Being you, reader, a wise person you are going to keep a copy of your entire music library in some external storage device.
Here is a simple example on how to keep your external mirror synchronised with your music library:
SOURCE="/my/library/" DEST="/media/myDevice/my/mirror/library/" rsync -ruv --progress --size-only "$SOURCE" "$DEST" | tee sync.log
-r : recursively sync the subfolders;
-u : skip files that are newer on the destination folder (update);
-v : increase verbosity (print the files that are copied to the destination);
–progress : show a progress bar;
–size-only : skip files with same size.
This way, a new music file that you added in SOURCE is copied in DEST if not present, or if newer and of a different size of the corresponding file (song..) in DEST.
For a two-way synchronisation, just repeat the rsync instruction reversing the order of “$SOURCE” and “$DEST”.