How to extract duplicate rows

chromatin · February 26, 2011, 12:55am

Hi! I have a file as below:

line1
line2
line2
line3
line3
line3
line4
line4
line4
line4

I would like to extract duplicate lines (not unique, triplicate or quadruplicate lines). Output will be as below:

line2
line2

I would appreciate if anyone can help. Thanks.

pravin27 · February 26, 2011, 1:39am

If your file is sorted then try below.

awk '{if(a!=$0){if(i==2){print a"\n"a}a=$0;i=0;i++}else{++i;a=$0}}' infile

chromatin · February 26, 2011, 1:47am

thanks pravin27,
it works great.

kurumi · February 26, 2011, 1:54am

$ ruby -ne 'BEGIN{h={}};(h[$_] ||= []) << $_;END{h.each{|k,v| puts v.join() if v.size==2}}' file
line2
line2

yinyuemi · February 26, 2011, 2:13am

awk '{++a[$0];b[$0]=b[$0]?b[$0]"\n"$0:$0}END{ for (i in a) {if(a==2) print b}}' file