AWK(1) User Commands AWK(1)

NAME

awk - pattern scanning and processing language

SYNOPSIS

/usr/bin/awk [-F ERE] [-v assignment] 'program' | -f progfile...
[argument]...


/usr/bin/nawk [-F ERE] [-v assignment] 'program' | -f progfile...
[argument]...


/usr/xpg4/bin/awk [-F ERE] [-v assignment]... 'program' | -f progfile...
[argument]...

DESCRIPTION

NOTE: The nawk command is now the system default awk for illumos.

The /usr/bin/awk and /usr/xpg4/bin/awk utilities execute programs written
in the awk programming language, which is specialized for textual data
manipulation. A awk program is a sequence of patterns and corresponding
actions. The string specifying program must be enclosed in single quotes
(') to protect it from interpretation by the shell. The sequence of
pattern - action statements can be specified in the command line as
program or in one, or more, file(s) specified by the -fprogfile option.
When input is read that matches a pattern, the action associated with the
pattern is performed.


Input is interpreted as a sequence of records. By default, a record is a
line, but this can be changed by using the RS built-in variable. Each
record of input is matched to each pattern in the program. For each
pattern matched, the associated action is executed.


The awk utility interprets each input record as a sequence of fields
where, by default, a field is a string of non-blank characters. This
default white-space field delimiter (blanks and/or tabs) can be changed
by using the FS built-in variable or the -FERE option. The awk utility
denotes the first field in a record $1, the second $2, and so forth. The
symbol $0 refers to the entire record; setting any other field causes the
reevaluation of $0. Assigning to $0 resets the values of all fields and
the NF built-in variable.

OPTIONS

The following options are supported:

-F ERE
Define the input field separator to be the extended
regular expression ERE, before any input is read (can be
a character).


-f progfile
Specifies the pathname of the file progfile containing a
awk program. If multiple instances of this option are
specified, the concatenation of the files specified as
progfile in the order specified is the awk program. The
awk program can alternatively be specified in the
command line as a single argument.


-v assignment
The assignment argument must be in the same form as an
assignment operand. The assignment is of the form
var=value, where var is the name of one of the variables
described below. The specified assignment occurs before
executing the awk program, including the actions
associated with BEGIN patterns (if any). Multiple
occurrences of this option can be specified.


-safe
When passed to awk, this flag will prevent the program
from opening new files or running child processes. The
ENVIRON array will also not be initialized.

OPERANDS

The following operands are supported:

program
If no -f option is specified, the first operand to awk is the
text of the awk program. The application supplies the program
operand as a single argument to awk. If the text does not end
in a newline character, awk interprets the text as if it did.


argument
Either of the following two types of argument can be
intermixed:

file
A pathname of a file that contains the input to
be read, which is matched against the set of
patterns in the program. If no file operands
are specified, or if a file operand is -, the
standard input is used.


assignment
An operand that begins with an underscore or
alphabetic character from the portable
character set, followed by a sequence of
underscores, digits and alphabetics from the
portable character set, followed by the =
character specifies a variable assignment
rather than a pathname. The characters before
the = represent the name of a awk variable. If
that name is a awk reserved word, the behavior
is undefined. The characters following the
equal sign is interpreted as if they appeared
in the awk program preceded and followed by a
double-quote (") character, as a STRING token,
except that if the last character is an
unescaped backslash, it is interpreted as a
literal backslash rather than as the first
character of the sequence \.. The variable is
assigned the value of that STRING token. If the
value is considered a numeric string, the
variable is assigned its numeric value. Each
such variable assignment is performed just
before the processing of the following file, if
any. Thus, an assignment before the first file
argument is executed after the BEGIN actions
(if any), while an assignment after the last
file argument is executed before the END
actions (if any). If there are no file
arguments, assignments are executed before
processing the standard input.

INPUT FILES

Input files to the awk program from any of the following sources:

o any file operands or their equivalents, achieved by modifying
the awk variables ARGV and ARGC

o standard input in the absence of any file operands

o arguments to the getline function


must be text files. Whether the variable RS is set to a value other than
a newline character or not, for these files, implementations support
records terminated with the specified separator up to {LINE_MAX} bytes
and can support longer records.


If -f progfile is specified, the files named by each of the progfile
option-arguments must be text files containing an awk program.


The standard input are used only if no file operands are specified, or if
a file operand is -.

EXTENDED DESCRIPTION

A 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. Pattern-action statements are separated by a
semicolon or by a newline.


A missing pattern matches any record of input, and a missing action is
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) is processed 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, evaluating each pattern
in the program in the order of occurrence, and executing the action
associated with each pattern that matches the current record. The action
for a matching pattern is executed before evaluating subsequent patterns.
Last, the actions associated with all END patterns is executed in the
order they occur in the program.

Expressions in awk

Expressions describe computations used in patterns and actions. In the
following table, valid expression operations are given in groups from
highest precedence first to lowest precedence last, with equal-precedence
operators grouped between horizontal lines. In expression evaluation,
where the grammar is formally ambiguous, higher precedence operators are
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).


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 Multi-dimension array numeric left
array membership
-------------------------------------------------------------------------------
expr && expr Logical AND numeric left
-------------------------------------------------------------------------------
expr || expr Logical OR numeric left
-------------------------------------------------------------------------------
expr1 ? expr2 Conditional expression type of selected right
: expr3 expr2 or expr3
-------------------------------------------------------------------------------
lvalue ^= expr Exponentiation numeric right
assignment
lvalue %= expr Modulus assignment numeric right
lvalue *= expr Multiplication numeric right
assignment
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 is
implicitly converted to the type needed for the context in which it is
used. A string value is converted to a numeric value by the equivalent
of the following calls:

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


A numeric value that is exactly equal to the value of an integer is
converted to a string by the equivalent of a call to the sprintf function
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
is 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.


A string value is considered to be a numeric string in the following
case:

1. Any leading and trailing blank characters is ignored.

2. If the first unignored character is a + or -, it is ignored.

3. If the remaining unignored characters would be lexically
recognized as a NUMBER token, the string is considered a
numeric string.


If a - character is ignored in the above steps, the numeric value of the
numeric string is the negation of the numeric value of the recognized
NUMBER token. Otherwise the numeric value of the numeric string is the
numeric value of the recognized NUMBER token. Whether or not a string is
a numeric string is 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:

o the first subexpression of a conditional expression.

o an expression operated on by logical NOT, logical AND, or
logical OR.

o the second expression of a for statement.

o the expression of an if statement.

o the expression of the while clause in either a while or do ...
while statement.

o an expression used as a pattern (as in Overall Program
Structure).


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 changes 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 as an expression, as described in the grammar. Unsubscripted array
names are used in only the following contexts:

o a parameter in a function definition or function call.

o the NAME token following any use of the keyword in.


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 is 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 are equivalent:

var[expr1, expr2, ... exprn]
var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]


A multi-dimensioned index used with the in operator must be put in
parentheses. The in operator, which tests for the existence of a
particular array element, does not create the element if it does not
exist. Any other reference to a non-existent array element automatically
creates it.

Variables and Special Variables

Variables can be used in an awk program by referencing them. With the
exception of function parameters, they are not explicitly declared.
Uninitialized scalar variables and array elements have both a numeric
value of zero and a string value of the empty string.


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. Uninitialized
variables or string values need not be converted to numeric values in
this context. New field variables are created by assigning a value to
them. References to non-existent fields (that is, fields after $NF)
produce the null string. However, assigning to a non-existent field (for
example, $(NF+2) = 5) increases 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 has 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, is considered a numeric string (see Expressions in
awk above), the field variable also has the numeric value of the numeric
string.


/usr/bin/awk, /usr/xpg4/bin/awk
awk sets the following special variables that are supported by both
/usr/bin/awk and /usr/xpg4/bin/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 treats the next
non-null element of ARGV, up to the current value of ARGC-1,
inclusive, as the name of the next input file. Setting an
element of ARGV to null means that it is not treated as an
input file. The name - indicates the standard input. If an
argument matches the format of an assignment operand, this
argument is 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). The default is
%.6g.


ENVIRON
The variable ENVIRON is an array representing the value of
the environment. 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, the array element
also has its numeric value.

In all cases where awk behavior 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 is the environment at
the time awk began executing.


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 retains 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 "%.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; a space character
by default.


ORS
The print 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 produce empty records at the beginning or
end of input, and the field separator is always newline, no
matter what the value of FS.


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 \034.


/usr/bin/awk
The following variable is supported for /usr/bin/awk only:

RT
The record terminator for the most recent record read. For
most records this will be the same value as RS. At the end of
a file with no trailing separator value, though, this will be
set to the empty string ("").

Regular Expressions

The awk utility makes use of the extended regular expression notation
(see regex(7)) except that it allows the use of C-language conventions to
escape special characters within the EREs, namely \\, \a, \b, \f, \n, \r,
\t, \v, and those specified in the following table. These escape
sequences are 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 the regular expression
requires escaping as shown in the table below:


Escape Sequence Description Meaning
----------------------------------------------------------------------
\" Backslash quotation-mark Quotation-mark character
----------------------------------------------------------------------
\/ Backslash slash Slash character
----------------------------------------------------------------------
\ddd A backslash character The character encoded by
followed by the longest the one-, two- or
sequence of one, two, or three-digit octal
three octal-digit integer. Multi-byte
characters (01234567). characters require
If all of the digits are multiple, concatenated
0, (that is, escape sequences,
representation of the including the leading \
NULL character), the for each byte.
behavior is undefined.
----------------------------------------------------------------------
\c A backslash character Undefined
followed by any
character not described
in this table or special
characters (\\, \a, \b,
\f, \n, \r, \t, \v).


A regular expression can be matched against a specific field or string by
using one of the two regular expression matching operators, ~ and !~.
These operators 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 evaluates to the value 1,
and the !~ expression evaluates to the value 0. If the regular expression
does not match the string, the ~ expression evaluates to the value 0, and
the !~ expression evaluates to the value 1. If the right-hand operand is
any expression other than the lexical token ERE, the string value of the
expression is interpreted as an extended regular expression, including
the escape conventions described above. Notice that these same escape
conventions also are applied in the determining the value of a string
literal (the lexical token STRING), and is 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 is 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) is interpreted as extended
regular expressions. These can be either ERE tokens or arbitrary
expressions, and are interpreted in the same manner as the right-hand
side of the ~ or !~ operator.


An extended regular expression can be used to separate fields by using
the -F ERE option or by assigning a string containing the expression to
the built-in variable FS. The default value of the FS variable is a
single space character. The following describes FS behavior:

1. If FS is a single character:

o If FS is the space character, skip leading and trailing
blank characters; fields are delimited by sets of one or
more blank characters.

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

2. Otherwise, the string value of FS is considered to be an
extended regular expression. Each occurrence of a sequence
matching the extended regular expression delimits fields.


Except in the gsub, match, split, and sub built-in functions, regular
expression matching is 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 matches 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 are based on text strings. So, any character
(including the newline character and the record separator) can be
embedded in the pattern and an appropriate pattern matches any character.
However, in all awk regular expression matching, the use of one or more
NULL 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 recognizes two special patterns, BEGIN and END. Each
BEGIN pattern is matched once and its associated action executed before
the first record of input is read (except possibly by use of the getline
function in a prior BEGIN action) and before command line assignment is
done. Each END pattern is matched once and its associated action executed
after the last record of input has been read. These two patterns have
associated actions.


BEGIN and END do not combine with other patterns. Multiple BEGIN and END
patterns are allowed. The actions associated with the BEGIN patterns are
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 exits 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 is read before the
statements in the END actions are executed.

Expression Patterns

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

Pattern Ranges

A pattern range consists of two expressions separated by a comma. In this
case, the action is 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. A statement can be one of the
following:

if ( expression ) statement [ else statement ]
while ( expression ) statement
do statement while ( expression )
for ( expression ; expression ; expression ) statement
for ( var in array ) statement
delete array[subscript] #delete an array element
delete array #delete all elements within an array
break
continue
{ [ statement ] ... }
expression # commonly variable = expression
print [ expression-list ] [ >expression ]
printf format [ ,expression-list ] [ >expression ]
next # skip remaining patterns on this input line
nextfile # skip remaining patterns on this input file
exit [expr] # skip the rest of the input; exit status is expr
return [expr]


Any single statement can be replaced by a statement list enclosed in
braces. The statements are terminated by newline characters or
semicolons, and are executed sequentially in the order that they appear.


The next statement causes 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 nextfile statement is similar to next, but also skips all other
records in the current file, and moves on to processing the next input
file if available (or exits the program if there are none). (Note that
this keyword is not supported by /usr/xpg4/bin/awk.)


The exit statement invokes 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 terminates
the program without further execution of END actions. If an expression
is specified in an exit statement, its numeric value is the exit status
of awk, unless subsequent errors are encountered or a subsequent exit
statement with an expression is executed.

Output Statements

Both print and printf statements write to standard output by default.
The output is written to the location specified by output_redirection if
one is supplied, as follows:

> expression>> expression| expression


In all cases, the expression is 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 is opened, creating it if necessary and using the
first form, truncating the file. The output then is appended to the file.
As long as the file remains open, subsequent calls in which expression
evaluates to the same string value simply appends output to the file. The
file remains open until the close function, which is called with an
expression that evaluates to the same string value.


The third form writes output onto a stream piped to the input of a
command. The stream is created if no stream is currently open with the
value of expression as its command name. The stream created is
equivalent to one created by a call to the popen(3C) 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 writes output to the
existing stream. The stream remains open until the close function is
called with an expression that evaluates to the same string value. At
that time, the stream is closed as if by a call to the pclose function.


These output statements take a comma-separated list of expression s
referred 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 writes 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 is taken as strings,
being converted if necessary; with the exception that the printf format
in OFMT is used instead of the value in CONVFMT. An empty expression list
stands for the whole input record ($0).


The printf statement produces output based on a notation similar to the
File Format Notation used to describe file formats in this document
Output is 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 is 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, is treated as an ordinary character that is
copied to the output.

2. If the character set contains a Delta character and that
character appears in the format string, it is treated as an
ordinary character that is copied to the output.

3. The escape sequences beginning with a backslash character is
treated as sequences of ordinary characters that are copied to
the output. Note that these same sequences is interpreted
lexically by awk when they appear in literal strings, but they
is not 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 is fetched and its numeric value taken as
the field width or precision.

5. The implementation does not precede or follow output from the
d or u conversion specifications with blank characters not
specified by the format string.

6. The implementation does not precede output from the o
conversion specification with leading zeros not specified by
the format string.

7. For the c conversion specification: if the argument has a
numeric value, the character whose encoding is that value is
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 is 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 is evaluated. With the exception
of the c conversion, the value is converted to the appropriate
type for the conversion specification.

9. If there are insufficient expression arguments to satisfy all
the conversion specifications in the format string, the
behavior is undefined.

10. If any character sequence in the format string begins with a %
character, but does not form a valid conversion specification,
the behavior is unspecified.


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

Functions

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

Arithmetic Functions

The arithmetic functions, except for int, are based on 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 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.


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 is 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 is
returned.

String Functions

The string functions in the following list shall be supported. Although
the 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 replaces 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[([v])]

Given no argument, this function returns the length of the whole
record, $0. If given an array as an argument (and using
/usr/bin/awk), then this returns the number of elements it contains.
Otherwise, this function interprets the argument as a string
(performing any needed conversions) and returns its length in
characters.


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 is set to the starting position (which is
the same as the returned value), zero if no match is found; RLENGTH
is 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 is done with the extended regular expression
fs or with the field separator FS if fs is not given. Each array
element has 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; the array element also has 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 is
replaced by the string from in that matches the regular expression.
An ampersand preceded with a backslash ( \ ) is interpreted as the
literal ampersand character. An occurrence of two consecutive
backslashes is interpreted as just a single literal backslash
character. Any other occurrence of a backslash (for example,
preceding any other character) is treated as a literal backslash
character. If repl is a string literal, 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 the behavior is undefined. If in is omitted, awk uses
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 missing, the length of the substring is
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 is replaced in the returned string by
the lower-case letter specified by the mapping. Other characters in s
are 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 is replaced in the returned string by
the upper-case 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 below.


Input/Output and General Functions
The input/output and general functions are:

close(expression)
Close the file or pipe opened by a print or
printf statement or a call to getline with the
same string-valued expression. If the close
was successful, the function returns 0;
otherwise, it returns non-zero.


fflush(expression)
Flush any buffered output for the file or pipe
opened by a print or printf statement or a
call to getline with the same string-valued
expression. If the flush was successful, the
function returns 0; otherwise, it returns EOF.
If no arguments or the empty string ("") are
given, then all open files will be flushed.
(Note that fflush is supported in /usr/bin/awk
only.)


expression|getline[var]
Read a record of input from a stream piped
from the output of a command. The stream is
created if no stream is currently open with
the value of expression as its command name.
The stream created is 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 reads subsequent
records from the file. The stream remains open
until the close function is called with an
expression that evaluates to the same string
value. At that time, the stream is closed as
if by a call to the pclose function. If var is
missing, $0 and NF is set. Otherwise, var is
set.

The getline operator can form ambiguous
constructs when there are operators that are
not in parentheses (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
all such uses of portable applications must be
put in parentheses properly.


getline
Set $0 to the next input record from the
current input file. This form of getline sets
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
sets the FNR and NR variables.


getline [var] < expression
Read the next record of input from a named
file. The expression is evaluated to produce a
string that is used as a full pathname. If the
file of that name is not currently open, it is
opened. As long as the stream remains open,
subsequent calls in which expression evaluates
to the same string value reads subsequent
records from the file. The file remains open
until the close function is called with an
expression that evaluates to the same string
value. If var is missing, $0 and NF is set.
Otherwise, var is set.

The getline operator can form ambiguous
constructs when there are binary operators
that are not in parentheses (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 all such uses of portable
applications must be put in parentheses
properly.


system(expression)
Execute the command given by expression in a
manner equivalent to the system(3C) function
and return the exit status of the command.


All forms of getline return 1 for successful input, 0 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
The awk language also provides user-defined functions. Such functions can
be defined as:

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 is global.


Function arguments can be either scalars or arrays; the behavior 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 are passed by value if
scalar and by reference if array name. Argument names are local to the
function; all other variable names are global. The same name is not 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 are initialized 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 are initialized as empty
arrays. If more arguments are supplied in a function call than are in the
function definition, the behavior 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 are 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 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.

USAGE

The index, length, match, and substr functions should not be confused
with similar functions in the ISO C standard; the awk versions deal with
characters, while the ISO C standard deals with bytes.


Because the concatenation operation is represented by adjacent
expressions rather than an explicit operator, it is often necessary to
use parentheses to enforce the proper evaluation precedence.


See largefile(7) for the description of the behavior of awk when
encountering files greater than or equal to 2 Gbyte (2^31 bytes).

EXAMPLES

The awk program specified in the command line is most easily specified
within single-quotes (for example, 'program') for applications using sh,
because awk programs commonly contain characters that are special to the
shell, including double-quotes. In the cases where a awk program contains
single-quote characters, it is usually easiest to specify most of the
program as strings within single-quotes concatenated by the shell with
quoted single-quote characters. For example:

awk '/'\''/ { print "quote:", $0 }'


prints all lines from the standard input containing a single-quote
character, prefixed with quote:.


The following are examples of simple awk programs:

Example 1: Write to the standard output all input lines for which field 3

is greater than 5:

$3 > 5

Example 2: Write every tenth line:

(NR % 10) == 0

Example 3: Write any line with a substring matching the regular

expression:

/(G|D)(2[0-9][[:alpha:]]*)/

Example 4: Print any line with a substring containing a G or D, followed

by a sequence of digits and characters:


This example uses character classes digit and alpha to match language-
independent digit and alphabetic characters, respectively.


/(G|D)([[:digit:][:alpha:]]*)/

Example 5: Write any line in which the second field matches the regular

expression and the fourth field does not:

$2 ~ /xyz/ && $4 !~ /xyz/

Example 6: Write any line in which the second field contains a backslash:

$2 ~ /\\/

Example 7: Write any line in which the second field contains a backslash

(alternate method):


Notice that backslash escapes are interpreted twice, once in lexical
processing of the string and once in processing the regular expression.


$2 ~ "\\\\"

Example 8: Write the second to the last and the last field in each line,

separating the fields by a colon:

{OFS=":";print $(NF-1), $NF}

Example 9: Write the line number and number of fields in each line:

The three strings representing the line number, the colon and the number
of fields are concatenated and that string is written to standard output.


{print NR ":" NF}

Example 10: Write lines longer than 72 characters:

{length($0) > 72}

Example 11: Write first two fields in opposite order separated by the OFS:

{ print $2, $1 }

Example 12: Same, with input fields separated by comma or space and tab

characters, or both:

BEGIN { FS = ",[\t]*|[\t]+" }
{ print $2, $1 }

Example 13: Add up first column, print sum and average:

{s += $1 }
END {print "sum is ", s, " average is", s/NR}

Example 14: Write fields in reverse order, one per line (many lines out

for each line in):

{ for (i = NF; i > 0; --i) print $i }

Example 15: Write all lines between occurrences of the strings "start" and

"stop":

/start/, /stop/

Example 16: Write all lines whose first field is different from the

previous one:

$1 != prev { print; prev = $1 }

Example 17: Simulate the echo command:

BEGIN {
for (i = 1; i < ARGC; ++i)
printf "%s%s", ARGV[i], i==ARGC-1?"\n":""
}

Example 18: Write the path prefixes contained in the PATH environment

variable, one per line:

BEGIN {
n = split (ENVIRON["PATH"], path, ":")
for (i = 1; i <= n; ++i)
print path[i]
}

Example 19: Print the file "input", filling in page numbers starting at 5:

If there is a file named input containing page headers of the form


Page#


and a file named program that contains


/Page/{ $2 = n++; }
{ print }


then the command line


awk -f program n=5 input


prints the file input, filling in page numbers starting at 5.

ENVIRONMENT VARIABLES

See environ(7) for descriptions of the following environment variables
that affect execution: LC_COLLATE, LC_CTYPE, LC_MESSAGES, and NLSPATH.

LC_NUMERIC
Determine the radix character used when interpreting
numeric input, performing conversions between numeric and
string values and formatting numeric output. Regardless of
locale, the period character (the decimal-point character
of the POSIX locale) is the decimal-point character
recognized in processing awk programs (including
assignments in command-line arguments).

EXIT STATUS

The following exit values are returned:

0
All input files were processed successfully.


>0
An error occurred.


The exit status can be altered within the program by using an exit
expression.

SEE ALSO

ed(1), egrep(1), grep(1), lex(1), oawk(1), sed(1), popen(3C), printf(3C),
system(3C), XPG4(7), attributes(7), environ(7), largefile(7), regex(7)


Aho, A. V., B. W. Kernighan, and P. J. Weinberger, The AWK Programming
Language, Addison-Wesley, 1988.

DIAGNOSTICS

If any file operand is specified and the named file cannot be accessed,
awk writes a diagnostic message to standard error and terminate without
any further action.


If the program specified by either the program operand or a progfile
operand is not a valid awk program (as specified in EXTENDED
DESCRIPTION), the behavior is undefined.

NOTES

Input white space is not preserved on output if fields are involved.


There are no explicit conversions between numbers and strings. To force
an expression to be treated as a number add 0 to it; to force it to be
treated as a string concatenate the null string ("") to it.

June 13, 2021 AWK(1)