dunstontc/tapl

The Awk Programming Language

The AWK Programming Language

Contents

• Chapter 1: An Awk Tutorial
  • 1.1 Getting Started
  • 1.2 Simple Output
  • 1.3 Fancier Output
  • 1.4 Selection
  • 1.5 Computing with AWK
  • 1.6 Control-Flow Statements
  • 1.7 Arrays
  • 1.8 A Handfull of Useful "One-liners"
  • 1.9 What Next?
• Chapter 2: The Awk Language
  • 2.1 Patterns
  • 2.2 Actions
  • 2.3 User-Defined Functions
  • 2.4 Output
  • 2.5 Input
  • 2.6 Interaction with Other Programs
• Chapter 3: Data Processing
• Chapter 4: Reports and Databases
• Chapter 5: Processing Words
• Chapter 6: Litile Languages
• Chapter 7: Experiments with Algorithms
• Chapter 8: Epilog

Chapter 2: The Awk Language

Comparison Operators (Table 2-1)

Operator	Meaning
<	less than
<=	less than or equal
==	equal to
!=	not equal to
>=	greater than or equal to
>	greater than
~	matched by
!~	not matched by

Escape Sequences (Table 2-2)

Sequence	Meaning
\b	backspace
\f	formfeed
\n	newline (line feed)
\r	carriage return
\t	tab
\ddd	octal value ddd, where ddd is 1 to 3 digits between 0 and 7
\c	any other character c literally (e.g., \\ for backslash, \" for ")

Regular Expressions (Table 2-3)

Expression	Matches
c	the nonmetacharacter c
\c	escape sequence or literal character c
^	beginning of string
$	end of string
.	any character
[c1c2...]	any character in c1c2...
[^c1c2...]	any character not in c1c2...
[c1-c2]	any character in the range beginning with c1 and ending with c2
[^c1-c2]	any character not in the range c1 to c2
r1|r2	any string matched by r1 or r2
(r1)(r2)	any string xy where r1 matches x and r2 matches y; parentheses not needed around arguments with no alternations
(r)*	zero or more consecutive strings matched by r
(r)+	one or more consecutive strings matched by r
(r)?	zero or one string matched by r parentheses not needed around basic regular expressions
(r)	any string matched by r

Patterns (Table 2-4)

Pattern	Example	Matches
BEGIN	BEGIN	before any input has been read
END	END	after all input has been read
expression	$3 < 100	lines in which third field is less than 100
string-matching	/Asia/	lines that contain Asia
compound	$3 < 100 && $4 == "Asia"	lines in which third field is less than 100 and fourth field is Asia
range	NR==10, NR==20	tenth to twentieth lines of input inclusive

Built-in Variables (Table 2-5)

Variable	Meaning	Default
ARGC	number of command-line arguments	-
ARGV	array of command-line arguments	-
FILENAME	name of current input file	-
FNR	record number in current file	-
FS	controls the input field separator	" "
NF	number of fields in current record	-
NR	number of records read so far	-
OFMT	output format for numbers	"%.6g"
OFS	output field separator	" "
ORS	output record separator	"\n"
RLENGTH	length of string matched by match function	-
RS	controls the input record separator	"\n"
RSTART	start of string matched by match function	-
SUBSEP	subscript separator	"\034"

Built-in Arithmetic Functions (Table 2-6)

Function	Value Returned
atan2(y,x)	arctangent of y/x in the range -π to π
cos(x)	cosine of x, with x in radians
exp(x)	exponential function of x, e**x
int(x)	integer part of x; truncated towards 0 when x > 0
log(x)	natural (base e) logarithm of x
rand()	random number r, where 0 <= r < 1
sin(x)	sine of x, with x in radians
sqrt(x)	square root of x
srand(x)	x is new seed for rand()

Built-in String Functions (Table 2-7)

Function	Description
gsub(r,s)	substitute s for r globally in $0, return number of substitutions made
gsub(r,s,t)	substitutes for r globally in string t, return number of substitutions made
index(s,t)	return first position of string t in s, or 0 if t is not present
length(s)	return number of characters in s
match(s,r)	test whether s contains a substring matched by r, return index or 0; sets RSTART and RLENGTH
split(s,a)	split s into array a on FS, return number of fields
split(s,a,fs)	splits into array a on field separator fs, return number of fields
sprintf(fmt,expr-list)	return expr-list formatted according to format string fmt
sub(r,s)	substitute s for the leftmost longest substring of $0 matched by r, return number of substitutions made
sub(r,s,t)	substitute s for the leftmost longest substring of t matched by r, return number of substitutions made
substr(s,p)	return suffix of s starting at position p
substr(s,p,n)	return substring of s of length n starting at position p

Expression Operators (Table 2-8)

Operation	Operators	Example	Meaning of Example
assignment	= += -= *= /= %= ^=	x *= 2	x = x * 2
conditional	?:	x?y:z	if x is true then y else z
logical OR	||	x || y	1 if x or y are true, 0 otherwise
logical AND	&&	x && y	1 if x and y are true, 0 otherwise
array membership	in	i in a	1 if a[i] exists, 0 otherwise
matching	~ !~	$1 ~ /x/	1 if the first field contains x, 0 otherwise
relational	< <= == != >= >	x == y	1 id x is equal to y, 0 otherwise
concatenation		"a" "bc"	"abc"; there is no explicit concatenation operator
add, subtract	+ -	x + y	sum of x and y
multiply, divide, mod	* / %	x % y	remainder of x divided by y
unary plus and minus	+ -	-x	negated value of x
logical NOT	!	!$1	1 if $1 is zero or null, 0 otherwise
exponentiation	^	x ^ y	x**y (x raised to y)
increment, decrement	++ --	++x, x++	add 1 to x
field	$	$i+1	value of the ith field, plus 1
grouping	( )	($i)++	add 1 to value of the ith field

printf Format-control Characters (Table 2-9)

Character	Print Expression As
c	ASCII character
d	decimal integer
e	[-]d.ddddddE[+-]dd
f	[-]ddd.dddddd
g	e or f conversion, whichever is shorter, with nonsignificant zeros suppressed
o	unsigned octal number
s	string
x	unsigned hexadecimal number
%	print a %; no argument is consumed

Examples of printf Specifications (Table 2-10)

fmt	$1	printf(fmt, $1)
%c	97	a
%d	97.5	97
%5d	97.5	97
%e	97.5	9.750000e+01
%f	97.5	97.500000
%7.2f	97.5	97.50
%q	97.5	97.5
%.6g	97.5	97.5
%o	97	141
%06o	97	000141
%x	97	61
|%s|	January	|January|
|%10s|	January	| January|
|%-10s|	January	|January |
|%.3s|	January	|Jan|
|%10.3s|	January	| Jan|
|%-10.3s|	January	|Jan |
%%	January	%

getline Function (Table 2-11)

Expression	Sets
getline	$0, NF, NR, FNR
getline var	var, NR, FNR
getline <file	$0, NF
getline var <file	var
cmd | getline	$0, NF
cmd | getline var	var

Control-Flow Statements

• { statements }
  • statement grouping
• if (expression) statement
  • if expression is true, execute statement
• if (expression) statement_1 else statement_2
  • if expression is true, execute statement_1 otherwise execute statement 2
• while (expression) statement
  • if expression is true, execute statement, then repeat
• for (expression_1; expression_2 ; expression_3) statement
  • equivalent to expression_1; while (expression_2) { statement; expression_3 }
• for (variable in array) statement
  • execute statement with variable set to each subscript in array in turn
• do statement while (expression)
  • execute statement; if expression is true, repeat
• break
  • immediately leave innermost enclosing while, for or do
• continue
  • start next iteration of innermost enclosing while, for or do
• next
  • start next iterc1tion of main input loop
• exit
• exit expression
  • go immediately to the END action; if within the END action, exit program entirely. Return expression as program status.

---

Terms

Term	Definition
pattern-action statement
field
match	The precise meaning of the word "match" depends on the pattern in question
Bailing out	No recovery was attempted.
Built-In Variables
Field Variables