demo_system_putenv Program

      program demo_system_putenv
      use M_system, only : system_putenv
      use iso_c_binding
      implicit none
      integer :: ierr
         !
         write(*,'(a)')'no environment variables containing "GRU":'
         call execute_command_line('env|grep GRU')
         !
         call system_putenv('GRU=this is the value',ierr)
         write(*,'(a,i0)')'now "GRU" should be defined: ',ierr
         call execute_command_line('env|grep GRU')
         !
         call system_putenv('GRU2=this is the second value',ierr)
         write(*,'(a,i0)')'now "GRU" and "GRU2" should be defined: ',ierr
         call execute_command_line('env|grep GRU')
         !
         call system_putenv('GRU2',ierr)
         call system_putenv('GRU',ierr)
         write(*,'(a,i0)')&
              & 'should be gone, varies with different putenv(3c): ',ierr
         call execute_command_line('env|grep GRU')
         write(*,'(a)')&
              & 'system_unsetenv(3f) is a better way to remove variables'
         !
      end program demo_system_putenv