
Though if your shell scripting allows you can use UFT-8 strings directly, though unless you are doing direct 'non-shell-parsed' calls to command from a API like perl, that will probably not work.
The better method to handle utf-8 binary text is either from a file or stdin using the '@' string escape.
Converting unicodes into utf-8 is best done though other programs, like the GNU printf (under linux) or the perl printf.
See IM Examples.
http://imagemagick.org/Usage/text/#unicode