Is UNIX(TM) Multi-User?

[Thomas Mitchell]

>| | is unix REALLY a multi-user operating system? ...
     because it is a multi-tasking o/s?

Yes, not because it is multi-tasking but because of ownership.

>|   Any multitasking o/s can be a multiuser system, provided that the user
>|   agent (shell or whatever) is a normal process.

No must have ownership or all tasks look the same.  Without
ownership it is only multi-tasking.

Well -- multitasking and multiuser.  Each person has a differing
concept about multi everything (user, tasking, processor).

In a multiuser OS there should be the concept of ownership.
However, the hooks of ownership need not be available to the user.  
A multiuser OS also need not be multitasking.  As I thought about
all the permutations it became useful to write them down.

Start with a single user OS. (loader)

Then a multitasking single user OS. (multi-loader)

Then move to a virtual operating system which permits any user to
believe that he is the only user on the machine.  The OS imposes
virtual (solid) walls between one user and another.  If this is a
multi user environment we may have multiple users on different
terminals.  No user can see, feel or in any way learn about the
other.  When one user is 'idle' the OS places an idle loop in his
time slice so even a stop watch cannot see the other user.

The simplest of this type of virtual environment would permit the
user a single task.

Up scale from this each user could have the option of multiple
processes (tasks).

Down scale,  only one of many could be active at any time. Say
the machine has one console.  When I sit down and login  there is
no clue that you were there yesterday.

Then begin to provide tools to share resources.  
	Visible ownership advisory
	Visible ownership enforced
	Invisible ownership enforced
	Resource locking advisory
	Resource locking enforced
	Quotas (processor, disk, I/O, printer paper)
	Communication 
	Services -- virtual users (lpr)

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Unix is not the OS of the future.  It is the environment the
OS of the future will be written in.

-- 
Thomas P. Mitchell (mitch at stride1.Stride.COM)
Phone: (702)322-6868	TWX: 910-395-6073	FAX: (702)322-7975
MicroSage Computer Systems Inc.
Opinions expressed are probably mine.