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UNIX in a Nutshell: System V Edition, 3rd Edition

UNIX in a Nutshell: System V Edition, 3rd Edition

By Arnold Robbins
3rd Edition September 1999
1-56592-427-4, Order Number: 4274
616 pages, $29.95

Chapter 11 The awk Programming Language

Contents:
Conceptual Overview
Command-Line Syntax
Patterns and Procedures
Built-in Variables
Operators
Variables and Array Assignments
User-Defined Functions
Group Listing of awk Functions and Commands
Implementation Limits
Alphabetical Summary of Functions and Commands

11.1 Conceptual Overview

awk is a pattern-matching program for processing files, especially when they are databases. The new version of awk, called nawk, provides additional capabilities.[1] Every modern Unix system comes with a version of new awk, and its use is recommended over old awk.

[1] It really isn't so new. The additional features were added in 1984, and it was first shipped with System V Release 3.1 in 1987. Nevertheless, the name was never changed on most systems.

Different systems vary in what the two versions are called. Some have oawk and awk, for the old and new versions, respectively. Others have awk and nawk. Still others only have awk, which is the new version. This example shows what happens if your awk is the old one:

$ awk 1 /dev/null
awk: syntax error near line 1
awk: bailing out near line 1

awk exits silently if it is the new version.

Source code for the latest version of awk, from Bell Labs, can be downloaded starting at Brian Kernighan's home page: http://cm.bell-labs.com/~bwk. Michael Brennan's mawk is available via anonymous FTP from ftp://ftp.whidbey.net/pub/brennan/mawk1.3.3.tar.gz. Finally, the Free Software Foundation has a version of awk called gawk, available from ftp://gnudist.gnu.org/gnu/gawk/gawk-3.0.4.tar.gz. All three programs implement "new" awk. Thus, references below such as "nawk only," apply to all three. gawk has additional features.

With original awk, you can:

With nawk, you can also:

In addition, with GNU awk (gawk), you can:

11.2 Command-Line Syntax

The syntax for invoking awk has two forms:

awk  [options]  'script'  var=value  file(s)
awk  [options]  -f scriptfile  var=value  file(s)

You can specify a script directly on the command line, or you can store a script in a scriptfile and specify it with -f. nawk allows multiple -f scripts. Variables can be assigned a value on the command line. The value can be a literal, a shell variable ($name), or a command substitution (`cmd`), but the value is available only after the BEGIN statement is executed.

awk operates on one or more files. If none are specified (or if - is specified), awk reads from the standard input.

The recognized options are:

-Ffs

Set the field separator to fs. This is the same as setting the system variable FS. Original awk allows the field separator to be only a single character. nawk allows fs to be a regular expression. Each input line, or record, is divided into fields by whitespace (blanks or tabs) or by some other user-definable record separator. Fields are referred to by the variables $1, $2,..., $n. $0 refers to the entire record.

-v var=value

Assign a value to variable var. This allows assignment before the script begins execution (available in nawk only).

To print the first three (colon-separated) fields of each record on separate lines:

awk -F: '{ print $1; print $2; print $3 }' /etc/passwd

More examples are shown in the section "Simple Pattern-Procedure Examples."

11.3 Patterns and Procedures

awk scripts consist of patterns and procedures:

pattern  { procedure }

Both are optional. If pattern is missing, { procedure } is applied to all lines; if { procedure } is missing, the matched line is printed.

11.3.1 Patterns

A pattern can be any of the following:

/regular expression/
relational expression
pattern-matching expression
BEGIN
END

Except for BEGIN and END, patterns can be combined with the Boolean operators || (or), && (and), and ! (not). A range of lines can also be specified using comma-separated patterns:

pattern,pattern

11.3.2 Procedures

Procedures consist of one or more commands, functions, or variable assignments, separated by newlines or semicolons, and contained within curly braces. Commands fall into five groups:

11.3.3 Simple Pattern-Procedure Examples

11.4 Built-in Variables

Version Variable Description
awk FILENAME

Current filename

  FS

Field separator (a space)

  NF

Number of fields in current record

  NR

Number of the current record

  OFMT

Output format for numbers ("%.6g") and for conversion to string

  OFS

Output field separator (a space)

  ORS

Output record separator (a newline)

  RS

Record separator (a newline)

  $0

Entire input record

  $n

nth field in current record; fields are separated by FS

nawk ARGC

Number of arguments on command line

  ARGV

An array containing the command-line arguments, indexed from 0 to ARGC - 1

  CONVFMT

String conversion format for numbers ("%.6g") (POSIX)

  ENVIRON

An associative array of environment variables

  FNR

Like NR, but relative to the current file

  RLENGTH

Length of the string matched by match() function

  RSTART

First position in the string matched by match() function

  SUBSEP

Separator character for array subscripts ("\034")

gawk ARGIND

Index in ARGV of current input file

  ERRNO

A string indicating the error when a redirection fails for getline or if close() fails

  FIELDWIDTHS

A space-separated list of field widths to use for splitting up the record, instead of FS

  IGNORECASE

When true, all regular expression matches, string comparisons, and calls to index()s ignore case

  RT

The text matched by RS, which can be a regular expression in gawk

11.5 Operators

The following table lists the operators, in order of increasing precedence, that are available in awk. Note: while ** and **= are common extensions, they are not part of POSIX awk.

Symbol Meaning

= += -= *= /= %= ^= **=

Assignment
?: C conditional expression (nawk only)
|| Logical OR (short-circuit)
&& Logical AND (short-circuit)
in Array membership (nawk only)
~ !~ Match regular expression and negation
< <= > >= != == Relational operators
(blank) Concatenation
+ - Addition, subtraction
* / % Multiplication, division, and modulus (remainder)
+ - ! Unary plus and minus, and logical negation
^ ** Exponentiation
++ -- Increment and decrement, either prefix or postfix
$ Field reference

11.6 Variables and Array Assignments

Variables can be assigned a value with an = sign. For example:

FS = ","

Expressions using the operators +, -, /, and % (modulo) can be assigned to variables.

Arrays can be created with the split() function (see below), or they can simply be named in an assignment statement. Array elements can be subscripted with numbers (array[1], ..., array[n]) or with strings. Arrays subscripted by strings are called associative arrays.[2] For example, to count the number of widgets you have, you could use the following script:

[2] In fact, all arrays in awk are associative; numeric subscripts are converted to strings before using them as array subscripts. Associative arrays are one of awk's most powerful features.

/widget/ { count["widget"]++ }		Count widgets
END      { print count["widget"] }		Print the count

You can use the special for loop to read all the elements of an associative array:

for (item in array)
	process array[item]

The index of the array is available as item, while the value of an element of the array can be referenced as array[item].

You can use the operator in to see if an element exists by testing to see if its index exists (nawk only):

if (index in array)
	...

This sequence tests that array[index] exists, but you cannot use it to test the value of the element referenced by array[index].

You can also delete individual elements of the array using the delete statement (nawk only).

11.6.1 Escape Sequences

Within string and regular expression constants, the following escape sequences may be used. Note: The \x escape sequence is a common extension; it is not part of POSIX awk.

Sequence Meaning Sequence Meaning
\a Alert (bell) \v Vertical tab
\b Backspace \\ Literal backslash
\f Form feed \nnn Octal value nnn
\n Newline \xnn Hexadecimal value nn
\r Carriage return \" Literal double quote (in strings)
\t Tab \/ Literal slash (in regular expressions)

11.7 User-Defined Functions

nawk allows you to define your own functions. This makes it easy to encapsulate sequences of steps that need to be repeated into a single place, and reuse the code from anywhere in your program. Note: for user-defined functions, no space is allowed between the function name and the left parenthesis when the function is called.

The following function capitalizes each word in a string. It has one parameter, named input, and five local variables, which are written as extra parameters.

# capitalize each word in a string
function capitalize(input,    result, words, n, i, w)
{
	result = ""
	n = split(input, words, " ")
	for (i = 1; i <= n; i++) {
		w = words[i]
		w = toupper(substr(w, 1, 1)) substr(w, 2)
		if (i > 1)
			result = result " "
		result = result w
	}
	return result
}

# main program, for testing
{ print capitalize($0) }

With this input data:

A test line with words and numbers like 12 on it.

This program produces:

A Test Line With Words And Numbers Like 12 On It.

11.8 Group Listing of awk Functions and Commands

The following table classifies awk functions and commands.

Arithmetic String Control Flow I/O Time Program-
Functions Functions Statements Processing Functions ming
atan2[3] gensub[4] break close[3] strftime[4] delete[3]
cos[3] gsub[3] continue fflush[5] systime[4] function[3]
exp index do/while[3] getline[3]   system[3]
int length exit next    
log match[3] for nextfile[5]    
rand[3] split if print    
sin[3] sprintf return[3] printf    
sqrt sub[3] while      
srand[3] substr        
  tolower[3]        
  toupper[3]        

[3] Available in nawk.

[4] Available in gawk.

[5] Available in Bell Labs awk and gawk.

11.9 Implementation Limits

Many versions of awk have various implementation limits, on things such as:

gawk does not have limits on any of these items, other than those imposed by the machine architecture and/or the operating system.

11.10 Alphabetical Summary of Functions and Commands

The following alphabetical list of keywords and functions includes all that are available in awk, nawk, and gawk. nawk includes all old awk functions and keywords, plus some additional ones (marked as {N}). gawk includes all nawk functions and keywords, plus some additional ones (marked as {G}). Items marked with {B} are available in the Bell Labs awk. Items that aren't marked with a symbol are available in all versions.

atan2

atan2(y, x)

Return the arctangent of y/x in radians. {N}

break

break

Exit from a while, for, or do loop.

close

close(filename-expr)
close(command-expr)

In most implementations of awk, you can have only 10 files open simultaneously and one pipe. Therefore, nawk provides a close function that allows you to close a file or a pipe. It takes as an argument the same expression that opened the pipe or file. This expression must be identical, character by character, to the one that opened the file or pipe; even whitespace is significant. {N}

continue

continue

Begin next iteration of while, for, or do loop.

cos

cos(x)

Return the cosine of x, an angle in radians. {N}

delete

delete array[element]
delete array

Delete element from array. The brackets are typed literally. The second form is a common extension, which deletes all elements of the array at one shot. {N}

do

do
    statement
while (expr)

Looping statement. Execute statement, then evaluate expr and, if true, execute statement again. A series of statements must be put within braces. {N}

exit

exit [expr]

Exit from script, reading no new input. The END procedure, if it exists, will be executed. An optional expr becomes awk's return value.

exp

exp(x)

Return exponential of x (ex).

fflush

fflush([output-expr])

Flush any buffers associated with open output file or pipe output-expr. {B}

gawk extends this function. If no output-expr is supplied, it flushes standard output. If output-expr is the null string (""), it flushes all open files and pipes. {G}

for

for (init-expr; test-expr; incr-expr)
    statement

C-style looping construct. init-expr assigns the initial value of a counter variable. test-expr is a relational expression that is evaluated each time before executing the statement. When test-expr is false, the loop is exited. incr-expr increments the counter variable after each pass. All the expressions are optional. A missing test-expr is considered to be true. A series of statements must be put within braces.

for

for (item in array)
    statement

Special loop designed for reading associative arrays. For each element of the array, the statement is executed; the element can be referenced by array[item]. A series of statements must be put within braces.

function

function name(parameter-list) {
    statements
}

Create name as a user-defined function consisting of awk statements that apply to the specified list of parameters. No space is allowed between name and the left paren when the function is called. {N}

getline

getline [var] [< file]
    or
command | getline [var]

Read next line of input. Original awk doesn't support the syntax to open multiple input streams. The first form reads input from file; the second form reads the output of command. Both forms read one record at a time, and each time the statement is executed, it gets the next record of input. The record is assigned to $0 and is parsed into fields, setting NF, NR and FNR. If var is specified, the result is assigned to var, and $0 and NF aren't changed. Thus, if the result is assigned to a variable, the current record doesn't change. getline is actually a function and returns 1 if it reads a record successfully, 0 if end-of-file is encountered, and -1 if it's otherwise unsuccessful. {N}

gensub

gensub(r, s, h [, t])

General substitution function. Substitute s for matches of the regular expression r in the string t. If h is a number, replace the hth match. If it is "g" or "G", substitute globally. If t is not supplied, $0 is used. Return the new string value. The original t is not modified. (Compare gsub and sub.) {G}

gsub

gsub(r, s [, t])

Globally substitute s for each match of the regular expression r in the string t. If t is not supplied, defaults to $0. Return the number of substitutions. {N}

if

if (condition)
    statement
[else
    statement]

If condition is true, do statement(s); otherwise do statement in the optional else clause. The condition can be an expression using any of the relational operators <, <=, ==, !=, >=, or >, as well as the array membership operator in, and the pattern-matching operators ~ and !~ (e.g., if ($1 ~ /[Aa].*/)). A series of statements must be put within braces. Another if can directly follow an else in order to produce a chain of tests or decisions.

index

index(str, substr)

Return the position (starting at 1) of substr in str, or zero if substr is not present in str.

int

int(x)

Return integer value of x by truncating any fractional part.

length

length([arg])

Return length of arg, or the length of $0 if no argument.

log

log(x)

Return the natural logarithm (base e) of x.

match

match(s, r)

Function that matches the pattern, specified by the regular expression r, in the string s, and returns either the position in s, where the match begins, or 0 if no occurrences are found. Sets the values of RSTART and RLENGTH to the start and length of the match, respectively. {N}

next

next

Read next input line and start new cycle through pattern/procedures statements.

nextfile

nextfile

Stop processing the current input file and start new cycle through pattern/procedures statements, beginning with the first record of the next file. {B} {G}

print

print [ output-expr[, ...]] [ dest-expr ]

Evaluate the output-expr and direct it to standard output, followed by the value of ORS. Each comma-separated output-expr is separated in the output by the value of OFS. With no output-expr, print $0.

Output Redirections

dest-expr is an optional expression that directs the output to a file or pipe.

> file

Directs the output to a file, overwriting its previous contents.

>> file

Appends the output to a file, preserving its previous contents. In both cases, the file is created if it does not already exist.

| command

Directs the output as the input to a Unix command.

Be careful not to mix > and >> for the same file. Once a file has been opened with >, subsequent output statements continue to append to the file until it is closed.

Remember to call close() when you have finished with a file or pipe. If you don't, eventually you will hit the system limit on the number of simultaneously open files.

printf

printf(format [, expr-list ]) [ dest-expr ]

An alternative output statement borrowed from the C language. It can produce formatted output and also output data without automatically producing a newline. format is a string of format specifications and constants. expr-list is a list of arguments corresponding to format specifiers. See print for a description of dest-expr.

format follows the conventions of the C-language printf(3S) library function. Here are a few of the most common formats:

%s

A string.

%d

A decimal number.

%n.mf

A floating-point number; n = total number of digits. m = number of digits after decimal point.

%[-]nc

n specifies minimum field length for format type c, while - left-justifies value in field; otherwise, value is right-justified.

Like any string, format can also contain embedded escape sequences: \n (newline) or \t (tab) being the most common. Spaces and literal text can be placed in the format argument by quoting the entire argument. If there are multiple expressions to be printed, there should be multiple formats specified.

Example

Using the script:

{ printf("The sum on line %d is %.0f.\n", NR, $1+$2) }

The following input line:

5   5

produces this output, followed by a newline:

The sum on line 1 is 10.

rand

rand()

Generate a random number between 0 and 1. This function returns the same series of numbers each time the script is executed, unless the random number generator is seeded using srand(). {N}

return

return [expr]

Used within a user-defined function to exit the function, returning value of expr. The return value of a function is undefined if expr is not provided. {N}

sin

sin(x)

Return the sine of x, an angle in radians. {N}

split

split(string, array [, sep])

Split string into elements of array array[1],...,array[n]. The string is split at each occurrence of separator sep. If sep is not specified, FS is used. The number of array elements created is returned.

sprintf

sprintf(format [, expressions])

Return the formatted value of one or more expressions, using the specified format (see printf). Data is formatted but not printed. {N}

sqrt

sqrt(arg)

Return square root of arg.

srand

srand([expr])

Use optional expr to set a new seed for the random number generator. Default is the time of day. Return value is the old seed. {N}

strftime

strftime([format [,timestamp]])

Format timestamp according to format. Return the formatted string. The timestamp is a time-of-day value in seconds since midnight, January 1, 1970, UTC. The format string is similar to that of sprintf. (See the Example for systime.) If timestamp is omitted, it defaults to the current time. If format is omitted, it defaults to a value that produces output similar to that of date. {G}

sub

sub(r, s [, t])

Substitute s for first match of the regular expression r in the string t. If t is not supplied, defaults to $0. Return 1 if successful; 0 otherwise. {N}

substr

substr(string, beg [, len])

Return substring of string at beginning position beg and the characters that follow to maximum specified length len. If no length is given, use the rest of the string.

system

system(command)

Function that executes the specified command and returns its status. The status of the executed command typically indicates success or failure. A value of 0 means that the command executed successfully. A nonzero value indicates a failure of some sort. The documentation for the command you're running will give you the details.

The output of the command is not available for processing within the awk script. Use command | getline to read the output of a command into the script. {N}

systime

systime()

Return a time-of-day value in seconds since midnight, January 1, 1970, UTC. {G}

Example

Log the start and end times of a data-processing program:

BEGIN {
	now = systime()
	mesg = strftime("Started at %m/%d/%Y %H:%M:%S", now)
	print mesg
}
process data ...
END {
	now = systime()
	mesg = strftime("Ended at %m/%d/%Y %H:%M:%S", now)
	print mesg
}

tolower

tolower(str)

Translate all uppercase characters in str to lowercase and return the new string.[6] {N}

[6] Very early versions of nawk don't support tolower() and toupper(). However, they are now part of the POSIX specification for awk, and are included in the SVR4 nawk.

toupper

toupper(str)

Translate all lowercase characters in str to uppercase and return the new string. {N}

while

while (condition)
    statement

Do statement while condition is true (see if for a description of allowable conditions). A series of statements must be put within braces.

11.10.1 printf Formats

Format specifiers for printf and sprintf have the following form:

%[flag][width][.precision]letter

The control letter is required. The format conversion control letters are as follows.

Character Description
c ASCII character
d Decimal integer
i Decimal integer (added in POSIX)
e Floating-point format ([-]d.precisione[+-]dd)
E Floating-point format ([-]d.precisionE[+-]dd)
f Floating-point format ([-]ddd.precision)
g e or f conversion, whichever is shortest, with trailing zeros removed
G E or f conversion, whichever is shortest, with trailing zeros removed
o Unsigned octal value
s String
x Unsigned hexadecimal number; uses a-f for 10 to 15
X Unsigned hexadecimal number; uses A-F for 10 to 15
% Literal %

The optional flag is one of the following.

Character Description
-

Left-justify the formatted value within the field.

space

Prefix positive values with a space and negative values with a minus.

+

Always prefix numeric values with a sign, even if the value is positive.

#

Use an alternate form: %o has a preceding 0; %x and %X are prefixed with 0x and 0X, respectively; %e, %E, and %f always have a decimal point in the result; and %g and %G do not have trailing zeros removed.

0

Pad output with zeros, not spaces. This happens only when the field width is wider than the converted result.

The optional width is the minimum number of characters to output. The result will be padded to this size if it is smaller. The 0 flag causes padding with zeros; otherwise, padding is with spaces.

The precision is optional. Its meaning varies by control letter, as shown in this table.

Conversion Precision Means
%d, %i, %o The minimum number of digits to print
%u, %x, %X
%e, %E, %f The number of digits to the right of the decimal point
%g, %G The maximum number of significant digits
%s The maximum number of characters to print

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