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



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

NAME
       awk - pattern scanning and processing language

SYNOPSIS
       awk [-F ERE][-v assignment] ... program [argument ...]

       awk [-F ERE] -f progfile ...  [-v assignment] ...[argument ...]


DESCRIPTION
       The  awk  utility shall execute programs written in the awk programming language, which is specialized for tex-
       tual 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 <new-
       line>, 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>s. This default white-space field delimiter can be changed by using the FS built-in vari-
       able or -F ERE. The awk utility shall denote the first field in a record $1, the second $2, and so on. The sym-
       bol $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 IEEE Std 1003.1-2001, Section 12.2, Utility
       Syntax Guidelines.

       The following options shall be supported:

       -F  ERE
              Define the input field separator to be the extended regular expression ERE, before any  input  is  read;
              see Regular Expressions .

       -f  progfile
              Specify  the  pathname  of  the  file  progfile containing an awk program. 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 argu-
              ment.

       -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  appli-
              cation shall supply the program operand as a single argument to awk. If the text does not end in a <new-
              line>, 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 IEEE Std 1003.1-2001, 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 equal 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  assign-
              ment  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 '-' ; see  the
       INPUT  FILES section. If the awk program contains no actions and no patterns, but is otherwise a valid awk pro-
       gram, 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the prece-
              dence 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 ele-
              ments 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  exam-
              ple,  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  map-
              ping 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 writ-
              ten 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  process-
              ing 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 IEEE Std 1003.1-2001, 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 oper-
       ations are given in groups from highest precedence first to lowest precedence last, with equal-precedence oper-
       ators 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: 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             Pre-increment             Numeric          N/A
                       -- lvalue             Pre-decrement             Numeric          N/A
                       lvalue ++             Post-increment            Numeric          N/A
                       lvalue --             Post-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 con-
       texts, 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 by the equivalent of the following calls to func-
       tions defined by the ISO C standard:


              setlocale(LC_NUMERIC, "");
              numeric_value = atof(string_value);

       A numeric value that is exactly equal to the value of an integer (see 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 argu-
       ment.  Any  other numeric value shall be converted to a string by the equivalent of a call to the sprintf func-
       tion 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  IEEE Std 1003.1-2001  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 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>s are discarded.


        * If the first non- <blank> is '+' or '-', it is discarded.


        * Changing each occurrence of the decimal point character from the current locale to a period.


       If  a  '-'  character is ignored in the preceding description, 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
       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  identi-
       fiers,  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-dimen-
       sional, 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 follow-
       ing two index operations shall be equivalent:


              var[expr1, expr2, ... exprn]


              var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]

       The application shall ensure that a multi-dimensioned index used with the in operator is parenthesized. The  in
       operator,  which  tests for the existence of a particular array element, shall not cause that element to exist.
       Any other reference to a nonexistent array element shall automatically create it.

       Comparisons (with the '<', "<=", "!=", "==", '>', and  ">="  operators)  shall  be  made  numerically  if  both
       operands are numeric, if one is numeric and the other has a string value that is a numeric string, or if one is
       numeric and the other has the uninitialized value.  Otherwise,  operands  shall  be  converted  to  strings  as
       required  and  a string comparison shall be made using the locale-specific collation sequence. The value of the
       comparison expression shall be 1 if the relation is true, or 0 if the relation is false.

   Variables and Special Variables
       Variables can be used in an awk program by referencing them.  With the exception of  function  parameters  (see
       User-Defined  Functions  ),  they  are  not explicitly declared. Function parameter names shall be local to the
       function; all other variable names shall be global. The same name shall not be used as both a function  parame-
       ter  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  argu-
       ment 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 IEEE Std 1003.1-2001. 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  con-
              sisting  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 separation; <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 <new-
              line> 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
       IEEE Std 1003.1-2001,  Section  9.4, Extended Regular Expressions) except that it shall allow the use of C-lan-
       guage conventions for escaping special characters within the EREs, as specified in the table in the Base  Defi-
       nitions  volume  of IEEE Std 1003.1-2001, 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>s and string constants can contain
       <newline>s, 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: 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
                                    by the longest sequence of     is represented by the one,
                                    one, two, or three octal-digit two, or three-digit octal
                                    characters (01234567). If all  integer. Multi-byte characters
                                    of the digits are 0 (that is,  require multiple, concatenated
                                    representation of the NUL      escape sequences of this type,
                                    character), the behavior is    including the leading '\' for
                                    undefined.                     each byte.
                           \c       A backslash character followed Undefined
                                    by any character not described
                                    in this table or in the table
                                    in the Base Definitions volume
                                    of IEEE Std 1003.1-2001, Chap-
                                    ter 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  expres-
       sion  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
       IEEE Std 1003.1-2001,  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 "!~" oper-
       ator  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 Func-
       tions  )  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 using the -F ERE  option  or  by  assigning  a
       string  containing  the expression to the built-in variable FS. 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>s; fields shall be delimited by sets of one or  more
               <blank>s.


            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>s 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 get-
       line function-see Input/Output and General Functions - in a prior BEGIN action) and before command line assign-
       ment  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 exe-
       cuted. 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 braces. The application shall ensure that statements in a statement list are
       separated by <newline>s or semicolons. 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 Concepts
       Derived from the ISO C Standard ), except that the Boolean expressions shall be treated as described in Expres-
       sions 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,  trun-
       cating  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
       IEEE Std 1003.1-2001  with  the  value of expression as the command argument and a value of w as the mode argu-
       ment. 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 IEEE Std 1003.1-2001.

       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 sepa-
       rated by the current output field separator (see variable OFS above), and terminated by the output record sepa-
       rator  (see  variable ORS above). All expression arguments shall be taken as strings, being converted if neces-
       sary; 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  IEEE Std 1003.1-2001  (see  the  Base  Definitions  volume  of
       IEEE Std 1003.1-2001, 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 charac-
           ters that are copied to the output. Note that these same sequences shall be interpreted  lexically  by  awk
           when they appear in literal strings, but they shall not be treated specially by the printf statement.


        4. A  field  width  or precision can be specified as the '*' character instead of a digit string. In this case
           the next argument from the expression list shall be fetched and its numeric value taken as the field  width
           or precision.


        5. The  implementation shall not precede or follow output from the d or u conversion specifier characters with
           <blank>s not specified by the format string.


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


        7. For the c conversion specifier character: if the argument has a numeric value, the character whose encoding
           is that value shall be output. If the value is zero or is not the encoding of any character in the  charac-
           ter  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 evalu-
           ated. With the exception of the c conversion specifier character, the value shall be  converted  (according
           to the rules specified in Expressions in awk ) to the appropriate type for the conversion specification.


        9. If  there  are insufficient expression arguments to satisfy all the conversion specifications in the format
           string, the behavior is undefined.


       10. If any character sequence in the format string begins with a '%' character, but does not form a valid  con-
           version 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 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 back-
              slashes 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 charac-
              ter. 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  unde-
              fined. 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 omit-
              ted, 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  com-
              mand  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 evalu-
              ates 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 concate-
       nate) 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 IEEE Std 1003.1-2001 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>s, 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>s 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 Grammar Conventions
       . A valid program can be represented as the non-terminal symbol program in  the  grammar.  This  formal  syntax
       shall take precedence over the preceding text syntax description.


              %token NAME NUMBER STRING ERE
              %token FUNC_NAME   /* Name followed by '(' without white space. */


              /* Keywords  */
              %token       Begin   End
              /*          'BEGIN' 'END'                            */


              %token       Break   Continue   Delete   Do   Else
              /*          'break' 'continue' 'delete' 'do' 'else'  */


              %token       Exit   For   Function   If   In
              /*          'exit' 'for' 'function' 'if' 'in'        */


              %token       Next   Print   Printf   Return   While
              /*          'next' 'print' 'printf' 'return' 'while' */


              /* Reserved function names */
              %token BUILTIN_FUNC_NAME
                          /* One token for the following:
                           * atan2 cos sin exp log sqrt int rand srand
                           * gsub index length match split sprintf sub
                           * substr tolower toupper close system
                           */
              %token GETLINE
                          /* Syntactically different from other built-ins. */


              /* Two-character tokens. */
              %token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
              /*     '+='       '-='       '*='       '/='       '%='       '^=' */


              %token OR   AND  NO_MATCH   EQ   LE   GE   NE   INCR  DECR  APPEND
              /*     '||' '&&' '!~' '==' '<=' '>=' '!=' '++'  '--'  '>>'   */


              /* One-character tokens. */
              %token '{' '}' '(' ')' '[' ']' ',' ';' NEWLINE
              %token '+' '-' '*' '%' '^' '!' '>' '<' '|' '?' ':' '~' '$' '='


              %start program
              %%


              program          : item_list
                               | actionless_item_list
                               ;


              item_list        : newline_opt
                               | actionless_item_list item terminator
                               | item_list            item terminator
                               | item_list          action terminator
                               ;


              actionless_item_list : item_list            pattern terminator
                               | actionless_item_list pattern terminator
                               ;


              item             : pattern action
                               | Function NAME      '(' param_list_opt ')'
                                     newline_opt action
                               | Function FUNC_NAME '(' param_list_opt ')'
                                     newline_opt action
                               ;


              param_list_opt   : /* empty */
                               | param_list
                               ;


              param_list       : NAME
                               | param_list ',' NAME
                               ;


              pattern          : Begin
                               | End
                               | expr
                               | expr ',' newline_opt expr
                               ;


              action           : '{' newline_opt                             '}'
                               | '{' newline_opt terminated_statement_list   '}'
                               | '{' newline_opt unterminated_statement_list '}'
                               ;


              terminator       : terminator ';'
                               | terminator NEWLINE
                               |            ';'
                               |            NEWLINE
                               ;


              terminated_statement_list : terminated_statement
                               | terminated_statement_list terminated_statement
                               ;


              unterminated_statement_list : unterminated_statement
                               | terminated_statement_list unterminated_statement
                               ;


              terminated_statement : action newline_opt
                               | If '(' expr ')' newline_opt terminated_statement
                               | If '(' expr ')' newline_opt terminated_statement
                                     Else newline_opt terminated_statement
                               | While '(' expr ')' newline_opt terminated_statement
                               | For '(' simple_statement_opt ';'
                                    expr_opt ';' simple_statement_opt ')' newline_opt
                                    terminated_statement
                               | For '(' NAME In NAME ')' newline_opt
                                    terminated_statement
                               | ';' newline_opt
                               | terminatable_statement NEWLINE newline_opt
                               | terminatable_statement ';'     newline_opt
                               ;


              unterminated_statement : terminatable_statement
                               | If '(' expr ')' newline_opt unterminated_statement
                               | If '(' expr ')' newline_opt terminated_statement
                                    Else newline_opt unterminated_statement
                               | While '(' expr ')' newline_opt unterminated_statement
                               | For '(' simple_statement_opt ';'
                                expr_opt ';' simple_statement_opt ')' newline_opt
                                    unterminated_statement
                               | For '(' NAME In NAME ')' newline_opt
                                    unterminated_statement
                               ;


              terminatable_statement : simple_statement
                               | Break
                               | Continue
                               | Next
                               | Exit expr_opt
                               | Return expr_opt
                               | Do newline_opt terminated_statement While '(' expr ')'
                               ;


              simple_statement_opt : /* empty */
                               | simple_statement
                               ;


              simple_statement : Delete NAME '[' expr_list ']'
                               | expr
                               | print_statement
                               ;


              print_statement  : simple_print_statement
                               | simple_print_statement output_redirection
                               ;


              simple_print_statement : Print  print_expr_list_opt
                               | Print  '(' multiple_expr_list ')'
                               | Printf print_expr_list
                               | Printf '(' multiple_expr_list ')'
                               ;


              output_redirection : '>'    expr
                               | APPEND expr
                               | '|'    expr
                               ;


              expr_list_opt    : /* empty */
                               | expr_list
                               ;


              expr_list        : expr
                               | multiple_expr_list
                               ;


              multiple_expr_list : expr ',' newline_opt expr
                               | multiple_expr_list ',' newline_opt expr
                               ;


              expr_opt         : /* empty */
                               | expr
                               ;


              expr             : unary_expr
                               | non_unary_expr
                               ;


              unary_expr       : '+' expr
                               | '-' expr
                               | unary_expr '^'      expr
                               | unary_expr '*'      expr
                               | unary_expr '/'      expr
                               | unary_expr '%'      expr
                               | unary_expr '+'      expr
                               | unary_expr '-'      expr
                               | unary_expr          non_unary_expr
                               | unary_expr '<'      expr
                               | unary_expr LE       expr
                               | unary_expr NE       expr
                               | unary_expr EQ       expr
                               | unary_expr '>'      expr
                               | unary_expr GE       expr
                               | unary_expr '~'      expr
                               | unary_expr NO_MATCH expr
                               | unary_expr In NAME
                               | unary_expr AND newline_opt expr
                               | unary_expr OR  newline_opt expr
                               | unary_expr '?' expr ':' expr
                               | unary_input_function
                               ;


              non_unary_expr   : '(' expr ')'
                               | '!' expr
                               | non_unary_expr '^'      expr
                               | non_unary_expr '*'      expr
                               | non_unary_expr '/'      expr
                               | non_unary_expr '%'      expr
                               | non_unary_expr '+'      expr
                               | non_unary_expr '-'      expr
                               | non_unary_expr          non_unary_expr
                               | non_unary_expr '<'      expr
                               | non_unary_expr LE       expr
                               | non_unary_expr NE       expr
                               | non_unary_expr EQ       expr
                               | non_unary_expr '>'      expr
                               | non_unary_expr GE       expr
                               | non_unary_expr '~'      expr
                               | non_unary_expr NO_MATCH expr
                               | non_unary_expr In NAME
                               | '(' multiple_expr_list ')' In NAME
                               | non_unary_expr AND newline_opt expr
                               | non_unary_expr OR  newline_opt expr
                               | non_unary_expr '?' expr ':' expr
                               | NUMBER
                               | STRING
                               | lvalue
                               | ERE
                               | lvalue INCR
                               | lvalue DECR
                               | INCR lvalue
                               | DECR lvalue
                               | lvalue POW_ASSIGN expr
                               | lvalue MOD_ASSIGN expr
                               | lvalue MUL_ASSIGN expr
                               | lvalue DIV_ASSIGN expr
                               | lvalue ADD_ASSIGN expr
                               | lvalue SUB_ASSIGN expr
                               | lvalue '=' expr
                               | FUNC_NAME '(' expr_list_opt ')'
                                    /* no white space allowed before '(' */
                               | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                               | BUILTIN_FUNC_NAME
                               | non_unary_input_function
                               ;


              print_expr_list_opt : /* empty */
                               | print_expr_list
                               ;


              print_expr_list  : print_expr
                               | print_expr_list ',' newline_opt print_expr
                               ;


              print_expr       : unary_print_expr
                               | non_unary_print_expr
                               ;


              unary_print_expr : '+' print_expr
                               | '-' print_expr
                               | unary_print_expr '^'      print_expr
                               | unary_print_expr '*'      print_expr
                               | unary_print_expr '/'      print_expr
                               | unary_print_expr '%'      print_expr
                               | unary_print_expr '+'      print_expr
                               | unary_print_expr '-'      print_expr
                               | unary_print_expr          non_unary_print_expr
                               | unary_print_expr '~'      print_expr
                               | unary_print_expr NO_MATCH print_expr
                               | unary_print_expr In NAME
                               | unary_print_expr AND newline_opt print_expr
                               | unary_print_expr OR  newline_opt print_expr
                               | unary_print_expr '?' print_expr ':' print_expr
                               ;


              non_unary_print_expr : '(' expr ')'
                               | '!' print_expr
                               | non_unary_print_expr '^'      print_expr
                               | non_unary_print_expr '*'      print_expr
                               | non_unary_print_expr '/'      print_expr
                               | non_unary_print_expr '%'      print_expr
                               | non_unary_print_expr '+'      print_expr
                               | non_unary_print_expr '-'      print_expr
                               | non_unary_print_expr          non_unary_print_expr
                               | non_unary_print_expr '~'      print_expr
                               | non_unary_print_expr NO_MATCH print_expr
                               | non_unary_print_expr In NAME
                               | '(' multiple_expr_list ')' In NAME
                               | non_unary_print_expr AND newline_opt print_expr
                               | non_unary_print_expr OR  newline_opt print_expr
                               | non_unary_print_expr '?' print_expr ':' print_expr
                               | NUMBER
                               | STRING
                               | lvalue
                               | ERE
                               | lvalue INCR
                               | lvalue DECR
                               | INCR lvalue
                               | DECR lvalue
                               | lvalue POW_ASSIGN print_expr
                               | lvalue MOD_ASSIGN print_expr
                               | lvalue MUL_ASSIGN print_expr
                               | lvalue DIV_ASSIGN print_expr
                               | lvalue ADD_ASSIGN print_expr
                               | lvalue SUB_ASSIGN print_expr
                               | lvalue '=' print_expr
                               | FUNC_NAME '(' expr_list_opt ')'
                                   /* no white space allowed before '(' */
                               | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                               | BUILTIN_FUNC_NAME
                               ;


              lvalue           : NAME
                               | NAME '[' expr_list ']'
                               | '$' expr
                               ;


              non_unary_input_function : simple_get
                               | simple_get '<' expr
                               | non_unary_expr '|' simple_get
                               ;


              unary_input_function : unary_expr '|' simple_get
                               ;


              simple_get       : GETLINE
                               | GETLINE lvalue
                               ;


              newline_opt      : /* empty */
                               | newline_opt NEWLINE
                               ;

       This grammar has several ambiguities that shall be resolved as follows:

        * Operator precedence and associativity shall be as described in 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.)


       One  convention that might not be obvious from the formal grammar is where <newline>s are acceptable. There are
       several obvious placements such as terminating a statement, and a backslash can be used  to  escape  <newline>s
       between any lexical tokens. In addition, <newline>s without backslashes can follow a comma, an open brace, log-
       ical 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  speci-
           fied in the table in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 5, File Format Notation (
           '\\', '\a', '\b', '\f', '\n', '\r', '\t', '\v' ). In addition,  the  escape  sequences  in  Expressions  in
           Decreasing  Precedence  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 IEEE Std 1003.1-2001, Chapter 5,  File
           Format  Notation. In addition, the escape sequences in Expressions in Decreasing Precedence 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  be  equivalent  to
           either  of  the  tokens  floating-constant or integer-constant as specified by the ISO C standard, with the
           following exceptions:

            a. An integer constant cannot begin with 0x or include the hexadecimal digits 'a',  'b',  'c',  'd',  'e',
               'f', 'A', 'B', 'C', 'D', 'E', or 'F' .


            b. The value of an integer constant beginning with 0 shall be taken in decimal rather than octal.


            c. An integer constant cannot include a suffix ( 'u', 'U', 'l', or 'L' ).


            d. A floating constant cannot include a suffix ( 'f', 'F', 'l', or 'L' ).


       If the value is too large or too small to be representable (see 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  Defini-
           tions volume of IEEE Std 1003.1-2001, Section 6.1, Portable Character Set), beginning with an underscore or
           alphabetic, 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, fol-
           lowed 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 stan-
       dard; 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.

EXAMPLES
       The  awk program specified in the command line is most easily specified within single-quotes (for example, pro-
       grams 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>s and <tab>s, 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.


       The overall awk syntax has always been based on the C language, with a few features from the shell command lan-
       guage  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  sev-
       eral  C-language  conventions that are not in awk. One of the notable ones is the comma operator, which is com-
       monly 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 IEEE Std 1003.1-2001 (see the Base Definitions volume
       of IEEE Std 1003.1-2001, Section 9.4, Extended Regular Expressions).  The main extensions are internationaliza-
       tion 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  incon-
       sistency  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
       IEEE Std 1003.1-2001  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 IEEE Std 1003.1-2001.

       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 slashes.

       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 documenta-
       tion 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  deter-
       mine  whether a string or numeric comparison is performed. From the perspective of an application writer, 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. IEEE Std 1003.1-2001 ensures that
       comparisons will be numeric if necessary. With historical implementations, the following program:


              BEGIN {
                  OFMT = "%e"
                  print 3.14
                  OFMT = "%f"
                  print 3.14
              }

       would  output  "3.140000e+00"  twice,  because  in  the second print statement the constant "3.14" would have a
       string value from the previous conversion. IEEE Std 1003.1-2001 requires that the output of  the  second  print
       statement  be  "3.140000"  .  The behavior of historical implementations was seen as too unintuitive and unpre-
       dictable.

       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 histor-
       ical 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>s are acceptable. There are several obvious placements such
       as terminating a statement, and a backslash can be used to escape <newline>s between  any  lexical  tokens.  In
       addition,  <newline>s without backslashes 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, mak-
       ing it match a text file in form. There is no way for an application or test suite to determine whether a  lit-
       eral <newline> is added or whether awk simply acts as if it did.

       IEEE Std 1003.1-2001  requires several changes from historical implementations in order to support internation-
       alization. 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 charac-
       ter 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 exe-
       cutes in the environment-specified locale after an explicit call to setlocale(). However, adding such an elabo-
       rate new feature to the awk language was seen as inappropriate for IEEE Std 1003.1-2001. It is possible to exe-
       cute 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 unde-
       fined 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 wor-
       thy  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  ini-
       tial  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 ambi-
       guity  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 docu-
       mented  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 charac-
       ters 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 descrip-
       tion  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, "\\\\&" ). 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 defi-
       nition of an uninitialized value in Variables and Special Variables is necessary to describe the  value  placed
       on uninitialized variables and on fields that are valid (for example, < $NF) but have no characters in them and
       to describe how these variables are to be used in comparisons. A valid field, such as $1, that has  no  charac-
       ters  in  it can be obtained from an input line of "\t\t" when FS= '\t' . Historically, the comparison ( $1<10)
       was done numerically after evaluating $1 to the value zero.

       The phrase "... also shall have the numeric value of the numeric string" was removed from several  sections  of
       the  ISO POSIX-2:1993  standard  because is specifies an unnecessary implementation detail. It is not necessary
       for IEEE Std 1003.1-2001 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.

       The description of numeric string processing is based on the behavior of the atof() function in the ISO C stan-
       dard. While it is not a requirement for an implementation to use this function, many historical implementations
       of  awk  do.  In the ISO C standard, floating-point constants use a period as a decimal point character for the
       language itself, independent of the current locale, but the atof() function and the associated  strtod()  func-
       tion use the decimal point character of the current locale when converting strings to numeric values. Similarly
       in awk, floating-point constants in an awk script use a period independent of the locale, but input strings use
       the decimal point character of the locale.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Grammar  Conventions,  grep,  lex,  sed,  the  System  Interfaces volume of IEEE Std 1003.1-2001, atof(), exec,
       popen(), setlocale(), strtod()

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 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 Stan-
       dard, 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 .



IEEE/The Open Group                  2003                              AWK(1P)