Merge two files on Linux keeping only lines that appear in both files

Question

In Linux, how can I merge two files and only keep lines that have a match in both files?

Each line is separated by a newline (\n).

So far, I found to sort it, then use comm -12. Is this the best approach (assuming it's correct)?

fileA contains

aaa
bbb
ccc
ddd

fileB contains

aaa
ddd
eee

and I'd like a new file to contain

aaa
ddd

See http://stackoverflow.com/questions/746458/how-to-show-lines-in-common-reverse-diff — elmov, Aug 18 '13 at 17:12
If sorting is an option (or the files are already sorted), then `sort` plus `comm` is the way to go. — Jonathan Leffler, Aug 18 '13 at 17:25
See also [Unix command to find lines common in two files](http://stackoverflow.com/questions/373810/unix-command-to-find-lines-common-in-two-files/373819#373819). — Jonathan Leffler, Aug 18 '13 at 17:54

score 2 · Accepted Answer · edited Aug 18 '13 at 18:00

2

Provided both of your two input files are lexicographically sorted, you can indeed use comm:

$ comm -12 fileA fileB > fileC

If that's not the case, you should sort your input files first:

$ comm -12 <(sort fileA) <(sort fileB) > fileC

edited Aug 18 '13 at 18:00

Jonathan Leffler

answered Aug 18 '13 at 17:57

elmov

1 Answers1