http://www.loveunix.net/html/200407/33920.html
-r Copies file hierarchies under the file or directory specified by the SourceFile or SourceDirectory parameter (recursive copy). The -r flag processes special files in the same manner as regular files.
-R Copies file hierarchies under the regular files and directories from the directory specified by the SourceFile or SourceDirectory parameter to the directory specified by the TargetDirectory parameter. Special file types, such as first-in, first-out (FIFO) files and block and character device files, are re-created instead of copied. Symbolic links are followed unless the -h flag is specified. (The -R flag is preferred to the -r flag.)
cp -r 会把所有source当作普通文件（regular文件）；而cp -r 对特殊文件（管道文件，块设备文件，字符设备文件）会进行创建操作，而不是拷贝,比如FIFO文件或/dev/zero的时候， 就必须用-R， 否则， 他们没区别 。