That's not as weird as it might sound (perhaps you knew this). He used /bin/sh until bfox wrote bash.

But on ITS, the PDP-10 OS we mostly used until the mid 80s, DDT (the debugger) was the shell. There were no core dumps for interactive programs; if your program halted abnormally you were already in the debugger so it was no big deal. The Lispm, of course, worked this way too although the debugger was more sophisticated than DDT.