Ambiguous arguments

Problem:

Ambiguity leads to different behaviour depending on the state of the filesystem.

Example

$ touch src
$ mv src dst

1. If 'dst' does not exist, 'src' will be renamed 'dst'
2. If 'dst' is a file, 'src' will be renamed 'dst', deleting the existing file
3. If 'dst' is a directory, the file 'src' will be renamed to 'dst/src'
4. If 'dst/src' is a directory, an error will occur

Note that there are 3 different outcomes. This makes it difficult to write scripts that work reliably. This gets even worse when 'src' is a directory, as demonstrated here:

$ rm -rf dst
$ mkdir src
$ mv src dst
$ mkdir src
$ mv src dst

If 'dst' doesn't exist, the first mv command will rename 'src' to 'dst'. The second mv command will create dst/src ! If you replace 'mkdir' with a command that creates a tree of files (like wget --mirror) suddenly instead of having a newer version, you now have two copies. The sharp edge here is that the second mv succeeds, which can trick you into thinking that it did case #1 above, not #3. Only if you run 'mkdir;mv' a third time, it will result in an error.

Solving

Notice that whenever you run ls -al' you see two entries: . and .. ? If you use these as your destination, your command becomes unambiguous. For example:

$ mkdir src
$ mv src dst/.

1. If 'dst' does not exist, an error will occur
2. If 'dst' is a file, an error will occur
3. If 'dst' is a directory and dst/src is a file, an error will occur ("cannot overwrite directory ... with non-directory")
4. If 'dst' is a directory, it will create the directory dst/src/

$ update my tree of files in src/...
$ rsync -n --archive --delete src/. dst/.

Since both src and dst arguments end in "/." it's completely unambiguous; you're making one directory tree mimic another. It will correctly grab all dot files or anything with a weird name; no need to try and create a glob for files like ".foo.rc". The src is guaranteed to be a directory, dst is guaranteed to be a directory. Zero ambiguity.

1. You may need to add a mkdir to create your target directory.
2. You can't mv a directory specified by ".". However, you can specify the source by name e.g. 'src' without the trailing "/.". Mostly it is the destination directory that requires "/." to be unambiguous.
3. Renaming files or directories is not possible. Rename the file/directory before you move it, or instead respecify and move the contents

Conclusion