Given two directory trees, how can I find out which files differ by content? [closed]

Try:

diff --brief --recursive dir1/ dir2/

Or alternatively, with the short flags -qr:

diff -qr dir1/ dir2/

If you also want to see differences for files that may not exist in either directory:

diff --brief --recursive --new-file dir1/ dir2/  # with long options
diff -qrN dir1/ dir2/                            # with short flag aliases