    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.

       Commands and descriptions:

            afni    : AFNI program: the main GUI for viewing volume 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 afni
       help for ctrl-z: not ready
       help for bg:     not ready
       help for &:     not ready

       A. Go to the AFNI_data6/afni directory and run afni.

             cd AFNI_data6/afni
             afni

          As was seen earlier, there are many AFNI datasets in this directory.
          When the afni GUI is opened, it shows us something.  But this is about
          Unix, so we do not care what it shows.

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

          Locate the terminal window from which 'afni' 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 'afni' program is running in the foreground of the
          terminal shell.  The shell is occupied by afni.

       C. Suspend afni.

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

             ctrl-z

          The terminal window shows 'Suspended'.  That refers to afni program,
          meaning that afni 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 afni program is no long occupying our
          terminal shell.  A prompt may or may not show up when suspending afni,
          so we started off with a couple of Enter key strokes, just to see a
          prompt.

       E. Try to use the afni GUI.

          Move the afni windows around, possibly over other afni windows.

          The windows basically get erased, in that they are not re-drawn by the
          window manager.  This shows us that 'afni' is no longer running (since
          it was suspended).

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

          Find the terminal window again and type the 'bg' command (for background).
          Watch the afni GUI while this is done.

             bg

          Notice that afni 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 'afni' in the backgrond to begin with.  Then ctrl-z
          and 'bg' would not be needed.

          Start by closing the current afni GUI.  Press the red 'done' button
          twice to close afni (twice within 5 seconds).

          Then start afni, putting the & character after the command.

             afni &

          Now afni is in the background to begin with.  We can use afni 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.