AWK one-liner collection

I love perl and I use it for most scripts but nothing beats awk on the commandline. AWK is a pattern matching and string processing language named after the surnames of the original authors: Alfred Aho, Peter Weinberger and Brian Kernighan.

Print selected fields

Split up the lines of the file file.txt with ":" (colon) separated fields and print the second field ($2) of each line:
awk -F":" '{print $2}' file.txt

Same as above but print only output if the second field ($2) exists and is not empty:
awk -F":" '{if ($2)print $2}' file.txt

Print selected fields from each line separated by a dash:
awk -F: '{ print $1 "-" $4 "-" $6 }' file.txt

Print the last field in each line:
awk -F: '{ print $NF }' file.txt

Print every line and delete the second field:
awk '{ $2 = ""; print }' file.txt

Good to know:

Print matching lines

Print field number two ($2) only on lines matching "some regexp":
awk -F":" '/some regexp/{print $2}' file.txt

Print lines matching "regexp a" and lines matching "regexp b" but the later ones are printed without newline (note the printf):
awk '/regexp a/{print};/regexp b/{printf $0}' file.txt

Print field number two ($2) only on lines not matching "some regexp":
awk -F":" '!/some regexp/{print $2}' file.txt or awk -F":" '/some regexp/{next;}{print $2}' file.txt

Print field number two ($2) only on lines matching "some regexp" otherwise print field number three ($3):
awk -F":" '/some regexp/{print $2;next}{print $3}' file.txt
The "next" command causes awk to continue with the next line and execute "{print $3}" only for non matching lines. This is like

Print the next two (i=2) lines after the line matching regexp:
awk '/regexp/{i=2;next;}{if(i){i--; print;}}' file.txt

Print the line and the next two (i=2) lines after the line matching regexp:
awk '/regexp/{i=2+1;}{if(i){i--; print;}}' file.txt

Print the lines from a file starting at the line matching "start" until the line matching "stop":
awk '/start/,/stop/' file.txt

Regexp syntax:

Insert string after the matching line

This inserts a new line after the matching line:
awk '/regexp/{print $0; print "text inserted after matching line";next}{print}' file.txt
$0 is the line where the search pattern "regexp" matches without the newline at the end. The awk print command prints the string and appends a new line.

This appends a string to the matching line:
awk '/regexp/{print $0 "text appended at end of the matching line";next}{print}' file.txt

If matching "do A" else "do B"

awk '/regexp/{A-here;next}{B-here}' file.txt Example: awk '/regexp/{gsub(/string/,"replacement");print $1;next}{print;}' file.txt
The example would print lines that do not match unchanged (action B is just "print;") while on lines that match /regexp/ it would replace /string/ by replacement and print the first element ($1).

Replacement for some common unix commands (useful in a non unix environment)

Count lines (wc -l):
awk 'END{print NR}'

Search for matching lines (egrep regexp):
awk '/regexp/'

Print non matching lines (egrep -v regexp):
awk '!/regexp/'

Print matching lines and ignore case (egrep -i regexp):
awk 'BEGIN {IGNORECASE=1};/regexp/'

Number lines (cat -n):
awk '{print FNR "\t" $0}'

Remove duplicate consecutive lines (uniq):
awk 'a !~ $0{print}; {a=$0}'

Print first 5 lines of file (head -5):
awk 'NR < 6'

Number non empty lines

This prints all lines and adds a line number to non empty lines:
awk '/^..*$/{ print NR ":" $0 ;next}{print}' file.txt

Substitute foo for bar on lines matching regexp

awk '/regexp/{gsub(/foo/, "bar")};{print}' file.txt

Delete trailing white space (spaces, tabs)

awk '{sub(/[ \t]*$/, "");print}' file.txt

Delete leading white space

awk '{sub(/^[ \t]+/, ""); print}' file.txt

Add some characters at the beginning of matching lines

Add ++++ at lines matching regexp.
awk '/regexp/{sub(/^/, "++++"); print;next;}{print}' file.txt

Color gcc warnings in red

gcc -Wall main.c |& awk '/: warning:/{print "\x1B[01;31m" $0 "\x1B[m";next;}{print}'
The "\x1B" means the ascii character with hex number 1B (ESC).

Print only lines of less than 80 characters

awk 'length < 80' file.txt

Renaming files with AWK

You can use awk to generate shell commands such as e.g mv commands to rename files according to a given recipe. I suggest to always print the commands before piping them to sh in order to execute them. A small typo can have very significant side effect so double check what would happen by printing the commands first.

Rename all .MP3 file to be lower case:
ls *.MP3 | awk '{ printf("mv \"%s\" \"%s\"\n", $0, tolower($0)) }' the above will just print what would happen to really execute it you run: ls *.MP3 | awk '{ printf("mv \"%s\" \"%s\"\n", $0, tolower($0)) }' | sh

Substitute a regexp pattern with a given replacement string. We can e.g replace " " (spaces in the file names) by "-":
ls | awk '{ printf("mv \"%s\" \"%s\"\n", $0, gensub(/ +/,"-","g")) }' the above will just print what would happen to really execute it you run: ls | awk '{ printf("mv \"%s\" \"%s\"\n", $0, gensub(/ +/,"-","g")) }' | sh
The gensub function reads the strings from $0 (=current line) and returns the modified string. The third argument, the "g", means to find and replace everywhere (globally) on the current line.

AWK as a command-line calculator

This prints 5:
awk 'BEGIN{print 3+4/2}'

This prints 1.41421:
awk 'BEGIN{print sqrt(2)}'

This prints 1.41421:
awk 'BEGIN{print 2^(1/2)}'

This prints 3.141592653589793 (PI with a 15 digits behind the decimal point):
awk 'BEGIN{printf "%.15f\n",4*atan2(1,1)}'

Print decimal number as hex (this prints 0x20):
awk 'BEGIN{printf "0x%x\n", 32}'

Convert hex string to decimal (this prints 32):
awk 'BEGIN{print strtonum(0x20)}'

Math operators in gnu awk:

AWK minimal web server

You can't write a web server as a reasonable one-liner in AWK, you can do that with netcat but there are some cases where you don't have a real web server and you don't have netcat but you have a very basic shell environment and that does usally include awk. Here is a web server that allows you to serve files at port 8080 (or any port, just change the number):
#!/usr/bin/gawk -f
if (ARGC < 2) { print "Usage: wwwawk  file.html"; exit 0 }
	Concnt = 1;
        while (1) {
        RS = ORS = "\r\n";
        HttpService = "/inet/tcp/8080/0/0";
        getline Dat < ARGV[1];
        Datlen = length(Dat) + length(ORS);
        while (HttpService |& getline ){
		if (ERRNO) { print "Connection error: " ERRNO; exit 1}
                print "client: " $0;
                if ( length($0) < 1 ) break;
        print "HTTP/1.1 200 OK"             |& HttpService;
        print "Content-Type: text/html"     |& HttpService;
        print "Server: wwwawk/1.0"          |& HttpService;
        print "Connection: close"           |& HttpService;
        print "Content-Length: " Datlen ORS |& HttpService;
        print Dat                           |& HttpService;
        print "OK: served file " ARGV[1] ", count " Concnt;
Copy this code and save it into a file called wwwawk and then make it executable with "chmod 755 wwwawk". Now take some file (e.g somefile.html) and you can serve it via that little web server:
chmod 755 wwwawk ./wwwawk somefile.html from another shell: curl http://localhost:8080 or lynx http://localhost:8080 or firefox -new-tab http://localhost:8080
This is e.g. a great way to serve kickstart files for automated Linux installations. Note that this awk web server requires gawk. Most linux distributions use gawk by default except for raspberry pi which uses mawk and mawk does not support network connections.

References, other nice pages about AWK

© Guido Socher, Please enable Javascript to see email address
License: CC BY 4.0,