I had two folders in the same directory and I wanted to move one into other with the command mv foo /foo2. But now foo has vanished. 
Moved folder into other has now disappeared
directory
If the above command terminated without a Permission Denied error, it means you had root privileges when you ran it and it actually moved the directory foo into the directory /foo2, which is not foo2 in the current directory, but foo2 under the root directory (/).You'll want to domv /foo2 foo2in the directory where you issued the original command.