Convert all item files in the current directory to yaml and back

for x in *.uti; do
  nwn-gff -i"$x" -o"$x.yml"
done

for x in *.uti.yml; do
  nwn-gff -i"$x" -o"$(basename $x .yml)"
done

A Makefile that rebuilds only changed resources

This can be done with a GNU Makefile too, with the added benefit that it will keep track of changes automagically and only rebuild those resources that you have changed (saving you a lot of time).

yml = $(wildcard *.yml)
gff = $(basename $(yml))

all: $(gff)

% : %.yml
    nwn-gff -i $< -o $@