Comparing and Merging Files
18.1.3 Files that are Neither Directories Nor Regular Files
Some files are neither directories nor regular files: they are unusual
files like symbolic links, device special files, named pipes, and
diff treats symbolic links like regular files;
it treats other special files like regular files if they are specified
at the top level, but simply reports their presence when comparing
directories. This means that
patch cannot represent changes
to such files. For example, if you change which file a symbolic link
diff outputs the difference between the two files,
instead of the change to the symbolic link.
diff should optionally report changes to special files specially,
patch should be extended to understand these extensions.