mpirun -np 2 hellompirun is not part of the standard, but some version of it is common with several MPI implementations. The version shown here is for the MPICH implementation of MPI.
127 Just as Fortran does not specify how Fortran programs are started, MPI does not specify how MPI programs are started.
127 The option -t shows the commands that mpirun would execute; you can use this to find out how mpirun starts programs on yor system. The option -help shows all options to mpirun.