How to flatten a directory on Linux and MacOS

If you want to flatten a directory with lots of deeply nested files (for example ./2012/06/09/images/previews/200x200/image1.jpg becomes ./image1.jpg), you can run this simple command:

find target/ -mindepth 2 -type f -exec mv -i '{}' target/ ';'

All the files in target's subdirectories will be moved directly under target. If multiple files have the same name (target/hello.txt, target/backup/hello.txt, target/hello/english/hello.txt), you will be asked to overwrite them:

overwrite ./hello.txt? (y/n [n])

The default option is "no", so you can just hold the Enter key to say no to all overwrites.