子文件夹按字母排倒序，只保留前5个，其他的删除。
find path/to/folder/ -mindepth 1 -maxdepth 1 -type d | head -n -5 | xargs rm -rf
-find: the unix command for finding files / directories / links etc.
-/path/to/base/dir: the directory to start your search in.
-mindepth 1 -maxdepth 1 only search the first sub folder
-type d: only find directories
-head -n -5: Filter out all lines except the last 5
-xargs rm -rf: remove each given folder.