In modern bash, this case is already protected with the "noclobber" option (enabled with "set -C" or "set -o noclobber"). It is described in the "REDIRECTION" section of bash(1), subsection "Redirecting Output".

It fails ">" redirections with an error message when it would clobber an existing file (you probably wanted to append to the file wiht ">>"). If you really intended this behavior, the new redirection operator '>|' has the old behavior.

    $ set -o noclobber
    $ echo foo > bar
    $ cat bar
    foo
    $ cat bar > bar
    bash: bar: cannot overwrite existing file
    $ echo baz > bar
    bash: bar: cannot overwrite existing file
    $ echo baz >| bar
    $ cat bar
    baz

edit: fixed typo