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Before we dive in to actually using Unix, there's one last fundamental concept that I want to make sure that you understand, because we're going to be using it a few times later on and that is the difference between the kernel and the shells. So the kernel is the core of the operating system in Unix. It's what takes care of allocating time and memory to program, really doing sort of very fundamental root level management of how programs go about doing their thing. Mac OS X uses the Mach kernel inside Darwin to do that. So if you ever hear someone refer to Mach kernel, that's what they are referring to, is this very central part of the Unix operating system that really just takes care of how the operating system handles very basic tasks like memory management.
Then outside of that there is the shell. The shell is the outer layer of the operating system. That's what we see when we open up a Terminal window. We're working in the shell. It interacts with the user and we can think of it as our working environment. The shell will send requests to the kernel. The kernel will then do its thing and results will then be returned back to the shell so that we can interact with them again. Mac OS X by default uses what's called the Bash shell, B-A-S-H, but it includes other choices as well. Let's talk about what some of those other choices are, because it also gives you a little bit more of the history of Unix as well.
The popular shells are sh, which is the very first one. It's called the Thompson Shell and it was created in 1971 and that was the main working environment that people had for a long time in Unix. In 1977 something called the Bourne Shell was created and that was a replacement for the Thompson Shell. So it completely replaced it. It has the same name, sh, short for shell, and it just replaced the old one. Starting though in 1979, people said "Hey, instead of replacing shells all the time, let's get people choices. Let's have different shells and then if someone loves one shell or another, they can just switch between them." They started having different names at that point.
We have csh for C Shell, a little bit of programmer humor there. Then we have the Tabbed C Shell in 1979, tcsh, the Korn Shell, ksh, then we have the Bourne-Again Shell, again another bit of programmer humor. Take off the Bourne Shell, this is the Bourne-Again Shell, which is bash, and that's the one that we have by default. And then the Z Shell. It's one of the most recent. There are a lot of others. They almost all end in sh. You can go some place like Wikipedia to get a full list of all of those, but these are the main ones and these are all installed on Mac OS X. So we have the choice.
These are all there waiting for us to use. Actually I think that back. Thompson Shell has gone. It's been completely replaced by the Bourne Shell, but the other ones are all there on the Mac for us to choose from. For beginners, the difference between these shells is tiny. A lot of the difference between them is going to be the features that are available to high-end users for task like advanced shell scripting. For now, stick with bash to learn. You can always switch later, but I want you to understand that you are working in the bash shell when you log in. Now you may be tempted to think that a shell is just like an operating system, because we can pick between these different ones, that it somehow like the difference between Windows and Mac or something. It's not.
It's just a working environment that we can choose and move between and in fact we can move from inside one shell into another shell without leaving the old one. We can nest them inside each other. It's just like moving into another working layer. Let's go to the Terminal and take a closer look. So here I am in my Terminal, and you see up at the top here it actually tells me that I'm in the Bash Shell. It's right there at the top. That's a nice giveaway. I can also find out what the login shell, the one that is going to put me in by default is, with echo $SHELL.
That's like I can know it is an environment variable. It has the $ in all capitals. And it says "Oh, bin and bash. That's what I am going to start you up in when I first launch." That's actually changeable here in Terminal > Preferences, under Startup. Shells open with the default login shell or we can pick something else. So if we want to launch it with a different shell, we can do that there. Now again, that's the login shell that we're seeing. If we want to see the shell that we're in right now, it's echo $0. That's the shell that we are working in right now. Not the login shell, the shell that's going to be run when we launch this, but the shell that we're in.
Why that's important is because we can go inside other shells. Let's take an example. Let's go into the Tabbed C Shell, tcsh. Just type it at the command line and boom! We changed shells. Now just to show you the difference, let's do echo $SHELL and you'll see that we still get bash. If we do echo $0, now you can see that we are inside the Tabbed C Shell. Just to show you again, let's try a few other. Let's go bash. Now I am inside bash. Let's do csh. Now I am inside the C Shell. sh, now I am inside the Bourne Shell.
Now to get back out of each of these, you just type exit and you can see that the prompt changes slightly each time as I back out, until finally I get back to my original prompt and now I know that I am back at the Bash Shell. So as you can see, you can move between these. You are just moving into a new working layer, a new working environment. You won't need to do this; you can just be in bash all the time. But I just want you to know that that's where you are, that you are inside bash, and that as an advanced user you do have these other options. So if you hear people talking about, "Well, I know how to do this in the Z shell, but I don't know how to do it in bash," that's what they are talking about.
They are talking about these different working environments.
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