thanks a lot, IOhannes !
your first answer pointed to the right direction. i found the solution after digging a bit more in the archives:
for non-native executables (FFPLAY in my case) you need to provide absolute paths in the shell scripts.
so, while a shell script that reads:
ffplay -i MYSTREAM
works ok when run from the terminal, a shell script called from [shell] needs to be:
/usr/local/bin/ffplay -i MYSTREAM
thought so, but where to look ? since the [shell] operation is in the background i don't really have a terminal to check for errors.
start Pd from the terminal. then watch the terminal where you started Pd from.
also very helpful !
best
oliver