Un*x does not do that, you need 'mv -f file1 file2' to force the mv on existing file.

No, you don't.

$ echo foo >foo
$ echo bar >bar
$ mv foo bar
$ cat bar
foo
$ uname -a
Linux raven 2.6.22-8-generic #1 SMP Thu Jul 12 15:59:45 GMT 2007 i686 
+GNU/Linux
$ mv --version
mv (GNU coreutils) 5.97
[download]

and from the info page:

If a destination file exists but is normally unwritable, standard input is a terminal, and the `-f' or `--force' option is not given, `mv' prompts the user for whether to replace the file. (You might own the file, or have write permission on its directory.) If the response is not affirmative, the file is skipped.