Tvheadend

I am trying to define a mux invoking a bash script of mine, but the debug log shows the following:

2017-05-xx xx:xx:xx.389 spawn: bash: cannot set terminal process group (27532): Invalid argument
2017-05-xx xx:xx:xx.389 spawn: bash: no job control in this shell

What are the limitations when it comes to the external commands that can be called via pipe:// ?

TIA