    Basic Unix 2. Shifting a process between the foreground and the background

       A foreground process is one that occupies your shell (terminal window),
       meaning that any new commands that are typed have no effect until the
       previous command is finished.  This is as we might expect, but can be
       confusing when we run long lasting programs, such as the 'afni' GUI.

       Note: afni can no longer be used as the example for this, since it creates
             a child process and returns the user to an active command prompt.  So
             suma will now be used for this demonstration.

       Commands and descriptions:

            suma    : AFNI program: the main GUI for viewing surface data
            ctrl-z  : keystroke: suspend the foreground process
            bg      : put the recently suspended process in the background
            &       : run a program in the background to begin with

       help for suma
       help for ctrl-z
       help for bg:     not ready
       help for &:      not ready

       A. Start suma (from any directory).  It will display a random dataset.

             suma

          When the suma GUI is opened, it shows us something (a random internal
          dataset of its choosing), which will vary.  But this is about Unix, so
          we do not care what it shows.

          To see that suma is active, use the left mouse button to "grab" the
          surface data and rotate it around.

       B. Try to type commands in the original terminal window.

          Locate the terminal window from which 'suma' was launched.  Try to
          enter some commands into it.

             <Enter>
             <Enter>
             ls
             pwd
             <Enter>

          We get no prompt and the commands have no effect.

          That is because the 'suma' program is running in the foreground of the
          terminal shell.  The shell is occupied by suma.

       C. Suspend suma.

          In the terminal window, hold the ctrl key and press 'z'.

             ctrl-z

          The terminal window shows 'Suspended'.  That refers to the suma program,
          meaning that suma is no longer running, but the terminal is available.

       D. Repeat of B: try to type commands in the terminal window.

             <Enter>
             <Enter>
             ls
             pwd

          This time the commands work.  The suma program is no long occupying the
          terminal shell.

       E. Try to use the suma GUI.

          Try to use the left mouse button to grab and rotate the surface again.

          Next grab a different window (maybe the terminal or web browser), move
          it to partially obscure the suma window, and then move it back away.

          The windows basically get erased, maybe leaving copies of the obscuring
          image in the suma window.  The suma image is no longer getting re-drawn
          by the window manager.  This shows us that 'suma' is no longer running
          (since it was suspended).

       F. Put suma into the background, then try to use it again.

          Find the same terminal window again and type the 'bg' command (for
          background).  Watch the suma GUI when you hit Enter.

             bg

          Notice that suma started running again, as soon as the 'bg' command was
          entered.  Also, notice that we can still enter commands into the terminal
          window.  So not only is afni running, but we have access to the shell.

       G. Be efficient.  Run 'suma' in the background to begin with.  Then ctrl-z
          and 'bg' would not be needed.

          Start by closing the current suma GUI.  Use the 'x' in the upper corner
          of the window, or hit <Esc> in the suma image window, and click on
          Yes to close (or just hit <Enter>).

          Now re-start suma, putting the & character after the command.

             suma &

          Now suma is in the background to begin with.  We can use suma and we can
          type commands in the terminal window.

    Comment(s):

       c1. After starting some program, it can be suspended and put into the
           background by using first ctrl-z and then the bg command.

       c2. A program can be put directly into the background using '&'.

       c3. If a program terminates quickly, such as 'ls', we do not really care.