bpipe how to get the output path when the -d was defined

[fengzhang]

My pipe file saves as hello.pipe in path of "~/test" and it looks :
hello = { exec "echo a > a.txt; echo Hello | cat $input.txt - > $output.txt" }
It was run using command bpipe run -d ~/ hello.pipe test.txt .
Now I get the test.hello.txt at ~/ as expected, but the a.txt file is at ~/test directory. How could I get the output path that was defined by the parameter -d, so I can move the a.txt to ~/ directory.

Thanks.