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Originally Posted by ALon
Now, considering UNIX...
At university there are about 50 machines in the lab. Each is a box, with a label on it stating its hostname e.g abc.def.ac.uk.
So I am assuming that each is a machine in its own right, yes? i.e. if we were to open the box, inside each machine will have its own CPU, own RAM, own motherboard etc. Is this correct?
Yes. They're ordinary computers running some variety of UNIX.
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Now how does the UNIX bit come in? It says that UNIX is supposed to be "multi user, multi tasking, time sharing". Does this simply mean that some other user too can remotely log into that box and use it to do their work?
If the machine is configured to allow this, then yes -- absolutely. The computers at my university had a similar arrangement which I often used for just that, though the university placed careful restrictions on who was allowed to login from where.
May I shift your perspective a little? Try imagining it, not as a "computer" in the Windows sense, but a machine that sits there and runs "jobs". When you run a program, that makes a new job, and you can have many jobs running at the same time belonging to many different users. It keeps careful track of which users are allowed to run which jobs and access which files but beyond that, the fact that you're the one sitting in the chair in front of it is largely incidental. You could remove the keyboard and monitor and just use it completely remotely -- I do for my UNIX machine, which is handy since it's a web server hundreds of kilometers away from me.
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Now for the other question. If I sit at machine 1, and create a file, and then I log out, and log in on machine 5, then that file is visible. Does this mean that when I created that file, it was not created on the harddrive of machine 1, but rather on a big hard drive inside the server room?
Quite probably.
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So, I am very confused here. There seem to be many directory structures, one somewhere in a server room which stores everything, and one on each box where the UNIX O/S is stored.
UNIX does not have drive letters, as you may have noticed. Instead it has one(one per computer!) giant directory tree starting from one "root partition" -- that's going to be the hard drive of the computer you're sitting at -- and attaches or "mounts" more drives to different directories. Usually, the /etc/fstab file tells a UNIX computer which drives to attach where, when. Also try 'df' in a shell to show you what disks are mounted where at that particular moment.
I've seen something like your setup before. How it works is, each individual computer has it's own independent disk space used for system files(stored on it's root partition), so it can boot up independently. But the directories
you use, something like /home/username/, get "mounted" to the big server through the network. When you login, do things, and logout, you're running programs on that particular machine, but all your data and settings gets saved to the server. So no matter which one you login to, you get the same files and settings.