confused

under /etc there are two directories related to hotplug:

/etc/hotplug
/etc/hotplug.d

in /etc/hotplug.d/net
I run
# ./50-ifup.hotplug
./50-ifup.hotplug: line 18: INTERFACE: Bad invocation: $INTERFACE is not set

It's 10 minutes before I throw this condemned japanese trash out of my window.

anyhow thank you very much