awk - Man Page

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 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 4-1: Expressions in Decreasing Precedence in awk

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 either by the equivalent of the following calls to functions defined by the ISO C standard:

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

or by converting the initial portion of the string to type double representation as follows:

A numeric value that is exactly equal to the value of an integer (see Section 1.1.2, 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 POSIX.1-2017 specifies no explicit conversions between numbers and strings. An application can force an expression to be treated as a number by adding zero to it, or can force it to be treated as a string by concatenating the null string ("") to it.

A string value shall be considered a numeric string if it comes from one of the following:

1. Field variables
2. Input from the getline() function
3. FILENAME
4. ARGV array elements
5. ENVIRON array elements
6. Array elements created by the split() function
7. A command line variable assignment
8. Variable assignment from another numeric string variable

and an implementation-dependent condition corresponding to either case (a) or (b) below is met.

a.

After the equivalent of the following calls to functions defined by the ISO C standard, string_value_end would differ from string_value, and any characters before the terminating null character in string_value_end would be <blank> characters:

char *string_value_end;
setlocale(LC_NUMERIC, "");
numeric_value = strtod (string_value, &string_value_end);

b.

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> characters are discarded.

--

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

--

Each occurrence of the decimal point character from the current locale is changed to a <period>.

In case (a) the numeric value of the numeric string shall be the value that would be returned by the strtod() call. In case (b) if the first non-<blank> is '-', 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 Section 1.1.2, 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 as follows:

*

For the "!=" and "==" operators, the strings should be compared to check if they are identical but may be compared using the locale-specific collation sequence to check if they collate equally.

*

For the other operators, the strings shall be compared 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 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 POSIX.1-2017. 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 separator; <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.

The awk utility shall make use of the extended regular expression notation (see the Base Definitions volume of POSIX.1-2017, 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 POSIX.1-2017, 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> characters and string constants can contain <newline> characters, 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 4-2: Escape Sequences in awk

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 POSIX.1-2017, 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 assigning a string containing the expression to the built-in variable FS, either directly or as a consequence of using the -F sepstring option. The default value of the FS variable shall be a single <space>. The following describes FS behavior:

1. If FS is a null string, the behavior is unspecified.

2. If FS is a single character:

   a.

   If FS is <space>, skip leading and trailing <blank> and <newline> characters; fields shall be delimited by sets of one or more <blank> or <newline> characters.

   b.

   Otherwise, if FS is any other character c, fields shall be delimited by each single occurrence of c.

3. Otherwise, the string value of FS shall be considered to be an extended regular expression. Each occurrence of a sequence matching the extended regular expression shall delimit fields.

Except for the '~' and "!~" operators, and in the gsub, match, split, and sub built-in functions, ERE matching shall be based on input records; that is, record separator characters (the first character of the value of the variable RS, <newline> by default) cannot be embedded in the expression, and no expression shall match the record separator character. If the record separator is not <newline>, <newline> characters 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.

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.

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.

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.

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.

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 curly braces. The application shall ensure that statements in a statement list are separated by <newline> or <semicolon> characters. 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 Section 1.1.2, 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.

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 POSIX.1-2017 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 POSIX.1-2017.

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 POSIX.1-2017 (see the Base Definitions volume of POSIX.1-2017, Chapter 5, File Format Notation). Output shall be produced as specified with the first expression argument as the string format and subsequent expression arguments as the strings arg1 to argn, inclusive, with the following exceptions:

1. The format shall be an actual character string rather than a graphical representation. Therefore, it cannot contain empty character positions. The <space> in the format string, in any context other than a flag of a conversion specification, shall be treated as an ordinary character that is copied to the output.
2. If the character set contains a '' character and that character appears in the format string, it shall be treated as an ordinary character that is copied to the output.
3. The escape sequences beginning with a <backslash> character shall be treated as sequences of ordinary characters that are copied to the output. Note that these same sequences shall be interpreted lexically by awk when they appear in literal strings, but they shall not be treated specially by the printf statement.
4. A field width or precision can be specified as the '*' character instead of a digit string. In this case the next argument from the expression list shall be fetched and its numeric value taken as the field width or precision.
5. The implementation shall not precede or follow output from the d or u conversion specifier characters with <blank> characters not specified by the format string.
6. The implementation shall not precede output from the o conversion specifier character with leading zeros not specified by the format string.
7. For the c conversion specifier character: if the argument has a numeric value, the character whose encoding is that value shall be output. If the value is zero or is not the encoding of any character in the character set, the behavior is undefined. If the argument does not have a numeric value, the first character of the string value shall be output; if the string does not contain any characters, the behavior is undefined.
8. For each conversion specification that consumes an argument, the next expression argument shall be evaluated. With the exception of the c conversion specifier character, the value shall be converted (according to the rules specified in Expressions in awk) to the appropriate type for the conversion specification.
9. If there are insufficient expression arguments to satisfy all the conversion specifications in the format string, the behavior is undefined.
10. If any character sequence in the format string begins with a '%' character, but does not form a valid conversion specification, the behavior is unspecified.

Both print and printf can output at least {LINE_MAX} bytes.

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

The arithmetic functions, except for int, shall be based on the ISO C standard (see Section 1.1.2, 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 [-π,π].

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.

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 <backslash> characters 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.

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 POSIX.1-2017 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> characters, represent different files.

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> characters 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.

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 Section 1.3, 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
                 | item_list item
                 ;

item_list        : /* empty */
                 | item_list item terminator
                 ;

item             : action
                 | pattern action
                 | normal_pattern
                 | 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          : normal_pattern
                 | special_pattern
                 ;

normal_pattern   : expr
                 | expr ',' newline_opt expr
                 ;

special_pattern  : Begin
                 | End
                 ;

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

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 Table 4-1, 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.)

Each expression in an awk program shall conform to the precedence and associativity rules, even when this is not needed to resolve an ambiguity. For example, because '$' has higher precedence than '++', the string "$x++--" is not a valid awk expression, even though it is unambiguously parsed by the grammar as "$(x++)--".

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 <backslash> characters 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 }

The lexical conventions for awk programs, with respect to the preceding grammar, shall be as follows:

1. Except as noted, awk shall recognize the longest possible token or delimiter beginning at a given point.
2. A comment shall consist of any characters beginning with the <number-sign> character and terminated by, but excluding the next occurrence of, a <newline>. Comments shall have no effect, except to delimit lexical tokens.
3. The <newline> shall be recognized as the token NEWLINE.
4. A <backslash> character immediately followed by a <newline> shall have no effect.
5. The token STRING shall represent a string constant. A string constant shall begin with the character '"'. Within a string constant, a <backslash> character shall be considered to begin an escape sequence as specified in the table in the Base Definitions volume of POSIX.1-2017, Chapter 5, File Format Notation ('\\', '\a', '\b', '\f', '\n', '\r', '\t', '\v'). In addition, the escape sequences in Table 4-2, Escape Sequences in awk shall be recognized. A <newline> shall not occur within a string constant. A string constant shall be terminated by the first unescaped occurrence of the character '"' after the one that begins the string constant. The value of the string shall be the sequence of all unescaped characters and values of escape sequences between, but not including, the two delimiting '"' characters.
6. The token ERE represents an extended regular expression constant. An ERE constant shall begin with the <slash> character. Within an ERE constant, a <backslash> character shall be considered to begin an escape sequence as specified in the table in the Base Definitions volume of POSIX.1-2017, Chapter 5, File Format Notation. In addition, the escape sequences in Table 4-2, Escape Sequences in awk shall be recognized. The application shall ensure that a <newline> does not occur within an ERE constant. An ERE constant shall be terminated by the first unescaped occurrence of the <slash> character after the one that begins the ERE constant. The extended regular expression represented by the ERE constant shall be the sequence of all unescaped characters and values of escape sequences between, but not including, the two delimiting <slash> characters.
7. A <blank> shall have no effect, except to delimit lexical tokens or within STRING or ERE tokens.

8. The token NUMBER shall represent a numeric constant. Its form and numeric value shall either be equivalent to the decimal-floating-constant token as specified by the ISO C standard, or it shall be a sequence of decimal digits and shall be evaluated as an integer constant in decimal. In addition, implementations may accept numeric constants with the form and numeric value equivalent to the hexadecimal-constant and hexadecimal-floating-constant tokens as specified by the ISO C standard.

   If the value is too large or too small to be representable (see Section 1.1.2, Concepts Derived from the ISO C Standard), the behavior is undefined.

9. A sequence of underscores, digits, and alphabetics from the portable character set (see the Base Definitions volume of POSIX.1-2017, Section 6.1, Portable Character Set), beginning with an <underscore> or alphabetic character, shall be considered a word.

10. The following words are keywords that shall be recognized as individual tokens; the name of the token is the same as the keyword:

    BEGIN break continue	delete do else	END exit for	function getline if	in next print	printf return while

11. 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.

12. The token NAME shall consist of a word that is not a keyword or a name of a built-in function and is not followed immediately (without any delimiters) by the '(' character.
13. The token FUNC_NAME shall consist of a word that is not a keyword or a name of a built-in function, followed immediately (without any delimiters) by the '(' character. The '(' character shall not be included as part of the token.

14. The following two-character sequences shall be recognized as the named tokens:

    Token Name	Sequence	Token Name	Sequence
    ADD_ASSIGN	+=	NO_MATCH	!~
    SUB_ASSIGN	-=	EQ	==
    MUL_ASSIGN	*=	LE	<=
    DIV_ASSIGN	/=	GE	>=
    MOD_ASSIGN	%=	NE	!=
    POW_ASSIGN	^=	INCR	++
    OR	||	DECR	--
    AND	&&	APPEND	>>

15. The following single characters shall be recognized as tokens whose names are the character:

    <newline> { } ( ) [ ] , ; + - * % ^ ! > < | ? : ~ $ =

There is a lexical ambiguity between the token ERE and the tokens '/' and DIV_ASSIGN. When an input sequence begins with a <slash> character in any syntactic context where the token '/' or DIV_ASSIGN could appear as the next token in a valid program, the longer of those two tokens that can be recognized shall be recognized. In any other syntactic context where the token ERE could appear as the next token in a valid program, the token ERE shall be recognized.