Date   : Fri, 20 Jul 1984 06:48:00 MDT (Fri)
From   : Richard Conn <RCONN@Simtel20.ARPA>
Subject: SHSET

       Well, the new feature (actually, utility pair) is in place,
and it works quite nicely.  I thought I'd describe the concept to you
and see what people think about it.

       ZCPR3 supports a concept (taken from AT&T's UNIX) which I call
shells.  The UNIX shell accepts a command line, interprets it,
suspends itself and allows the command line to execute, and then
resumes.  The ZCPR3 shell, since we have a single-process (simple) OS,
simply accepts a command line, interprets it, terminates (leaving
messages about as to how ZCPR3 CP should reinvoke it) and allows the
command line to execute, and then is resumed by the ZCPR3 CP.  The
bottom line is that, once a shell is running, whenever the ZCPR3 CP
would print its prompt (DU:DIR>, like the CP/M D> prompt), the shell
runs instead (no prompt appears from the ZCPR3 CP), and whatever the
shell does becomes the user's interface to his system.  Shells can be
nested up to N levels deep (I recommend 4), so one shell can run
another can run another ... .  Examples of supplied ZCPR3 shells are
MENU (which prints a menu and allows the user to select a menu item
with a single keystroke), VFILER (which shows a file directory, allows
the user to move the cursor around and manipulate files [copy, delete,
rename, etc]), VMENU (which is a cross between VFILER [with the
directory display and arrow] and MENU [with the menu display and
selection]), and SH (which allows named variable sets and substitution
in various forms).

       The new SHSET command (which is ONLY 1K in size) now exists
and it allows ANYTHING to become a shell, whether is knows about ZCPR3
or not.  The syntax is:

               SHSET cmd1;cmd2;...

and, like a shell, when the ZCPR3 CP is going to print its prompt,
it realizes that a shell has been defined, but instead of running just
one program as a shell, it runs a command sequence.  Along with SHSET
is CMD, another 1K utility, which prompts the user for a command line
and builds the new command line into the command line buffer in place
of itself, allowing execution to resume with the command line the user
just typed followed by any commands which originally followed CMD.
Now commands like the following can be created:

       SHSET WS;CMD

Each time the ZCPR3 CP is going to print a prompt, Word Star is run
instead.  The users does what he wishes inside of Word Star, and, when
he exits, CMD is run.  CMD will prompt him for a line:

       CMD DU:DIR> user input goes here

The user's input is entered, it is executed, and then the shell is
reinvoked, running Word Star again.  To get out of the loop, the handy
ZCPR3 Phase 1 command SHCTRL can be run.  The command SHCTRL POP will
pop the shell stack one level, terminating the WS;CMD sequence, and
invoking the next lower shell, whether it is VFILER, VMENU, or the
ZCPR3 CP itself.

       One feature I plan to zip into CMD this evening is sending
output to a register to indicate whether an input line was entered.
If this is done, ZEX could be run on top of the new SHSET shell:

               SHSET CMD;IF 9 0;WS;FI

would establish the new shell to be:

               CMD - input a command line from the user and run it in
                       place of CMD in the above script
                       (if the command line was empty, set reg 9 to
                       0, else set reg 9 to 1)
               IF 9 0;WS;FI - if reg 9 = 0, run WS (no command was
                       input, so run WS) 
               go back and run CMD again after WS exits or the
                       command line is finished

       If ZEX was running, ZEX would see the prompt and provide
input.  The line would be non-empty, so reg 9 = 1, we run the line,
and then run CMD again.  If ZEX was not running, the user could either
input his own line (to be run immediately) or strike return (to enter
WS).

       Of course, in the above example, WS could be anything, such as
DBASE II, if the user desired [this capability has no limitation that
I can see].

       Note that all of this was done without any modification to the
ZCPR3 CP (as would have to have been done with ZCPR2).  All that was
needed were two little 1K COM files.  The concept of CMD is still
firming up, but this is the idea (altho I may use the ERROR message
instead of Reg 9 - have not decided yet).  SHSET and CMD are basically
running now.  Also note that standard ZCPR3 shells, such as VFILER,
already allow ZEX to run on top of them and the user can issue any
command line he wishes from within the shell (VFILER has a Z command
which prompts for command line, stores it, and exits, allowing the
command to run and VFILER be reinvoked when the ZCPR3 CP decides to
run the shell at the top of the shell stack).

       Comments?

               Rick