shell script question

[Jeff Elkins <jeffelkins@earthlink.net>]

I'm trying to write a script to change spaces in a filename to the underscore 
character:

#!/bin/sh
for i in *; do
        if test -f $i; then
          mv $i `echo $i | tr '" "' '_'`
    fi
done

When run, it gives me the error "too many args in line four."

How can I fix this?

Thanks,

Jeff Elkins