Compare the output of two commands

..without creating a temporary file:

diff -y <(ls First_directory) <(ls Second_directory)

The -y is just to show a side-by-side comparison. Very handy! And the result?

[jorge@j-laptop temp]$ diff -y <(ls First_directory) <(ls Second_directory)
common.txt                          common.txt
first_file.txt                            | second_file.txt

Thanks to my colleague Ingvar for showing me this!