And, of course, the single awk statement in:

Code:

ls dir1 dir2 | awk '{if ($0 in x) print; else x[$0]}'

can be split into two awk statements (one with a default action and one with a default pattern) producing the same output with less text:

Code:

ls dir1 dir2 | awk '$0 in x; {x[$0]}'

But, of course, if instead of printing each line that has appeared more than once you just want to print each line (once) that appears two or more times, you would need to keep a count, but the code is still short:

Code:

ls dir1 dir2 dir3 ...| awk 'x[$0]++ == 1'