tool that compares the files in a source directory against a destination directory, and runs an executable when the destination file is missing or older.
convert -v -i /path/to/source -o /path/to/destination -e ext <executable> %i %o
-v be verbose about decisions being made
-i path to source directory
-o path to destination directory
-e substitute this extension for the destination file (e.g. m4v)
%i substitute source filename here
%o substitute destination filename here