root function.
srand(_e_x_p_r) use _e_x_p_r as a new seed for the random number
generator. If no _e_x_p_r is provided, the time of
day will be used. The return value is the pre-
vious 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 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.
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.
sprintf(_f_m_t, _e_x_p_r-_l_i_s_t) prints _e_x_p_r-_l_i_s_t according to _f_m_t,
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(_s_t_r) returns a copy of the string _s_t_r,
with all the upper-case characters
in _s_t_r translated to their
corresponding lower-case counter-
parts. Non-alphabetic characters
are left unchanged.
toupper(_s_t_r) returns a copy of the string _s_t_r,
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GAWK(1) Utility Commands GAWK(1)
with all the lower-case characters
in _s_t_r 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, _g_a_w_k 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(_f_o_r_m_a_t, _t_i_m_e_s_t_a_m_p)
formats _t_i_m_e_s_t_a_m_p according to the specification
in _f_o_r_m_a_t. The _t_i_m_e_s_t_a_m_p should be of the same
form as returned by systime(). If _t_i_m_e_s_t_a_m_p 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
_s_t_r_f_t_i_m_e(3) and a man page for it are shipped with
_g_a_w_k; if that version was used to build _g_a_w_k, then
all of the conversions described in that man page
are available to _g_a_w_k.
String Constants
String constants in AWK are sequences of characters enclosed
between double quotes ("). Within strings, certain _e_s_c_a_p_e
_s_e_q_u_e_n_c_e_s 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 new line.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\x_h_e_x _d_i_g_i_t_s
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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.
\_d_d_d 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).
FUNCTIONS
Functions in AWK are defined as follows:
function _n_a_m_e(_p_a_r_a_m_e_t_e_r _l_i_s_t) { _s_t_a_t_e_m_e_n_t_s }
Functions are executed when called from within the action
parts of regular pattern-action statements. 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.
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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
_e_g_r_e_p(1)
_T_h_e _A_W_K _P_r_o_g_r_a_m_m_i_n_g _L_a_n_g_u_a_g_e, Alfred V. Aho, Brian W. Ker-
nighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-
201-07981-X.
_T_h_e _G_A_W_K _M_a_n_u_a_l, Edition 0.14, published by the Free
Software Foundation, 1992.
POSIX COMPATIBILITY
A primary goal for _g_a_w_k is compatibility with the POSIX
standard, as well as with the latest version of UNIX _a_w_k.
To this end, _g_a_w_k incorporates the following user visible
features which are not described in the AWK book, but are
part of _a_w_k in System V Release 4, 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 _a_w_k 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 _b_e_f_o_r_e the BEGIN block was run. Applications
came to depend on this ``feature.'' When _a_w_k 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 develop-
ers.)
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The -W option for implementation specific features is from
the POSIX standard.
When processing arguments, _g_a_w_k uses the special option
``--'' to signal the end of arguments, and warns about, but
otherwise ignores, undefined options.
The AWK book does not define the return value of srand().
The System V Release 4 version of UNIX _a_w_k (and the POSIX
standard) has it return the seed it was using, to allow
keeping track of random number sequences. Therefore srand()
in _g_a_w_k also returns its current seed.
Other new features are: The use of multiple -f options (from
MKS _a_w_k); the ENVIRON array; the \a, and \v escape sequences
(done originally in _g_a_w_k 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
_G_a_w_k has some extensions to POSIX _a_w_k. They are described
in this section. All the extensions described here can be
disabled by invoking _g_a_w_k with the -W compat option.
The following features of _g_a_w_k are not available in POSIX
_a_w_k.
o+ The \x escape sequence.
o+ The systime() and strftime() functions.
o+ The special file names available for I/O redirection
are not recognized.
o+ The IGNORECASE variable and its side-effects are not
available.
o+ The FIELDWIDTHS variable and fixed width field split-
ting.
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 next file to abandon processing of the
current input file.
The AWK book does not define the return value of the close()
function. _G_a_w_k's close() returns the value from _f_c_l_o_s_e(3),
or _p_c_l_o_s_e(3), when closing a file or pipe, respectively.
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When _g_a_w_k is invoked with the -W compat option, if the _f_s
argument to the -F option is ``t'', then FS will be set to
the tab character. Since this is a rather ugly special
case, it is not the default behavior. This behavior also
does not occur if -Wposix has been specified.
HISTORICAL FEATURES
There are two features of historical AWK implementations
that _g_a_w_k supports. 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 stan-
dard, and _g_a_w_k will issue a warning about its use if -Wlint
is specified on the command line.
The other feature is the use of the continue statement out-
side the body of a while, for, or do loop. Traditional AWK
implementations have treated such usage as equivalent to the
next statement. _G_a_w_k will support this usage if -Wposix has
not been specified.
BUGS
The -F option is not necessary given the command line vari-
able assignment feature; it remains only for backwards com-
patibility.
VERSION INFORMATION
This man page documents _g_a_w_k, version 2.14.
For the 2.14 version of _g_a_w_k, the -c, -V, -C, -a, and -e
options of the 2.11 version are recognized. However, _g_a_w_k
will print a warning message, and these options will go away
in the 2.15 version.
AUTHORS
The original version of UNIX _a_w_k 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 _g_a_w_k, to be compatible with the original version
of _a_w_k distributed in Seventh Edition UNIX. John Woods con-
tributed a number of bug fixes. David Trueman, with
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contributions from Arnold Robbins, made _g_a_w_k compatible with
the new version of UNIX _a_w_k.
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.
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs provided valuable assistance
during testing and debugging. We thank him.
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