Asynchronous jobs are tricky. For now, we disable them behind a flag in [set]. 

  $ msh -c "echo hello &"
  You are using asynchronous operators and [set -o async] has not been called.
  [1]

But we do have some support for them.

  $ cat > msh.sh << EOF
  > set -o async
  > sleep 10000 & echo hello
  > kill -9 \$!
  > EOF

  $ cat > test.sh << EOF
  > sleep 10000 & echo hello
  > kill -9 \$!
  > EOF

  $ sh test.sh
  hello
  $ msh msh.sh
  hello