The Awk Text Processing Language

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28 April 2015, 11:36pm
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Awk is a unix tool, or programming language designed to process and transform text files which are arranged in a series of 'fields' (chunks of text separated by spaces, or any other delimiter) and records. This document is mainly about the 'mawk' variant of 'awk'.

find out the version of mawk

 mawk -W version

more awk one liners

Gotchas ‹↑›

[+] The so-called "gotchas" are small but potentially frustating problems which arise and which stop a program from working or which make the awk program work in an unexpected way. Gotcha derives from the contraction of the english phrase "got you".

00- On a unix system the awk phrase <<awk "{print $1}">> doesnt work as expected because the unix (bash) shell expands or "interpolates" the "$1" variable. It is necessary to write "awk '{print $1}'" -

BEGIN and and variables such as FS must be uppercase

 begin{FS=","}{print $2}    No!! this doesnt work

Simple Usage ‹↑›

simple usage of awk on different operating systems.

     Unix: awk '/pattern/ {print "$1"}'    # standard Unix shells
  DOS/Win: awk '/pattern/ {print "$1"}'    # compiled with DJGPP, Cygwin
           awk "/pattern/ {print \"$1\"}"  # GnuWin32, UnxUtils, Mingw

Users of MS-DOS or Microsoft Windows must remember that the percent sign (%) is used to indicate environment variables, so this symbol must be doubled (%%) to yield a single percent sign visible to awk.

run an awk script

 cat file1 | awk -f a.awk > file2
 awk -f a.awk file1 > file2   the same

Strings ‹↑›

Concatenation Of String ‹↑›

Concatenation is the fancy term for joining 2 strings (bits of text) together.

print the first two columns of the space/tab delimited file 'data.txt'

 awk '{print $1 $2}' data.txt  $1 and $2 are printed with no space between
 awk '{print $1$2}' data.txt    the same, at least on my mawk version
 awk '{print $1 $2;}' data.txt    the same again
 awk '{print $1 "" $2}' data.txt  the same again, but why would you?

awk doesnt have variable 'interpolation' in strings

 awk '{print "$1 ..."}' data.txt   this prints '$1 ...' literally

print the first column of 'data.txt' with 3 dots '...' appended to it

 awk '{print $1 "..."}' data.txt

append a string to itself (string concatenation)

  s = s "xxx";  this appends 3 x's to the end of the string 's'

Matching Patterns ‹↑›

determine if the variable "s" contains the letter "r"

 s ~ /r/

print the first field of each line if it does *not* contain "a" or "b"

 $1 !~ /(a|b)/ { print $1 }
 $1 !~ /[ab]/ { print $1 }     the same

add an "X" between every letter of every line

 { gsub(//, "X") ; print }

split the string "s" into the array A using the pattern "r"

 split(s, A, r)

Printing Strings ‹↑›

make a multiline string..

    print "\
     <html> \n\
     <head> \n\

print multiple expressions

 print "variable a is " a "."

Its not possible to break printing expressions across lines

    print "variable a is"
          a ".";
this doesnt work, at least not with mawk 1.3.3

Newlines ‹↑›

display the file 'days.txt' with all newline characters removed

 awk '{ printf "%s", $0 }' days.txt
 cat days.txt | awk '{ printf "%s", $0 }'  the same

display 'days.txt' with newline characters replaced with spaces

 awk '{ printf "%s ", $0 }' days.txt
 cat days.txt | awk '{ printf "%s ", $0 }'

Arrays ‹↑›

delete an array called record

 delete record

assign a value to an associative style array

 a["cars"] = 3

Regular Expressions ‹↑›

regular expression meta-characters: ^ $ . [ ] | ( ) * + ?

print all lines which start with an awk identifier

    BEGIN { identifier = "[_a-zA-Z][_a-zA-Z0-9]*" } 
    $0 ~ "^" identifier

Case Insensitive Matching ‹↑›

use tolower

 tolower($0) ~ /bhp/ {print $0}

set the ignorecase var

 BEGIN {IGNORECASE=1} /bhp/ {print $0}

Loops ‹↑›

loop through each field of each record

 awk '{ for(i = 1 ; i <= NF ; i++)  print $i }' /usr/share/dict/words

print each element of an array

 for ( i in aa ) print aa[i]

Splitting Data Fields ‹↑›

# The field separator variable FS is interpreted as a regular # expression

split fields with any character followed by a colon ":" character

 BEGIN {FS=".:"}

split quoted comma delimited fields (csv)

 BEGIN {FS="\" *, *\""}

helpful man pages for awk
man gawk - the gnu awk man page
man ed - contains regular expression examples
man mawk - contains good examples
man regex - regular expression syntax

Range Of Fields ‹↑›

Awk has no simple way to print a range of fields such as $[1-4] A 'for' loop must be used to loop through the range and print each one. One may use cut instead

use 'cut' to print fields 1 to 5 from a comma delimited file

 cut -d, -f1-5

Awk One Line Recipes ‹↑›

These one line scripts were taken from

30 April 2008, by Eric Pement - eric [at], version 0.27

Latest version of the Eric Pement one line scripts (in English)
Chinese version of these one line scripts

File Spacing ‹↑›

double space a file

 awk '1;{print ""}'
 awk 'BEGIN{ORS="\n\n"};1'   another way

double space a file which already has blank lines in it. Output file should contain no more than one blank line between lines of text. NOTE: On Unix systems, DOS lines which have only CRLF (\r\n) are often treated as non-blank, and thus 'NF' alone will return TRUE.

 awk 'NF{print $0 "\n"}'

triple space a file

 awk '1;{print "\n"}'

Summing Numeric Columns ‹↑›

sum up all the numbers in column 2 and print out the total at the end

 awk '{ a+=$2 } END { print "total=" a }' data.txt

sum a column between 2 lines in a file (with help from sed)

 sed -n '/#1/,/#2/p' data.txt | awk -F, '{a+=$2; print $2, a}' | less

Line Numbering ‹↑›

precede each line by its line number FOR THAT FILE (left alignment). Using a tab (\t) instead of space will preserve margins.

 awk '{print FNR "\t" $0}' files*

precede each line by its line number FOR ALL FILES TOGETHER, with tab.

 awk '{print NR "\t" $0}' files*

number each line of a file (number on left, right-aligned) Double the percent signs if typing from the DOS command prompt.

 awk '{printf("%5d : %s\n", NR,$0)}'

number each line of file, but only print numbers if line is not blank Remember caveats about Unix treatment of \r (mentioned above)

 awk 'NF{$0=++a " :" $0};1'
 awk '{print (NF? ++a " :" :"") $0}'

count lines (emulates "wc -l")

 awk 'END{print NR}'

print the sums of the fields of every line

 awk '{s=0; for (i=1; i<=NF; i++) s=s+$i; print s}'

add all fields in all lines and print the sum

 awk '{for (i=1; i<=NF; i++) s=s+$i}; END{print s}'

print every line after replacing each field with its absolute value

 awk '{for (i=1; i<=NF; i++) if ($i < 0) $i = -$i; print }'
 awk '{for (i=1; i<=NF; i++) $i = ($i < 0) ? -$i : $i; print }'

print the total number of fields ("words") in all lines

 awk '{ total = total + NF }; END {print total}' file

print the total number of lines that contain "Beth"

 awk '/Beth/{n++}; END {print n+0}' file

print the largest first field and the line that contains it Intended for finding the longest string in field #1

 awk '$1 > max {max=$1; maxline=$0}; END{ print max, maxline}'

The Number Of Fields ‹↑›

print the number of fields in each line, followed by the line

 awk '{ print NF ":" $0 } '

print the last field of each line

 awk '{ print $NF }'

print the last field of the last line

 awk '{ field = $NF }; END{ print field }'

print every line with more than 4 fields

 awk 'NF > 4'

print every line where the value of the last field is > 4

 awk '$NF > 4'

String Creation ‹↑›

create a string of a specific length (e.g., generate 513 spaces)

 awk 'BEGIN{while (a++<513) s=s " "; print s}'

insert a string of specific length at a certain character position Example: insert 49 spaces after column #6 of each input line. gawk --re-interval 'BEGIN{while(a++<49)s=s " "};{sub(/^.{6}/,"&" s)};1'

Array Creation ‹↑›

These next 2 entries are not one-line scripts, but the technique is so handy that it merits inclusion here.

create an array named "month", indexed by numbers, so that month[1] is 'Jan', month[2] is 'Feb', month[3] is 'Mar' and so on.

 split("Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec", month, " ")

create an array named "mdigit", indexed by strings, so that mdigit["Jan"] is 1, mdigit["Feb"] is 2, etc. Requires "month" array

 for (i=1; i<=12; i++) mdigit[month[i]] = i

Text Conversion And Substitution ‹↑›

IN UNIX ENVIRONMENT: convert DOS newlines (CR/LF) to Unix format

 awk '{sub(/\r$/,"")};1'   # assumes EACH line ends with Ctrl-M

IN UNIX ENVIRONMENT: convert Unix newlines (LF) to DOS format

 awk '{sub(/$/,"\r")};1'

IN DOS ENVIRONMENT: convert Unix newlines (LF) to DOS format

 awk 1

IN DOS ENVIRONMENT: convert DOS newlines (CR/LF) to Unix format Cannot be done with DOS versions of awk, other than gawk

 gawk -v BINMODE="w" '1' infile >outfile

Use "tr" instead.

 tr -d \r <infile >outfile         GNU tr version 1.22 or higher 

delete leading whitespace (spaces, tabs) from front of each line

 awk '{sub(/^[ \t]+/, "")};1'     aligns all text flush left

delete trailing whitespace (spaces, tabs) from end of each line

 awk '{sub(/[ \t]+$/, "")};1'

delete BOTH leading and trailing whitespace from each line

 awk '{gsub(/^[ \t]+|[ \t]+$/,"")};1'
 awk '{$1=$1};1'           # also removes extra space between fields

insert 5 blank spaces at beginning of each line (make page offset)

 awk '{sub(/^/, "     ")};1'

align all text flush right on a 79-column width

 awk '{printf "%79s\n", $0}' file*

center all text on a 79-character width

 awk '{l=length();s=int((79-l)/2); printf "%"(s+l)"s\n",$0}' file*

substitute (find and replace) "foo" with "bar" on each line

 awk '{sub(/foo/,"bar")}; 1'           # replace only 1st instance
 gawk '{$0=gensub(/foo/,"bar",4)}; 1'  # replace only 4th instance
 awk '{gsub(/foo/,"bar")}; 1'          # replace ALL instances in a line

substitute "foo" with "bar" ONLY for lines which contain "baz"

 awk '/baz/{gsub(/foo/, "bar")}; 1'

substitute "foo" with "bar" EXCEPT for lines which contain "baz"

 awk '!/baz/{gsub(/foo/, "bar")}; 1'

change "scarlet" or "ruby" or "puce" to "red"

 awk '{gsub(/scarlet|ruby|puce/, "red")}; 1'

reverse order of lines (emulates "tac")

 awk '{a[i++]=$0} END {for (j=i-1; j>=0;) print a[j--] }' file*

if a line ends with a backslash, append the next line to it (fails if there are multiple lines ending with backslash...)

 awk '/\\$/ {sub(/\\$/,""); getline t; print $0 t; next}; 1' file*

print and sort the login names of all users

 awk -F ":" '{print $1 | "sort" }' /etc/passwd

Rearranging Fields Or Columns ‹↑›

print the first 2 fields, in opposite order, of every line

 awk '{print $2, $1}' file

switch the first 2 fields of every line

 awk '{temp = $1; $1 = $2; $2 = temp}' file

print every line, deleting the second field of that line

 awk '{ $2 = ""; print }'

print in reverse order the fields of every line

 awk '{for (i=NF; i>0; i--) printf("%s ",$i);print ""}' file

concatenate every 5 lines of input, using a comma separator between fields

 awk 'ORS=NR%5?",":"\n"' file

Selective Printing Of Certain Lines ‹↑›

print first 10 lines of file (emulates behavior of "head")

 awk 'NR < 11'

print first line of file (emulates "head -1")

 awk 'NR>1{exit};1'

print the last 2 lines of a file (emulates "tail -2")

 awk '{y=x "\n" $0; x=$0};END{print y}'

print the last line of a file (emulates "tail -1")

 awk 'END{print}'

print only lines which match regular expression (emulates "grep")

 awk '/regex/'

print only lines which do NOT match regex (emulates "grep -v")

 awk '!/regex/'

print any line where field #5 is equal to "abc123"

awk built in variables
ARGC - number of command line arguments.
ARGV - array of command line arguments 0..ARGC-1.
CONVFMT - format for conversion of numbers to string default "%.6g".
ENVIRON - array indexed by environment variables. An environment string var=value is stored as ENVIRON[var] = value.
FILENAME - name of the current input file.
FNR - current record number in FILENAME.
FS - splits records into fields as a regular expression.
NF - number of fields in the current record.
NR - current record number in the total input stream.
OFMT - format for printing numbers; initially = "%.6g".
OFS - inserted between fields on output initially = " ".
ORS - terminates each record on output initially = "\n".
RLENGTH - length set by the last call to the built-in function match().
RS - input record separator initially = "\n".
RSTART - index set by the last call to match().
SUBSEP - used to build multiple array subscripts initially = "\034".

print only those lines where field #5 is NOT equal to "abc123" This will also print lines which have less than 5 fields.

 awk '$5 != "abc123"'
 awk '$5 == "abc123"'

matching a field against a regular expression

 awk '$7  ~ /^[a-f]/'    # print line if field #7 matches regex
 awk '$7 !~ /^[a-f]/'    # print line if field #7 does NOT match regex

print the line immediately before a regex, but not the line containing the regex

 awk '/regex/{print x};{x=$0}'
 awk '!($5 == "abc123")'

print the line immediately after a regex, but not the line containing the regex

 awk '/regex/{getline;print}'

grep for AAA and BBB and CCC (in any order on the same line)

 awk '/AAA/ && /BBB/ && /CCC/'

grep for AAA and BBB and CCC (in that order)

 awk '/AAA.*BBB.*CCC/'

print only lines of 65 characters or longer

 awk 'length > 64'

print only lines of less than 65 characters

 awk 'length < 64'

print section of file from regular expression to end of file

 awk '/regex/,0'
 awk '/regex/,EOF'

print section of file based on line numbers (lines 8-12, inclusive)

 awk '/regex/{print (NR==1 ? "match on line 1" : x)};{x=$0}'

print line number 52

 awk 'NR==8,NR==12'
 awk 'NR==52'

print section of file between two regular expressions (inclusive)

 awk '/Iowa/,/Montana/'             # case sensitive

Selective Deletion Of Certain Lines ‹↑›

delete ALL blank lines from a file (same as "grep '.' ")

 awk NF
 awk '/./'

remove duplicate, consecutive lines (emulates "uniq")

 awk 'a !~ $0; {a=$0}'

remove duplicate, nonconsecutive lines

 awk '!a[$0]++'                     # most concise script
 awk '!($0 in a){a[$0];print}'      # most efficient script

Pipe Awk Output To The Shell ‹↑›

This technique allows each line generated by an awk script to be executed by the shell

move files to the "iraf" folder and add .dat to the names

 ls junk* | awk '{print "mv "$0" ../iraf/"$0".dat"}' | sh

More One Line Examples ‹↑›

Print first two fields in opposite order

 awk '{ print $2, $1 }' file

Print lines longer than 72 characters

 awk 'length > 72' file

Print length of string in 2nd column

 awk '{print length($2)}' file

Add up first column, print sum and average

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

Print fields in reverse order

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

Print the last line

     {line = $0}
 END {print line}

Print the total number of lines that contain the word Pat

 /Pat/ {nlines = nlines + 1}
 END {print nlines}

Print all lines between start/stop pairs

 awk '/start/, /stop/' file

Print all lines whose first field is different from previous one

 awk '$1 != prev { print; prev = $1 }' file

Print column 3 if column 1 > column 2

 awk '$1 > $2 {print $3}' file

Print line if column 3 > column 2

 awk '$3 > $2' file

Count number of lines where col 3 > col 1

 awk '$3 > $1 {print i + "1"; i++}' file

Print sequence number and then column 1 of file

 awk '{print NR, $1}' file

Print every line after erasing the 2nd field

 awk '{$2 = ""; print}' file

Print hi 28 times

 yes | head -28 | awk '{ print "hi" }'

Print hi.0010 to hi.0099 (NOTE IRAF USERS!)

 yes | head -90 | awk '{printf("hi00%2.0f \n", NR+9)}'

Print out 4 random numbers between 0 and 1

 yes | head -4 | awk '{print rand()}'

Print out 40 random integers modulo 5

 yes | head -40 | awk '{print int(100*rand()) % 5}'

Replace every field by its absolute value

 { for (i = 1; i <= NF; i=i+1) if ($i < 0) $i = -$i print}

Field Delimiter ‹↑›

The field delimiter or separator, determines how awk divides up each line of the text file into 'fields' or 'columns' which can then be accessed with the $1, $2, ... variables. The delimiter can be a regular expression (unlike 'cut' for example)

The default awk field delimiter is a single space " " or a tab.

use '|' as the field delimiter and print the 4th field

 awk -F"|" '{print $4}' filename
 awk -F'|' '{print $4}' filename           the same
 awk -F\| '{print $4}' filename            should work
 awk  'BEGIN {FS="|"} {print $4}' filename the same

set the field delimiter to be a comma followed by a space, print 2nd field

 awk -F', ' '{print $2}' data.txt

set the field delimiter to be a comma followed by any number of spaces

 awk -F', *' '{print $2}' data.txt
 awk 'BEGIN{FS=", *"}{print $2}' data.txt     the same
 awk 'BEGIN{FS=", *";};{print $2;}' data.txt  the same again

set the field delimiter to be the double quote character

 awk -F'"' '{print $2}' data.txt
 awk -F\" '{print $2}' data.txt    the same

set the field delimiter to be any number of '+' plus signs

 awk -F'\+*' '{print $2}' data.txt

set the field delimiter to a space following by one or more '*' star signs

 awk -F' \*+' '{print $2}' data.txt

Some looping commands Remove a bunch of print jobs from the queue

BEGIN{ for (i=875;i>833;i--){ printf "lprm -Plw %d\n", i } exit } ,,,

 awk 'NR==52 {print;exit}'          # more efficient on large files

Find maximum and minimum values present in column 1

example format strings for 'printf'
e.g. printf("howdy %-8s What it is bro. %.2f\n" $1, $2*$3)
%s - string
%-8s - 8 character string left justified
%.2f - number with 2 places after .
%6.2f - field 6 chars with 2 chars after .
\n - newline
\t - tab
$1 >= m {m = $1} $1 <= p {p = $1} END { print "Max = " m, " Min = " p } ,,,

Example of defining variables, multiple commands on one line

NR == 1 {m=$1 ; p=$1}
$4 != prev {print preva, prevb, prev, sum/n; n=0; sum=0; prev = $4; preva = $1; prevb = $2}
NR == 1 {prev=$4; preva = $1; prevb = $2; n=0; sum=0}
END {print preva, prevb, prev, sum/n} ,,,

Example of defining and using a function, inserting values into an array and doing integer arithmetic mod(n). This script finds the number of days elapsed since Jan 1, 1901. (from

h3) function daynum(y, m, d, days, i, n)

$4 == prev {n++; sum=sum+$5/$6}
split("31 28 31 30 31 30 31 31 30 31 30 31", days) # 365 days a year, plus one for each leap year n = (y-1901) * 365 + int((y-1901)/4)
{ # 1 == Jan 1, 1901
days[2]++ for (i = 1; i < m; i++) n += days[i] return n + d } { print daynum($1, $2, $3) } ,,,

Example of using substrings substr($2,9,7) picks out characters 9 thru 15 of column 2

{print "imarith", substr($2,1,7) " - " $3, "out."substr($2,5,3)} {print "imarith", substr($2,9,7) " - " $3, "out."substr($2,13,3)} {print "imarith", substr($2,17,7) " - " $3, "out."substr($2,21,3)} {print "imarith", substr($2,25,7) " - " $3, "out."substr($2,29,3)} ,,,

Password Generation With Awk ‹↑›

A useful capability of awk may be in the generation of password 'dictionary' files, for the use with security auditing programs ('password crackers') such as "john" and "aircrack-ng". Awk may be used to amplify and multiplex a text password dictionary file.

print each line joining to the preceding line

 awk '{print last$0; last=$0}' /usr/share/dict/words | less

as above but with all punctuation removed

 awk '{gsub(/[[:punct:]]/,"");print last$0; last=$0}' /usr/share/dict/words | less

Software Written With Awk ‹↑› a very good command line script to access Google translate

Alternatives To Awk ‹↑›

awk is particularly good handling 'csv' (comma separated values) data or other tabular style data.

cut - perl - sed -

Books About Awk ‹↑›

"sed & awk, 2nd Edition," by Dale Dougherty and Arnold Robbins (O'Reilly, 1997)

"UNIX Text Processing," by Dale Dougherty and Tim O'Reilly (Hayden Books, 1987)

"GAWK: Effective awk Programming," 3d edition, by Arnold D. Robbins (O'Reilly, 2003) or at

"Mastering Regular Expressions, 3d edition" by Jeffrey Friedl (O'Reilly, 2006).

The info and manual ("man") pages on Unix systems may be helpful (try "man awk", "man nawk", "man gawk", "man regexp", or the section on regular expressions in "man ed").

Awk Contributors ‹↑›

Peter S. Tillier (U.K.); Daniel Jana; Yisu Dong

History ‹↑›

Awk was created by Aho, W? and Kernighan. Mr Aho has written some dense books about computer science. Brian Kernighan has been an important figure in the early development of Unix. Kernighan co-authored the first 'c' book- the C programming language, he also has maintained nroff (used for man pages) as well as other obscure unix tools.

Notes ‹↑›

Convert numbers to SI notation

 $ awk '{ split(sprintf("%1.3e", $1), b, "e"); p = substr("yzafpnum_kMGTPEZY", (b[2]/3)+9, 1); o = sprintf("%f", b[1] * (10 ^ (b[2]%3))); gsub(/\./, p, o); print substr( gensub(/_[[:digit:]]*/, "", "g", o), 1, 4); }' < test.dat


if (y % 4 == 0) # leap year from 1901 to 2099