Question

I have multiple programs which each produce lines of output. How do I concatenate those outputs, and then only return one copy of each line which repeated at least once? In other words, I want to return the set intersection of all response lines.

for example:

$ progA
9
13
14
15
$ progA --someFlag
13
14
15
100
$ progB
14
15
-42
$ magicFunction 'progA' 'progA --someFlag' 'progB'
14
15

This doesn’t have to be a function per se. I just wanted a unix command-line way.

Answer 1

The solutions above don’t really compute the set intersection of all 3 outputs. uniq -d will also output lines which are output only by 2 of the 3 programs.
Here’s my take on it:

progA | sort > f1; progA --someFlag | sort > f2; progB | sort > f3; comm -1 -2 f1 f2 | comm -1 -2 f3 -; rm f[123]