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Kernel and shells

From: Unix for Mac OS X Users

Video: Kernel and shells

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.

Kernel and shells

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.

Show transcript

This video is part of

Image for Unix for Mac OS X Users
Unix for Mac OS X Users

82 video lessons · 25593 viewers

Kevin Skoglund
Author

 
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  1. 3m 57s
    1. Introduction
      1m 14s
    2. Using the exercise files
      2m 43s
  2. 32m 2s
    1. What is Unix?
      7m 27s
    2. The terminal application
      4m 23s
    3. Logging in and using the command prompt
      5m 19s
    4. Command structure
      5m 22s
    5. Kernel and shells
      5m 25s
    6. Unix manual pages
      4m 6s
  3. 15m 58s
    1. The working directory
      2m 49s
    2. Listing files and directories
      3m 59s
    3. Moving around the filesystem
      4m 58s
    4. Filesystem organization
      4m 12s
  4. 1h 4m
    1. Naming files
      5m 41s
    2. Creating files
      2m 19s
    3. Unix text editors
      6m 39s
    4. Reading files
      5m 35s
    5. Reading portions of files
      3m 27s
    6. Creating directories
      2m 40s
    7. Moving and renaming files and directories
      8m 32s
    8. Copying files and directories
      3m 7s
    9. Deleting files and directories
      3m 38s
    10. Finder aliases in Unix
      4m 10s
    11. Hard links
      5m 30s
    12. Symbolic links
      6m 36s
    13. Searching for files and directories
      6m 32s
  5. 34m 58s
    1. Who am I?
      4m 3s
    2. Unix groups
      1m 52s
    3. File and directory ownership
      6m 41s
    4. File and directory permissions
      4m 27s
    5. Setting permissions using alpha notation
      6m 49s
    6. Setting permissions using octal notation
      3m 49s
    7. The root user
      1m 57s
    8. sudo and sudoers
      5m 20s
  6. 52m 34s
    1. Command basics
      4m 4s
    2. The PATH variable
      4m 13s
    3. System information commands
      3m 40s
    4. Disk information commands
      6m 8s
    5. Viewing processes
      5m 0s
    6. Monitoring processes
      3m 36s
    7. Stopping processes
      3m 19s
    8. Text file helpers
      6m 50s
    9. Utility programs
      7m 28s
    10. Using the command history
      8m 16s
  7. 20m 39s
    1. Standard input and standard output
      1m 24s
    2. Directing output to a file
      4m 13s
    3. Appending to a file
      2m 44s
    4. Directing input from a file
      5m 28s
    5. Piping output to input
      4m 40s
    6. Suppressing output
      2m 10s
  8. 41m 28s
    1. Profile, login, and resource files
      9m 11s
    2. Setting command aliases
      6m 59s
    3. Setting and exporting environment variables
      4m 54s
    4. Setting the PATH variable
      6m 10s
    5. Configuring history with variables
      6m 17s
    6. Customizing the command prompt
      6m 5s
    7. Logout file
      1m 52s
  9. 1h 25m
    1. grep: Searching for matching expressions
      5m 21s
    2. grep: Multiple files, other input
      4m 28s
    3. grep: Coloring matched text
      2m 57s
    4. Introduction to regular expressions
      3m 22s
    5. Regular expressions: Basic syntax
      3m 19s
    6. Using regular expressions with grep
      5m 20s
    7. tr: Translating characters
      8m 17s
    8. tr: Deleting and squeezing characters
      5m 30s
    9. sed: Stream editor
      7m 45s
    10. sed: Regular expressions and back-references
      7m 8s
    11. cut: Cutting select text portions
      7m 42s
    12. diff: Comparing files
      4m 35s
    13. diff: Alternative formats
      4m 30s
    14. xargs: Passing argument lists to commands
      7m 25s
    15. xargs: Usage examples
      7m 59s
  10. 42m 25s
    1. Finder integration
      4m 45s
    2. Clipboard integration
      5m 5s
    3. Screen capture
      3m 42s
    4. Shut down, reboot, and sleep
      3m 34s
    5. Text to speech
      2m 36s
    6. Spotlight integration: Searching metadata
      3m 41s
    7. Spotlight integration: Metadata attributes
      4m 24s
    8. Using AppleScript
      5m 23s
    9. System configurations: Viewing and setting
      5m 51s
    10. System configurations: Examples
      3m 24s
  11. 1m 26s
    1. Conclusion
      1m 26s

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