The ATTACH command connects your input stream to another process.
    You can use the ATTACH command to change control from one
    subprocess to another subprocess or to the parent process.

    When you enter the ATTACH command, the parent process goes into
    hibernation and your input stream connects to the specified
    destination process. You can use the ATTACH command to connect
    to a subprocess that is part of a current job (left hibernating
    as a result of the SPAWN/WAIT command or another ATTACH command)
    as long as the connection is valid. No connection can be made to
    the current process, to a process that is not part of the current
    job, or to a process that does not exist. If you attempt any of
    these connections, you receive an error message.

    You can also use the ATTACH command in conjunction with the
    SPAWN/WAIT command to return to a parent process without
    terminating the created subprocess. See the description of the
    SPAWN command for more details.