$ sbx cp $CAPTURE/work/in.txt m101-demo:/tmp/in.txt
[exit 0]

$ sbx exec m101-demo sh -c 'cat /tmp/in.txt; printf out > /tmp/out.txt'
in
[exit 0]

$ sbx cp m101-demo:/tmp/out.txt $CAPTURE/work/out.txt
[exit 0]

$ cat $CAPTURE/work/out.txt
out
[exit 0]

$ sbx cp m101-demo:/tmp/out.txt m101-demo-2:/tmp/x
error: copying between sandboxes is not supported
  try: sbx cp --help
[exit 1]