C     This program reads a real number and prints it back out.  The
C     program also remembers the "state" it is in and changes state if
C     there was an error reading the input.  The total number of states
C     is specified by the variable "numsts".
C.....
      program shell
      implicit none
      
      real a
      integer state, numsts, num
C.....      
C     a      = variable that is set by read operation
C     state  = current "state" or mode of the program
C     numsts = the total number of possible states (the state numbers go
C              from zero to numsts-1)
C     num    = status of the read operation stored here
C.....
      state  = 0
      numsts = 5

 10   read(5,*,iostat=num) a
        if (num .eq. 0) then
           print *, 'my state is ',state,' and you entered ', a
        elseif (num .lt. 0) then
           stop
        else
           state = mod(state+1,numsts)
           print *, 'I''ve changed state to state ',state
        endif
      goto 10

      stop
      end