How to decide that mv moves into a directory rather than replacing directory?

mv a b attempts to move a into b if b is a directory or a symlink to a directory. Otherwise, it will rename (or copy and delete if on different file systems) a to b.

To get a consistent result to move a file into a directory, you can do:

mv file dir/

or

mv file dir/.

If dir doesn't exist, you'll get an error, and it won't rename file to dir.

If, on the other hand, you want to do a rename without having to worry if the destination file exists as a directory or not, with GNU mv, you can do:

mv -T file dest

In that case, file will not be moved into dest if dest is a directory. However, if both file and dest are directories and dest is empty, file will be renamed to dest (and the original dest removed). If both are directories and dest is non-empty, mv -T will complain.

Same, if file and dest are not directories (and that includes symlinks including symlinks to directories), file will be renamed to dest (though you will get a prompt if you don't have write permission to dest), and the original dest will be removed. There's a difference with mv file dest in the case where dest is a symlink to a directory. With -T, file is renamed to dest, but without -T, file is moved into the directory pointed to by dest.

So to sum up, after mv -T file dest, either file will have been renamed to dest or you'll get an error message (or a prompt). If the command succeeded, the original dest, if it existed beforehand will have been removed.

This is entirely dependent of if there was already a directory named '/hello2' already in existence or not.

If /hello2 exists AND is a directory, then move will always move /hello to /hello/hello2

If /hello2 does not exist, then move will always rename /hello to /hello2

If /hello2 exists AND is a file, you will get an error, "cannot overwrite non-directory 'hello2' with directory 'hello'.

I tried these

mkdir test1
mv -v test1 test2
output:`test1' -> `test2'
mkdir test1
mv -v test2 test1
output: `test2' -> `test1/test2'
touch test2
output:mv: cannot overwrite non-directory `test2' with directory `test1'

hope this explains everything. -v is verbose mode.