awk is a very versatile programming language for working on text files. It is more powerful than sed but less complex than C. It is an excellent filter and report writer. In this class I will go over the elements and features of gawk, (the Free Software foundation version of awk), examples and a few one-liners.

1. awk – Essentials and Examples
1
Logan Palanisamy

2. Agenda
2
 Elements of awk
 Optional Bio Break
 Examples and one-liners
 Q&A

3. What is awk
3
 An acronym of the last names of the three authors
 General purpose pattern-scanning and processing
language
 Used for filtering, transforming and reporting
 More advanced than sed, but less complicated than
C; less cryptic than Perl.
 gawk, nawk

4. awk syntax
4
 awk [-Ffield_sep] 'cmd' infile(s)
 awk [-Ffield_sep] –f cmd_file infile(s)
 infile can be the output of pipeline.
 Space is the default field_sep

5. awk mechanics
5
 [pattern] [{action} …]
 Input files processed a line at a time
 Every line is processed if there is no pattern
 Lines split into fields based on field-sep
 Print is the default action.
 Input files not affected in anyway

6. Field Names
6
 Lines split into fields based on field-sep
 $0 represents the whole line
 $1, $2, … $n represent different fields
 Field names could be used as variables

7. Built-in variables.
7
Variable Explanation
FS Field separator variable for input lines. Defaults to space or tab
NR Number of input lines processed so far
NF Number of fields in the current input line
FILENAME Name of the current input file
OFMT Default format for output numbers
OFS Output field separator. Defaults to space
ORS Output record separator. Defaults to new-line character
RS Input Record Separator. Defaults to new-line character.
FNR Same as NR; but gets reset after each file unlike NR
RSTART, Variables set by the match() function which indicates where the
RLENGTH match starts and how long the match is
SUBSEP Subscript separator. Used in multi-dimensional arrays

8. Operators
8
Operator Explanation
+, -, *, / Addition, Subtraction, Multiplication, Division,
% Remainder/Modulo operation
++ Unary increment (var++ same as var=var+1)
-- Unary decrement
^ or ** Exponentaion
+=, -=, *=, /=, Assignment operator preceded by arithmetic operation (var+=5
%= same as var=var+5)
No operator String concatenation (newstr=“new” $3)
?: Ternary operator (expr1 ? expr2 : expr3)

9. Relational Operators.
9
Operator Explanation
== Equality operator
!= Not equal to
< Less than
<= Less than or equal to
> Greater than
>= Greater than equal to
~ Contains regular expression
!~ Doesn‟t contain regular expression

10. awk patterns
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 Can match either particular lines or ranges of lines
 Regular expression patterns
 Relational expression patterns
 BEGIN and END patterns

11. Regular Expressions
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Meta character Meaning
. Matches any single character except newline
* Matches zero or more of the character preceding it
e.g.: bugs*, table.*
^ Denotes the beginning of the line. ^A denotes lines starting
with A
$ Denotes the end of the line. :$ denotes lines ending with :
Escape character (., *, [, , etc)
[] matches one or more characters within the brackets. e.g.
[aeiou], [a-z], [a-zA-Z], [0-9], [[:alpha:]], [a-z?,!]
[^] matches any characters others than the ones inside brackets.
eg. ^[^13579] denotes all lines not starting with odd numbers,
[^02468]$ denotes all lines not ending with even numbers
<, > Matches characters at the beginning or end of words

12. Extended Regular Expressions
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Meta character Meaning
| alternation. e.g.: ho(use|me), the(y|m), (they|them)
+ one or more occurrences of previous character. a+ is same as
aa*)
? zero or one occurrences of previous character.
{n} exactly n repetitions of the previous char or group
{n,} n or more repetitions of the previous char or group
{n, m} n to m repetitions of previous char or group. For the above
four –re-interval option needs to be specified
(....) Used for grouping

13. Regular Expressions – Examples
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Example Meaning
.{10,} 10 or more characters. Curly braces have to
escaped
[0-9]{3}-[0-9]{2}-[0-9]{4} Social Security number
([2-9][0-9]{2})[0-9]{3}-[0- Phone number (xxx)yyy-zzzz
9]{4}
[0-9]{3}[ ]*[0-9]{3} Postal code in India
[0-9]{5}(-[0-9]{4})? US ZIP Code with optional four-digit extension

14. Regular Expression Patterns.
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Example Explanation
awk „/pat1/‟ infile Same as grep „pat1‟ infile
awk „/pat1/, /pat2/‟ infile Print all lines between pat1 and pat2 repetitively
awk „/pat1|pat2/‟ infile Print lines that have either pat1 or pat2
awk „/pat1.*pat2/‟ infile Print lines that have pat1 followed by pat2 with
something or nothing in between

15. Relational Expression Patterns.
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Example Explanation
awk „$1==“USA”‟ infile Print the line if the first field is USA
awk „$2 !=“xyz”‟ infile Print all lines whose second field is not “xyz”
awk „$2 < $3‟ infile Print all lines whose third field is greater than the
second
awk „$5 ~ /USA/‟ infile Print if the fifth field contains USA
awk „$5 !~ /USA/‟ infile Print if the fifth field doesn‟t contain USA
awk „NF == 5‟ infile Print lines that have five fields
awk „NR == 5, NR==10‟ Print lines 5 to 10
infile
awk „NR%5==0‟ infile Print every fifth line (% is the modulo operator)
awk „NR%5‟ infile Print everything other than every fifth line
awk „$NF ~ /pat1/‟ infile Print if the last field contains pat1

16. awk compound-patterns
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 Compound patterns formed with Boolean operations
(&&, ||, !), and range patterns
 pat1 && pat2 (compound AND)
 pat1 || pat2 (compound OR)
 !pat1 (Negation)
 pat1, pat2 (range pattern)

17. Compound Pattern Examples
17
Example Explanation
awk „/pat1/ && $1==“str1”‟ infile Print lines that have pat1 and whose first
field equals str1
awk „/pat1/ || $2 >= 10‟ infile Print lines that have pat1 OR whose second
field is greater than or equal to 10
awk „!/pat1/‟ infile Same as grep –v “pat1” infile
awk „NF >=3 && NF <=6‟ infile Print lines that have between 3 and six
fields
awk „/pat1/ || /pat2/‟ infile Same as awk „/pat1|pat2/‟ infile
awk „/pat1/, /pat2/‟ infile Print all lines between pat1 and pat2
repetitively
awk „!/pat1|pat2/‟ infile Print lines that have neither pat1 nor pat2
awk „NR > 30 && $1 ~ /pat1|pat2/‟ Print lines beyond 30 that have first field
infile containing either pat1 or pat2

18. Compound Pattern Examples
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Example Explanation
awk „/pat1/&&/pat2/‟ infile Print lines that have both pat1 and pat2.
awk „/pat1.*pat2/‟ infile How is this different from the one above?
awk „NR<10 || NR>20‟ infile Print all lines except lines 10 to 20
awk „!(NR >=10 && NR<=20)‟ infile Print lines between 10 and 20. Same as awk
‘NR==10, NR==20’ infile

19. BEGIN and END patterns
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 BEGIN allows actions before any lines are processed.
 END allows actions after all lines have been
processed
 Either or both optional
BEGIN {action}
[Pattern] {action}
END {action}

20. BEGIN
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 Use BEGIN to:
 Set initial values for variables
 Print headings
 Set internal field separator (same as –F on command line)
awk „BEGIN {FS=“:”; print “File name”, FILENAME}‟ file2 file2

21. END
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 Use END to:
 Perform any final calculations
 Print report footers.
 Do any thing that must be done after all lines have been
processed.
awk „END {print NR}‟ file2 file2

22. Creating Actions
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 Actions consist of one or more statements separated
by semicolon, newline, or a right-brace.
 Types of statements:
 Assignment statement (e.g.var1=1)
 Flow-control statements
 Print control statement

23. Flow-control statements
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Statement Explanation
if (conditional) Perform statement_list1 if conditional is true.
{statement_list1} Otherwise statement_list2 if specified
[else {statement_listt2}]
while (conditional) Perform statement_list while conditional is true
{statement_list}
for Perform int_expr firt. While conditional_expr is
(int_expr;conditional_expr true, perform statement_list and execute ctrl_expr.
;ctrl_expr) {statement_list}
break Break from the containing loop and continue with
the next statement
continue Go to the next iteration of the containing loop without
executing the remaining statements in loop
next Skip remaining patterns on this line
exit Skip the rest of the input and go to the END pattern
if one exists or exit.

24. Print-control statements
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Statement Explanation
print [expression_list] Print the expression on stdout unless redirected to
[>filename] filename.
printf format [, Prints the output as specified in format (like printf
expression_list] in C). Has a rich set of format specifiers.
[>filename]

25. Variables
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 Provide power and flexibility
 Formed with letters, numbers and underscore
character.
 Can be of either string or numeric type
 No need to declare or initialize.
 Type implied by the assignment. No $ in front of
variables. (e.g. var1=10; job_type=„clerk‟)
 Field names ($1, $2, ..$n) are special form of
variables. Can be used like any other variable.

26. Arrays
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 One-dimensional arrays: array_name[index]
 Index can be either numeric or string. Starts with 1
if numeric
 No special declaration needed. Simply assign
values to an array element.
 No set size. Limited only by the amount of memory
on the machine.
 phone[“home”], phone[“mobile”], phone[var1],
phone[$1], ranks[1]

27. Multi-Dimensional arrays
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 Arrays are one-dimensional.
 Array_name[1,2] not supported
 Concatenate the subscripts to form a string which
could be used as the index:
array_name[1”,”2]
Space is the concatenation operator. “1,2”, a three character
string is the index.
 Use SUBSEP, subscript separator, variable to
eliminate the need to have double quotes around
the comma.

28. Built-in functions
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Function Explanation
cos(awk_expr) Cosine of awk_expr
exp(awk_expr) Returns the exponential of awk_expr (as in e raised to the
power of awk_expr)
index(str1, str2) Returns the position of strt2 in str1.
length(str) Returns the length of str
log(awk_expr) Base-e log of awk_expr
sin(awk_expr) Sine of awk_expr
sprintf(frmt, awk_expr) Returns the value of awk_expr formatted as per frmt
sqrt(awk_expr) Square root of awk_expr
split(str, array, [field_sep]) Splits a string into its elements and stores into an array
substr(str, start, length) Returns a substring of str starting at position “start” for
“length” characters.
toupper(), tolower() Useful when doing case-insensitive searches

29. Built-in functions contd.
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Function Explanation
sub(pat1, “pat2”, [string]) Substitute the first occurrence of pat1 with pat2 in string.
String by default is the entire line
gsub(pat1, “pat2”, [string]) Same as above, but replace all occurrences of pat1 with
pat2.
match(string, pat1) Finds the regular expression pat1, and sets two special
variables (RSTART, RLENGTH) that indicate where the
regular expression begins and ends
systime() returns the current time of day as the number of seconds
since Midnight, January 1, 1970

30. Case Insensitive Match
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 Case insensitive match:
 awk „BEGIN {ignorecase=1} /PAT1/‟
 awk „tolower($0) ~ /pat1/ …‟

31. User-Defined functions
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Gawk allows user defined functions
#!/usr/bin/gawk -f
{
if (NF != 4) {
error("Expected 4 fields");
} else {
print;
}
}
function error ( message ) {
if (FILENAME != "-") {
printf("%s: ", FILENAME) > "/dev/tty";
}
printf("line # %d, %s, line: %sn", NR, message, $0) >> "/dev/tty";
}

32. Very Simple Examples
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 Find the average filesize in a directory
 Find the users without password
 Convert String to Word (string2word.awk)
 List the file count and size for each user
(cnt_and_size.awk)

33. Awk one-liners
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Example Explanation
awk‟{print $NF}‟ infile Print the last field in each line
awk‟{print $(NF-1)}‟ infile Print the field before the last field. What would
happen if () are removed? What happens if there is
only one field
awk‟NF‟ infile Print only non-blank lines. Same as awk „/./‟
awk „{print length, $0)‟ infile Print each line preceded by its length.
awk „BEGIN {while Print 1 to 10
(++x<11) print x}‟
awk „BEGIN {for (i=10; Print 10 to 50 in increments of 4
i<=50; i+=4) print i}‟
awk „{print; print “”}‟ infile Add a blank line after every line
awk „{print; if (NF>0) print Add a blank line after every non-blank line
“”}‟ infile

34. Awk one-liners
34
Example Explanation
awk‟NF !=0 {++cnt} END Count the number of non-blank lines
{print cnt}‟ infile
ls –l | awk „NR>1 {s+=$5} Return the average file size in a directory
END {print “Average:”
s/(NR-1)}‟
awk „/pat1/?/pat2/:/pat3/‟ uses ternary operator ?: Equivalent to awk „/pat1/ && /pat2/ ||
pat3‟ except for lines containing both pat1 and pat3
infile
awk „NF<10?/pat1/:/pat2/‟ Use pat1 if number of fields is less than 10;
infile otherwise use pat2
awk „ORS=NR%3?” ”:”n”‟ Join three adjacent lines. ORS is the output record
infile separator
awk „ORS=NR%3?”t”:”n” Print the first field three to row. ORS is the output
{print $1}‟ infile record separator
awk „FNR < 11‟ f1, f2, f3 Concatenate the first 10 lines of f1, f2, and f3.

35. Awk one-liners
35
Example Explanation
awk „length < 81‟ Print lines that are shorter than 81 characters
awk „/pat1/, 0‟ Print all lines between the line containing pat1 and
end of file
awk „NR==10, 0‟ Print lines 10 to the end of file. The end condition
“0” represents “false”.
awk '{ sub(/^[ t]+/, ""); Trim the leading tabs or spaces. Called ltrim
print }'
awk '{ sub(/[ t]+$/, ""); Trim the trailing tabs or spaces. Called rtrim
print }'
awk '{ gsub(/^[ t]+|[ Trim the white spaces on both sides
t]+$/, ""); print }'

36. Awk one-liners
36
Example Explanation
awk '/pat1/ { gsub(/pat2/, Replace pat2 with “str” on lines containing pat1
“str") }; { print }'
awk '{ $NF = ""; print }' Delete the last field on each line

37. Awk one-liners
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 http://www.catonmat.net/blog/awk-one-liners-
explained-part-one/

38. Translators
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 awk2c – Translates awk programs to C
 awk2p – Translates awk progrms to Perl

39. References
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 sed & awk by Dale Dougherty & Arnold Robins
 http://www.grymoire.com/Unix/Awk.html
 http://www.vectorsite.net/tsawk.html
 http://snap.nlc.dcccd.edu/reference/awkref/gawk
_toc.html

40. Q&A
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 devel-awk@yahoo-inc.com

41. Unanswered questions
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 How to print lines that are outside a block of lines?
(print lines that are not enclosed by /pat1/,/pat2/
 Does awk support grouping and back-referencing
(e.g. identify adjacent duplicate words)?