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awk: skenování a zpracování jazyka

Originální popis anglicky: awk - pattern scanning and processing language

Návod, kniha: POSIX Programmer's Manual

STRUČNĚ

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

POPIS / INSTRUKCE

The awk utility shall execute programs written in the awk programming language, which is specialized for textual data manipulation. An awk program is a sequence of patterns and corresponding actions. When input is read that matches a pattern, the action associated with that pattern is carried out.
Input shall be interpreted as a sequence of records. By default, a record is a line, less its terminating <newline>, but this can be changed by using the RS built-in variable. Each record of input shall be matched in turn against each pattern in the program. For each pattern matched, the associated action shall be executed.
The awk utility shall interpret each input record as a sequence of fields where, by default, a field is a string of non- <blank>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-evaluation of $0. Assigning to $0 shall reset the values of all other fields and the NF built-in variable.

OPTIONS

The awk utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-F  ERE
Define the input field separator to be the extended regular expression ERE, before any input is read; see Regular Expressions .
-f  progfile
Specify the pathname of the file progfile containing an awk program. If multiple instances of this option are specified, the concatenation of the files specified as progfile in the order specified shall be the awk program. The awk program can alternatively be specified in the command line as a single argument.
-v  assignment
The application shall ensure that the assignment argument is in the same form as an assignment operand. The specified variable assignment shall occur prior to executing the awk program, including the actions associated with BEGIN patterns (if any). Multiple occurrences of this option can be specified.
 

OPERANDS

The following operands shall be supported:
program
If no -f option is specified, the first operand to awk shall be the text of the awk program. The application shall supply the program operand as a single argument to awk. If the text does not end in a <newline>, awk shall interpret the text as if it did.
argument
Either of the following two types of argument can be intermixed:
file
A pathname of a file that contains the input to be read, which is matched against the set of patterns in the program. If no file operands are specified, or if a file operand is '-' , the standard input shall be used.
assignment
An operand that begins with an underscore or alphabetic character from the portable character set (see the table in the Base Definitions volume of IEEE Std 1003.1-2001, Section 6.1, Portable Character Set), followed by a sequence of underscores, digits, and alphabetics from the portable character set, followed by the '=' character, shall specify a variable assignment rather than a pathname. The characters before the '=' represent the name of an awk variable; if that name is an awk reserved word (see Grammar ) the behavior is undefined. The characters following the equal sign shall be interpreted as if they appeared in the awk program preceded and followed by a double-quote ( ' )' character, as a STRING token (see Grammar ), except that if the last character is an unescaped backslash, it shall be interpreted as a literal backslash rather than as the first character of the sequence "\"" . The variable shall be assigned the value of that STRING token and, if appropriate, shall be considered a numeric string (see Expressions in awk ), the variable shall also be assigned its numeric value. Each such variable assignment shall occur just prior to the processing of the following file, if any. Thus, an assignment before the first file argument shall be executed after the BEGIN actions (if any), while an assignment after the last file argument shall occur before the END actions (if any). If there are no file arguments, assignments shall be executed before processing the standard input.
 
 

STDIN

The standard input shall be used only if no file operands are specified, or if a file operand is '-' ; see the INPUT FILES section. If the awk program contains no actions and no patterns, but is otherwise a valid awk program, standard input and any file operands shall not be read and awk shall exit with a return status of zero.

INPUT FILES

Input files to the awk program from any of the following sources shall be text files:
*
Any file operands or their equivalents, achieved by modifying the awk variables ARGV and ARGC
*
Standard input in the absence of any file operands
*
Arguments to the getline function
Whether the variable RS is set to a value other than a <newline> or not, for these files, implementations shall support records terminated with the specified separator up to {LINE_MAX} bytes and may support longer records.
If -f progfile is specified, the application shall ensure that the files named by each of the progfile option-arguments are text files and their concatenation, in the same order as they appear in the arguments, is an awk program.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of awk:
LANG
Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
LC_ALL
If set to a non-empty string value, override the values of all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements within regular expressions and in comparisons of string values.
LC_CTYPE
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments and input files), the behavior of character classes within regular expressions, the identification of characters as letters, and the mapping of uppercase and lowercase characters for the toupper and tolower functions.
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
LC_NUMERIC
Determine the radix character used when interpreting numeric input, performing conversions between numeric and string values, and formatting numeric output. Regardless of locale, the period character (the decimal-point character of the POSIX locale) is the decimal-point character recognized in processing awk programs (including assignments in command line arguments).
NLSPATH
Determine the location of message catalogs for the processing of LC_MESSAGES .
PATH
Determine the search path when looking for commands executed by system(expr), or input and output pipes; see the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables.
 
In addition, all environment variables shall be visible via the awk variable ENVIRON.

ASYNCHRONOUS EVENTS

Default.

STDOUT

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

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

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

EXTENDED DESCRIPTION

Overall Program Structure

An awk program is composed of pairs of the form:
 
pattern { action }
Either the pattern or the action (including the enclosing brace characters) can be omitted.
A missing pattern shall match any record of input, and a missing action shall be equivalent to:
 
{ print }
Execution of the awk program shall start by first executing the actions associated with all BEGIN patterns in the order they occur in the program. Then each file operand (or standard input if no files were specified) shall be processed in turn by reading data from the file until a record separator is seen ( <newline> by default). Before the first reference to a field in the record is evaluated, the record shall be split into fields, according to the rules in Regular Expressions , using the value of FS that was current at the time the record was read. Each pattern in the program then shall be evaluated in the order of occurrence, and the action associated with each pattern that matches the current record executed. The action for a matching pattern shall be executed before evaluating subsequent patterns. Finally, the actions associated with all END patterns shall be executed in the order they occur in the program.

Expressions in awk

Expressions describe computations used in patterns and actions. In the following table, valid expression operations are given in groups from highest precedence first to lowest precedence last, with equal-precedence operators grouped between horizontal lines. In expression evaluation, where the grammar is formally ambiguous, higher precedence operators shall be evaluated before lower precedence operators. In this table expr, expr1, expr2, and expr3 represent any expression, while lvalue represents any entity that can be assigned to (that is, on the left side of an assignment operator). The precise syntax of expressions is given in Grammar .
 
 
Table: 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 expression shall be implicitly converted to the type needed for the context in which it is used. A string value shall be converted to a numeric value by the equivalent of the following calls to 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 resulting string would lexically be recognized as a NUMBER token as described by the lexical conventions in Grammar :
*
All leading and trailing <blank>s are discarded.
*
If the first non- <blank> is '+' or '-' , it is discarded.
*
Changing each occurrence of the decimal point character from the current locale to a period.
If a '-' character is ignored in the preceding description, the numeric value of the numeric string shall be the negation of the numeric value of the recognized NUMBER token. Otherwise, the numeric value of the numeric string shall be the numeric value of the recognized NUMBER token. Whether or not a string is a numeric string shall be relevant only in contexts where that term is used in this section.
When an expression is used in a Boolean context, if it has a numeric value, a value of zero shall be treated as false and any other value shall be treated as true. Otherwise, a string value of the null string shall be treated as false and any other value shall be treated as true. A Boolean context shall be one of the following:
*
The first subexpression of a conditional expression
*
An expression operated on by logical NOT, logical AND, or logical OR
*
The second expression of a for statement
*
The expression of an if statement
*
The expression of the while clause in either a while or do... while statement
*
An expression used as a pattern (as in Overall Program Structure)
All arithmetic shall follow the semantics of floating-point arithmetic as specified by the ISO C standard (see Concepts Derived from the ISO C Standard ).
The value of the expression:
 
expr1 ^ expr2
shall be equivalent to the value returned by the ISO C standard function call:
 
pow(expr1, expr2)
The expression:
 
lvalue ^= expr
shall be equivalent to the ISO C standard expression:
 
lvalue = pow(lvalue, expr)
except that lvalue shall be evaluated only once. The value of the expression:
 
expr1 % expr2
shall be equivalent to the value returned by the ISO C standard function call:
 
fmod(expr1, expr2)
The expression:
 
lvalue %= expr
shall be equivalent to the ISO C standard expression:
 
lvalue = fmod(lvalue, expr)
except that lvalue shall be evaluated only once.
Variables and fields shall be set by the assignment statement:
 
lvalue = expression
and the type of expression shall determine the resulting variable type. The assignment includes the arithmetic assignments ( "+=" , "-=" , "*=" , "/=" , "%=" , "^=" , "++" , "--" ) all of which shall produce a numeric result. The left-hand side of an assignment and the target of increment and decrement operators can be one of a variable, an array with index, or a field selector.
The awk language supplies arrays that are used for storing numbers or strings. Arrays need not be declared. They shall initially be empty, and their sizes shall change dynamically. The subscripts, or element identifiers, are strings, providing a type of associative array capability. An array name followed by a subscript within square brackets can be used as an lvalue and thus as an expression, as described in the grammar; see Grammar . Unsubscripted array names can be used in only the following contexts:
*
A parameter in a function definition or function call
*
The NAME token following any use of the keyword in as specified in the grammar (see Grammar ); if the name used in this context is not an array name, the behavior is undefined
A valid array index shall consist of one or more comma-separated expressions, similar to the way in which multi-dimensional arrays are indexed in some programming languages. Because awk arrays are really one-dimensional, such a comma-separated list shall be converted to a single string by concatenating the string values of the separate expressions, each separated from the other by the value of the SUBSEP variable. Thus, the following two index operations shall be equivalent:
 
var[expr1, expr2, ... exprn]
 
var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]
The application shall ensure that a multi-dimensioned index used with the in operator is parenthesized. The in operator, which tests for the existence of a particular array element, shall not cause that element to exist. Any other reference to a nonexistent array element shall automatically create it.
Comparisons (with the '<' , "<=" , "!=" , "==" , '>' , and ">=" operators) shall be made numerically if both operands are numeric, if one is numeric and the other has a string value that is a numeric string, or if one is numeric and the other has the uninitialized value. Otherwise, operands shall be converted to strings as required and a string comparison shall be made using the locale-specific collation sequence. The value of the comparison expression shall be 1 if the relation is true, or 0 if the relation is false.

Variables and Special Variables

Variables can be used in an awk program by referencing them. With the exception of function parameters (see User-Defined Functions ), they are not explicitly declared. Function parameter names shall be local to the function; all other variable names shall be global. The same name shall not be used as both a function parameter name and as the name of a function or a special awk variable. The same name shall not be used both as a variable name with global scope and as the name of a function. The same name shall not be used within the same scope both as a scalar variable and as an array. Uninitialized variables, including scalar variables, array elements, and field variables, shall have an uninitialized value. An uninitialized value shall have both a numeric value of zero and a string value of the empty string. Evaluation of variables with an uninitialized value, to either string or numeric, shall be determined by the context in which they are used.
Field variables shall be designated by a '$' followed by a number or numerical expression. The effect of the field number expression evaluating to anything other than a non-negative integer is unspecified; uninitialized variables or string values need not be converted to numeric values in this context. New field variables can be created by assigning a value to them. References to nonexistent fields (that is, fields after $ NF), shall evaluate to the uninitialized value. Such references shall not create new fields. However, assigning to a nonexistent field (for example, $( NF+2)=5) shall increase the value of NF; create any intervening fields with the uninitialized value; and cause the value of $0 to be recomputed, with the fields being separated by the value of OFS. Each field variable shall have a string value or an uninitialized value when created. Field variables shall have the uninitialized value when created from $0 using FS and the variable does not contain any characters. If appropriate, the field variable shall be considered a numeric string (see Expressions in awk ).
Implementations shall support the following other special variables that are set by awk:
ARGC
The number of elements in the ARGV array.
ARGV
An array of command line arguments, excluding options and the program argument, numbered from zero to ARGC-1.
The arguments in ARGV can be modified or added to; ARGC can be altered. As each input file ends, awk shall treat the next non-null element of ARGV, up to the current value of ARGC-1, inclusive, as the name of the next input file. Thus, setting an element of ARGV to null means that it shall not be treated as an input file. The name '-' indicates the standard input. If an argument matches the format of an assignment operand, this argument shall be treated as an assignment rather than a file argument.
CONVFMT
The printf format for converting numbers to strings (except for output statements, where OFMT is used); "%.6g" by default.
ENVIRON
An array representing the value of the environment, as described in the exec functions defined in the System Interfaces volume of 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 variables (including the environment of any commands that awk executes via the system function or via pipeline redirections with the print statement, the printf statement, or the getline function), the environment used shall be the environment at the time awk began executing; it is implementation-defined whether any modification of ENVIRON affects this environment.
FILENAME
A pathname of the current input file. Inside a BEGIN action the value is undefined. Inside an END action the value shall be the name of the last input file processed.
FNR
The ordinal number of the current record in the current file. Inside a BEGIN action the value shall be zero. Inside an END action the value shall be the number of the last record processed in the last file processed.
FS
Input field separator regular expression; a <space> by default.
NF
The number of fields in the current record. Inside a BEGIN action, the use of NF is undefined unless a getline function without a var argument is executed previously. Inside an END action, NF shall retain the value it had for the last record read, unless a subsequent, redirected, getline function without a var argument is performed prior to entering the END action.
NR
The ordinal number of the current record from the start of input. Inside a BEGIN action the value shall be zero. Inside an END action the value shall be the number of the last record processed.
OFMT
The printf format for converting numbers to strings in output statements (see Output Statements ); "%.6g" by default. The result of the conversion is unspecified if the value of OFMT is not a floating-point format specification.
OFS
The print statement output field separation; <space> by default.
ORS
The print statement output record separator; a <newline> by default.
RLENGTH
The length of the string matched by the match function.
RS
The first character of the string value of RS shall be the input record separator; a <newline> by default. If RS contains more than one character, the results are unspecified. If RS is null, then records are separated by sequences consisting of a <newline> plus one or more blank lines, leading or trailing blank lines shall not result in empty records at the beginning or end of the input, and a <newline> shall always be a field separator, no matter what the value of FS is.
RSTART
The starting position of the string matched by the match function, numbering from 1. This shall always be equivalent to the return value of the match function.
SUBSEP
The subscript separator string for multi-dimensional arrays; the default value is implementation-defined.
 

Regular Expressions

The awk utility shall make use of the extended regular expression notation (see the Base Definitions volume of 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 <newline>s and string constants can contain <newline>s, so even the "\n" sequence is valid in awk EREs. Using a slash character within an ERE requires the escaping shown in the following table.
 
 
 
Table: Escape Sequences in awk
 
Escape    
Sequence Description Meaning
\" Backslash quotation-mark Quotation-mark character
\/ Backslash slash Slash character
\ddd A backslash character followed by the longest sequence of one, two, or three octal-digit characters (01234567). If all of the digits are 0 (that is, representation of the NUL character), the behavior is undefined. The character whose encoding is represented by the one, two, or three-digit octal integer. Multi-byte characters require multiple, concatenated escape sequences of this type, including the leading '\' for each byte.
\c A backslash character followed by any character not described in this table or 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' ). Undefined
A regular expression can be matched against a specific field or string by using one of the two regular expression matching operators, '~' and "!~" . These operators shall interpret their right-hand operand as a regular expression and their left-hand operand as a string. If the regular expression matches the string, the '~' expression shall evaluate to a value of 1, and the "!~" expression shall evaluate to a value of 0. (The regular expression matching operation is as defined by the term matched in the Base Definitions volume of IEEE Std 1003.1-2001, Section 9.1, Regular Expression Definitions, where a match occurs on any part of the string unless the regular expression is limited with the circumflex or dollar sign special characters.) If the regular expression does not match the string, the '~' expression shall evaluate to a value of 0, and the "!~" expression shall evaluate to a value of 1. If the right-hand operand is any expression other than the lexical token ERE, the string value of the expression shall be interpreted as an extended regular expression, including the escape conventions described above. Note that these same escape conventions shall also be applied in determining the value of a string literal (the lexical token STRING), and thus shall be applied a second time when a string literal is used in this context.
When an ERE token appears as an expression in any context other than as the right-hand of the '~' or "!~" operator or as one of the built-in function arguments described below, the value of the resulting expression shall be the equivalent of:
 
$0 ~ /ere/
The ere argument to the gsub, match, sub functions, and the fs argument to the split function (see String Functions ) shall be interpreted as extended regular expressions. These can be either ERE tokens or arbitrary expressions, and shall be interpreted in the same manner as the right-hand side of the '~' or "!~" operator.
An extended regular expression can be used to separate fields by using the -F ERE option or by assigning a string containing the expression to the built-in variable FS. The default value of the FS variable shall be a single <space>. The following describes FS behavior:
1.
If FS is a null string, the behavior is unspecified.
2.
If FS is a single character:
a.
If FS is <space>, skip leading and trailing <blank>s; fields shall be delimited by sets of one or more <blank>s.
b.
Otherwise, if FS is any other character c, fields shall be delimited by each single occurrence of c.
3.
Otherwise, the string value of FS shall be considered to be an extended regular expression. Each occurrence of a sequence matching the extended regular expression shall delimit fields.
Except for the '~' and "!~" operators, and in the gsub, match, split, and sub built-in functions, ERE matching shall be based on input records; that is, record separator characters (the first character of the value of the variable RS, <newline> by default) cannot be embedded in the expression, and no expression shall match the record separator character. If the record separator is not <newline>, <newline>s embedded in the expression can be matched. For the '~' and "!~" operators, and in those four built-in functions, ERE matching shall be based on text strings; that is, any character (including <newline> and the record separator) can be embedded in the pattern, and an appropriate pattern shall match any character. However, in all awk ERE matching, the use of one or more NUL characters in the pattern, input record, or text string produces undefined results.

Patterns

A pattern is any valid expression, a range specified by two expressions separated by a comma, or one of the two special patterns BEGIN or END.

Special Patterns

The awk utility shall recognize two special patterns, BEGIN and END. Each BEGIN pattern shall be matched once and its associated action executed before the first record of input is read (except possibly by use of the getline function-see Input/Output and General Functions - in a prior BEGIN action) and before command line assignment is done. Each END pattern shall be matched once and its associated action executed after the last record of input has been read. These two patterns shall have associated actions.
BEGIN and END shall not combine with other patterns. Multiple BEGIN and END patterns shall be allowed. The actions associated with the BEGIN patterns shall be executed in the order specified in the program, as are the END actions. An END pattern can precede a BEGIN pattern in a program.
If an awk program consists of only actions with the pattern BEGIN, and the BEGIN action contains no getline function, awk shall exit without reading its input when the last statement in the last BEGIN action is executed. If an awk program consists of only actions with the pattern END or only actions with the patterns BEGIN and END, the input shall be read before the statements in the END actions are executed.

Expression Patterns

An expression pattern shall be evaluated as if it were an expression in a Boolean context. If the result is true, the pattern shall be considered to match, and the associated action (if any) shall be executed. If the result is false, the action shall not be executed.

Pattern Ranges

A pattern range consists of two expressions separated by a comma; in this case, the action shall be performed for all records between a match of the first expression and the following match of the second expression, inclusive. At this point, the pattern range can be repeated starting at input records subsequent to the end of the matched range.

Actions

An action is a sequence of statements as shown in the grammar in Grammar . Any single statement can be replaced by a statement list enclosed in braces. The application shall ensure that statements in a statement list are separated by <newline>s or semicolons. Statements in a statement list shall be executed sequentially in the order that they appear.
The expression acting as the conditional in an if statement shall be evaluated and if it is non-zero or non-null, the following statement shall be executed; otherwise, if else is present, the statement following the else shall be executed.
The if, while, do... while, for, break, and continue statements are based on the ISO C standard (see Concepts Derived from the ISO C Standard ), except that the Boolean expressions shall be treated as described in Expressions in awk , and except in the case of:
 
for (variable in array)
which shall iterate, assigning each index of array to variable in an unspecified order. The results of adding new elements to array within such a for loop are undefined. If a break or continue statement occurs outside of a loop, the behavior is undefined.
The delete statement shall remove an individual array element. Thus, the following code deletes an entire array:
 
for (index in array) delete array[index]
The next statement shall cause all further processing of the current input record to be abandoned. The behavior is undefined if a next statement appears or is invoked in a BEGIN or END action.
The exit statement shall invoke all END actions in the order in which they occur in the program source and then terminate the program without reading further input. An exit statement inside an END action shall terminate the program without further execution of END actions. If an expression is specified in an exit statement, its numeric value shall be the exit status of awk, unless subsequent errors are encountered or a subsequent exit statement with an expression is executed.

Output Statements

Both print and printf statements shall write to standard output by default. The output shall be written to the location specified by output_redirection if one is supplied, as follows:
 
> expression>> expression| expression
In all cases, the expression shall be evaluated to produce a string that is used as a pathname into which to write (for '>' or ">>" ) or as a command to be executed (for '|' ). Using the first two forms, if the file of that name is not currently open, it shall be opened, creating it if necessary and using the first form, truncating the file. The output then shall be appended to the file. As long as the file remains open, subsequent calls in which expression evaluates to the same string value shall simply append output to the file. The file remains open until the close function (see Input/Output and General Functions ) is called with an expression that evaluates to the same string value.
The third form shall write output onto a stream piped to the input of a command. The stream shall be created if no stream is currently open with the value of expression as its command name. The stream created shall be equivalent to one created by a call to the popen() function defined in the System Interfaces volume of 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 statements shall take a comma-separated list of expressions referred to in the grammar by the non-terminal symbols expr_list, print_expr_list, or print_expr_list_opt. This list is referred to here as the expression list, and each member is referred to as an expression argument.
The print statement shall write the value of each expression argument onto the indicated output stream separated by the current output field separator (see variable OFS above), and terminated by the output record separator (see variable ORS above). All expression arguments shall be taken as strings, being converted if necessary; this conversion shall be as described in Expressions in awk , with the exception that the printf format in OFMT shall be used instead of the value in CONVFMT. An empty expression list shall stand for the whole input record ($0).
The printf statement shall produce output based on a notation similar to the File Format Notation used to describe file formats in this volume of IEEE Std 1003.1-2001 (see the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 5, File Format Notation). Output shall be produced as specified with the first expression argument as the string format and subsequent expression arguments as the strings arg1 to argn, inclusive, with the following exceptions:
1.
The format shall be an actual character string rather than a graphical representation. Therefore, it cannot contain empty character positions. The <space> in the format string, in any context other than a flag of a conversion specification, shall be treated as an ordinary character that is copied to the output.
2.
If the character set contains a ' ' character and that character appears in the format string, it shall be treated as an ordinary character that is copied to the output.
3.
The escape sequences beginning with a backslash character shall be treated as sequences of ordinary characters that are copied to the output. Note that these same sequences shall be interpreted lexically by awk when they appear in literal strings, but they shall not be treated specially by the printf statement.
4.
A field width or precision can be specified as the '*' character instead of a digit string. In this case the next argument from the expression list shall be fetched and its numeric value taken as the field width or precision.
5.
The implementation shall not precede or follow output from the d or u conversion specifier characters with <blank>s not specified by the format string.
6.
The implementation shall not precede output from the o conversion specifier character with leading zeros not specified by the format string.
7.
For the c conversion specifier character: if the argument has a numeric value, the character whose encoding is that value shall be output. If the value is zero or is not the encoding of any character in the character set, the behavior is undefined. If the argument does not have a numeric value, the first character of the string value shall be output; if the string does not contain any characters, the behavior is undefined.
8.
For each conversion specification that consumes an argument, the next expression argument shall be evaluated. With the exception of the c conversion specifier character, the value shall be converted (according to the rules specified in Expressions in awk ) to the appropriate type for the conversion specification.
9.
If there are insufficient expression arguments to satisfy all the conversion specifications in the format string, the behavior is undefined.
10.
If any character sequence in the format string begins with a '%' character, but does not form a valid conversion specification, the behavior is unspecified.
Both print and printf can output at least {LINE_MAX} bytes.

Functions

The awk language has a variety of built-in functions: arithmetic, string, input/output, and general.

Arithmetic Functions

The arithmetic functions, except for int, shall be based on the ISO C standard (see Concepts Derived from the ISO C Standard ). The behavior is undefined in cases where the ISO C standard specifies that an error be returned or that the behavior is undefined. Although the grammar (see Grammar ) permits built-in functions to appear with no arguments or parentheses, unless the argument or parentheses are indicated as optional in the following list (by displaying them within the "[]" brackets), such use is undefined.
atan2(y,x)
Return arctangent of y/x in radians in the range [-pi,pi].
cos(x)
Return cosine of x, where x is in radians.
sin(x)
Return sine of x, where x is in radians.
exp(x)
Return the exponential function of x.
log(x)
Return the natural logarithm of x.
sqrt(x)
Return the square root of x.
int(x)
Return the argument truncated to an integer. Truncation shall be toward 0 when x>0.
rand()
Return a random number n, such that 0<=n<1.
srand([expr])
Set the seed value for rand to expr or use the time of day if expr is omitted. The previous seed value shall be returned.
 

String Functions

The string functions in the following list shall be supported. Although the grammar (see Grammar ) permits built-in functions to appear with no arguments or parentheses, unless the argument or parentheses are indicated as optional in the following list (by displaying them within the "[]" brackets), such use is undefined.
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