At https://github.com/hudson/hudson/blob/master/core/src/main/java/hudson/tasks/Shell.java#L139 the Shell task attempts to detect Windows, and if so, use "sh" instead of "/bin/sh". The problem is that this detection takes place on the Master, instead of on the Slave that is actually going to run the task. If this were fixed to detect correctly, lots of people's workarounds of globally setting the shell as plain "sh" could go away.