AWK(1)                     POSIX Programmer's Manual                    AWK(1)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 44--11:: 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 44--22:: 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 dd or
           uu conversion specifier characters with <blank> characters not
           specified by the format string.

        6. The implementation shall not precede output from the oo conversion
           specifier character with leading zeros not specified by the format
           string.

        7. For the cc 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 cc 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
                 [-<pi>,<pi>].

       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 ""33..1144"" would have a string value from the
       previous conversion. POSIX.1-2008 requires that the output of the
       second print statement be ""33..114400000000"".  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
       $$11, that has no characters in it can be obtained from an input line of
       "\t\t" when FS='\t'.  Historically, the comparison ($$11<<10) was done
       numerically after evaluating $$11 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 .



IEEE/The Open Group                  2017                               AWK(1)
