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AWK(1P) POSIX Programmer's Manual AWK(1P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
awk — pattern scanning and processing language
awk [−F sepstring] [−v assignment]... program [argument...]
awk [−F sepstring] −f progfile [−f progfile]... [−v assignment]...
[argument...]
The awk utility shall execute programs written in the awk programming
language, which is specialized for textual data manipulation. An awk
program is a sequence of patterns and corresponding actions. When
input is read that matches a pattern, the action associated with that
pattern is carried out.
Input shall be interpreted as a sequence of records. By default, a
record is a line, less its terminating <newline>, but this can be
changed by using the RS built-in variable. Each record of input shall
be matched in turn against each pattern in the program. For each
pattern matched, the associated action shall be executed.
The awk utility shall interpret each input record as a sequence of
fields where, by default, a field is a string of non-<blank>
non-<newline> characters. This default <blank> and <newline> field
delimiter can be changed by using the FS built-in variable or the −F
sepstring option. The awk utility shall denote the first field in a
record $1, the second $2, and so on. The symbol $0 shall refer to the
entire record; setting any other field causes the re-evaluation of
$0. Assigning to $0 shall reset the values of all other fields and
the NF built-in variable.
The awk utility shall conform to the Base Definitions volume of
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
−F sepstring
Define the input field separator. This option shall be
equivalent to:
-v FS=sepstring
except that if −F sepstring and −v FS=sepstring are both
used, it is unspecified whether the FS assignment resulting
from −F sepstring is processed in command line order or is
processed after the last −v FS=sepstring. See the
description of the FS built-in variable, and how it is
used, in the EXTENDED DESCRIPTION section.
−f progfile
Specify the pathname of the file progfile containing an awk
program. A pathname of '−' shall denote the standard input.
If multiple instances of this option are specified, the
concatenation of the files specified as progfile in the
order specified shall be the awk program. The awk program
can alternatively be specified in the command line as a
single argument.
−v assignment
The application shall ensure that the assignment argument
is in the same form as an assignment operand. The specified
variable assignment shall occur prior to executing the awk
program, including the actions associated with BEGIN
patterns (if any). Multiple occurrences of this option can
be specified.
The following operands shall be supported:
program If no −f option is specified, the first operand to awk
shall be the text of the awk program. The application shall
supply the program operand as a single argument to awk. If
the text does not end in a <newline>, awk shall interpret
the text as if it did.
argument Either of the following two types of argument can be
intermixed:
file A pathname of a file that contains the input to
be read, which is matched against the set of
patterns in the program. If no file operands are
specified, or if a file operand is '−', the
standard input shall be used.
assignment
An operand that begins with an <underscore> or
alphabetic character from the portable character
set (see the table in the Base Definitions volume
of POSIX.1‐2008, Section 6.1, Portable Character
Set), followed by a sequence of underscores,
digits, and alphabetics from the portable
character set, followed by the '=' character,
shall specify a variable assignment rather than a
pathname. The characters before the '='
represent the name of an awk variable; if that
name is an awk reserved word (see Grammar) the
behavior is undefined. The characters following
the <equals-sign> shall be interpreted as if they
appeared in the awk program preceded and followed
by a double-quote ('"') character, as a STRING
token (see Grammar), except that if the last
character is an unescaped <backslash>, it shall
be interpreted as a literal <backslash> rather
than as the first character of the sequence "\"".
The variable shall be assigned the value of that
STRING token and, if appropriate, shall be
considered a numeric string (see Expressions in
awk), the variable shall also be assigned its
numeric value. Each such variable assignment
shall occur just prior to the processing of the
following file, if any. Thus, an assignment
before the first file argument shall be executed
after the BEGIN actions (if any), while an
assignment after the last file argument shall
occur before the END actions (if any). If there
are no file arguments, assignments shall be
executed before processing the standard input.
The standard input shall be used only if no file operands are
specified, or if a file operand is '−', or if a progfile option-
argument is '−'; see the INPUT FILES section. If the awk program
contains no actions and no patterns, but is otherwise a valid awk
program, standard input and any file operands shall not be read and
awk shall exit with a return status of zero.
Input files to the awk program from any of the following sources
shall be text files:
* Any file operands or their equivalents, achieved by modifying the
awk variables ARGV and ARGC
* Standard input in the absence of any file operands
* Arguments to the getline function
Whether the variable RS is set to a value other than a <newline> or
not, for these files, implementations shall support records
terminated with the specified separator up to {LINE_MAX} bytes and
may support longer records.
If −f progfile is specified, the application shall ensure that the
files named by each of the progfile option-arguments are text files
and their concatenation, in the same order as they appear in the
arguments, is an awk program.
The following environment variables shall affect the execution of
awk:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions
volume of POSIX.1‐2008, Section 8.2, Internationalization
Variables for the precedence of internationalization
variables used to determine the values of locale
categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges,
equivalence classes, and multi-character collating elements
within regular expressions and in comparisons of string
values.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte
as opposed to multi-byte characters in arguments and input
files), the behavior of character classes within regular
expressions, the identification of characters as letters,
and the mapping of uppercase and lowercase characters for
the toupper and tolower functions.
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
LC_NUMERIC
Determine the radix character used when interpreting
numeric input, performing conversions between numeric and
string values, and formatting numeric output. Regardless of
locale, the <period> character (the decimal-point character
of the POSIX locale) is the decimal-point character
recognized in processing awk programs (including
assignments in command line arguments).
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
PATH Determine the search path when looking for commands
executed by system(expr), or input and output pipes; see
the Base Definitions volume of POSIX.1‐2008, Chapter 8,
Environment Variables.
In addition, all environment variables shall be visible via the awk
variable ENVIRON.
Default.
The nature of the output files depends on the awk program.
The standard error shall be used only for diagnostic messages.
The nature of the output files depends on the awk program.
Overall Program Structure
An awk program is composed of pairs of the form:
pattern { action }
Either the pattern or the action (including the enclosing brace
characters) can be omitted.
A missing pattern shall match any record of input, and a missing
action shall be equivalent to:
{ print }
Execution of the awk program shall start by first executing the
actions associated with all BEGIN patterns in the order they occur in
the program. Then each file operand (or standard input if no files
were specified) shall be processed in turn by reading data from the
file until a record separator is seen (<newline> by default). Before
the first reference to a field in the record is evaluated, the record
shall be split into fields, according to the rules in Regular
Expressions, using the value of FS that was current at the time the
record was read. Each pattern in the program then shall be evaluated
in the order of occurrence, and the action associated with each
pattern that matches the current record executed. The action for a
matching pattern shall be executed before evaluating subsequent
patterns. Finally, the actions associated with all END patterns shall
be executed in the order they occur in the program.
Expressions in awk
Expressions describe computations used in patterns and actions. In
the following table, valid expression operations are given in groups
from highest precedence first to lowest precedence last, with equal-
precedence operators grouped between horizontal lines. In expression
evaluation, where the grammar is formally ambiguous, higher
precedence operators shall be evaluated before lower precedence
operators. In this table expr, expr1, expr2, and expr3 represent any
expression, while lvalue represents any entity that can be assigned
to (that is, on the left side of an assignment operator). The
precise syntax of expressions is given in Grammar.
Table 4-1: Expressions in Decreasing Precedence in awk
┌─────────────────────┬─────────────────────────┬────────────────┬──────────────┐
│ Syntax │ Name │ Type of Result │Associativity │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│( expr ) │Grouping │Type of expr │N/A │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│$expr │Field reference │String │N/A │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│lvalue ++ │Post-increment │Numeric │N/A │
│lvalue −− │Post-decrement │Numeric │N/A │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│++ lvalue │Pre-increment │Numeric │N/A │
│−− lvalue │Pre-decrement │Numeric │N/A │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr ^ expr │Exponentiation │Numeric │Right │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│! expr │Logical not │Numeric │N/A │
│+ expr │Unary plus │Numeric │N/A │
│− expr │Unary minus │Numeric │N/A │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr * expr │Multiplication │Numeric │Left │
│expr / expr │Division │Numeric │Left │
│expr % expr │Modulus │Numeric │Left │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr + expr │Addition │Numeric │Left │
│expr − expr │Subtraction │Numeric │Left │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr expr │String concatenation │String │Left │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr < expr │Less than │Numeric │None │
│expr <= expr │Less than or equal to │Numeric │None │
│expr != expr │Not equal to │Numeric │None │
│expr == expr │Equal to │Numeric │None │
│expr > expr │Greater than │Numeric │None │
│expr >= expr │Greater than or equal to │Numeric │None │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr ~ expr │ERE match │Numeric │None │
│expr !~ expr │ERE non-match │Numeric │None │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr in array │Array membership │Numeric │Left │
│( index ) in array │Multi-dimension array │Numeric │Left │
│ │membership │ │ │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr && expr │Logical AND │Numeric │Left │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr || expr │Logical OR │Numeric │Left │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│expr1 ? expr2 : expr3│Conditional expression │Type of selected│Right │
│ │ │expr2 or expr3 │ │
├─────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│lvalue ^= expr │Exponentiation assignment│Numeric │Right │
│lvalue %= expr │Modulus assignment │Numeric │Right │
│lvalue *= expr │Multiplication assignment│Numeric │Right │
│lvalue /= expr │Division assignment │Numeric │Right │
│lvalue += expr │Addition assignment │Numeric │Right │
│lvalue −= expr │Subtraction assignment │Numeric │Right │
│lvalue = expr │Assignment │Type of expr │Right │
└─────────────────────┴─────────────────────────┴────────────────┴──────────────┘
Each expression shall have either a string value, a numeric value, or
both. Except as stated for specific contexts, the value of an
expression shall be implicitly converted to the type needed for the
context in which it is used. A string value shall be converted to a
numeric value either by the equivalent of the following calls to
functions defined by the ISO C standard:
setlocale(LC_NUMERIC, "");
numeric_value = atof(string_value);
or by converting the initial portion of the string to type double
representation as follows:
The input string is decomposed into two parts: an initial,
possibly empty, sequence of white-space characters (as
specified by isspace()) and a subject sequence interpreted as
a floating-point constant.
The expected form of the subject sequence is an optional '+'
or '−' sign, then a non-empty sequence of digits optionally
containing a <period>, then an optional exponent part. An
exponent part consists of 'e' or 'E', followed by an optional
sign, followed by one or more decimal digits.
The sequence starting with the first digit or the <period>
(whichever occurs first) is interpreted as a floating constant
of the C language, and if neither an exponent part nor a
<period> appears, a <period> is assumed to follow the last
digit in the string. If the subject sequence begins with a
minus-sign, the value resulting from the conversion is
negated.
A numeric value that is exactly equal to the value of an integer (see
Section 1.1.2, Concepts Derived from the ISO C Standard) shall be
converted to a string by the equivalent of a call to the sprintf
function (see String Functions) with the string "%d" as the fmt
argument and the numeric value being converted as the first and only
expr argument. Any other numeric value shall be converted to a string
by the equivalent of a call to the sprintf function with the value of
the variable CONVFMT as the fmt argument and the numeric value being
converted as the first and only expr argument. The result of the
conversion is unspecified if the value of CONVFMT is not a floating-
point format specification. This volume of POSIX.1‐2008 specifies no
explicit conversions between numbers and strings. An application can
force an expression to be treated as a number by adding zero to it,
or can force it to be treated as a string by concatenating the null
string ("") to it.
A string value shall be considered a numeric string if it comes from
one of the following:
1. Field variables
2. Input from the getline() function
3. FILENAME
4. ARGV array elements
5. ENVIRON array elements
6. Array elements created by the split() function
7. A command line variable assignment
8. Variable assignment from another numeric string variable
and an implementation-dependent condition corresponding to either
case (a) or (b) below is met.
a. After the equivalent of the following calls to functions defined
by the ISO C standard, string_value_end would differ from
string_value, and any characters before the terminating null
character in string_value_end would be <blank> characters:
char *string_value_end;
setlocale(LC_NUMERIC, "");
numeric_value = strtod (string_value, &string_value_end);
b. After all the following conversions have been applied, the
resulting string would lexically be recognized as a NUMBER token
as described by the lexical conventions in Grammar:
-- All leading and trailing <blank> characters are discarded.
-- If the first non-<blank> is '+' or '−', it is discarded.
-- Each occurrence of the decimal point character from the
current locale is changed to a <period>.
In case (a) the numeric value of the numeric string shall be the
value that would be returned by the strtod() call. In case (b) if the
first non-<blank> is '−', the numeric value of the numeric string
shall be the negation of the numeric value of the recognized NUMBER
token; otherwise, the numeric value of the numeric string shall be
the numeric value of the recognized NUMBER token. Whether or not a
string is a numeric string shall be relevant only in contexts where
that term is used in this section.
When an expression is used in a Boolean context, if it has a numeric
value, a value of zero shall be treated as false and any other value
shall be treated as true. Otherwise, a string value of the null
string shall be treated as false and any other value shall be treated
as true. A Boolean context shall be one of the following:
* The first subexpression of a conditional expression
* An expression operated on by logical NOT, logical AND, or logical
OR
* The second expression of a for statement
* The expression of an if statement
* The expression of the while clause in either a while or
do...while statement
* An expression used as a pattern (as in Overall Program Structure)
All arithmetic shall follow the semantics of floating-point
arithmetic as specified by the ISO C standard (see Section 1.1.2,
Concepts Derived from the ISO C Standard).
The value of the expression:
expr1 ^ expr2
shall be equivalent to the value returned by the ISO C standard
function call:
pow(expr1, expr2)
The expression:
lvalue ^= expr
shall be equivalent to the ISO C standard expression:
lvalue = pow(lvalue, expr)
except that lvalue shall be evaluated only once. The value of the
expression:
expr1 % expr2
shall be equivalent to the value returned by the ISO C standard
function call:
fmod(expr1, expr2)
The expression:
lvalue %= expr
shall be equivalent to the ISO C standard expression:
lvalue = fmod(lvalue, expr)
except that lvalue shall be evaluated only once.
Variables and fields shall be set by the assignment statement:
lvalue = expression
and the type of expression shall determine the resulting variable
type. The assignment includes the arithmetic assignments ("+=", "−=",
"*=", "/=", "%=", "^=", "++", "−−") all of which shall produce a
numeric result. The left-hand side of an assignment and the target of
increment and decrement operators can be one of a variable, an array
with index, or a field selector.
The awk language supplies arrays that are used for storing numbers or
strings. Arrays need not be declared. They shall initially be empty,
and their sizes shall change dynamically. The subscripts, or element
identifiers, are strings, providing a type of associative array
capability. An array name followed by a subscript within square
brackets can be used as an lvalue and thus as an expression, as
described in the grammar; see Grammar. Unsubscripted array names can
be used in only the following contexts:
* A parameter in a function definition or function call
* The NAME token following any use of the keyword in as specified
in the grammar (see Grammar); if the name used in this context is
not an array name, the behavior is undefined
A valid array index shall consist of one or more <comma>-separated
expressions, similar to the way in which multi-dimensional arrays are
indexed in some programming languages. Because awk arrays are really
one-dimensional, such a <comma>-separated list shall be converted to
a single string by concatenating the string values of the separate
expressions, each separated from the other by the value of the SUBSEP
variable. Thus, the following two index operations shall be
equivalent:
var[expr1, expr2, ... exprn]
var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]
The application shall ensure that a multi-dimensioned index used with
the in operator is parenthesized. The in operator, which tests for
the existence of a particular array element, shall not cause that
element to exist. Any other reference to a nonexistent array element
shall automatically create it.
Comparisons (with the '<', "<=", "!=", "==", '>', and ">=" operators)
shall be made numerically if both operands are numeric, if one is
numeric and the other has a string value that is a numeric string, or
if one is numeric and the other has the uninitialized value.
Otherwise, operands shall be converted to strings as required and a
string comparison shall be made using the locale-specific collation
sequence. The value of the comparison expression shall be 1 if the
relation is true, or 0 if the relation is false.
Variables and Special Variables
Variables can be used in an awk program by referencing them. With the
exception of function parameters (see User-Defined Functions), they
are not explicitly declared. Function parameter names shall be local
to the function; all other variable names shall be global. The same
name shall not be used as both a function parameter name and as the
name of a function or a special awk variable. The same name shall not
be used both as a variable name with global scope and as the name of
a function. The same name shall not be used within the same scope
both as a scalar variable and as an array. Uninitialized variables,
including scalar variables, array elements, and field variables,
shall have an uninitialized value. An uninitialized value shall have
both a numeric value of zero and a string value of the empty string.
Evaluation of variables with an uninitialized value, to either string
or numeric, shall be determined by the context in which they are
used.
Field variables shall be designated by a '$' followed by a number or
numerical expression. The effect of the field number expression
evaluating to anything other than a non-negative integer is
unspecified; uninitialized variables or string values need not be
converted to numeric values in this context. New field variables can
be created by assigning a value to them. References to nonexistent
fields (that is, fields after $NF), shall evaluate to the
uninitialized value. Such references shall not create new fields.
However, assigning to a nonexistent field (for example, $(NF+2)=5)
shall increase the value of NF; create any intervening fields with
the uninitialized value; and cause the value of $0 to be recomputed,
with the fields being separated by the value of OFS. Each field
variable shall have a string value or an uninitialized value when
created. Field variables shall have the uninitialized value when
created from $0 using FS and the variable does not contain any
characters. If appropriate, the field variable shall be considered a
numeric string (see Expressions in awk).
Implementations shall support the following other special variables
that are set by awk:
ARGC The number of elements in the ARGV array.
ARGV An array of command line arguments, excluding options and
the program argument, numbered from zero to ARGC−1.
The arguments in ARGV can be modified or added to; ARGC can
be altered. As each input file ends, awk shall treat the
next non-null element of ARGV, up to the current value of
ARGC−1, inclusive, as the name of the next input file.
Thus, setting an element of ARGV to null means that it
shall not be treated as an input file. The name '−'
indicates the standard input. If an argument matches the
format of an assignment operand, this argument shall be
treated as an assignment rather than a file argument.
CONVFMT The printf format for converting numbers to strings (except
for output statements, where OFMT is used); "%.6g" by
default.
ENVIRON An array representing the value of the environment, as
described in the exec functions defined in the System
Interfaces volume of POSIX.1‐2008. The indices of the array
shall be strings consisting of the names of the environment
variables, and the value of each array element shall be a
string consisting of the value of that variable. If
appropriate, the environment variable shall be considered a
numeric string (see Expressions in awk); the array element
shall also have its numeric value.
In all cases where the behavior of awk is affected by
environment variables (including the environment of any
commands that awk executes via the system function or via
pipeline redirections with the print statement, the printf
statement, or the getline function), the environment used
shall be the environment at the time awk began executing;
it is implementation-defined whether any modification of
ENVIRON affects this environment.
FILENAME A pathname of the current input file. Inside a BEGIN action
the value is undefined. Inside an END action the value
shall be the name of the last input file processed.
FNR The ordinal number of the current record in the current
file. Inside a BEGIN action the value shall be zero. Inside
an END action the value shall be the number of the last
record processed in the last file processed.
FS Input field separator regular expression; a <space> by
default.
NF The number of fields in the current record. Inside a BEGIN
action, the use of NF is undefined unless a getline
function without a var argument is executed previously.
Inside an END action, NF shall retain the value it had for
the last record read, unless a subsequent, redirected,
getline function without a var argument is performed prior
to entering the END action.
NR The ordinal number of the current record from the start of
input. Inside a BEGIN action the value shall be zero.
Inside an END action the value shall be the number of the
last record processed.
OFMT The printf format for converting numbers to strings in
output statements (see Output Statements); "%.6g" by
default. The result of the conversion is unspecified if the
value of OFMT is not a floating-point format specification.
OFS The print statement output field separator; <space> by
default.
ORS The print statement output record separator; a <newline> by
default.
RLENGTH The length of the string matched by the match function.
RS The first character of the string value of RS shall be the
input record separator; a <newline> by default. If RS
contains more than one character, the results are
unspecified. If RS is null, then records are separated by
sequences consisting of a <newline> plus one or more blank
lines, leading or trailing blank lines shall not result in
empty records at the beginning or end of the input, and a
<newline> shall always be a field separator, no matter what
the value of FS is.
RSTART The starting position of the string matched by the match
function, numbering from 1. This shall always be equivalent
to the return value of the match function.
SUBSEP The subscript separator string for multi-dimensional
arrays; the default value is implementation-defined.
Regular Expressions
The awk utility shall make use of the extended regular expression
notation (see the Base Definitions volume of POSIX.1‐2008, Section
9.4, Extended Regular Expressions) except that it shall allow the use
of C-language conventions for escaping special characters within the
EREs, as specified in the table in the Base Definitions volume of
POSIX.1‐2008, Chapter 5, File Format Notation ('\\', '\a', '\b',
'\f', '\n', '\r', '\t', '\v') and the following table; these escape
sequences shall be recognized both inside and outside bracket
expressions. Note that records need not be separated by <newline>
characters and string constants can contain <newline> characters, so
even the "\n" sequence is valid in awk EREs. Using a <slash>
character within an ERE requires the escaping shown in the following
table.
Table 4-2: Escape Sequences in awk
┌─────────┬────────────────────────────────────┬────────────────────────────────────┐
│ Escape │ │ │
│Sequence │ Description │ Meaning │
├─────────┼────────────────────────────────────┼────────────────────────────────────┤
│\" │ <backslash> <quotation-mark> │ <quotation-mark> character │
├─────────┼────────────────────────────────────┼────────────────────────────────────┤
│\/ │ <backslash> <slash> │ <slash> character │
├─────────┼────────────────────────────────────┼────────────────────────────────────┤
│\ddd │ A <backslash> character followed │ The character whose encoding is │
│ │ by the longest sequence of one, │ represented by the one, two, or │
│ │ two, or three octal-digit │ three-digit octal integer. Multi- │
│ │ characters (01234567). If all of │ byte characters require multiple, │
│ │ the digits are 0 (that is, │ concatenated escape sequences of │
│ │ representation of the NUL │ this type, including the leading │
│ │ character), the behavior is │ <backslash> for each byte. │
│ │ undefined. │ │
├─────────┼────────────────────────────────────┼────────────────────────────────────┤
│\c │ A <backslash> character followed │ Undefined │
│ │ by any character not described in │ │
│ │ this table or in the table in the │ │
│ │ Base Definitions volume of │ │
│ │ POSIX.1‐2008, Chapter 5, File │ │
│ │ Format Notation ('\\', '\a', '\b', │ │
│ │ '\f', '\n', '\r', '\t', '\v'). │ │
└─────────┴────────────────────────────────────┴────────────────────────────────────┘
A regular expression can be matched against a specific field or
string by using one of the two regular expression matching operators,
'~' and "!~". These operators shall interpret their right-hand
operand as a regular expression and their left-hand operand as a
string. If the regular expression matches the string, the '~'
expression shall evaluate to a value of 1, and the "!~" expression
shall evaluate to a value of 0. (The regular expression matching
operation is as defined by the term matched in the Base Definitions
volume of POSIX.1‐2008, Section 9.1, Regular Expression Definitions,
where a match occurs on any part of the string unless the regular
expression is limited with the <circumflex> or <dollar-sign> special
characters.) If the regular expression does not match the string, the
'~' expression shall evaluate to a value of 0, and the "!~"
expression shall evaluate to a value of 1. If the right-hand operand
is any expression other than the lexical token ERE, the string value
of the expression shall be interpreted as an extended regular
expression, including the escape conventions described above. Note
that these same escape conventions shall also be applied in
determining the value of a string literal (the lexical token STRING),
and thus shall be applied a second time when a string literal is used
in this context.
When an ERE token appears as an expression in any context other than
as the right-hand of the '~' or "!~" operator or as one of the built-
in function arguments described below, the value of the resulting
expression shall be the equivalent of:
$0 " " /ere/
The ere argument to the gsub, match, sub functions, and the fs
argument to the split function (see String Functions) shall be
interpreted as extended regular expressions. These can be either ERE
tokens or arbitrary expressions, and shall be interpreted in the same
manner as the right-hand side of the '~' or "!~" operator.
An extended regular expression can be used to separate fields by
assigning a string containing the expression to the built-in variable
FS, either directly or as a consequence of using the −F sepstring
option. The default value of the FS variable shall be a single
<space>. The following describes FS behavior:
1. If FS is a null string, the behavior is unspecified.
2. If FS is a single character:
a. If FS is <space>, skip leading and trailing <blank> and
<newline> characters; fields shall be delimited by sets of
one or more <blank> or <newline> characters.
b. Otherwise, if FS is any other character c, fields shall be
delimited by each single occurrence of c.
3. Otherwise, the string value of FS shall be considered to be an
extended regular expression. Each occurrence of a sequence
matching the extended regular expression shall delimit fields.
Except for the '~' and "!~" operators, and in the gsub, match, split,
and sub built-in functions, ERE matching shall be based on input
records; that is, record separator characters (the first character of
the value of the variable RS, <newline> by default) cannot be
embedded in the expression, and no expression shall match the record
separator character. If the record separator is not <newline>,
<newline> characters embedded in the expression can be matched. For
the '~' and "!~" operators, and in those four built-in functions, ERE
matching shall be based on text strings; that is, any character
(including <newline> and the record separator) can be embedded in the
pattern, and an appropriate pattern shall match any character.
However, in all awk ERE matching, the use of one or more NUL
characters in the pattern, input record, or text string produces
undefined results.
Patterns
A pattern is any valid expression, a range specified by two
expressions separated by a comma, or one of the two special patterns
BEGIN or END.
Special Patterns
The awk utility shall recognize two special patterns, BEGIN and END.
Each BEGIN pattern shall be matched once and its associated action
executed before the first record of input is read—except possibly by
use of the getline function (see Input/Output and General Functions)
in a prior BEGIN action—and before command line assignment is done.
Each END pattern shall be matched once and its associated action
executed after the last record of input has been read. These two
patterns shall have associated actions.
BEGIN and END shall not combine with other patterns. Multiple BEGIN
and END patterns shall be allowed. The actions associated with the
BEGIN patterns shall be executed in the order specified in the
program, as are the END actions. An END pattern can precede a BEGIN
pattern in a program.
If an awk program consists of only actions with the pattern BEGIN,
and the BEGIN action contains no getline function, awk shall exit
without reading its input when the last statement in the last BEGIN
action is executed. If an awk program consists of only actions with
the pattern END or only actions with the patterns BEGIN and END, the
input shall be read before the statements in the END actions are
executed.
Expression Patterns
An expression pattern shall be evaluated as if it were an expression
in a Boolean context. If the result is true, the pattern shall be
considered to match, and the associated action (if any) shall be
executed. If the result is false, the action shall not be executed.
Pattern Ranges
A pattern range consists of two expressions separated by a comma; in
this case, the action shall be performed for all records between a
match of the first expression and the following match of the second
expression, inclusive. At this point, the pattern range can be
repeated starting at input records subsequent to the end of the
matched range.
Actions
An action is a sequence of statements as shown in the grammar in
Grammar. Any single statement can be replaced by a statement list
enclosed in curly braces. The application shall ensure that
statements in a statement list are separated by <newline> or
<semicolon> characters. Statements in a statement list shall be
executed sequentially in the order that they appear.
The expression acting as the conditional in an if statement shall be
evaluated and if it is non-zero or non-null, the following statement
shall be executed; otherwise, if else is present, the statement
following the else shall be executed.
The if, while, do...while, for, break, and continue statements are
based on the ISO C standard (see Section 1.1.2, Concepts Derived from
the ISO C Standard), except that the Boolean expressions shall be
treated as described in Expressions in awk, and except in the case
of:
for (variable in array)
which shall iterate, assigning each index of array to variable in an
unspecified order. The results of adding new elements to array within
such a for loop are undefined. If a break or continue statement
occurs outside of a loop, the behavior is undefined.
The delete statement shall remove an individual array element. Thus,
the following code deletes an entire array:
for (index in array)
delete array[index]
The next statement shall cause all further processing of the current
input record to be abandoned. The behavior is undefined if a next
statement appears or is invoked in a BEGIN or END action.
The exit statement shall invoke all END actions in the order in which
they occur in the program source and then terminate the program
without reading further input. An exit statement inside an END action
shall terminate the program without further execution of END actions.
If an expression is specified in an exit statement, its numeric value
shall be the exit status of awk, unless subsequent errors are
encountered or a subsequent exit statement with an expression is
executed.
Output Statements
Both print and printf statements shall write to standard output by
default. The output shall be written to the location specified by
output_redirection if one is supplied, as follows:
> expression
>> expression
| expression
In all cases, the expression shall be evaluated to produce a string
that is used as a pathname into which to write (for '>' or ">>") or
as a command to be executed (for '|'). Using the first two forms, if
the file of that name is not currently open, it shall be opened,
creating it if necessary and using the first form, truncating the
file. The output then shall be appended to the file. As long as the
file remains open, subsequent calls in which expression evaluates to
the same string value shall simply append output to the file. The
file remains open until the close function (see Input/Output and
General Functions) is called with an expression that evaluates to the
same string value.
The third form shall write output onto a stream piped to the input of
a command. The stream shall be created if no stream is currently open
with the value of expression as its command name. The stream created
shall be equivalent to one created by a call to the popen() function
defined in the System Interfaces volume of POSIX.1‐2008 with the
value of expression as the command argument and a value of w as the
mode argument. As long as the stream remains open, subsequent calls
in which expression evaluates to the same string value shall write
output to the existing stream. The stream shall remain open until the
close function (see Input/Output and General Functions) is called
with an expression that evaluates to the same string value. At that
time, the stream shall be closed as if by a call to the pclose()
function defined in the System Interfaces volume of POSIX.1‐2008.
As described in detail by the grammar in Grammar, these output
statements shall take a <comma>-separated list of expressions
referred to in the grammar by the non-terminal symbols expr_list,
print_expr_list, or print_expr_list_opt. This list is referred to
here as the expression list, and each member is referred to as an
expression argument.
The print statement shall write the value of each expression argument
onto the indicated output stream separated by the current output
field separator (see variable OFS above), and terminated by the
output record separator (see variable ORS above). All expression
arguments shall be taken as strings, being converted if necessary;
this conversion shall be as described in Expressions in awk, with the
exception that the printf format in OFMT shall be used instead of the
value in CONVFMT. An empty expression list shall stand for the whole
input record ($0).
The printf statement shall produce output based on a notation similar
to the File Format Notation used to describe file formats in this
volume of POSIX.1‐2008 (see the Base Definitions volume of
POSIX.1‐2008, Chapter 5, File Format Notation). Output shall be
produced as specified with the first expression argument as the
string format and subsequent expression arguments as the strings arg1
to argn, inclusive, with the following exceptions:
1. The format shall be an actual character string rather than a
graphical representation. Therefore, it cannot contain empty
character positions. The <space> in the format string, in any
context other than a flag of a conversion specification, shall be
treated as an ordinary character that is copied to the output.
2. If the character set contains a '' character and that character
appears in the format string, it shall be treated as an ordinary
character that is copied to the output.
3. The escape sequences beginning with a <backslash> character shall
be treated as sequences of ordinary characters that are copied to
the output. Note that these same sequences shall be interpreted
lexically by awk when they appear in literal strings, but they
shall not be treated specially by the printf statement.
4. A field width or precision can be specified as the '*' character
instead of a digit string. In this case the next argument from
the expression list shall be fetched and its numeric value taken
as the field width or precision.
5. The implementation shall not precede or follow output from the d
or u conversion specifier characters with <blank> characters not
specified by the format string.
6. The implementation shall not precede output from the o conversion
specifier character with leading zeros not specified by the
format string.
7. For the c conversion specifier character: if the argument has a
numeric value, the character whose encoding is that value shall
be output. If the value is zero or is not the encoding of any
character in the character set, the behavior is undefined. If the
argument does not have a numeric value, the first character of
the string value shall be output; if the string does not contain
any characters, the behavior is undefined.
8. For each conversion specification that consumes an argument, the
next expression argument shall be evaluated. With the exception
of the c conversion specifier character, the value shall be
converted (according to the rules specified in Expressions in
awk) to the appropriate type for the conversion specification.
9. If there are insufficient expression arguments to satisfy all the
conversion specifications in the format string, the behavior is
undefined.
10. If any character sequence in the format string begins with a '%'
character, but does not form a valid conversion specification,
the behavior is unspecified.
Both print and printf can output at least {LINE_MAX} bytes.
Functions
The awk language has a variety of built-in functions: arithmetic,
string, input/output, and general.
Arithmetic Functions
The arithmetic functions, except for int, shall be based on the ISO C
standard (see Section 1.1.2, Concepts Derived from the ISO C
Standard). The behavior is undefined in cases where the ISO C
standard specifies that an error be returned or that the behavior is
undefined. Although the grammar (see Grammar) permits built-in
functions to appear with no arguments or parentheses, unless the
argument or parentheses are indicated as optional in the following
list (by displaying them within the "[]" brackets), such use is
undefined.
atan2(y,x)
Return arctangent of y/x in radians in the range [−π,π].
cos(x) Return cosine of x, where x is in radians.
sin(x) Return sine of x, where x is in radians.
exp(x) Return the exponential function of x.
log(x) Return the natural logarithm of x.
sqrt(x) Return the square root of x.
int(x) Return the argument truncated to an integer. Truncation
shall be toward 0 when x>0.
rand() Return a random number n, such that 0≤n<1.
srand([expr])
Set the seed value for rand to expr or use the time of day
if expr is omitted. The previous seed value shall be
returned.
String Functions
The string functions in the following list shall be supported.
Although the grammar (see Grammar) permits built-in functions to
appear with no arguments or parentheses, unless the argument or
parentheses are indicated as optional in the following list (by
displaying them within the "[]" brackets), such use is undefined.
gsub(ere, repl[, in])
Behave like sub (see below), except that it shall replace
all occurrences of the regular expression (like the ed
utility global substitute) in $0 or in the in argument,
when specified.
index(s, t)
Return the position, in characters, numbering from 1, in
string s where string t first occurs, or zero if it does
not occur at all.
length[([s])]
Return the length, in characters, of its argument taken as
a string, or of the whole record, $0, if there is no
argument.
match(s, ere)
Return the position, in characters, numbering from 1, in
string s where the extended regular expression ere occurs,
or zero if it does not occur at all. RSTART shall be set to
the starting position (which is the same as the returned
value), zero if no match is found; RLENGTH shall be set to
the length of the matched string, −1 if no match is found.
split(s, a[, fs ])
Split the string s into array elements a[1], a[2], ...,
a[n], and return n. All elements of the array shall be
deleted before the split is performed. The separation shall
be done with the ERE fs or with the field separator FS if
fs is not given. Each array element shall have a string
value when created and, if appropriate, the array element
shall be considered a numeric string (see Expressions in
awk). The effect of a null string as the value of fs is
unspecified.
sprintf(fmt, expr, expr, ...)
Format the expressions according to the printf format given
by fmt and return the resulting string.
sub(ere, repl[, in ])
Substitute the string repl in place of the first instance
of the extended regular expression ERE in string in and
return the number of substitutions. An <ampersand> ('&')
appearing in the string repl shall be replaced by the
string from in that matches the ERE. An <ampersand>
preceded with a <backslash> shall be interpreted as the
literal <ampersand> character. An occurrence of two
consecutive <backslash> characters shall be interpreted as
just a single literal <backslash> character. Any other
occurrence of a <backslash> (for example, preceding any
other character) shall be treated as a literal <backslash>
character. Note that if repl is a string literal (the
lexical token STRING; see Grammar), the handling of the
<ampersand> character occurs after any lexical processing,
including any lexical <backslash>-escape sequence
processing. If in is specified and it is not an lvalue (see
Expressions in awk), the behavior is undefined. If in is
omitted, awk shall use the current record ($0) in its
place.
substr(s, m[, n ])
Return the at most n-character substring of s that begins
at position m, numbering from 1. If n is omitted, or if n
specifies more characters than are left in the string, the
length of the substring shall be limited by the length of
the string s.
tolower(s)
Return a string based on the string s. Each character in s
that is an uppercase letter specified to have a tolower
mapping by the LC_CTYPE category of the current locale
shall be replaced in the returned string by the lowercase
letter specified by the mapping. Other characters in s
shall be unchanged in the returned string.
toupper(s)
Return a string based on the string s. Each character in s
that is a lowercase letter specified to have a toupper
mapping by the LC_CTYPE category of the current locale is
replaced in the returned string by the uppercase letter
specified by the mapping. Other characters in s are
unchanged in the returned string.
All of the preceding functions that take ERE as a parameter expect a
pattern or a string valued expression that is a regular expression as
defined in Regular Expressions.
Input/Output and General Functions
The input/output and general functions are:
close(expression)
Close the file or pipe opened by a print or printf
statement or a call to getline with the same string-valued
expression. The limit on the number of open expression
arguments is implementation-defined. If the close was
successful, the function shall return zero; otherwise, it
shall return non-zero.
expression | getline [var]
Read a record of input from a stream piped from the output
of a command. The stream shall be created if no stream is
currently open with the value of expression as its command
name. The stream created shall be equivalent to one created
by a call to the popen() function with the value of
expression as the command argument and a value of r as the
mode argument. As long as the stream remains open,
subsequent calls in which expression evaluates to the same
string value shall read subsequent records from the stream.
The stream shall remain open until the close function is
called with an expression that evaluates to the same string
value. At that time, the stream shall be closed as if by a
call to the pclose() function. If var is omitted, $0 and NF
shall be set; otherwise, var shall be set and, if
appropriate, it shall be considered a numeric string (see
Expressions in awk).
The getline operator can form ambiguous constructs when
there are unparenthesized operators (including concatenate)
to the left of the '|' (to the beginning of the expression
containing getline). In the context of the '$' operator,
'|' shall behave as if it had a lower precedence than '$'.
The result of evaluating other operators is unspecified,
and conforming applications shall parenthesize properly all
such usages.
getline Set $0 to the next input record from the current input
file. This form of getline shall set the NF, NR, and FNR
variables.
getline var
Set variable var to the next input record from the current
input file and, if appropriate, var shall be considered a
numeric string (see Expressions in awk). This form of
getline shall set the FNR and NR variables.
getline [var] < expression
Read the next record of input from a named file. The
expression shall be evaluated to produce a string that is
used as a pathname. If the file of that name is not
currently open, it shall be opened. As long as the stream
remains open, subsequent calls in which expression
evaluates to the same string value shall read subsequent
records from the file. The file shall remain open until the
close function is called with an expression that evaluates
to the same string value. If var is omitted, $0 and NF
shall be set; otherwise, var shall be set and, if
appropriate, it shall be considered a numeric string (see
Expressions in awk).
The getline operator can form ambiguous constructs when
there are unparenthesized binary operators (including
concatenate) to the right of the '<' (up to the end of the
expression containing the getline). The result of
evaluating such a construct is unspecified, and conforming
applications shall parenthesize properly all such usages.
system(expression)
Execute the command given by expression in a manner
equivalent to the system() function defined in the System
Interfaces volume of POSIX.1‐2008 and return the exit
status of the command.
All forms of getline shall return 1 for successful input, zero for
end-of-file, and −1 for an error.
Where strings are used as the name of a file or pipeline, the
application shall ensure that the strings are textually identical.
The terminology ``same string value'' implies that ``equivalent
strings'', even those that differ only by <space> characters,
represent different files.
User-Defined Functions
The awk language also provides user-defined functions. Such functions
can be defined as:
function name([parameter, ...]) { statements }
A function can be referred to anywhere in an awk program; in
particular, its use can precede its definition. The scope of a
function is global.
Function parameters, if present, can be either scalars or arrays; the
behavior is undefined if an array name is passed as a parameter that
the function uses as a scalar, or if a scalar expression is passed as
a parameter that the function uses as an array. Function parameters
shall be passed by value if scalar and by reference if array name.
The number of parameters in the function definition need not match
the number of parameters in the function call. Excess formal
parameters can be used as local variables. If fewer arguments are
supplied in a function call than are in the function definition, the
extra parameters that are used in the function body as scalars shall
evaluate to the uninitialized value until they are otherwise
initialized, and the extra parameters that are used in the function
body as arrays shall be treated as uninitialized arrays where each
element evaluates to the uninitialized value until otherwise
initialized.
When invoking a function, no white space can be placed between the
function name and the opening parenthesis. Function calls can be
nested and recursive calls can be made upon functions. Upon return
from any nested or recursive function call, the values of all of the
calling function's parameters shall be unchanged, except for array
parameters passed by reference. The return statement can be used to
return a value. If a return statement appears outside of a function
definition, the behavior is undefined.
In the function definition, <newline> characters shall be optional
before the opening brace and after the closing brace. Function
definitions can appear anywhere in the program where a pattern-action
pair is allowed.
Grammar
The grammar in this section and the lexical conventions in the
following section shall together describe the syntax for awk
programs. The general conventions for this style of grammar are
described in Section 1.3, Grammar Conventions. A valid program can
be represented as the non-terminal symbol program in the grammar.
This formal syntax shall take precedence over the preceding text
syntax description.
%token NAME NUMBER STRING ERE
%token FUNC_NAME /* Name followed by '(' without white space. */
/* Keywords */
%token Begin End
/* 'BEGIN' 'END' */
%token Break Continue Delete Do Else
/* 'break' 'continue' 'delete' 'do' 'else' */
%token Exit For Function If In
/* 'exit' 'for' 'function' 'if' 'in' */
%token Next Print Printf Return While
/* 'next' 'print' 'printf' 'return' 'while' */
/* Reserved function names */
%token BUILTIN_FUNC_NAME
/* One token for the following:
* atan2 cos sin exp log sqrt int rand srand
* gsub index length match split sprintf sub
* substr tolower toupper close system
*/
%token GETLINE
/* Syntactically different from other built-ins. */
/* Two-character tokens. */
%token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
/* '+=' '−=' '*=' '/=' '%=' '^=' */
%token OR AND NO_MATCH EQ LE GE NE INCR DECR APPEND
/* '||' '&&' '!~' '==' '<=' '>=' '!=' '++' '−−' '>>' */
/* One-character tokens. */
%token '{' '}' '(' ')' '[' ']' ',' ';' NEWLINE
%token '+' '−' '*' '%' '^' '!' '>' '<' '|' '?' ':' ' " " ' '$' '='
%start program
%%
program : item_list
| actionless_item_list
;
item_list : newline_opt
| actionless_item_list item terminator
| item_list item terminator
| item_list action terminator
;
actionless_item_list : item_list pattern terminator
| actionless_item_list pattern terminator
;
item : pattern action
| Function NAME '(' param_list_opt ')'
newline_opt action
| Function FUNC_NAME '(' param_list_opt ')'
newline_opt action
;
param_list_opt : /* empty */
| param_list
;
param_list : NAME
| param_list ',' NAME
;
pattern : Begin
| End
| expr
| expr ',' newline_opt expr
;
action : '{' newline_opt '}'
| '{' newline_opt terminated_statement_list '}'
| '{' newline_opt unterminated_statement_list '}'
;
terminator : terminator ';'
| terminator NEWLINE
| ';'
| NEWLINE
;
terminated_statement_list : terminated_statement
| terminated_statement_list terminated_statement
;
unterminated_statement_list : unterminated_statement
| terminated_statement_list unterminated_statement
;
terminated_statement : action newline_opt
| If '(' expr ')' newline_opt terminated_statement
| If '(' expr ')' newline_opt terminated_statement
Else newline_opt terminated_statement
| While '(' expr ')' newline_opt terminated_statement
| For '(' simple_statement_opt ';'
expr_opt ';' simple_statement_opt ')' newline_opt
terminated_statement
| For '(' NAME In NAME ')' newline_opt
terminated_statement
| ';' newline_opt
| terminatable_statement NEWLINE newline_opt
| terminatable_statement ';' newline_opt
;
unterminated_statement : terminatable_statement
| If '(' expr ')' newline_opt unterminated_statement
| If '(' expr ')' newline_opt terminated_statement
Else newline_opt unterminated_statement
| While '(' expr ')' newline_opt unterminated_statement
| For '(' simple_statement_opt ';'
expr_opt ';' simple_statement_opt ')' newline_opt
unterminated_statement
| For '(' NAME In NAME ')' newline_opt
unterminated_statement
;
terminatable_statement : simple_statement
| Break
| Continue
| Next
| Exit expr_opt
| Return expr_opt
| Do newline_opt terminated_statement While '(' expr ')'
;
simple_statement_opt : /* empty */
| simple_statement
;
simple_statement : Delete NAME '[' expr_list ']'
| expr
| print_statement
;
print_statement : simple_print_statement
| simple_print_statement output_redirection
;
simple_print_statement : Print print_expr_list_opt
| Print '(' multiple_expr_list ')'
| Printf print_expr_list
| Printf '(' multiple_expr_list ')'
;
output_redirection : '>' expr
| APPEND expr
| '|' expr
;
expr_list_opt : /* empty */
| expr_list
;
expr_list : expr
| multiple_expr_list
;
multiple_expr_list : expr ',' newline_opt expr
| multiple_expr_list ',' newline_opt expr
;
expr_opt : /* empty */
| expr
;
expr : unary_expr
| non_unary_expr
;
unary_expr : '+' expr
| '−' expr
| unary_expr '^' expr
| unary_expr '*' expr
| unary_expr '/' expr
| unary_expr '%' expr
| unary_expr '+' expr
| unary_expr '−' expr
| unary_expr non_unary_expr
| unary_expr '<' expr
| unary_expr LE expr
| unary_expr NE expr
| unary_expr EQ expr
| unary_expr '>' expr
| unary_expr GE expr
| unary_expr '~' expr
| unary_expr NO_MATCH expr
| unary_expr In NAME
| unary_expr AND newline_opt expr
| unary_expr OR newline_opt expr
| unary_expr '?' expr ':' expr
| unary_input_function
;
non_unary_expr : '(' expr ')'
| '!' expr
| non_unary_expr '^' expr
| non_unary_expr '*' expr
| non_unary_expr '/' expr
| non_unary_expr '%' expr
| non_unary_expr '+' expr
| non_unary_expr '−' expr
| non_unary_expr non_unary_expr
| non_unary_expr '<' expr
| non_unary_expr LE expr
| non_unary_expr NE expr
| non_unary_expr EQ expr
| non_unary_expr '>' expr
| non_unary_expr GE expr
| non_unary_expr '~' expr
| non_unary_expr NO_MATCH expr
| non_unary_expr In NAME
| '(' multiple_expr_list ')' In NAME
| non_unary_expr AND newline_opt expr
| non_unary_expr OR newline_opt expr
| non_unary_expr '?' expr ':' expr
| NUMBER
| STRING
| lvalue
| ERE
| lvalue INCR
| lvalue DECR
| INCR lvalue
| DECR lvalue
| lvalue POW_ASSIGN expr
| lvalue MOD_ASSIGN expr
| lvalue MUL_ASSIGN expr
| lvalue DIV_ASSIGN expr
| lvalue ADD_ASSIGN expr
| lvalue SUB_ASSIGN expr
| lvalue '=' expr
| FUNC_NAME '(' expr_list_opt ')'
/* no white space allowed before '(' */
| BUILTIN_FUNC_NAME '(' expr_list_opt ')'
| BUILTIN_FUNC_NAME
| non_unary_input_function
;
print_expr_list_opt : /* empty */
| print_expr_list
;
print_expr_list : print_expr
| print_expr_list ',' newline_opt print_expr
;
print_expr : unary_print_expr
| non_unary_print_expr
;
unary_print_expr : '+' print_expr
| '−' print_expr
| unary_print_expr '^' print_expr
| unary_print_expr '*' print_expr
| unary_print_expr '/' print_expr
| unary_print_expr '%' print_expr
| unary_print_expr '+' print_expr
| unary_print_expr '−' print_expr
| unary_print_expr non_unary_print_expr
| unary_print_expr '~' print_expr
| unary_print_expr NO_MATCH print_expr
| unary_print_expr In NAME
| unary_print_expr AND newline_opt print_expr
| unary_print_expr OR newline_opt print_expr
| unary_print_expr '?' print_expr ':' print_expr
;
non_unary_print_expr : '(' expr ')'
| '!' print_expr
| non_unary_print_expr '^' print_expr
| non_unary_print_expr '*' print_expr
| non_unary_print_expr '/' print_expr
| non_unary_print_expr '%' print_expr
| non_unary_print_expr '+' print_expr
| non_unary_print_expr '−' print_expr
| non_unary_print_expr non_unary_print_expr
| non_unary_print_expr '~' print_expr
| non_unary_print_expr NO_MATCH print_expr
| non_unary_print_expr In NAME
| '(' multiple_expr_list ')' In NAME
| non_unary_print_expr AND newline_opt print_expr
| non_unary_print_expr OR newline_opt print_expr
| non_unary_print_expr '?' print_expr ':' print_expr
| NUMBER
| STRING
| lvalue
| ERE
| lvalue INCR
| lvalue DECR
| INCR lvalue
| DECR lvalue
| lvalue POW_ASSIGN print_expr
| lvalue MOD_ASSIGN print_expr
| lvalue MUL_ASSIGN print_expr
| lvalue DIV_ASSIGN print_expr
| lvalue ADD_ASSIGN print_expr
| lvalue SUB_ASSIGN print_expr
| lvalue '=' print_expr
| FUNC_NAME '(' expr_list_opt ')'
/* no white space allowed before '(' */
| BUILTIN_FUNC_NAME '(' expr_list_opt ')'
| BUILTIN_FUNC_NAME
;
lvalue : NAME
| NAME '[' expr_list ']'
| '$' expr
;
non_unary_input_function : simple_get
| simple_get '<' expr
| non_unary_expr '|' simple_get
;
unary_input_function : unary_expr '|' simple_get
;
simple_get : GETLINE
| GETLINE lvalue
;
newline_opt : /* empty */
| newline_opt NEWLINE
;
This grammar has several ambiguities that shall be resolved as
follows:
* Operator precedence and associativity shall be as described in
Table 4-1, Expressions in Decreasing Precedence in awk.
* In case of ambiguity, an else shall be associated with the most
immediately preceding if that would satisfy the grammar.
* In some contexts, a <slash> ('/') that is used to surround an ERE
could also be the division operator. This shall be resolved in
such a way that wherever the division operator could appear, a
<slash> is assumed to be the division operator. (There is no
unary division operator.)
Each expression in an awk program shall conform to the precedence and
associativity rules, even when this is not needed to resolve an
ambiguity. For example, because '$' has higher precedence than '++',
the string "$x++−−" is not a valid awk expression, even though it is
unambiguously parsed by the grammar as "$(x++)−−".
One convention that might not be obvious from the formal grammar is
where <newline> characters are acceptable. There are several obvious
placements such as terminating a statement, and a <backslash> can be
used to escape <newline> characters between any lexical tokens. In
addition, <newline> characters without <backslash> characters can
follow a comma, an open brace, logical AND operator ("&&"), logical
OR operator ("||"), the do keyword, the else keyword, and the closing
parenthesis of an if, for, or while statement. For example:
{ print $1,
$2 }
Lexical Conventions
The lexical conventions for awk programs, with respect to the
preceding grammar, shall be as follows:
1. Except as noted, awk shall recognize the longest possible token
or delimiter beginning at a given point.
2. A comment shall consist of any characters beginning with the
<number-sign> character and terminated by, but excluding the next
occurrence of, a <newline>. Comments shall have no effect,
except to delimit lexical tokens.
3. The <newline> shall be recognized as the token NEWLINE.
4. A <backslash> character immediately followed by a <newline> shall
have no effect.
5. The token STRING shall represent a string constant. A string
constant shall begin with the character '"'. Within a string
constant, a <backslash> character shall be considered to begin an
escape sequence as specified in the table in the Base Definitions
volume of POSIX.1‐2008, Chapter 5, File Format Notation ('\\',
'\a', '\b', '\f', '\n', '\r', '\t', '\v'). In addition, the
escape sequences in Table 4-2, Escape Sequences in awk shall be
recognized. A <newline> shall not occur within a string constant.
A string constant shall be terminated by the first unescaped
occurrence of the character '"' after the one that begins the
string constant. The value of the string shall be the sequence of
all unescaped characters and values of escape sequences between,
but not including, the two delimiting '"' characters.
6. The token ERE represents an extended regular expression constant.
An ERE constant shall begin with the <slash> character. Within an
ERE constant, a <backslash> character shall be considered to
begin an escape sequence as specified in the table in the Base
Definitions volume of POSIX.1‐2008, Chapter 5, File Format
Notation. In addition, the escape sequences in Table 4-2, Escape
Sequences in awk shall be recognized. The application shall
ensure that a <newline> does not occur within an ERE constant. An
ERE constant shall be terminated by the first unescaped
occurrence of the <slash> character after the one that begins the
ERE constant. The extended regular expression represented by the
ERE constant shall be the sequence of all unescaped characters
and values of escape sequences between, but not including, the
two delimiting <slash> characters.
7. A <blank> shall have no effect, except to delimit lexical tokens
or within STRING or ERE tokens.
8. The token NUMBER shall represent a numeric constant. Its form and
numeric value shall either be equivalent to the decimal-floating-
constant token as specified by the ISO C standard, or it shall be
a sequence of decimal digits and shall be evaluated as an integer
constant in decimal. In addition, implementations may accept
numeric constants with the form and numeric value equivalent to
the hexadecimal-constant and hexadecimal-floating-constant tokens
as specified by the ISO C standard.
If the value is too large or too small to be representable (see
Section 1.1.2, Concepts Derived from the ISO C Standard), the
behavior is undefined.
9. A sequence of underscores, digits, and alphabetics from the
portable character set (see the Base Definitions volume of
POSIX.1‐2008, Section 6.1, Portable Character Set), beginning
with an <underscore> or alphabetic character, shall be considered
a word.
10. The following words are keywords that shall be recognized as
individual tokens; the name of the token is the same as the
keyword:
BEGIN delete END function in printf
break do exit getline next return
continue else for if print while
11. The following words are names of built-in functions and shall be
recognized as the token BUILTIN_FUNC_NAME:
atan2 gsub log split sub toupper
close index match sprintf substr
cos int rand sqrt system
exp length sin srand tolower
The above-listed keywords and names of built-in functions are
considered reserved words.
12. The token NAME shall consist of a word that is not a keyword or a
name of a built-in function and is not followed immediately
(without any delimiters) by the '(' character.
13. The token FUNC_NAME shall consist of a word that is not a keyword
or a name of a built-in function, followed immediately (without
any delimiters) by the '(' character. The '(' character shall not
be included as part of the token.
14. The following two-character sequences shall be recognized as the
named tokens:
┌───────────┬──────────┬────────────┬──────────┐
│Token Name │ Sequence │ Token Name │ Sequence │
├───────────┼──────────┼────────────┼──────────┤
│ADD_ASSIGN │ += │ NO_MATCH │ !~ │
│SUB_ASSIGN │ −= │ EQ │ == │
│MUL_ASSIGN │ *= │ LE │ <= │
│DIV_ASSIGN │ /= │ GE │ >= │
│MOD_ASSIGN │ %= │ NE │ != │
│POW_ASSIGN │ ^= │ INCR │ ++ │
│OR │ || │ DECR │ −− │
│AND │ && │ APPEND │ >> │
└───────────┴──────────┴────────────┴──────────┘
15. The following single characters shall be recognized as tokens
whose names are the character:
<newline> { } ( ) [ ] , ; + − * % ^ ! > < | ? : " " $ =
There is a lexical ambiguity between the token ERE and the tokens '/'
and DIV_ASSIGN. When an input sequence begins with a <slash>
character in any syntactic context where the token '/' or DIV_ASSIGN
could appear as the next token in a valid program, the longer of
those two tokens that can be recognized shall be recognized. In any
other syntactic context where the token ERE could appear as the next
token in a valid program, the token ERE shall be recognized.
The following exit values shall be returned:
0 All input files were processed successfully.
>0 An error occurred.
The exit status can be altered within the program by using an exit
expression.
If any file operand is specified and the named file cannot be
accessed, awk shall write a diagnostic message to standard error and
terminate without any further action.
If the program specified by either the program operand or a progfile
operand is not a valid awk program (as specified in the EXTENDED
DESCRIPTION section), the behavior is undefined.
The following sections are informative.
The index, length, match, and substr functions should not be confused
with similar functions in the ISO C standard; the awk versions deal
with characters, while the ISO C standard deals with bytes.
Because the concatenation operation is represented by adjacent
expressions rather than an explicit operator, it is often necessary
to use parentheses to enforce the proper evaluation precedence.
The awk program specified in the command line is most easily
specified within single-quotes (for example, 'program') for
applications using sh, because awk programs commonly contain
characters that are special to the shell, including double-quotes. In
the cases where an awk program contains single-quote characters, it
is usually easiest to specify most of the program as strings within
single-quotes concatenated by the shell with quoted single-quote
characters. For example:
awk '/'\''/ { print "quote:", $0 }'
prints all lines from the standard input containing a single-quote
character, prefixed with quote:.
The following are examples of simple awk programs:
1. Write to the standard output all input lines for which field 3 is
greater than 5:
$3 > 5
2. Write every tenth line:
(NR % 10) == 0
3. Write any line with a substring matching the regular expression:
/(G|D)(2[0−9][[:alpha:]]*)/
4. Print any line with a substring containing a 'G' or 'D', followed
by a sequence of digits and characters. This example uses
character classes digit and alpha to match language-independent
digit and alphabetic characters respectively:
/(G|D)([[:digit:][:alpha:]]*)/
5. Write any line in which the second field matches the regular
expression and the fourth field does not:
$2 " " /xyz/ && $4 ! " " /xyz/
6. Write any line in which the second field contains a <backslash>:
$2 " " /\\/
7. Write any line in which the second field contains a <backslash>.
Note that <backslash>-escapes are interpreted twice; once in
lexical processing of the string and once in processing the
regular expression:
$2 " " "\\\\"
8. Write the second to the last and the last field in each line.
Separate the fields by a <colon>:
{OFS=":";print $(NF−1), $NF}
9. Write the line number and number of fields in each line. The
three strings representing the line number, the <colon>, and the
number of fields are concatenated and that string is written to
standard output:
{print NR ":" NF}
10. Write lines longer than 72 characters:
length($0) > 72
11. Write the first two fields in opposite order separated by OFS:
{ print $2, $1 }
12. Same, with input fields separated by a <comma> or <space> and
<tab> characters, or both:
BEGIN { FS = ",[ \t]*|[ \t]+" }
{ print $2, $1 }
13. Add up the first column, print sum, and average:
{s += $1 }
END {print "sum is ", s, " average is", s/NR}
14. Write fields in reverse order, one per line (many lines out for
each line in):
{ for (i = NF; i > 0; −−i) print $i }
15. Write all lines between occurrences of the strings start and
stop:
/start/, /stop/
16. Write all lines whose first field is different from the previous
one:
$1 != prev { print; prev = $1 }
17. Simulate echo:
BEGIN {
for (i = 1; i < ARGC; ++i)
printf("%s%s", ARGV[i], i==ARGC−1?"\n":" ")
}
18. Write the path prefixes contained in the PATH environment
variable, one per line:
BEGIN {
n = split (ENVIRON["PATH"], path, ":")
for (i = 1; i <= n; ++i)
print path[i]
}
19. If there is a file named input containing page headers of the
form: Page #
and a file named program that contains:
/Page/ { $2 = n++; }
{ print }
then the command line:
awk −f program n=5 input
prints the file input, filling in page numbers starting at 5.
This description is based on the new awk, ``nawk'', (see the
referenced The AWK Programming Language), which introduced a number
of new features to the historical awk:
1. New keywords: delete, do, function, return
2. New built-in functions: atan2, close, cos, gsub, match, rand,
sin, srand, sub, system
3. New predefined variables: FNR, ARGC, ARGV, RSTART, RLENGTH,
SUBSEP
4. New expression operators: ?, :, ,, ^
5. The FS variable and the third argument to split, now treated as
extended regular expressions.
6. The operator precedence, changed to more closely match the C
language. Two examples of code that operate differently are:
while ( n /= 10 > 1) ...
if (!"wk" ~ /bwk/) ...
Several features have been added based on newer implementations of
awk:
* Multiple instances of −f progfile are permitted.
* The new option −v assignment.
* The new predefined variable ENVIRON.
* New built-in functions toupper and tolower.
* More formatting capabilities are added to printf to match the
ISO C standard.
The overall awk syntax has always been based on the C language, with
a few features from the shell command language and other sources.
Because of this, it is not completely compatible with any other
language, which has caused confusion for some users. It is not the
intent of the standard developers to address such issues. A few
relatively minor changes toward making the language more compatible
with the ISO C standard were made; most of these changes are based on
similar changes in recent implementations, as described above. There
remain several C-language conventions that are not in awk. One of
the notable ones is the <comma> operator, which is commonly used to
specify multiple expressions in the C language for statement. Also,
there are various places where awk is more restrictive than the C
language regarding the type of expression that can be used in a given
context. These limitations are due to the different features that the
awk language does provide.
Regular expressions in awk have been extended somewhat from
historical implementations to make them a pure superset of extended
regular expressions, as defined by POSIX.1‐2008 (see the Base
Definitions volume of POSIX.1‐2008, Section 9.4, Extended Regular
Expressions). The main extensions are internationalization features
and interval expressions. Historical implementations of awk have long
supported <backslash>-escape sequences as an extension to extended
regular expressions, and this extension has been retained despite
inconsistency with other utilities. The number of escape sequences
recognized in both extended regular expressions and strings has
varied (generally increasing with time) among implementations. The
set specified by POSIX.1‐2008 includes most sequences known to be
supported by popular implementations and by the ISO C standard. One
sequence that is not supported is hexadecimal value escapes beginning
with '\x'. This would allow values expressed in more than 9 bits to
be used within awk as in the ISO C standard. However, because this
syntax has a non-deterministic length, it does not permit the
subsequent character to be a hexadecimal digit. This limitation can
be dealt with in the C language by the use of lexical string
concatenation. In the awk language, concatenation could also be a
solution for strings, but not for extended regular expressions
(either lexical ERE tokens or strings used dynamically as regular
expressions). Because of this limitation, the feature has not been
added to POSIX.1‐2008.
When a string variable is used in a context where an extended regular
expression normally appears (where the lexical token ERE is used in
the grammar) the string does not contain the literal <slash>
characters.
Some versions of awk allow the form:
func name(args, ... ) { statements }
This has been deprecated by the authors of the language, who asked
that it not be specified.
Historical implementations of awk produce an error if a next
statement is executed in a BEGIN action, and cause awk to terminate
if a next statement is executed in an END action. This behavior has
not been documented, and it was not believed that it was necessary to
standardize it.
The specification of conversions between string and numeric values is
much more detailed than in the documentation of historical
implementations or in the referenced The AWK Programming Language.
Although most of the behavior is designed to be intuitive, the
details are necessary to ensure compatible behavior from different
implementations. This is especially important in relational
expressions since the types of the operands determine whether a
string or numeric comparison is performed. From the perspective of an
application developer, it is usually sufficient to expect intuitive
behavior and to force conversions (by adding zero or concatenating a
null string) when the type of an expression does not obviously match
what is needed. The intent has been to specify historical practice in
almost all cases. The one exception is that, in historical
implementations, variables and constants maintain both string and
numeric values after their original value is converted by any use.
This means that referencing a variable or constant can have
unexpected side-effects. For example, with historical implementations
the following program:
{
a = "+2"
b = 2
if (NR % 2)
c = a + b
if (a == b)
print "numeric comparison"
else
print "string comparison"
}
would perform a numeric comparison (and output numeric comparison)
for each odd-numbered line, but perform a string comparison (and
output string comparison) for each even-numbered line. POSIX.1‐2008
ensures that comparisons will be numeric if necessary. With
historical implementations, the following program:
BEGIN {
OFMT = "%e"
print 3.14
OFMT = "%f"
print 3.14
}
would output "3.140000e+00" twice, because in the second print
statement the constant "3.14" would have a string value from the
previous conversion. POSIX.1‐2008 requires that the output of the
second print statement be "3.140000". The behavior of historical
implementations was seen as too unintuitive and unpredictable.
It was pointed out that with the rules contained in early drafts, the
following script would print nothing:
BEGIN {
y[1.5] = 1
OFMT = "%e"
print y[1.5]
}
Therefore, a new variable, CONVFMT, was introduced. The OFMT variable
is now restricted to affecting output conversions of numbers to
strings and CONVFMT is used for internal conversions, such as
comparisons or array indexing. The default value is the same as that
for OFMT, so unless a program changes CONVFMT (which no historical
program would do), it will receive the historical behavior associated
with internal string conversions.
The POSIX awk lexical and syntactic conventions are specified more
formally than in other sources. Again the intent has been to specify
historical practice. One convention that may not be obvious from the
formal grammar as in other verbal descriptions is where <newline>
characters are acceptable. There are several obvious placements such
as terminating a statement, and a <backslash> can be used to escape
<newline> characters between any lexical tokens. In addition,
<newline> characters without <backslash> characters can follow a
comma, an open brace, a logical AND operator ("&&"), a logical OR
operator ("||"), the do keyword, the else keyword, and the closing
parenthesis of an if, for, or while statement. For example:
{ print $1,
$2 }
The requirement that awk add a trailing <newline> to the program
argument text is to simplify the grammar, making it match a text file
in form. There is no way for an application or test suite to
determine whether a literal <newline> is added or whether awk simply
acts as if it did.
POSIX.1‐2008 requires several changes from historical implementations
in order to support internationalization. Probably the most subtle of
these is the use of the decimal-point character, defined by the
LC_NUMERIC category of the locale, in representations of floating-
point numbers. This locale-specific character is used in recognizing
numeric input, in converting between strings and numeric values, and
in formatting output. However, regardless of locale, the <period>
character (the decimal-point character of the POSIX locale) is the
decimal-point character recognized in processing awk programs
(including assignments in command line arguments). This is
essentially the same convention as the one used in the ISO C
standard. The difference is that the C language includes the
setlocale() function, which permits an application to modify its
locale. Because of this capability, a C application begins executing
with its locale set to the C locale, and only executes in the
environment-specified locale after an explicit call to setlocale().
However, adding such an elaborate new feature to the awk language was
seen as inappropriate for POSIX.1‐2008. It is possible to execute an
awk program explicitly in any desired locale by setting the
environment in the shell.
The undefined behavior resulting from NULs in extended regular
expressions allows future extensions for the GNU gawk program to
process binary data.
The behavior in the case of invalid awk programs (including lexical,
syntactic, and semantic errors) is undefined because it was
considered overly limiting on implementations to specify. In most
cases such errors can be expected to produce a diagnostic and a non-
zero exit status. However, some implementations may choose to extend
the language in ways that make use of certain invalid constructs.
Other invalid constructs might be deemed worthy of a warning, but
otherwise cause some reasonable behavior. Still other constructs may
be very difficult to detect in some implementations. Also, different
implementations might detect a given error during an initial parsing
of the program (before reading any input files) while others might
detect it when executing the program after reading some input.
Implementors should be aware that diagnosing errors as early as
possible and producing useful diagnostics can ease debugging of
applications, and thus make an implementation more usable.
The unspecified behavior from using multi-character RS values is to
allow possible future extensions based on extended regular
expressions used for record separators. Historical implementations
take the first character of the string and ignore the others.
Unspecified behavior when split(string,array,<null>) is used is to
allow a proposed future extension that would split up a string into
an array of individual characters.
In the context of the getline function, equally good arguments for
different precedences of the | and < operators can be made.
Historical practice has been that:
getline < "a" "b"
is parsed as:
( getline < "a" ) "b"
although many would argue that the intent was that the file ab should
be read. However:
getline < "x" + 1
parses as:
getline < ( "x" + 1 )
Similar problems occur with the | version of getline, particularly in
combination with $. For example:
$"echo hi" | getline
(This situation is particularly problematic when used in a print
statement, where the |getline part might be a redirection of the
print.)
Since in most cases such constructs are not (or at least should not)
be used (because they have a natural ambiguity for which there is no
conventional parsing), the meaning of these constructs has been made
explicitly unspecified. (The effect is that a conforming application
that runs into the problem must parenthesize to resolve the
ambiguity.) There appeared to be few if any actual uses of such
constructs.
Grammars can be written that would cause an error under these
circumstances. Where backwards-compatibility is not a large
consideration, implementors may wish to use such grammars.
Some historical implementations have allowed some built-in functions
to be called without an argument list, the result being a default
argument list chosen in some ``reasonable'' way. Use of length as a
synonym for length($0) is the only one of these forms that is thought
to be widely known or widely used; this particular form is documented
in various places (for example, most historical awk reference pages,
although not in the referenced The AWK Programming Language) as
legitimate practice. With this exception, default argument lists
have always been undocumented and vaguely defined, and it is not at
all clear how (or if) they should be generalized to user-defined
functions. They add no useful functionality and preclude possible
future extensions that might need to name functions without calling
them. Not standardizing them seems the simplest course. The standard
developers considered that length merited special treatment, however,
since it has been documented in the past and sees possibly
substantial use in historical programs. Accordingly, this usage has
been made legitimate, but Issue 5 removed the obsolescent marking for
XSI-conforming implementations and many otherwise conforming
applications depend on this feature.
In sub and gsub, if repl is a string literal (the lexical token
STRING), then two consecutive <backslash> characters should be used
in the string to ensure a single <backslash> will precede the
<ampersand> when the resultant string is passed to the function. (For
example, to specify one literal <ampersand> in the replacement
string, use gsub(ERE, "\\&").)
Historically, the only special character in the repl argument of sub
and gsub string functions was the <ampersand> ('&') character and
preceding it with the <backslash> character was used to turn off its
special meaning.
The description in the ISO POSIX‐2:1993 standard introduced behavior
such that the <backslash> character was another special character and
it was unspecified whether there were any other special characters.
This description introduced several portability problems, some of
which are described below, and so it has been replaced with the more
historical description. Some of the problems include:
* Historically, to create the replacement string, a script could
use gsub(ERE, "\\&"), but with the ISO POSIX‐2:1993 standard
wording, it was necessary to use gsub(ERE, "\\\\&"). The
<backslash> characters are doubled here because all string
literals are subject to lexical analysis, which would reduce each
pair of <backslash> characters to a single <backslash> before
being passed to gsub.
* Since it was unspecified what the special characters were, for
portable scripts to guarantee that characters are printed
literally, each character had to be preceded with a <backslash>.
(For example, a portable script had to use gsub(ERE, "\\h\\i") to
produce a replacement string of "hi".)
The description for comparisons in the ISO POSIX‐2:1993 standard did
not properly describe historical practice because of the way numeric
strings are compared as numbers. The current rules cause the
following code:
if (0 == "000")
print "strange, but true"
else
print "not true"
to do a numeric comparison, causing the if to succeed. It should be
intuitively obvious that this is incorrect behavior, and indeed, no
historical implementation of awk actually behaves this way.
To fix this problem, the definition of numeric string was enhanced to
include only those values obtained from specific circumstances
(mostly external sources) where it is not possible to determine
unambiguously whether the value is intended to be a string or a
numeric.
Variables that are assigned to a numeric string shall also be treated
as a numeric string. (For example, the notion of a numeric string can
be propagated across assignments.) In comparisons, all variables
having the uninitialized value are to be treated as a numeric operand
evaluating to the numeric value zero.
Uninitialized variables include all types of variables including
scalars, array elements, and fields. The definition of an
uninitialized value in Variables and Special Variables is necessary
to describe the value placed on uninitialized variables and on fields
that are valid (for example, < $NF) but have no characters in them
and to describe how these variables are to be used in comparisons. A
valid field, such as $1, that has no characters in it can be obtained
from an input line of "\t\t" when FS='\t'. Historically, the
comparison ($1<10) was done numerically after evaluating $1 to the
value zero.
The phrase ``... also shall have the numeric value of the numeric
string'' was removed from several sections of the ISO POSIX‐2:1993
standard because is specifies an unnecessary implementation detail.
It is not necessary for POSIX.1‐2008 to specify that these objects be
assigned two different values. It is only necessary to specify that
these objects may evaluate to two different values depending on
context.
Historical implementations of awk did not parse hexadecimal integer
or floating constants like "0xa" and "0xap0". Due to an oversight,
the 2001 through 2004 editions of this standard required support for
hexadecimal floating constants. This was due to the reference to
atof(). This version of the standard allows but does not require
implementations to use atof() and includes a description of how
floating-point numbers are recognized as an alternative to match
historic behavior. The intent of this change is to allow
implementations to recognize floating-point constants according to
either the ISO/IEC 9899:1990 standard or ISO/IEC 9899:1999 standard,
and to allow (but not require) implementations to recognize
hexadecimal integer constants.
Historical implementations of awk did not support floating-point
infinities and NaNs in numeric strings; e.g., "−INF" and "NaN".
However, implementations that use the atof() or strtod() functions to
do the conversion picked up support for these values if they used a
ISO/IEC 9899:1999 standard version of the function instead of a
ISO/IEC 9899:1990 standard version. Due to an oversight, the 2001
through 2004 editions of this standard did not allow support for
infinities and NaNs, but in this revision support is allowed (but not
required). This is a silent change to the behavior of awk programs;
for example, in the POSIX locale the expression:
("-INF" + 0 < 0)
formerly had the value 0 because "−INF" converted to 0, but now it
may have the value 0 or 1.
None.
Section 1.3, Grammar Conventions, grep(1p), lex(1p), sed(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 5, File Format
Notation, Section 6.1, Portable Character Set, Chapter 8, Environment
Variables, Chapter 9, Regular Expressions, Section 12.2, Utility
Syntax Guidelines
The System Interfaces volume of POSIX.1‐2008, atof(3p), exec(1p),
isspace(3p), popen(3p), setlocale(3p), strtod(3p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 AWK(1P)
Pages that refer to this page: bc(1p), join(1p), printf(1p), sed(1p)