How can I show lines in common (reverse diff)?

Question

On *nix, you can use comm. The answer to the question is:

comm -1 -2 file1.sorted file2.sorted 
# where file1 and file2 are sorted and piped into *.sorted

Here's the full usage of comm:

comm [-1] [-2] [-3 ] file1 file2
-1 Suppress the output column of lines unique to file1.
-2 Suppress the output column of lines unique to file2.
-3 Suppress the output column of lines duplicated in file1 and file2. 

Also note that it is important to sort the files before using comm, as mentioned in the man pages.

I found this answer on a question listed as a duplicate. I find grep to be more administrator-friendly than comm, so if you just want the set of matching lines (useful for comparing CSV files, for instance) simply use

grep -F -x -f file1 file2

Or the simplified fgrep version:

fgrep -xf file1 file2

Plus, you can use file2* to glob and look for lines in common with multiple files, rather than just two.

Some other handy variations include

• -n flag to show the line number of each matched line
• -c to only count the number of lines that match
• -v to display only the lines in file2 that differ (or use diff).

Using comm is faster, but that speed comes at the expense of having to sort your files first. It isn't very useful as a 'reverse diff'.

It was asked here before: Unix command to find lines common in two files

You could also try with Perl (credit goes here):

perl -ne 'print if ($seen{$_} .= @ARGV) =~ /10$/'  file1 file2

I just learned the comm command from the answers, but I wanted to add something extra: if the files are not sorted, and you don't want to touch the original files, you can pipe the output of the sort command. This leaves the original files intact. It works in Bash, but I can't say about other shells.

comm -1 -2 <(sort file1) <(sort file2)

This can be extended to compare command output, instead of files:

comm -1 -2 <(ls /dir1 | sort) <(ls /dir2 | sort)