Things you should not do with /etc/network/interfaces

do_not_do_this_interfaces

##################################################################
# If you have not read the multiple disclaimers, do not do this. #
# It is supposed to be a joke, haha.                             #
##################################################################

This goofiness is valid:

/etc/network/interfaces:
for procif in /etc/network/interfaces.d/*; do
procifname="$(basename $procif)"
source "$procif" "$procifname"
done

/etc/network/interfaces.d/eth0:
ActiveIF=$1
allow-hotplug $ActiveIF
iface $ActiveIF inet dhcp

/etc/network/interfaces.d/eth1 -> eth0

=======================================

But it's easier to do:

/etc/network/interfaces:
for procifname in eth0 eth1 eth2 eth3; do

allow-hotplug "$procifname"
iface "$procifname" inet dhcp

done