Provided by: manpages-posix_2017a-2_all 
PROLOG
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.
NAME
awk — pattern scanning and processing language
SYNOPSIS
awk [-F sepstring] [-v assignment]... program [argument...]
awk [-F sepstring] -f progfile [-f progfile]... [-v assignment]...
[argument...]
DESCRIPTION
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.
OPTIONS
The awk utility shall conform to the Base Definitions volume of POSIX.1‐2017, 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.
OPERANDS
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.1‐2017, 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.
STDIN
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
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.
ENVIRONMENT VARIABLES
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.1‐2017, 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.1‐2017, Chapter
8, Environment Variables.
In addition, all environment variables shall be visible via the awk variable ENVIRON.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The nature of the output files depends on the awk program.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
The nature of the output files depends on the awk program.
EXTENDED DESCRIPTION
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 <hyphen-minus>, 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.1‐2017 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 as follows:
* For the "!=" and "==" operators, the strings should be compared to check if they are
identical but may be compared using the locale-specific collation sequence to check if
they collate equally.
* For the other operators, the strings shall be compared 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.1‐2017. 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.1‐2017, 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.1‐2017,
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.1‐2017, 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.1‐2017, 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.1‐2017 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.1‐2017.
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.1‐2017 (see the Base
Definitions volume of POSIX.1‐2017, 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.1‐2017 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
| item_list item
;
item_list : /* empty */
| item_list item terminator
;
item : action
| pattern action
| normal_pattern
| 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 : normal_pattern
| special_pattern
;
normal_pattern : expr
| expr ',' newline_opt expr
;
special_pattern : Begin
| End
;
action : '{' newline_opt '}'
| '{' newline_opt terminated_statement_list '}'
| '{' newline_opt unterminated_statement_list '}'
;
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.1‐2017, 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.1‐2017, 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.1‐2017, 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.
EXIT STATUS
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.
CONSEQUENCES OF ERRORS
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.
APPLICATION USAGE
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.
When using awk to process pathnames, it is recommended that LC_ALL, or at least LC_CTYPE
and LC_COLLATE, are set to POSIX or C in the environment, since pathnames can contain byte
sequences that do not form valid characters in some locales, in which case the utility's
behavior would be undefined. In the POSIX locale each byte is a valid single-byte
character, and therefore this problem is avoided.
On implementations where the "==" operator checks if strings collate equally, applications
needing to check whether strings are identical can use:
length(a) == length(b) && index(a,b) == 1
On implementations where the "==" operator checks if strings are identical, applications
needing to check whether strings collate equally can use:
a <= b && a >= b
EXAMPLES
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.
RATIONALE
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.
Earlier versions of this standard required implementations to support multiple adjacent
<semicolon>s, lines with one or more <semicolon> before a rule (pattern-action pairs), and
lines with only <semicolon>(s). These are not required by this standard and are
considered poor programming practice, but can be accepted by an implementation of awk as
an extension.
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.1‐2017, 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.
FUTURE DIRECTIONS
A future version of this standard may require the "!=" and "==" operators to perform
string comparisons by checking if the strings are identical (and not by checking if they
collate equally).
SEE ALSO
Section 1.3, Grammar Conventions, grep, lex, sed
The Base Definitions volume of POSIX.1‐2017, 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.1‐2017, atof(), exec, isspace(), popen(),
setlocale(), strtod()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 .