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:
-
Field variables
-
Input from the getline() function
-
FILENAME
-
ARGV array elements
-
ENVIRON array elements
-
Array elements created by the split() function
-
A command line variable assignment
-
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:
-
If FS is a null string, the behavior is unspecified.
-
If FS is a single character:
-
If FS is <space>, skip leading and trailing <blank>s; fields shall be delimited by sets of one or more
<blank>s.
-
Otherwise, if FS is any other character c, fields shall be delimited by each single occurrence of c.
-
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:
-
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.
-
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.
-
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.
-
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.
-
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.
-
The implementation shall not precede output from the o conversion specifier character with leading zeros not specified
by the format string.
-
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.
-
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.
-
If there are insufficient expression arguments to satisfy all the conversion specifications in the format string, the
behavior is undefined.
-
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 [-
,].
- cos(x)
- Return cosine of x, where x is in radians.
- sin(x)
- Return sine of x, where x is in radians.
- exp(x)
- Return the exponential function of x.
- log(x)
- Return the natural logarithm of x.
- sqrt(x)
- Return the square root of x.
- int(x)
- Return the argument truncated to an integer. Truncation shall be toward 0 when x>0.
- rand()
- Return a random number n, such that 0<=n<1.
- srand([expr])
- Set the seed value for rand to expr or use the time of day if expr is omitted. The previous seed value
shall be returned.
String Functions
The string functions in the following list shall be supported. Although the grammar (see Grammar
) permits built-in functions to appear with no arguments or parentheses, unless the argument or parentheses are indicated as
optional in the following list (by displaying them within the "[]" brackets), such use is undefined.
- gsub(ere, repl[, in])
-
Behave like sub (see below), except that it shall replace all occurrences of the regular expression (like the ed utility global substitute) in $0 or in the in argument, when specified.
- index(s, t)
- Return the position, in characters, numbering from 1, in string s where string t first occurs, or zero if it does
not occur at all.
- length[([s])]
- Return the length, in characters, of its argument taken as a string, or of the whole record, $0, if there is no argument.
- match(s, ere)
- Return the position, in characters, numbering from 1, in string s where the extended regular expression ere
occurs, or zero if it does not occur at all. RSTART shall be set to the starting position (which is the same as the returned
value), zero if no match is found; RLENGTH shall be set to the length of the matched string, -1 if no match is found.
- split(s, a[, fs ])
-
Split the string s into array elements a[1], a[2], ..., a[n], and return n. All elements
of the array shall be deleted before the split is performed. The separation shall be done with the ERE fs or with the field
separator FS if fs is not given. Each array element shall have a string value when created and, if appropriate, the
array element shall be considered a numeric string (see Expressions in awk ). The effect of a null
string as the value of fs is unspecified.
- sprintf(fmt, expr, expr, ...)
-
Format the expressions according to the printf format given by fmt and return the resulting string.
- sub(ere, repl[, in ])
-
Substitute the string repl in place of the first instance of the extended regular expression ERE in string in
and return the number of substitutions. An ampersand ( '&' ) appearing in the string repl shall be replaced by
the string from in that matches the ERE. An ampersand preceded with a backslash ( '\' ) shall be interpreted as the
literal ampersand character. An occurrence of two consecutive backslashes shall be interpreted as just a single literal backslash
character. Any other occurrence of a backslash (for example, preceding any other character) shall be treated as a literal backslash
character. Note that if repl is a string literal (the lexical token STRING; see Grammar ), the handling of the ampersand character occurs after any lexical processing, including any
lexical backslash escape sequence processing. If in is specified and it is not an lvalue (see Expressions in awk ), the behavior is undefined. If in is omitted, awk shall use the current
record ($0) in its place.
- substr(s, m[, n ])
-
Return the at most n-character substring of s that begins at position m, numbering from 1. If n is
omitted, or if n specifies more characters than are left in the string, the length of the substring shall be limited by the
length of the string s.
- tolower(s)
- Return a string based on the string s. Each character in s that is an uppercase letter specified to have a
tolower mapping by the LC_CTYPE category of the current locale shall be replaced in the returned string by the
lowercase letter specified by the mapping. Other characters in s shall be unchanged in the returned string.
- toupper(s)
- Return a string based on the string s. Each character in s that is a lowercase letter specified to have a
toupper mapping by the LC_CTYPE category of the current locale is replaced in the returned string by the uppercase
letter specified by the mapping. Other characters in s are unchanged in the returned string.
All of the preceding functions that take ERE as a parameter expect a pattern or a string valued expression that is a
regular expression as defined in Regular Expressions .
Input/Output and General Functions
The input/output and general functions are:
- close(expression)
-
Close the file or pipe opened by a print or printf statement or a call to getline with the same string-valued
expression. The limit on the number of open expression arguments is implementation-defined. If the close was
successful, the function shall return zero; otherwise, it shall return non-zero.
- expression | getline [var]
-
Read a record of input from a stream piped from the output of a command. The stream shall be created if no stream is currently open
with the value of expression as its command name. The stream created shall be equivalent to one created by a call to the popen() function with the value of expression as the command argument and a
value of r as the mode argument. As long as the stream remains open, subsequent calls in which expression
evaluates to the same string value shall read subsequent records from the stream. The stream shall remain open until the
close function is called with an expression that evaluates to the same string value. At that time, the stream shall be
closed as if by a call to the pclose() function. If var is omitted, $0 and
NF shall be set; otherwise, var shall be set and, if appropriate, it shall be considered a numeric string (see Expressions in awk ).
The getline operator can form ambiguous constructs when there are unparenthesized operators (including concatenate) to
the left of the '|' (to the beginning of the expression containing getline). In the context of the '$'
operator, '|' shall behave as if it had a lower precedence than '$' . The result of evaluating other operators is
unspecified, and conforming applications shall parenthesize properly all such usages.
- getline
- Set $0 to the next input record from the current input file. This form of getline shall set the NF, NR,
and FNR variables.
- getline var
- Set variable var to the next input record from the current input file and, if appropriate, var shall be
considered a numeric string (see Expressions in awk ). This form of getline shall set the
FNR and NR variables.
- getline [var] < expression
-
Read the next record of input from a named file. The expression shall be evaluated to produce a string that is used as a
pathname. If the file of that name is not currently open, it shall be opened. As long as the stream remains open, subsequent calls
in which expression evaluates to the same string value shall read subsequent records from the file. The file shall remain
open until the close function is called with an expression that evaluates to the same string value. If var is
omitted, $0 and NF shall be set; otherwise, var shall be set and, if appropriate, it shall be considered a numeric
string (see Expressions in awk ).
The getline operator can form ambiguous constructs when there are unparenthesized binary operators (including
concatenate) to the right of the '<' (up to the end of the expression containing the getline). The result of
evaluating such a construct is unspecified, and conforming applications shall parenthesize properly all such usages.
- system(expression)
-
Execute the command given by expression in a manner equivalent to the system()
function defined in the System Interfaces volume of IEEE Std 1003.1-2001 and return the exit status of the command.
All forms of getline shall return 1 for successful input, zero for end-of-file, and -1 for an error.
Where strings are used as the name of a file or pipeline, the application shall ensure that the strings are textually identical.
The terminology "same string value" implies that "equivalent strings", even those that differ only by <space>s, represent
different files.
User-Defined Functions
The awk language also provides user-defined functions. Such functions can be defined as:
function name([parameter, ...]) { statements }
A function can be referred to anywhere in an awk program; in particular, its use can precede its definition. The scope of
a function is global.
Function parameters, if present, can be either scalars or arrays; the behavior is undefined if an array name is passed as a
parameter that the function uses as a scalar, or if a scalar expression is passed as a parameter that the function uses as an
array. Function parameters shall be passed by value if scalar and by reference if array name.
The number of parameters in the function definition need not match the number of parameters in the function call. Excess formal
parameters can be used as local variables. If fewer arguments are supplied in a function call than are in the function definition,
the extra parameters that are used in the function body as scalars shall evaluate to the uninitialized value until they are
otherwise initialized, and the extra parameters that are used in the function body as arrays shall be treated as uninitialized
arrays where each element evaluates to the uninitialized value until otherwise initialized.
When invoking a function, no white space can be placed between the function name and the opening parenthesis. Function calls can
be nested and recursive calls can be made upon functions. Upon return from any nested or recursive function call, the values of all
of the calling function's parameters shall be unchanged, except for array parameters passed by reference. The return
statement can be used to return a value. If a return statement appears outside of a function definition, the behavior is
undefined.
In the function definition, <newline>s shall be optional before the opening brace and after the closing brace. Function
definitions can appear anywhere in the program where a pattern-action pair is allowed.
Grammar
The grammar in this section and the lexical conventions in the following section shall together describe the syntax for
awk programs. The general conventions for this style of grammar are described in Grammar Conventions . A valid program can be represented as the non-terminal symbol
program in the grammar. This formal syntax shall take precedence over the preceding text syntax description.
%token NAME NUMBER STRING ERE
%token FUNC_NAME /* Name followed by '(' without white space. */
/* Keywords */
%token Begin End
/* 'BEGIN' 'END' */
%token Break Continue Delete Do Else
/* 'break' 'continue' 'delete' 'do' 'else' */
%token Exit For Function If In
/* 'exit' 'for' 'function' 'if' 'in' */
%token Next Print Printf Return While
/* 'next' 'print' 'printf' 'return' 'while' */
/* Reserved function names */
%token BUILTIN_FUNC_NAME
/* One token for the following:
* atan2 cos sin exp log sqrt int rand srand
* gsub index length match split sprintf sub
* substr tolower toupper close system
*/
%token GETLINE
/* Syntactically different from other built-ins. */
/* Two-character tokens. */
%token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
/* '+=' '-=' '*=' '/=' '%=' '^=' */
%token OR AND NO_MATCH EQ LE GE NE INCR DECR APPEND
/* '||' '&&' '!˜' '==' '<=' '>=' '!=' '++' '--' '>>' */
/* One-character tokens. */
%token '{' '}' '(' ')' '[' ']' ',' ';' NEWLINE
%token '+' '-' '*' '%' '^' '!' '>' '<' '|' '?' ':' '˜' '$' '='
%start program
%%
program : item_list
| actionless_item_list
;
item_list : newline_opt
| actionless_item_list item terminator
| item_list item terminator
| item_list action terminator
;
actionless_item_list : item_list pattern terminator
| actionless_item_list pattern terminator
;
item : pattern action
| Function NAME '(' param_list_opt ')'
newline_opt action
| Function FUNC_NAME '(' param_list_opt ')'
newline_opt action
;
param_list_opt : /* empty */
| param_list
;
param_list : NAME
| param_list ',' NAME
;
pattern : Begin
| End
| expr
| expr ',' newline_opt expr
;
action : '{' newline_opt '}'
| '{' newline_opt terminated_statement_list '}'
| '{' newline_opt unterminated_statement_list '}'
;
terminator : terminator ';'
| terminator NEWLINE
| ';'
| NEWLINE
;
terminated_statement_list : terminated_statement
| terminated_statement_list terminated_statement
;
unterminated_statement_list : unterminated_statement
| terminated_statement_list unterminated_statement
;
terminated_statement : action newline_opt
| If '(' expr ')' newline_opt terminated_statement
| If '(' expr ')' newline_opt terminated_statement
Else newline_opt terminated_statement
| While '(' expr ')' newline_opt terminated_statement
| For '(' simple_statement_opt ';'
expr_opt ';' simple_statement_opt ')' newline_opt
terminated_statement
| For '(' NAME In NAME ')' newline_opt
terminated_statement
| ';' newline_opt
| terminatable_statement NEWLINE newline_opt
| terminatable_statement ';' newline_opt
;
unterminated_statement : terminatable_statement
| If '(' expr ')' newline_opt unterminated_statement
| If '(' expr ')' newline_opt terminated_statement
Else newline_opt unterminated_statement
| While '(' expr ')' newline_opt unterminated_statement
| For '(' simple_statement_opt ';'
expr_opt ';' simple_statement_opt ')' newline_opt
unterminated_statement
| For '(' NAME In NAME ')' newline_opt
unterminated_statement
;
terminatable_statement : simple_statement
| Break
| Continue
| Next
| Exit expr_opt
| Return expr_opt
| Do newline_opt terminated_statement While '(' expr ')'
;
simple_statement_opt : /* empty */
| simple_statement
;
simple_statement : Delete NAME '[' expr_list ']'
| expr
| print_statement
;
print_statement : simple_print_statement
| simple_print_statement output_redirection
;
simple_print_statement : Print print_expr_list_opt
| Print '(' multiple_expr_list ')'
| Printf print_expr_list
| Printf '(' multiple_expr_list ')'
;
output_redirection : '>' expr
| APPEND expr
| '|' expr
;
expr_list_opt : /* empty */
| expr_list
;
expr_list : expr
| multiple_expr_list
;
multiple_expr_list : expr ',' newline_opt expr
| multiple_expr_list ',' newline_opt expr
;
expr_opt : /* empty */
| expr
;
expr : unary_expr
| non_unary_expr
;
unary_expr : '+' expr
| '-' expr
| unary_expr '^' expr
| unary_expr '*' expr
| unary_expr '/' expr
| unary_expr '%' expr
| unary_expr '+' expr
| unary_expr '-' expr
| unary_expr non_unary_expr
| unary_expr '<' expr
| unary_expr LE expr
| unary_expr NE expr
| unary_expr EQ expr
| unary_expr '>' expr
| unary_expr GE expr
| unary_expr '˜' expr
| unary_expr NO_MATCH expr
| unary_expr In NAME
| unary_expr AND newline_opt expr
| unary_expr OR newline_opt expr
| unary_expr '?' expr ':' expr
| unary_input_function
;
non_unary_expr : '(' expr ')'
| '!' expr
| non_unary_expr '^' expr
| non_unary_expr '*' expr
| non_unary_expr '/' expr
| non_unary_expr '%' expr
| non_unary_expr '+' expr
| non_unary_expr '-' expr
| non_unary_expr non_unary_expr
| non_unary_expr '<' expr
| non_unary_expr LE expr
| non_unary_expr NE expr
| non_unary_expr EQ expr
| non_unary_expr '>' expr
| non_unary_expr GE expr
| non_unary_expr '˜' expr
| non_unary_expr NO_MATCH expr
| non_unary_expr In NAME
| '(' multiple_expr_list ')' In NAME
| non_unary_expr AND newline_opt expr
| non_unary_expr OR newline_opt expr
| non_unary_expr '?' expr ':' expr
| NUMBER
| STRING
| lvalue
| ERE
| lvalue INCR
| lvalue DECR
| INCR lvalue
| DECR lvalue
| lvalue POW_ASSIGN expr
| lvalue MOD_ASSIGN expr
| lvalue MUL_ASSIGN expr
| lvalue DIV_ASSIGN expr
| lvalue ADD_ASSIGN expr
| lvalue SUB_ASSIGN expr
| lvalue '=' expr
| FUNC_NAME '(' expr_list_opt ')'
/* no white space allowed before '(' */
| BUILTIN_FUNC_NAME '(' expr_list_opt ')'
| BUILTIN_FUNC_NAME
| non_unary_input_function
;
print_expr_list_opt : /* empty */
| print_expr_list
;
print_expr_list : print_expr
| print_expr_list ',' newline_opt print_expr
;
print_expr : unary_print_expr
| non_unary_print_expr
;
unary_print_expr : '+' print_expr
| '-' print_expr
| unary_print_expr '^' print_expr
| unary_print_expr '*' print_expr
| unary_print_expr '/' print_expr
| unary_print_expr '%' print_expr
| unary_print_expr '+' print_expr
| unary_print_expr '-' print_expr
| unary_print_expr non_unary_print_expr
| unary_print_expr '˜' print_expr
| unary_print_expr NO_MATCH print_expr
| unary_print_expr In NAME
| unary_print_expr AND newline_opt print_expr
| unary_print_expr OR newline_opt print_expr
| unary_print_expr '?' print_expr ':' print_expr
;
non_unary_print_expr : '(' expr ')'
| '!' print_expr
| non_unary_print_expr '^' print_expr
| non_unary_print_expr '*' print_expr
| non_unary_print_expr '/' print_expr
| non_unary_print_expr '%' print_expr
| non_unary_print_expr '+' print_expr
| non_unary_print_expr '-' print_expr
| non_unary_print_expr non_unary_print_expr
| non_unary_print_expr '˜' print_expr
| non_unary_print_expr NO_MATCH print_expr
| non_unary_print_expr In NAME
| '(' multiple_expr_list ')' In NAME
| non_unary_print_expr AND newline_opt print_expr
| non_unary_print_expr OR newline_opt print_expr
| non_unary_print_expr '?' print_expr ':' print_expr
| NUMBER
| STRING
| lvalue
| ERE
| lvalue INCR
| lvalue DECR
| INCR lvalue
| DECR lvalue
| lvalue POW_ASSIGN print_expr
| lvalue MOD_ASSIGN print_expr
| lvalue MUL_ASSIGN print_expr
| lvalue DIV_ASSIGN print_expr
| lvalue ADD_ASSIGN print_expr
| lvalue SUB_ASSIGN print_expr
| lvalue '=' print_expr
| FUNC_NAME '(' expr_list_opt ')'
/* no white space allowed before '(' */
| BUILTIN_FUNC_NAME '(' expr_list_opt ')'
| BUILTIN_FUNC_NAME
;
lvalue : NAME
| NAME '[' expr_list ']'
| '$' expr
;
non_unary_input_function : simple_get
| simple_get '<' expr
| non_unary_expr '|' simple_get
;
unary_input_function : unary_expr '|' simple_get
;
simple_get : GETLINE
| GETLINE lvalue
;
newline_opt : /* empty */
| newline_opt NEWLINE
;
This grammar has several ambiguities that shall be resolved as follows:
-
Operator precedence and associativity shall be as described in Expressions in Decreasing Precedence in awk .
-
In case of ambiguity, an else shall be associated with the most immediately preceding if that would satisfy the
grammar.
-
In some contexts, a slash ( '/' ) that is used to surround an ERE could also be the division operator. This shall be
resolved in such a way that wherever the division operator could appear, a slash is assumed to be the division operator. (There is
no unary division operator.)
One convention that might not be obvious from the formal grammar is where <newline>s are acceptable. There are several
obvious placements such as terminating a statement, and a backslash can be used to escape <newline>s between any lexical
tokens. In addition, <newline>s without backslashes can follow a comma, an open brace, 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:
-
Except as noted, awk shall recognize the longest possible token or delimiter beginning at a given point.
-
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.
-
The <newline> shall be recognized as the token NEWLINE.
-
A backslash character immediately followed by a <newline> shall have no effect.
-
The token STRING shall represent a string constant. A string constant shall begin with the character ' .' Within
a string constant, a backslash character shall be considered to begin an escape sequence as specified in the table in the Base
Definitions volume of 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.
-
The token ERE represents an extended regular expression constant. An ERE constant shall begin with the slash character.
Within an ERE constant, a backslash character shall be considered to begin an escape sequence as specified in the table in the Base
Definitions volume of IEEE Std 1003.1-2001, Chapter 5, File Format Notation. In
addition, the escape sequences in Expressions in Decreasing Precedence in awk shall be recognized. The application shall
ensure that a <newline> does not occur within an ERE constant. An ERE constant shall be terminated by the first unescaped
occurrence of the slash character after the one that begins the ERE constant. The extended regular expression represented by the
ERE constant shall be the sequence of all unescaped characters and values of escape sequences between, but not including, the two
delimiting slash characters.
-
A <blank> shall have no effect, except to delimit lexical tokens or within STRING or ERE tokens.
-
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:
-
An integer constant cannot begin with 0x or include the hexadecimal digits 'a' , 'b' , 'c' ,
'd' , 'e' , 'f' , 'A' , 'B' , 'C' , 'D' , 'E' , or
'F' .
-
The value of an integer constant beginning with 0 shall be taken in decimal rather than octal.
-
An integer constant cannot include a suffix ( 'u' , 'U' , 'l' , or 'L' ).
-
A floating constant cannot include a suffix ( 'f' , 'F' , 'l' , or 'L' ).
If the value is too large or too small to be representable (see Concepts Derived from
the ISO C Standard ), the behavior is undefined.
-
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), beginning
with an underscore or alphabetic, shall be considered a word.
-
The following words are keywords that shall be recognized as individual tokens; the name of the token is the same as the
keyword:
|
BEGIN
break
continue
|
delete
do
else
|
END
exit
for
|
function
getline
if
|
in
next
print
|
printf
return
while
|
-
The following words are names of built-in functions and shall be recognized as the token BUILTIN_FUNC_NAME:
|
atan2
close
cos
exp
|
gsub
index
int
length
|
log
match
rand
sin
|
split
sprintf
sqrt
srand
|
sub
substr
system
tolower
|
toupper
|
The above-listed keywords and names of built-in functions are considered reserved words.
-
The token NAME shall consist of a word that is not a keyword or a name of a built-in function and is not followed
immediately (without any delimiters) by the '(' character.
-
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.
-
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
|
>>
|
-
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.
This description is based on the new awk, "nawk", (see the referenced The AWK Programming Language), which
introduced a number of new features to the historical awk:
-
New keywords: delete, do, function, return
-
New built-in functions: atan2, close, cos, gsub, match, rand, sin,
srand, sub, system
-
New predefined variables: FNR, ARGC, ARGV, RSTART, RLENGTH, SUBSEP
-
New expression operators: ?, :, ,, ^
-
The FS variable and the third argument to split, now treated as extended regular expressions.
-
The operator precedence, changed to more closely match the C language. Two examples of code that operate differently are:
while ( n /= 10 > 1) ...
if (!"wk" ˜ /bwk/) ...
Several features have been added based on newer implementations of awk:
-
Multiple instances of -f progfile are permitted.
-
The new option -v assignment.
-
The new predefined variable ENVIRON.
-
New built-in functions toupper and tolower.
-
More formatting capabilities are added to printf to match the ISO C standard.
The overall awk syntax has always been based on the C language, with a few features from the shell command language and
other sources. Because of this, it is not completely compatible with any other language, which has caused confusion for some users.
It is not the intent of the standard developers to address such issues. A few relatively minor changes toward making the language
more compatible with the ISO C standard were made; most of these changes are based on similar changes in recent
implementations, as described above. There remain several C-language conventions that are not in awk. One of the notable
ones is the comma operator, which is commonly used to specify multiple expressions in the C language for statement. Also,
there are various places where awk is more restrictive than the C language regarding the type of expression that can be used
in a given context. These limitations are due to the different features that the awk language does provide.
Regular expressions in awk have been extended somewhat from historical implementations to make them a pure superset of
extended regular expressions, as defined by IEEE Std 1003.1-2001 (see the Base Definitions volume of
IEEE Std 1003.1-2001, Section 9.4, Extended Regular Expressions). The
main extensions are internationalization features and interval expressions. Historical implementations of awk have long
supported backslash escape sequences as an extension to extended regular expressions, and this extension has been retained despite
inconsistency with other utilities. The number of escape sequences recognized in both extended regular expressions and strings has
varied (generally increasing with time) among implementations. The set specified by IEEE Std 1003.1-2001 includes most
sequences known to be supported by popular implementations and by the ISO C standard. One sequence that is not supported is
hexadecimal value escapes beginning with '\x' . This would allow values expressed in more than 9 bits to be used within
awk as in the ISO C standard. However, because this syntax has a non-deterministic length, it does not permit the
subsequent character to be a hexadecimal digit. This limitation can be dealt with in the C language by the use of lexical string
concatenation. In the awk language, concatenation could also be a solution for strings, but not for extended regular
expressions (either lexical ERE tokens or strings used dynamically as regular expressions). Because of this limitation, the feature
has not been added to IEEE Std 1003.1-2001.
When a string variable is used in a context where an extended regular expression normally appears (where the lexical token ERE
is used in the grammar) the string does not contain the literal slashes.
Some versions of awk allow the form:
func name(args, ... ) { statements }
This has been deprecated by the authors of the language, who asked that it not be specified.
Historical implementations of awk produce an error if a next statement is executed in a BEGIN action, and
cause awk to terminate if a next statement is executed in an END action. This behavior has not been
documented, and it was not believed that it was necessary to standardize it.
The specification of conversions between string and numeric values is much more detailed than in the documentation of historical
implementations or in the referenced The AWK Programming Language. Although most of the behavior is designed to be
intuitive, the details are necessary to ensure compatible behavior from different implementations. This is especially important in
relational expressions since the types of the operands determine whether a string or numeric comparison is performed. From the
perspective of an application writer, it is usually sufficient to expect intuitive behavior and to force conversions (by adding
zero or concatenating a null string) when the type of an expression does not obviously match what is needed. The intent has been to
specify historical practice in almost all cases. The one exception is that, in historical implementations, variables and constants
maintain both string and numeric values after their original value is converted by any use. This means that referencing a variable
or constant can have unexpected side effects. For example, with historical implementations the following program:
{
a = "+2"
b = 2
if (NR % 2)
c = a + b
if (a == b)
print "numeric comparison"
else
print "string comparison"
}
would perform a numeric comparison (and output numeric comparison) for each odd-numbered line, but perform a string comparison
(and output string comparison) for each even-numbered line. IEEE Std 1003.1-2001 ensures that comparisons will be numeric
if necessary. With historical implementations, the following program:
BEGIN {
OFMT = "%e"
print 3.14
OFMT = "%f"
print 3.14
}
would output "3.140000e+00" twice, because in the second print statement the constant "3.14" would have
a string value from the previous conversion. IEEE Std 1003.1-2001 requires that the output of the second print
statement be "3.140000" . The behavior of historical implementations was seen as too unintuitive and unpredictable.
It was pointed out that with the rules contained in early drafts, the following script would print nothing:
BEGIN {
y[1.5] = 1
OFMT = "%e"
print y[1.5]
}
Therefore, a new variable, CONVFMT, was introduced. The OFMT variable is now restricted to affecting output
conversions of numbers to strings and CONVFMT is used for internal conversions, such as comparisons or array indexing. The
default value is the same as that for OFMT, so unless a program changes CONVFMT (which no historical program would
do), it will receive the historical behavior associated with internal string conversions.
The POSIX awk lexical and syntactic conventions are specified more formally than in other sources. Again the intent has
been to specify historical practice. One convention that may not be obvious from the formal grammar as in other verbal descriptions
is where <newline>s are acceptable. There are several obvious placements such as terminating a statement, and a backslash can
be used to escape <newline>s between any lexical tokens. In addition, <newline>s without backslashes can follow a
comma, an open brace, a logical AND operator ( "&&" ), a logical OR operator ( "||" ), the do
keyword, the else keyword, and the closing parenthesis of an if, for, or while statement. For
example:
{ print $1,
$2 }
The requirement that awk add a trailing <newline> to the program argument text is to simplify the grammar, 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 implementations in order to support
internationalization. Probably the most subtle of these is the use of the decimal-point character, defined by the LC_NUMERIC
category of the locale, in representations of floating-point numbers. This locale-specific character is used in recognizing numeric
input, in converting between strings and numeric values, and in formatting output. However, regardless of locale, the period
character (the decimal-point character of the POSIX locale) is the decimal-point character recognized in processing awk
programs (including assignments in command line arguments). This is essentially the same convention as the one used in the
ISO C standard. The difference is that the C language includes the setlocale() function, which permits an application to modify its locale. Because of this
capability, a C application begins executing with its locale set to the C locale, and only executes in the environment-specified
locale after an explicit call to setlocale(). However, adding such an elaborate
new feature to the awk language was seen as inappropriate for IEEE Std 1003.1-2001. It is possible to execute an
awk program explicitly in any desired locale by setting the environment in the shell.
The undefined behavior resulting from NULs in extended regular expressions allows future extensions for the GNU gawk
program to process binary data.
The behavior in the case of invalid awk programs (including lexical, syntactic, and semantic errors) is undefined because
it was considered overly limiting on implementations to specify. In most cases such errors can be expected to produce a diagnostic
and a non-zero exit status. However, some implementations may choose to extend the language in ways that make use of certain
invalid constructs. Other invalid constructs might be deemed worthy of a warning, but otherwise cause some reasonable behavior.
Still other constructs may be very difficult to detect in some implementations. Also, different implementations might detect a
given error during an initial parsing of the program (before reading any input files) while others might detect it when executing
the program after reading some input. Implementors should be aware that diagnosing errors as early as possible and producing useful
diagnostics can ease debugging of applications, and thus make an implementation more usable.
The unspecified behavior from using multi-character RS values is to allow possible future extensions based on extended
regular expressions used for record separators. Historical implementations take the first character of the string and ignore the
others.
Unspecified behavior when split( string, array, <null>) is used
is to allow a proposed future extension that would split up a string into an array of individual characters.
In the context of the getline function, equally good arguments for different precedences of the | and <
operators can be made. Historical practice has been that:
getline < "a" "b"
is parsed as:
( getline < "a" ) "b"
although many would argue that the intent was that the file ab should be read. However:
getline < "x" + 1
parses as:
getline < ( "x" + 1 )
Similar problems occur with the | version of getline, particularly in combination with $. For example:
$"echo hi" | getline
(This situation is particularly problematic when used in a print statement, where the |getline part might be a
redirection of the print.)
Since in most cases such constructs are not (or at least should not) be used (because they have a natural ambiguity for which
there is no conventional parsing), the meaning of these constructs has been made explicitly unspecified. (The effect is that a
conforming application that runs into the problem must parenthesize to resolve the ambiguity.) There appeared to be few if any
actual uses of such constructs.
Grammars can be written that would cause an error under these circumstances. Where backwards-compatibility is not a large
consideration, implementors may wish to use such grammars.
Some historical implementations have allowed some built-in functions to be called without an argument list, the result being a
default argument list chosen in some "reasonable" way. Use of length as a synonym for length($0) is the only one of
these forms that is thought to be widely known or widely used; this particular form is documented in various places (for example,
most historical awk reference pages, although not in the referenced The AWK Programming Language) as legitimate
practice. With this exception, default argument lists have always been undocumented and vaguely defined, and it is not at all clear
how (or if) they should be generalized to user-defined functions. They add no useful functionality and preclude possible future
extensions that might need to name functions without calling them. Not standardizing them seems the simplest course. The standard
developers considered that length merited special treatment, however, since it has been documented in the past and sees
possibly substantial use in historical programs. Accordingly, this usage has been made legitimate, but Issue 5 removed the
obsolescent marking for XSI-conforming implementations and many otherwise conforming applications depend on this feature.
In sub and gsub, if repl is a string literal (the lexical token STRING), then two consecutive
backslash characters should be used in the string to ensure a single backslash will precede the ampersand when the resultant string
is passed to the function. (For example, to specify one literal ampersand in the replacement string, use gsub( ERE,
"\\&" ).)
Historically the only special character in the repl argument of sub and gsub string functions was the
ampersand ( '&' ) character and preceding it with the backslash character was used to turn off its special
meaning.
The description in the ISO POSIX-2:1993 standard introduced behavior such that the backslash character was another special
character and it was unspecified whether there were any other special characters. This description introduced several portability
problems, some of which are described below, and so it has been replaced with the more historical description. Some of the problems
include:
-
Historically, to create the replacement string, a script could use gsub( ERE, "\\&" ), but with the
ISO POSIX-2:1993 standard wording, it was necessary to use gsub( ERE, "\\\\&" ). Backslash
characters are doubled here because all string literals are subject to lexical analysis, which would reduce each pair of backslash
characters to a single backslash before being passed to gsub.
-
Since it was unspecified what the special characters were, for portable scripts to guarantee that characters are printed
literally, each character had to be preceded with a backslash. (For example, a portable script had to use gsub( ERE,
"\\h\\i" ) to produce a replacement string of "hi" .)
The description for comparisons in the ISO POSIX-2:1993 standard did not properly describe historical practice because of
the way numeric strings are compared as numbers. The current rules cause the following code:
if (0 == "000")
print "strange, but true"
else
print "not true"
to do a numeric comparison, causing the if to succeed. It should be intuitively obvious that this is incorrect behavior,
and indeed, no historical implementation of awk actually behaves this way.
To fix this problem, the definition of numeric string was enhanced to include only those values obtained from specific
circumstances (mostly external sources) where it is not possible to determine unambiguously whether the value is intended to be a
string or a numeric.
Variables that are assigned to a numeric string shall also be treated as a numeric string. (For example, the notion of a numeric
string can be propagated across assignments.) In comparisons, all variables having the uninitialized value are to be treated as a
numeric operand evaluating to the numeric value zero.
Uninitialized variables include all types of variables including scalars, array elements, and fields. The definition of an
uninitialized value in Variables and Special Variables is necessary to describe the value placed on
uninitialized variables and on fields that are valid (for example, < $NF) but have no characters in them and to
describe how these variables are to be used in comparisons. A valid field, such as $1, that has no characters in it can be
obtained from an input line of "\t\t" when FS= '\t' . Historically, the comparison ( $1<10) was
done numerically after evaluating $1 to the value zero.
The phrase "... also shall have the numeric value of the numeric string" was removed from several sections of the
ISO POSIX-2:1993 standard because is specifies an unnecessary implementation detail. It is not necessary for
IEEE Std 1003.1-2001 to specify that these objects be assigned two different values. It is only necessary to specify that
these objects may evaluate to two different values depending on context.
The description of numeric string processing is based on the behavior of the atof()
function in the ISO C 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 constants use a period as a decimal point character
for the language itself, independent of the current locale, but the atof() function and
the associated strtod() 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.
![[Option Start]](../images/opt-start.gif)
Determine the location of message catalogs for the processing of LC_MESSAGES . ![[Option End]](../images/opt-end.gif)