That is what I did in the end I created a shell file. I could not get the bash -c to work. This solution probably won't work in Windows.
One thing that would work for this use case is if the scripting allowed for the interim storage of the result of a systemcommand call - some like this
%v = systemcommand:audtool arg1:get-volume
systemcommand:audtool arg1:set-volume arg2:%v - 1
Not sure about how the syntax will work but that would give us the start of the capability to read and set dmx channel values via these variables.
Ask a question about the usage of QLC+ with the Linux operating system