Bash: exclude unique lines from output

Question

How to exclude unique lines from output of sort?

Let's say I'm having the following text:

aa
ab
ax    (NOT dupplicated line)
aa
aa
ab
az    (NOT dupplicated line)
ay    (NOT dupplicated line)

and I want to remove non-duplicate lines from it:

aa
aa
aa
ab
ab

How would I do it using sort?

@BłażejMichalik: Googling for `unix sort keep only duplicates` found two solutions: [one on Stack Exchange](http://unix.stackexchange.com/questions/52534), [another on unix.com](http://www.unix.com/shell-programming-and-scripting/189519-bash-keep-only-duplicate-lines-file.html). Please read up on [how to ask a good question](http://stackoverflow.com/help/how-to-ask). — Tom Zych, Nov 16 '15 at 01:55
@TomZych Question though, how can it be that this question: http://stackoverflow.com/questions/9377040/remove-duplicate-entries-using-a-bash-script gets approved widely, while mine, being on the same context, and same level of research, gives me links to SO newbie guide? — Błażej Michalik, Nov 16 '15 at 02:09
@BłażejMichalik: That is a very good question. [I've asked about it on meta](http://meta.stackexchange.com/questions/269474); let's see what people think. — Tom Zych, Nov 16 '15 at 10:46

score 5 · Accepted Answer · answered Nov 15 '15 at 23:10

5

Use sort with uniq:

command | sort | uniq -D

-D, --all-repeated[=delimit-method]
print all duplicate lines delimit-method={none(default),prepend,separate} Delimiting is done with blank lines

answered Nov 15 '15 at 23:10

choroba

1 Answers1