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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 textual data  manipulation.  An  awk
        program is a sequence of patterns and corresponding actions. When input
        is read that matches a pattern, the action associated with that pattern
        is carried out.
 
        Input  shall  be  interpreted  as  a sequence of records. By default, a
        record is a line, less its  terminating  <newline>,  but  this  can  be
        changed  by  using the RS built-in variable. Each record of input shall
        be matched in turn against each pattern in the program. For  each  pat‐
        tern 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 variable or -F ERE. The awk utility  shall  denote  the  first
        field  in  a  record  $1, the second $2, and so on. The symbol $0 shall
        refer to the entire record; setting any other field causes the re-eval‐
        uation  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  Expres‐
               sions .
 
        -f  progfile
               Specify the pathname of the file progfile containing an awk pro‐
               gram. 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 alterna‐
               tively be specified in the command line as a single argument.
 
        -v  assignment
               The  application shall ensure that the assignment argument is in
               the same form as an assignment operand. The  specified  variable
               assignment  shall  occur  prior  to  executing  the awk program,
               including the actions associated with BEGIN patterns  (if  any).
               Multiple occurrences of this option can be specified.
 

OPERANDS

        The following operands shall be supported:
 
        program
               If  no -f option is specified, the first operand to awk shall be
               the text of the awk program. The application  shall  supply  the
               program  operand  as  a single argument to awk. If the text does
               not end in a <newline>, awk shall interpret the text  as  if  it
               did.
 
        argument
               Either of the following two types of argument can be intermixed:
 
        file
               A pathname of a file that contains the input to be  read,  which
               is  matched  against  the  set of patterns in the program. If no
               file operands are specified, or if a file operand is      -       ,  the
               standard input shall be used.
 
        assignment
               An  operand that begins with an underscore or alphabetic charac‐
               ter 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, fol‐
               lowed 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 vari‐
               able shall also be assigned its numeric value. Each  such  vari‐
               able  assignment shall occur just prior to the processing of the
               following file, if any. Thus, an  assignment  before  the  first
               file  argument  shall  be  executed  after the BEGIN actions (if
               any), while an assignment after the  last  file  argument  shall
               occur  before  the  END  actions  (if any). If there are no file
               arguments, assignments shall be executed before  processing  the
               standard input.
 

STDIN

        The  standard  input  shall be used only if no file operands are speci‐
        fied, 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 program, standard input and any file operands  shall  not  be
        read and awk shall exit with a return status of zero.
        Input  files to the awk program from any of the following sources shall
        be text files:
 
         * Any file operands or their equivalents, achieved  by  modifying  the
           awk variables ARGV and ARGC
 
         * Standard input in the absence of any file operands
 
         * Arguments to the getline function
 
        Whether  the  variable  RS  is set to a value other than a <newline> or
        not, for these files, implementations shall support records  terminated
        with  the  specified  separator  up to {LINE_MAX} bytes and may support
        longer records.
 
        If -f progfile is specified, the  application  shall  ensure  that  the
        files named by each of the progfile option-arguments are text files and
        their concatenation, in the same order as they appear in the arguments,
        is an awk program.
        The following environment variables shall affect the execution of awk:
 
        LANG   Provide  a  default value for the internationalization variables
               that are unset or null. (See  the  Base  Definitions  volume  of
               IEEE Std 1003.1-2001,  Section  8.2,  Internationalization Vari‐
               ables for the precedence of internationalization variables  used
               to determine the values of locale categories.)
 
        LC_ALL If  set  to a non-empty string value, override the values of all
               the other internationalization variables.
 
        LC_COLLATE
               Determine the locale for the  behavior  of  ranges,  equivalence
               classes,  and  multi-character collating elements within regular
               expressions and in comparisons of string values.
 
        LC_CTYPE
               Determine the locale for  the  interpretation  of  sequences  of
               bytes  of  text  data as characters (for example, single-byte as
               opposed to multi-byte characters in arguments and input  files),
               the  behavior  of  character classes within regular expressions,
               the identification of characters as letters, and the mapping  of
               uppercase  and  lowercase characters for the toupper and tolower
               functions.
 
        LC_MESSAGES
               Determine the locale that should be used to  affect  the  format
               and contents of diagnostic messages written to standard error.
 
        LC_NUMERIC
               Determine  the  radix  character  used when interpreting numeric
               input, performing conversions between numeric and string values,
               and  formatting numeric output. Regardless of locale, the period
               character (the decimal-point character of the POSIX  locale)  is
               the  decimal-point  character  recognized in processing awk pro‐
               grams (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  Defini‐
               tions  volume  of  IEEE Std 1003.1-2001,  Chapter 8, Environment
               Variables.
 
        In addition, all environment variables shall be  visible  via  the  awk
        variable ENVIRON.
        Default.
 

STDOUT

        The nature of the output files depends on the awk program.
 

STDERR

        The standard error shall be used only for diagnostic messages.
        The nature of the output files depends on the awk program.
    Overall Program Structure
        An awk program is composed of pairs of the form:
 
               pattern { action }
 
        Either the pattern or the action (including the enclosing brace charac‐
        ters) 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  pro‐
        gram. Then each file operand (or standard input if no files were speci‐
        fied) shall be processed in turn by reading data from the file until  a
        record separator is seen ( <newline> by default). Before the first ref‐
        erence 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 pat‐
        tern in the program then shall be evaluated in the order of occurrence,
        and the action associated with each pattern that  matches  the  current
        record  executed.  The  action for a matching pattern shall be executed
        before evaluating subsequent patterns. Finally, the actions  associated
        with  all END patterns shall be executed in the order they occur in the
        program.
 
    Expressions in awk
        Expressions describe computations used in patterns and actions.  In the
        following  table,  valid expression operations are given in groups from
        highest precedence first to lowest precedence last,  with  equal-prece‐
        dence operators grouped between horizontal lines. In expression evalua‐
        tion, where the grammar is formally ambiguous, higher precedence opera‐
        tors  shall be evaluated before lower precedence operators. In this ta‐
        ble 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 contexts, the value of  an  expres‐
        sion  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 functions 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  argument.  Any  other
        numeric  value  shall  be  converted to a string by the equivalent of a
        call to the sprintf function with the value of the variable CONVFMT  as
        the fmt argument and the numeric value being converted as the first and
        only expr argument. The result of the conversion is unspecified if  the
        value  of  CONVFMT  is  not a floating-point format specification. This
        volume  of  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  result‐
        ing 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  func‐
        tion 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  func‐
        tion call:
 
               fmod(expr1, expr2)
 
        The expression:
 
               lvalue %= expr
 
        shall be equivalent to the ISO C standard expression:
 
               lvalue = fmod(lvalue, expr)
 
        except that lvalue shall be evaluated only once.
 
        Variables and fields shall be set by the assignment statement:
 
               lvalue = expression
 
        and the type of expression shall determine the resulting variable type.
        The assignment includes the arithmetic assignments ( "+=" , "-=" , "*="
        ,  "/="  ,  "%="  ,  "^="  , "++" , "--" ) all of which shall produce a
        numeric result. The left-hand side of an assignment and the  target  of
        increment  and  decrement  operators can be one of a variable, an array
        with index, or a field selector.
 
        The awk language supplies arrays that are used for storing  numbers  or
        strings.  Arrays  need  not be declared. They shall initially be empty,
        and their sizes shall change dynamically. The  subscripts,  or  element
        identifiers,  are  strings, providing a type of associative array capa‐
        bility. An array name followed by a subscript  within  square  brackets
        can be used as an lvalue and thus as an expression, as described in the
        grammar; see Grammar . Unsubscripted array names can be  used  in  only
        the following contexts:
 
         * A parameter in a function definition or function call
 
         * The  NAME  token following any use of the keyword in as specified in
           the grammar (see Grammar ); if the name used in this context is  not
           an array name, the behavior is undefined
 
        A  valid  array  index  shall  consist  of  one or more comma-separated
        expressions, similar to the way in which multi-dimensional  arrays  are
        indexed  in  some programming languages.  Because awk arrays are really
        one-dimensional, such a comma-separated list shall be  converted  to  a
        single  string  by  concatenating  the  string  values  of the separate
        expressions, each separated from the other by the value of  the  SUBSEP
        variable.   Thus,  the  following two index operations shall be equiva‐
        lent:
 
               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 ">="  opera‐
        tors) 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 compar‐
        ison  shall  be  made using the locale-specific collation sequence. The
        value of the comparison expression shall be 1 if the relation is  true,
        or 0 if the relation is false.
 
    Variables and Special Variables
        Variables  can be used in an awk program by referencing them.  With the
        exception of function parameters (see User-Defined  Functions  ),  they
        are not explicitly declared. Function parameter names shall be local to
        the function; all other variable names shall be global. The  same  name
        shall  not be used as both a function parameter name and as the name of
        a function or a special awk variable. The same name shall not  be  used
        both  as  a  variable name with global scope and as the name of a func‐
        tion. 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 evalu‐
        ating 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 ref‐
        erences  shall  not create new fields. However, assigning to a nonexis‐
        tent 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 stan‐
        dard input. If an argument matches the format of an assignment operand,
        this  argument  shall  be  treated  as an assignment rather than a file
        argument.
 
        CONVFMT
               The printf format for converting numbers to strings (except  for
               output statements, where OFMT is used); "%.6g" by default.
 
        ENVIRON
               An array representing the value of the environment, as described
               in the exec functions defined in the System Interfaces volume of
               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 consisting of the
               value of that variable. If appropriate, the environment variable
               shall  be considered a numeric string (see Expressions in awk );
               the array element shall also have its numeric value.
 
        In all cases where the behavior of awk is affected by environment vari‐
        ables  (including the environment of any commands that awk executes via
        the system function or via pipeline redirections with the print  state‐
        ment,  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  pro‐
               cessed 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 pro‐
               cessed.
 
        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  <new‐
               line>  plus  one  or more blank lines, leading or trailing blank
               lines shall not result in empty records at the beginning or  end
               of the input, and a <newline> shall always be a field separator,
               no matter what the value of FS is.
 
        RSTART The starting position of the string matched by the  match  func‐
               tion,  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 nota‐
        tion (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-language conventions for escaping special  characters  within  the
        EREs,  as  specified  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      ) and the following table;
        these escape sequences shall be  recognized  both  inside  and  outside
        bracket  expressions.  Note that records need not be separated by <new‐
        line>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 expression matching operators,      ~       and
        "!~"  .  These  operators shall interpret their right-hand operand as a
        regular expression and their left-hand operand as a string. If the reg‐
        ular  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 circum‐
        flex 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 deter‐
        mining the value of a string literal (the lexical  token  STRING),  and
        thus  shall  be  applied a second time when a string literal is used in
        this context.
 
        When an ERE token appears as an expression in any context other than as
        the  right-hand  of  the      ~      or "!~" operator or as one of the built-in
        function arguments described below, the value of the resulting  expres‐
        sion shall be the equivalent of:
 
               $0 ~ /ere/
 
        The ere argument to the gsub, match, sub functions, and the fs argument
        to the split function (see String Functions ) shall be  interpreted  as
        extended  regular  expressions.  These  can  be  either  ERE  tokens or
        arbitrary expressions, and shall be interpreted in the same  manner  as
        the right-hand side of the      ~      or "!~" operator.
 
        An  extended regular expression can be used to separate fields by 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 embed‐
        ded 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 exe‐
        cuted before the first record of input is read (except possibly by  use
        of  the  getline function-see Input/Output and General Functions - in a
        prior BEGIN action) and before command line assignment  is  done.  Each
        END  pattern  shall  be matched once and its associated action executed
        after the last record of input has been read. These two patterns  shall
        have associated actions.
 
        BEGIN and END shall not combine with other patterns. Multiple BEGIN and
        END patterns shall be allowed. The actions associated  with  the  BEGIN
        patterns  shall  be  executed in the order specified in the program, as
        are the END actions. An END pattern can precede a BEGIN  pattern  in  a
        program.
 
        If  an awk program consists of only actions with the pattern BEGIN, and
        the BEGIN action contains no getline function, awk shall  exit  without
        reading  its  input when the last statement in the last BEGIN action is
        executed. If an awk program consists of only actions with  the  pattern
        END or only actions with the patterns BEGIN and END, the input shall be
        read before the statements in the END actions are executed.
 
    Expression Patterns
        An expression pattern shall be evaluated as if it were an expression in
        a  Boolean  context.  If  the  result  is  true,  the  pattern shall be
        considered to match, and the associated action (if any) shall  be  exe‐
        cuted. 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 Gram‐
        mar . 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  state‐
        ment 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 follow‐
        ing 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  Stan‐
        dard  ),  except  that  the  Boolean  expressions  shall  be treated as
        described in Expressions in awk , and except in the case of:
 
               for (variable in array)
 
        which shall iterate, assigning each index of array to  variable  in  an
        unspecified  order.  The results of adding new elements to array within
        such a for loop are undefined. If a break or continue statement  occurs
        outside of a loop, the behavior is undefined.
 
        The  delete  statement shall remove an individual array element.  Thus,
        the following code deletes an entire array:
 
               for (index in array)
                   delete array[index]
 
        The next statement shall cause all further processing  of  the  current
        input  record  to  be  abandoned.  The  behavior is undefined if a next
        statement appears or is invoked in a BEGIN or END action.
 
        The exit statement shall invoke all END actions in the order  in  which
        they occur in the program source and then terminate the program without
        reading further input. An exit statement inside  an  END  action  shall
        terminate  the  program without further execution of END actions. If an
        expression is specified in an exit statement, its numeric  value  shall
        be  the exit status of awk, unless subsequent errors are encountered or
        a subsequent exit statement with an expression is executed.
 
    Output Statements
        Both print and printf statements shall  write  to  standard  output  by
        default.  The output shall be written to the location specified by out‐
        put_redirection if one is supplied, as follows:
 
               > expression>> expression| expression
 
        In all cases, the expression shall be evaluated  to  produce  a  string
        that is used as a pathname into which to write (for      >      or ">>" ) or as
        a command to be executed (for      |      ). Using the first two forms, if  the
        file  of  that name is not currently open, it shall be opened, creating
        it if necessary and using the first form, truncating the file. The out‐
        put  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  argument. As long as the stream remains open, subsequent calls in
        which expression evaluates to the same string value shall write  output
        to  the  existing  stream. The stream shall remain open until the close
        function (see Input/Output and General Functions ) is  called  with  an
        expression  that evaluates to the same string value.  At that time, the
        stream shall be closed as if by a call to the pclose() function defined
        in the System Interfaces volume of IEEE Std 1003.1-2001.
 
        As  described in detail by the grammar in Grammar , these output state‐
        ments shall take a comma-separated list of expressions referred  to  in
        the  grammar by the non-terminal symbols expr_list, print_expr_list, or
        print_expr_list_opt. This list is referred to here  as  the  expression
        list, and each member is referred to as an expression argument.
 
        The  print  statement shall write the value of each expression argument
        onto the indicated output stream separated by the current output  field
        separator (see variable OFS above), and terminated by the output record
        separator (see variable ORS above). All expression arguments  shall  be
        taken  as  strings, being converted if necessary; this conversion shall
        be as described in Expressions in awk , with  the  exception  that  the
        printf format in OFMT shall be used instead of the value in CONVFMT. An
        empty expression list shall stand for the whole input record ($0).
 
        The printf statement shall produce output based on a  notation  similar
        to  the File Format Notation used to describe file formats in this vol‐
        ume  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 graph‐
            ical  representation.  Therefore, it cannot contain empty character
            positions. The <space> in the format string, in any  context  other
            than  a  flag of a conversion specification, shall be treated as an
            ordinary character that is copied to the output.
 
         2. If the character set contains a             character  and  that  character
            appears  in  the  format string, it shall be treated as an ordinary
            character that is copied to the output.
 
         3. The escape sequences beginning with a backslash character shall  be
            treated  as sequences of ordinary characters that are copied to the
            output. Note that these same sequences shall be  interpreted  lexi‐
            cally  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 charac‐
            ter in the character set, the behavior is undefined. If  the  argu‐
            ment  does  not  have  a  numeric value, the first character of the
            string value shall be output; if the string does  not  contain  any
            characters, the behavior is undefined.
 
         8. For  each  conversion  specification that consumes an argument, the
            next expression argument shall be evaluated. With the exception  of
            the  c conversion specifier character, the value shall be converted
            (according to the rules specified in Expressions in awk  )  to  the
            appropriate type for the conversion specification.
 
         9. If  there  are insufficient expression arguments to satisfy all the
            conversion specifications in the format  string,  the  behavior  is
            undefined.
 
        10. If  any  character  sequence in the format string begins with a      %     
            character, but does not form a valid conversion specification,  the
            behavior is unspecified.
 
        Both print and printf can output at least {LINE_MAX} bytes.
 
    Functions
        The  awk  language  has  a  variety  of built-in functions: arithmetic,
        string, input/output, and general.
 
    Arithmetic Functions
        The arithmetic functions, except for int, shall be based on  the  ISO C
        standard  (see 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 indi‐
        cated 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 num‐
               ber 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 backslashes shall be interpreted as just a  sin‐
               gle literal backslash character. Any other occurrence of a back‐
               slash (for example, preceding  any  other  character)  shall  be
               treated as a literal backslash character. Note that if repl is a
               string literal (the lexical token STRING;  see  Grammar  ),  the
               handling  of  the  ampersand  character occurs after any lexical
               processing, including any lexical backslash escape sequence pro‐
               cessing. If in is specified and it is not an lvalue (see Expres‐
               sions in awk ), the behavior is undefined. If in is omitted, awk
               shall use the current record ($0) in its place.
 
        substr(s, m[, n  ])
               Return  the  at  most  n-character substring of s that begins at
               position m, numbering from 1. If n is omitted, or if n specifies
               more  characters  than are left in the string, the length of the
               substring shall be limited by the length of the string s.
 
        tolower(s)
               Return a string based on the string s. Each character in s  that
               is  an  uppercase  letter specified to have a tolower mapping by
               the LC_CTYPE category of the current locale shall be replaced in
               the  returned  string  by  the lowercase letter specified by the
               mapping. Other  characters  in  s  shall  be  unchanged  in  the
               returned string.
 
        toupper(s)
               Return  a string based on the string s. Each character in s that
               is a lowercase letter specified to have a toupper mapping by the
               LC_CTYPE  category  of  the  current  locale  is replaced in the
               returned string by the uppercase letter specified  by  the  map‐
               ping.  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  implementa‐
               tion-defined.  If  the  close was successful, the function shall
               return zero; otherwise, it shall return non-zero.
 
        expression |  getline [var]
               Read a record of input from a stream piped from the output of  a
               command.   The stream shall be created if no stream is currently
               open with the value of  expression  as  its  command  name.  The
               stream  created  shall be equivalent to one created by a call to
               the popen() function with the value of expression as the command
               argument  and  a value of r as the mode argument. As long as the
               stream remains open, subsequent calls in which expression evalu‐
               ates to the same string value shall read subsequent records from
               the stream. The stream shall remain open until the  close  func‐
               tion  is  called  with  an expression that evaluates to the same
               string value. At that time, the stream shall be closed as if  by
               a  call  to  the pclose() function. If var is omitted, $0 and NF
               shall be set; otherwise, var shall be set and,  if  appropriate,
               it  shall be considered a numeric string (see Expressions in awk
               ).
 
        The getline operator can  form  ambiguous  constructs  when  there  are
        unparenthesized  operators  (including  concatenate) to the left of the
             |      (to the beginning of the expression  containing  getline).  In  the
        context  of  the       $       operator,       |      shall behave as if it had a lower
        precedence than      $      . The  result  of  evaluating  other  operators  is
        unspecified,  and  conforming  applications shall parenthesize properly
        all such usages.
 
        getline
               Set $0 to the next input record from  the  current  input  file.
               This form of getline shall set the NF, NR, and FNR variables.
 
        getline  var
               Set variable var to the next input record from the current input
               file and, if appropriate, var  shall  be  considered  a  numeric
               string (see Expressions in awk ). This form of getline shall set
               the FNR and NR variables.
 
        getline [var]  < expression
               Read the next record of input from a named file. The  expression
               shall  be  evaluated to produce a string that is used as a path‐
               name. 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 conform‐
        ing 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  com‐
               mand.
 
        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  applica‐
        tion shall ensure that the strings are textually identical.  The termi‐
        nology "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  particu‐
        lar,  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  func‐
        tion  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  follow‐
        ing  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, logical AND  operator  (
        "&&" ), logical OR operator ( "||" ), the do keyword, the else keyword,
        and the closing parenthesis of an if,  for,  or  while  statement.  For
        example:
 
               { print  $1,
                        $2 }
 
    Lexical Conventions
        The lexical conventions for awk programs, with respect to the preceding
        grammar, shall be as follows:
 
         1. Except as noted, awk shall recognize the longest possible token  or
            delimiter beginning at a given point.
 
         2. A comment shall consist of any characters beginning with the number
            sign character and terminated by, but excluding the next occurrence
            of,  a  <newline>. Comments shall have no effect, except to delimit
            lexical tokens.
 
         3. The <newline> shall be recognized as the token NEWLINE.
 
         4. A backslash character immediately followed  by  a  <newline>  shall
            have no effect.
 
         5. The  token  STRING shall represent a string constant. A string con‐
            stant shall begin with the character       .      Within a string constant,
            a  backslash  character  shall  be  considered  to  begin an escape
            sequence as specified in the table in the Base  Definitions  volume
            of  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  Prece‐
            dence 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 hex‐
                adecimal 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  Con‐
        cepts Derived from the ISO C Standard ), the behavior is undefined.
 
         9. A  sequence  of  underscores,  digits,  and  alphabetics  from  the
            portable  character  set  (see  the  Base  Definitions  volume   of
            IEEE Std 1003.1-2001,  Section 6.1, Portable Character Set), begin‐
            ning with an underscore or alphabetic, shall be considered a  word.
 
        10. The  following words are keywords that shall be recognized as indi‐
            vidual 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 consid‐
        ered 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 (with‐
            out any delimiters) by the      (      character.
 
        13. The token FUNC_NAME shall consist of a word that is not  a  keyword
            or a name of a built-in function, followed immediately (without any
            delimiters) by the      (      character. The      (      character  shall  not  be
            included as part of the token.
 
        14. The  following  two-character  sequences shall be recognized as the
            named tokens:
 
                       Token Name   Sequence   Token Name   Sequence
                       ADD_ASSIGN   +=         NO_MATCH     !~
                       SUB_ASSIGN   -=         EQ           ==
                       MUL_ASSIGN   *=         LE           <=
                       DIV_ASSIGN   /=         GE           >=
                       MOD_ASSIGN   %=         NE           !=
                       POW_ASSIGN   ^=         INCR         ++
                       OR           ||         DECR         --
                       AND          &&         APPEND       >>
 
        15. The following single characters shall be recognized as tokens whose
            names are the character:
 
            <newline> { } ( ) [ ] , ; + - * % ^ ! > < | ? : ~ $ =
 
        There  is  a lexical ambiguity between the token ERE and the tokens      /     
        and DIV_ASSIGN. When an input sequence begins with a slash character in
        any syntactic context where the token      /      or DIV_ASSIGN could appear as
        the next token in a valid program, the longer of those two tokens  that
        can  be  recognized shall be recognized. In any other syntactic context
        where the token ERE could appear as the next token in a valid  program,
        the token ERE shall be recognized.
        The following exit values shall be returned:
 
         0     All input files were processed successfully.
 
        >0     An error occurred.
 
        The  exit  status  can  be  altered within the program by using an exit
        expression.
        If any file operand is specified and the named file cannot be accessed,
        awk  shall  write  a diagnostic message to standard error and terminate
        without any further action.
 
        If the program specified by either the program operand  or  a  progfile
        operand  is  not  a  valid  awk  program  (as specified in the EXTENDED
        DESCRIPTION section), the behavior is undefined.
 
        The following sections are informative.
        The index, length, match, and substr functions should not  be  confused
        with  similar  functions  in  the ISO C standard; the awk versions deal
        with characters, while the ISO C standard deals with bytes.
 
        Because the concatenation operation is represented by adjacent  expres‐
        sions  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, programs commonly contain characters
        that are special to the shell, including double-quotes.  In  the  cases
        where  an  awk  program contains single-quote characters, it is usually
        easiest to specify most of the program as strings within  single-quotes
        concatenated  by  the  shell  with  quoted single-quote characters. For
        example:
 
               awk      /     \          / { print "quote:", $0 }     
 
        prints all lines from the  standard  input  containing  a  single-quote
        character, prefixed with quote:.
 
        The following are examples of simple awk programs:
 
         1. Write  to  the standard output all input lines for which field 3 is
            greater than 5:
 
            $3 > 5
 
         2. Write every tenth line:
 
            (NR % 10) == 0
 
         3. Write any line with a substring matching the regular expression:
 
            /(G|D)(2[0-9][[:alpha:]]*)/
 
         4. Print any line with a substring containing a      G      or      D      ,  followed
            by  a sequence of digits and characters.  This example uses charac‐
            ter 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 pro‐
            cessing of the string and once in processing  the  regular  expres‐
            sion:
 
            $2 ~ "\\\\"
 
         8. Write the second to the last and the last field in each line. Sepa‐
            rate 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 out‐
            put:
 
            {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 fea‐
        tures 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  lan‐
            guage.  Two examples of code that operate differently are:
 
            while ( n /= 10 > 1) ...
            if (!"wk" ~ /bwk/) ...
 
        Several features have been added based on newer implementations of awk:
 
         * Multiple instances of -f progfile are permitted.
 
         * The new option -v assignment.
 
         * The new predefined variable ENVIRON.
 
         * New built-in functions toupper and tolower.
 
         * More formatting capabilities are added to printf to match the  ISO C
           standard.
 
        The  overall awk syntax has always been based on the C language, with a
        few features from the shell command language and other sources. Because
        of this, it is not completely compatible with any other language, which
        has caused confusion for some users.  It is not the intent of the stan‐
        dard developers to address such issues.  A few relatively minor changes
        toward making the language more compatible with the ISO C standard were
        made;  most  of  these  changes  are based on similar changes in recent
        implementations, as described above. There  remain  several  C-language
        conventions  that  are not in awk. One of the notable ones is the comma
        operator, which is commonly used to specify multiple expressions in the
        C  language  for statement. Also, there are various places where awk is
        more restrictive than the C language regarding the type  of  expression
        that  can  be used in a given context. These limitations are due to the
        different features that the awk language does provide.
 
        Regular expressions in awk have been extended somewhat from  historical
        implementations  to  make  them  a  pure  superset  of extended regular
        expressions, as defined by IEEE Std 1003.1-2001 (see the  Base  Defini‐
        tions  volume  of  IEEE Std 1003.1-2001,  Section 9.4, Extended Regular
        Expressions).  The main extensions  are  internationalization  features
        and  interval expressions.  Historical implementations of awk have long
        supported backslash escape sequences as an extension to extended  regu‐
        lar expressions, and this extension has been retained despite inconsis‐
        tency 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-deter‐
        ministic 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,  con‐
        catenation  could  also be a solution for strings, but not for extended
        regular expressions (either lexical ERE tokens or strings used  dynami‐
        cally  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 doc‐
        umented, and it was not believed that it was necessary  to  standardize
        it.
 
        The  specification  of conversions between string and numeric values is
        much more detailed than in the documentation of historical  implementa‐
        tions or in the referenced The AWK Programming Language.  Although most
        of the behavior is designed to be intuitive, the details are  necessary
        to  ensure  compatible behavior from different implementations. This is
        especially important in relational expressions since the types  of  the
        operands determine whether a string or numeric comparison is performed.
        From the perspective of an application 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 histori‐
        cal practice in almost all cases. The one exception is that, in histor‐
        ical  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 state‐
        ment 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  implementa‐
        tions was seen as too unintuitive and unpredictable.
 
        It  was  pointed out that with the rules contained in early drafts, the
        following script would print nothing:
 
               BEGIN {
                   y[1.5] = 1
                   OFMT = "%e"
                   print y[1.5]
               }
 
        Therefore, a new variable, CONVFMT, was introduced. The  OFMT  variable
        is now restricted to affecting output conversions of numbers to strings
        and CONVFMT is used for internal conversions, such  as  comparisons  or
        array  indexing.  The  default  value  is the same as that for OFMT, so
        unless a program changes CONVFMT (which  no  historical  program  would
        do),  it  will receive the historical behavior associated with internal
        string conversions.
 
        The POSIX awk lexical and syntactic conventions are specified more for‐
        mally  than in other sources. Again the intent has been to specify his‐
        torical practice. One convention that may not be obvious from the  for‐
        mal  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 argu‐
        ment text is to simplify the grammar, making it match a  text  file  in
        form.  There  is  no  way for an application or test suite to determine
        whether a literal <newline> is added or whether awk simply acts  as  if
        it did.
 
        IEEE Std 1003.1-2001 requires several changes from historical implemen‐
        tations in order to support  internationalization.  Probably  the  most
        subtle  of  these is the use of the decimal-point character, defined by
        the LC_NUMERIC category of the locale, in representations of  floating-
        point  numbers.   This locale-specific character is used in recognizing
        numeric input, in converting between strings and numeric values, and in
        formatting  output. However, regardless of locale, the period character
        (the decimal-point character of the POSIX locale) is the  decimal-point
        character  recognized in processing awk programs (including assignments
        in command line arguments). This is essentially the same convention  as
        the  one  used in the ISO C standard. The difference is that the C lan‐
        guage includes the setlocale() function, which permits  an  application
        to  modify  its  locale.  Because  of  this capability, a C application
        begins executing with its locale set to the C locale, and only executes
        in  the  environment-specified  locale after an explicit call to setlo‐
        cale(). However, adding such an elaborate new feature to the  awk  lan‐
        guage  was seen as inappropriate for IEEE Std 1003.1-2001. It is possi‐
        ble to execute an awk program explicitly in any desired locale by  set‐
        ting the environment in the shell.
 
        The  undefined behavior resulting from NULs in extended regular expres‐
        sions allows future extensions for the  GNU  gawk  program  to  process
        binary data.
 
        The  behavior  in  the case of invalid awk programs (including lexical,
        syntactic, and semantic errors) is undefined because it was  considered
        overly  limiting  on  implementations  to  specify.  In most cases such
        errors can be expected to produce a diagnostic and a non-zero exit sta‐
        tus. However, some implementations may choose to extend the language in
        ways that make use of certain invalid constructs.  Other  invalid  con‐
        structs  might  be deemed worthy of a warning, but otherwise cause some
        reasonable behavior.  Still other constructs may be very  difficult  to
        detect  in some implementations.  Also, different implementations might
        detect a given error during an initial parsing of the  program  (before
        reading  any  input  files) while others might detect it when executing
        the program after reading some input. Implementors should be aware that
        diagnosing errors as early as possible and producing useful diagnostics
        can ease debugging of applications, and  thus  make  an  implementation
        more usable.
 
        The  unspecified  behavior  from  using multi-character RS values is to
        allow possible future extensions based on extended regular  expressions
        used  for  record separators. Historical implementations take the first
        character of the string and ignore the others.
 
        Unspecified behavior when split( string, array, <null>) is used  is  to
        allow  a proposed future extension that would split up a string into an
        array of individual characters.
 
        In the context of the getline function, equally good arguments for dif‐
        ferent  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 state‐
        ment, where the |getline part might be a redirection of the print.)
 
        Since in most cases such constructs are not (or at least should not) be
        used (because they have a natural ambiguity for which there is no  con‐
        ventional  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  circum‐
        stances.   Where  backwards-compatibility is not a large consideration,
        implementors may wish to use such grammars.
 
        Some historical implementations have allowed some built-in functions to
        be called without an argument list, the result being a default argument
        list chosen in some "reasonable" way. Use of length as  a  synonym  for
        length($0)  is the only one of these forms that is thought to be widely
        known or widely used; this particular form  is  documented  in  various
        places  (for example, most historical awk reference pages, although not
        in the referenced The AWK Programming Language) as legitimate practice.
        With  this  exception,  default argument lists have always been undocu‐
        mented 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.  Accord‐
        ingly,  this  usage  has  been made legitimate, but Issue 5 removed the
        obsolescent marking for XSI-conforming implementations and many  other‐
        wise conforming applications depend on this feature.
 
        In  sub  and  gsub,  if  repl  is  a  string literal (the lexical token
        STRING), then two consecutive backslash characters should  be  used  in
        the string to ensure a single backslash will precede the ampersand when
        the resultant string is passed to the function. (For example, to  spec‐
        ify  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  mean‐
        ing.
 
        The  description  in  the ISO POSIX-2:1993 standard introduced behavior
        such that the backslash character was another special character and  it
        was  unspecified  whether there were any other special characters. This
        description introduced several portability problems, some of which  are
        described  below,  and so it has been replaced with the more historical
        description. Some of the problems include:
 
         * Historically, to create the replacement string, a script  could  use
           gsub(  ERE, "\\&" ), but with the ISO POSIX-2:1993 standard wording,
           it was necessary to use gsub( ERE, "\\\\&" ).  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 evaluat‐
        ing to the numeric value zero.
 
        Uninitialized  variables  include  all  types  of  variables  including
        scalars, array elements, and fields. The definition of an uninitialized
        value  in  Variables and Special Variables is necessary to describe the
        value placed on uninitialized variables and on fields  that  are  valid
        (for example, < $NF) but have no characters in them and to describe how
        these variables are to be used in comparisons. A valid field,  such  as
        $1,  that has no characters in it can be obtained from an input line of
        "\t\t" when FS=      \t      . Historically, the comparison ( $1<10)  was  done
        numerically after evaluating $1 to the value zero.
 
        The  phrase  "...  also  shall  have  the  numeric value of the numeric
        string" was removed from several sections of the ISO POSIX-2:1993 stan‐
        dard  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  con‐
        text.
 
        The  description  of numeric string processing is based on the behavior
        of the atof() function in  the  ISO C  standard.  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  con‐
        stants  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() function 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.
        None.
        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, Standard for Information Technology
        -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
        Specifications  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 Standard, the original IEEE and The Open Group  Standard
        is  the  referee document. The original Standard can be obtained online
http://www.opengroup.org/unix/online.html .
 

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