Parts of a Computer

• Input (keyboard, mouse, network)
• Storage (disk drive aka filesystem)
• Memory (RAM)
• Processor (CPU)
• Output (screen, sound, network)

The Command Line

• the terminal is a window into which you can talk directly to your computer
  • aka console or command line or command prompt or shell

from WarGames, (1983)

• when you type into the terminal, you are issuing commands to the computer
• a CLI (Command Line Interface) is different from the GUI (Graphical User Interface) you are used to
  • a CLI is more primitive and more powerful than a GUI

Historical Terminal

Computers used to not have screens! They were connected to devices like this:

The Terminal app is a direct descendant of a TeleType printer or TTY.

When you type into the console and hit Enter you are pretending to type a line onto a TTY; the scrolling terminal is like a roll of printer paper.

See this twitter thread for more history and TTY pix.

picture of Teletype Corporation ASR-33 on display at the Computer History Museum by ArnoldReinhold [CC BY-SA 3.0] via Wikimedia

Lab: Opening the Terminal

• to open your Terminal:

  • Mac OS: launch the "Terminal" application
  • Windows: launch the "Cmder" application (pronounced "Commander"), make a new console (Ctrl-T) and select
  • Startup command: {bash::bash}
  • Startup directory: C:\Users\yourname
  • Windows (alternate): Launch the Windows Command Prompt (cmd.exe or PowerShell) -- but beware, the commands are slightly different than in bash

• Important: make your terminal as tall as possible and don't overlap windows

  • when reading a program's output you want to start reading at the top...
  • ...and if your window is too short then the top lines will scroll away and you will miss them
  • Make it look like this:

Lab: Interactive Calculator

1. open a terminal
2. look at the prompt -- it should end with a $ or > symbol
3. type node -- that's you commanding the computer to launch node
4. press the Return key (also called Enter)
5. see the > prompt
6. type 1 + 1
7. press the Return key again
8. see the 2

• Yay! Your computer is an expensive calculator!
• Bonus: what other math can you do?

Node is a JavaScript Engine

An "engine" is a type of program that either executes or empowers other programs.

NodeJS (aka node) is an engine that runs JavaScript programs -- either from files, or interactively from the command line.

A Tale of Two Prompts

WARNING: Before you start typing, look at the prompt!

• the shell is the command line that the terminal starts with
  • its prompt usually looks like this:
  • Davids-Macbook-Pro:~ David$ (Mac)
  • david@davidspc:~$ (Ubuntu Linux)
  • C:\Users\david> (Windows)
• node is a command line program that is launched from the shell
  • its prompt is usually >

From inside node, if you want to get back to the shell...

• type Ctrl-C twice
• or type .exit and Enter

When in doubt, try it out!

From now on, whenever you see text in the code font, try typing it into the terminal and see what happens! For example:

'pod' + 'cast'

If that doesn't print 'podcast', look at the prompt; you may be inside your shell instead of inside node.

Directories

• a directory is a location on your hard disk
  • also called a folder
• directories can contain files
• directories can also contain other directories (called subdirectories)

Where am I?

• Inside the Terminal, you are always "inside" a directory.
• It is very easy to get lost in a maze of directories.
• To find out which directory you are in, type: pwdReturn
  • This stands for "print working directory" (not "password").
  • Most of the time you can also look at the prompt to see what the current directory is.

Home Directory

• when you first open the Terminal you are in your HOME DIRECTORY
• if you store files directly in your home directory, it will soon get cluttered
• for this class, we recommend:
  • create a code directory inside your home directory
  • create a new directory inside code for each lesson or project

WARNING: On some windows systems, Command Prompt will open to C:\Windows\System32. You can get back to your home directory by typing cd %HOME%

Listing Directory Contents

• when you type ls ("list") it shows the contents of the current directory

  • On Windows you may need to type dir instead

• if you type ls -al ("list all long") it also shows hidden files and extra info like the modification date

  • On Windows you may need to type dir /A:SH instead

Lab: Home Decor

1. Using your GUI desktop, navigate to your home directory and open it in a desktop window.
   (Your desktop is called "Finder" in MacOS, or "Explorer" in Windows)
2. Using your terminal, list your home directory's contents.
3. Look carefully at them both. What's the same? What's different?

Making a directory

• when you type mkdir ("make directory") it creates a new subdirectory inside the current directory

mkdir code

Changing directories

• cd ("change dir") moves you into a different directory
• For example, cd code would move you into a subdirectory named code
• If you ever get lost, type cd all on its own and press the return key. This will send you back to your home directory.
  • (unix shell only, not Windows)

Basic Command Review (Unix)

• pwd ("print working dir") -- shows the name of the current directory
• ls ("list") -- shows the contents of the current directory
• mkdir ("make dir") -- creates a new subdirectory inside the current directory
• cd ("change dir") -- move into a different directory

These apply to Mac / Unix / Linux / bash

Basic Command Review (DOS)

• cd ("change dir") -- With no directory, it lists the current directory. Otherwise, it changes to the specified directory
• dir ("directory") -- shows the contents of the current directory
• mkdir ("make dir") -- creates a new subdirectory inside the current directory

These apply to Windows / DOS / PowerShell

LAB: make a subdirectory and then enter it

1. open Terminal or Command Prompt
2. Confirm that you are in your home directory
3. make a new subdirectory using mkdir code
4. change into that directory using cd code
5. make sure you're really there using pwd
   • On Windows use cd
6. list its contents using ls (and note that it's empty)
   • On Windows use dir

Files

• a file is a place on disk for storing stuff
• "stuff" here could be anything at all
  • documents, pictures, sounds, applications...
• every file lives inside a directory

Text Editor

• a text editor is a program that edits a text file
• a text editor is like a word processor
• but a text editor is not a word processor
• You probably have VS Code https://code.visualstudio.com/
  • others include TextMate, Notepad++, Sublime Text, Vim, Emacs, Atom
  • but NOT TextEdit or Wordpad or Microsoft Word

Source File

• source code is the essence of a program
• source files are text files that contain source code
• to RUN a JavaScript program you type node and then the name of the source file, like this:

$ node hello.js
Hello, World!

• The Recipe Metaphor
  • source file ≈ recipe
  • running a program ≈ cooking

LAB: Hello, World

1. Make sure you are in your code subdirectory using pwd or cd
2. Open this directory in your text editor
   • for VSCode, use code . ("code dot")
3. Create a file named hello.js using the File > New menu

4. Inside this file, put the following source code:

   console.log("Hello, World!");
   

5. Save the file

6. Switch back to the terminal (using Alt-Tab or Cmd-Tab or clicking)
   (If you are using VS Code, you can click Terminal → New Terminal for the built-in terminal panel)

7. Run this file using node hello.js

What happens? Is this what you expected?

LAB: Countdown

1. Inside your code directory, create a file named countdown.js

2. Inside this file, put the following source code:

   let count = 10;
   
   while (count > 0) {
     console.log(count + '...');
     count = count - 1;
   }
   
   console.log('Blastoff!');
   

3. Save the file

4. In your terminal, run node countdown.js

Analyzing Countdown

Consider this a sneak peek; we will cover all of these concepts in later lessons

Command-Line Shortcuts

These work in bash:

Also:

• you can use Esc instead of Alt for the above
• Esc-Backspace (delete previous word)
• up/down arrow (scroll through history)
• End and Home (jump to end or beginning of line)
• Tab for auto-completion of filenames (e.g. typing node hTab will emit node hello.js)

(image source: Clément Chastagnol)

LAB: Next Steps

Want to learn enough command line to be dangerous? Check out https://www.learnenough.com/command-line-tutorial by Michael Hartl (founder of Tau Day and all around solid geek).

Want to be a command-line hacker like in War Games? Play this game: http://overthewire.org/wargames/bandit/ where you use your real command line ssh tool to connect to sandboxed hosts and infiltrate them.

Want to learn the history of software user interfaces and operating systems? Read In The Beginning Was The Command Line by Neal Stephenson