for a variable z
z="I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII XIX XX"
echo "${z// /,}"
output is I,II,III,IV,V,VI,VII,VIII,IX,X,XI,XII,XIII,XIV,XV,XVI,XVII,XVIII,XIX,XX
OK.
but for
Y=(I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII XIX XX)
echo "${Y[@]// /,}"
output is I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII XVIII XIX XX