There seems to be no way to capture multi-line output in a shell variable. Any attempt to do so splits each line into a separate array element. In bash I'd simply put double-quotes around my $() invocation, but in fish you cannot quote a command substitution. This is rather unfortunate because it means I cannot capture multi-line output in a variable and send it back to a separate command without really weird contrivances, such as replacing all newlines with NUL when saving and reverting the process when emitting.