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 will match any record of input, and a missing action will be equivalent to an action that writes the matched record of input to standard output.
Execution of the
awk
program starts 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)
will be processed in turn by
reading data from the file until
a record separator is seen
(a newline character
by default),
splitting the current record into fields using the current value of
FS
according to the rules in
Expressions 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 membership | numeric | left |
| expr && expr | Logical AND | numeric | left |
| expr || expr | Logical OR | numeric | left |
| expr1 ? expr2 : expr3 | Conditional expression | type of selected expr2 or expr3 | right |
| 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 has either a string value, a numeric value or both. Except as stated for specific contexts, the value of an expression will be implicitly converted to the type needed for the context in which it is used. A string value will 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
will be converted to a string by the equivalent of a
call to the
sprintf
function (see
A string value will be considered to be a numeric string in the following case:
-
Any leading and trailing
blank characters
will be ignored.
-
If the first unignored character is a
"+" or "-", it will be ignored.
-
If the remaining unignored characters would be lexically recognised as a
NUMBER
token (as described by the lexical conventions in
Grammar ), the string will be considered a numeric string .
If a "-" character is ignored in the above steps, the numeric value of the numeric string will be the negation of the numeric value of the recognised NUMBER token. Otherwise the numeric value of the numeric string will be the numeric value of the recognised NUMBER token. Whether or not a string is a numeric string will 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 is treated as false and any other value is treated as true. Otherwise, a string value of the null string is treated as false and any other value is treated as true. A Boolean context is 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 will follow the semantics of floating-point arithmetic as specified by the ISO C standard.
The value of the expression:
expr1 ^ expr2
will be equivalent to the value returned by the ISO C standard
function call:
pow(expr1, expr2)
The expression:
lvalue ^= expr
will be equivalent to the ISO C standard expression:
lvalue = pow(lvalue, expr)
except that
lvalue
will be evaluated only once.
The value of the expression:
expr1 % expr2
will be equivalent to the value returned by the ISO C standard function call:
fmod(expr1, expr2)
The expression:
lvalue %= expr
will be equivalent to the ISO C standard expression:
lvalue = fmod(lvalue, expr)
except that
lvalue
will be evaluated only once.
Variables and fields will be set by the assignment statement:
lvalue = expression
and the type of
expression
will determine the resulting variable type.
The assignment includes the arithmetic assignments
(+=, -=, *=, /=, %=, ^=,
++, --) all of
which 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 are initially empty,
and their sizes will 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
-
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 behaviour is undefined.
A valid array index consists 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 will 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 will be equivalent:
var[expr1, expr2, ... exprn]
var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]
A multi-dimensioned index used with the in operator must be parenthesised. The in operator, which tests for the existence of a particular array element, will not cause that element to exist. Any other reference to a non-existent array element will automatically create it.
Comparisons (with the "<", "<=", "!=", "==", ">" and ">=" operators) will be made numerically if both operands are numeric or if one is numeric and the other has a string value that is a numeric string. Otherwise, operands will be converted to strings as required and a string comparison will be made using the locale-specific collation sequence. The value of the comparison expression will 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
Field variables are 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;
uninitialised 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 non-existent fields (that is, fields after
$NF),
will produce the null string.
However, assigning to a non-existent field
(for example, $(NF +2)
= 5)
will increase the value of
NF,
create any intervening fields with the null string as their values
and cause the value of
$0
to be recomputed, with the fields being separated by the value of
OFS.
Each field variable will have a string value when created.
If the string,
with any occurrence of the decimal-point character from the
current locale changed to a
period character,
would be considered a
numeric string
(see
Implementations 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 will 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 will 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 will 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
- The variable
ENVIRON
is an array representing the value of the environment,
as described in the XSH specification
under the
exec
functions.
The indices of the array are strings
consisting of the names of the environment variables,
and the value of each array element is
a string consisting of the value of that variable.
If the value of an environment variable is considered a
numeric string
(see
Expressions in awk ), the array element will also have its numeric value. In all cases where the behaviour 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 will be the environment at the time awk began executing; it is implementation-dependent 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 is 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 is zero. Inside an END action the value is the number of the last record processed in the last file processed.
- FS
Input field separator regular expression; a space character 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 will 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 is zero. Inside an END action the value is 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; a space character by default.
- ORS
- The print statement output record separator; a newline character by default.
- RLENGTH
- The length of the string matched by the match function.
- RS
- The first character of the string value of RS is the input record separator; a newline character by default. If RS contains more than one character, the results are unspecified. If RS is null, then records are separated by sequences of one or more blank lines, leading or trailing blank lines do not result in empty records at the beginning or end of the input, and a newline character is always 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 is always equivalent to the return value of the match function.
- SUBSEP
- The subscript separator string for multi-dimensional arrays; the default value is implementation-dependent.
Regular Expressions
| 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 behaviour is undefined. | The character whose encoding is represented by the one-, two- or three-digit octal integer. If the size of a byte on the system is greater than nine bits, the valid escape sequence used to represent a byte is implementation-dependent. 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 XBD specification, 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 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 will evaluate to a value of 1, and the !~ expression will evaluate to a value of 0. (The regular expression matching operation is as defined by the term matched in the XBD specification, 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 will evaluate to a value of 0, and the !~ expression will 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 will be interpreted as an extended regular expression, including the escape conventions described above. Note that these same escape conventions also will be applied in the determining the value of a string literal (the lexical token STRING), and thus will 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 will be the equivalent of:
$0 ~ /ere/
The
ere
argument to the
gsub,
match,
sub
functions, and the
fs
argument to the
split
function (see
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 will be a single space character. The following describes FS behaviour:
-
If
FS
is a single character:
-
If
FS
is the
space character,
skip leading and trailing
blank characters;
fields will be delimited by sets of one or more
blank characters.
-
Otherwise, if
FS
is any other character
c,
fields will be delimited by
each single occurrence of
c .
-
If
FS
is the
space character,
skip leading and trailing
blank characters;
fields will be delimited by sets of one or more
blank characters.
-
Otherwise,
the string value of
FS
will be considered to be an extended regular expression.
Each occurrence of
a sequence matching
the extended regular expression will delimit fields.
Except in the gsub, match, split and sub built-in functions, regular expression matching will be based on input records; that is, record separator characters (the first character of the value of the variable RS, a newline character by default) cannot be embedded in the expression, and no expression will match the record separator character. If the record separator is not a newline character, newline characters embedded in the expression can be matched. In those four built-in functions, regular expression matching will be based on text strings; that is, any character (including the newline character and the record separator) can be embedded in the pattern and an appropriate pattern will match any character. However, in all awk regular expression 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 comma, or one of the two special patterns BEGIN or END.Special Patterns
The awk utility recognises two special patterns, BEGIN and END. Each BEGIN pattern will be matched once and its associated action executed before the first record of input is read (except possibly by use of the getline function (seeBEGIN and END will not combine with other patterns. Multiple BEGIN and END patterns are allowed. The actions associated with the BEGIN patterns will 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 will 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 will be read before the statements in the END actions are executed.
Expression Patterns
An expression pattern will be evaluated as if it were an expression in a Boolean context. If the result is true, the pattern will be considered to match, and the associated action (if any) will be executed. If the result is false, the action will not be executed.Pattern Ranges
A pattern range consists of two expressions separated by a comma; in this case, the action will 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 inThe expression acting as the conditional in an if statement will be evaluated and if it is non-zero or non-null, the following statement will be executed; otherwise, if else is present, the statement following the else will be executed.
The
if,
while,
do
...
while,
for,
break
and
continue
statements are based on the ISO C standard,
except that the Boolean expressions are treated as described in
for (variable in array)
which will 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 behaviour is undefined.
The delete statement will remove an individual array element. Thus, the following code will delete an entire array:
for (index in array)
delete array[index]
The next statement will cause all further processing of the current input record to be abandoned. The behaviour is undefined if a next statement appears or is invoked in a BEGIN or END action.
The exit statement will 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 will terminate the program without further execution of END actions. If an expression is specified in an exit statement, its numeric value will be the exit status of awk, unless subsequent errors are encountered or a subsequent exit statement with an expression is executed.
Output Statements
> expression
>> expression
| expression
In all cases, the
expression
will be evaluated to produce a string that is used as a full pathname
to write into (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 will be opened, creating it if necessary and using the
first form, truncating the file.
The output then will be appended to the file.
As long as the file remains open,
subsequent calls in which
expression
evaluates to the same string value simply will append output to the file.
The file remains open until the
close
function (see
The third form will write output onto a stream
piped to the input of a command.
The stream will be created if no stream is currently open with the value of
expression
as its command name.
The stream created will be equivalent to one created by a call to the XSH specification
popen()
function 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 will write output to the existing stream.
The stream will remain open until the
close
function (see
As described in detail by the grammar in
The
print
statement will 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
will be taken as strings, being converted if necessary;
this conversion will be as described in
The printf statement will produce output based on a notation similar to the File Format Notation used to describe file formats in this specification (see the XBD specification, File Format Notation ). Output will 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
will be an actual character string rather than a graphical representation.
Therefore, it cannot contain empty character positions.
The space character in the
format
string, in any context other than a
flag
of a conversion specification,
will 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 will be treated as an
ordinary character that is copied to the output.
-
The
escape sequences
beginning with a backslash character will be treated as sequences of
ordinary characters that are copied to the output.
Note that these same sequences will be interpreted lexically by
awk
when they appear in literal strings, but they will 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 will be fetched
and its numeric value taken as the field width or precision.
-
The implementation will not precede or follow output from the
d
or
u
conversion specifications with
blank characters
not specified by the
format
string.
-
The implementation will not precede output from the
o
conversion specification with leading zeros not specified by the
format
string.
-
For the
c
conversion specification:
if the argument has a numeric value, the character whose encoding is that
value will be output.
If the value is zero or is not the encoding of any character in the
character set, the behaviour is undefined.
If the argument does not have a numeric value, the first character
of the string value will be output;
if the string does not contain any characters the behaviour is undefined.
-
For each conversion specification that consumes an argument,
the next expression argument will be evaluated.
With the exception of the
c
conversion, the value will 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 behaviour is undefined.
-
If any character sequence in the
format
string begins with a % character, but does not form a
valid conversion specification, the behaviour 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, are based on the ISO C standard. The behaviour is undefined in cases where the ISO C standard specifies that an error be returned or that the behaviour is undefined. Although the grammar (see- atan2(y,x)
- Return arctangent of y/x.
- 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)
- Truncate its argument to an integer. It will be truncated 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 will be returned.
String Functions
- gsub(ere, repl[,in])
- Behave like sub (see below), except that it will 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. The use of no argument and no parentheses with length is obsolescent in the ISO/IEC 9945-2:1993 standard; to be fully portable to POSIX systems, the application must use length($0) for the length of the whole record. However, XSI-conformant systems will continue to support this usage indefinitely.
- 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 will be set to the starting position (which is the same as the returned value), zero if no match is found; RLENGTH will 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 .
The separation will be done with the extended regular expression
fs
or with the field separator
FS
if
fs
is not given.
Each array element will have a string value when created.
If the string assigned to any array element,
with any occurrence of the decimal-point character from the
current locale changed to a
period character,
would be considered a
numeric string
(see
Expressions in awk ), the array element will also have the numeric value of the numeric string . 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
will be replaced by the string from
in
that matches the regular expression.
For each occurrence of backslash (\) encountered when scanning the
string
repl
from beginning to end, the next character is
taken literally and loses its special meaning (for example, \& will be
interpreted as a literal ampersand character).
Except for & and \,
it is unspecified what the special meaning of any such character is.
If
in
is specified and it is not an
lvalue
(see
Expressions in awk ), the behaviour is undefined. If in is omitted, awk will substitute in the current record ($0). - substr(s, m[,n])
- Return the at most n -character substring of s that begins at position m, numbering from 1. If n is missing, the length of the substring will 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 upper-case letter specified to have a tolower mapping by the LC_CTYPE category of the current locale will be replaced in the returned string by the lower-case letter specified by the mapping. Other characters in s will be unchanged in the returned string.
- toupper(s)
- Return a string based on the string s . Each character in s that is a lower-case letter specified to have a toupper mapping by the LC_CTYPE category of the current locale will be replaced in the returned string by the upper-case letter specified by the mapping. Other characters in s will be 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
Input/Output and General Functions
- 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-dependent. If the close was successful, the function will return zero; otherwise, it will return non-zero.
- expression | getline [var]
- Read a record of input from a stream piped from the output of a command. The stream will be created if no stream is currently open with the value of expression as its command name. The stream created will 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 will read subsequent records from the file. The stream will remain open until the close function is called with an expression that evaluates to the same string value. At that time, the stream will be closed as if by a call to the pclose() function. If var is missing, $0 and NF will be set; otherwise, var will be set. The getline operator can form ambiguous constructs when there are unparenthesised operators (including concatenate) to the left of the "|" (to the beginning of the expression containing getline). In the context of the "$" operator, "|" behaves as if it had a lower precedence than "$". The result of evaluating other operators is unspecified, and portable applications must parenthesise properly all such usages.
- getline
- Set $0 to the next input record from the current input file. This form of getline will set the NF, NR and FNR variables.
- getline var
- Set variable var to the next input record from the current input file. This form of getline will set the FNR and NR variables.
- getline [var] < expression
- Read the next record of input from a named file. The expression will be evaluated to produce a string that is used as a full pathname. If the file of that name is not currently open, it will be opened. As long as the stream remains open, subsequent calls in which expression evaluates to the same string value will read subsequent records from the file. The file will remain open until the close function is called with an expression that evaluates to the same string value. If var is missing, $0 and NF will be set; otherwise, var will be set. The getline operator can form ambiguous constructs when there are unparenthesised 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 portable applications must parenthesise properly all such usages.
- system(expression)
- Execute the command given by expression in a manner equivalent to the XSH specification system() function and return the exit status of the command.
All forms of getline will 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 strings must be textually identical. The terminology "same string value" implies that "equivalent strings", even those that differ only by space characters, represent different files.
User-defined Functions
function name(args,...) { 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 will be global.
Function arguments can be either scalars or arrays; the behaviour is undefined if an array name is passed as an argument that the function uses as a scalar, or if a scalar expression is passed as an argument that the function uses as an array. Function arguments will be passed by value if scalar and by reference if array name. Argument names will be local to the function; all other variable names will be global. The same name will not be used as both an argument name and as the name of a function or a special awk variable. The same name must not be used both as a variable name with global scope and as the name of a function. The same name must not be used within the same scope both as a scalar variable and as an array.
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 will be initialised with a string value of the null string and a numeric value of zero, and the extra parameters that are used in the function body as arrays will be initialised as empty arrays. If more arguments are supplied in a function call than are in the function definition, the behaviour is undefined.
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 will 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 behaviour is undefined.
In the function definition, newline characters are 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
%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 are resolved as follows:
-
Operator precedence and associativity are as described in
. -
In case of ambiguity, an
else
will 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 is 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 characters are acceptable. There are several obvious placements such as terminating a statement, and a backslash can be used to escape newline characters between any lexical tokens. In addition, newline characters without 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, are as follows:-
Except as noted,
awk
will recognise the longest possible
token or delimiter beginning at a given point.
-
A comment consists of any characters beginning with the
number sign character and terminated by, but excluding the next
occurrence of, a
newline
character.
Comments will have no effect, except to delimit lexical tokens.
-
The character
newline
will be recognised as the token
NEWLINE.
-
A backslash character immediately followed by a
newline
character will have no effect.
-
The token
STRING
represents a string constant.
A string constant begins with the character
.
Within a string
constant, a backslash character will be considered to begin an escape
sequence as specified in the table in
the XBD specification, File Format Notation
(\\,
\a,
\b,
\f,
\n,
\r,
\t,
\v).
In addition, the
escape sequences in
will be recognised. A newline character will not occur within a string constant. A string constant will be terminated by the first unescaped occurrence of the character after the one that begins the string constant. The value of the string will 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 begins with the slash character.
Within an ERE constant, a
backslash
character will be considered to begin
an escape sequence as specified in the table in
the XBD specification, File Format Notation .
In addition, the escape sequences in
will be recognised. A newline character must not occur within an ERE constant. An ERE constant will be terminated by the first unescaped occurrence of the slash character after the one that begins the string constant. The extended regular expression represented by the ERE constant will be the sequence of all unescaped characters and values of escape sequences between, but not including, the two delimiting slash characters. -
A
blank character
has no effect, except to delimit lexical tokens or within
STRING
or
ERE
tokens.
-
The token
NUMBER
represents a numeric constant.
Its form and numeric value are 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
will 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, the behaviour is undefined.
-
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.
-
A sequence of underscores, digits and alphabetics from the
portable character set (see
the XBD specification, Portable Character Set ),
beginning with an underscore
or alphabetic, will be considered a word.
-
The following words are keywords that will be recognised as
individual tokens; the name of the token is the same as the
keyword:
-
BEGIN delete for in printf END do function next return break else getline print while continue exit if
-
The following words are names of built-in functions and
will be recognised as the token
BUILTIN_FUNC_NAME:
-
atan2 index match sprintf substr close int rand sqrt system cos length sin srand tolower exp log split sub toupper gsub
The above-listed keywords and names of built-in functions are considered reserved words.
-
The token
NAME
consists 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
consists 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 will not be included as part of the token.
-
The following two-character sequences will be recognised 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 will be recognised 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 recognised will be recognised. In any other syntactic context where the token ERE could appear as the next token in a valid program, the token ERE will be recognised.