The Tilde (~) in Unix and URLs

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UNIX

In the Unix shell, the tilde (~) is an abbreviation for the path to your home directory. (It has other expansions but this the meaning when the tilde is used as a separate word.)

Tilde expansion is the process of converting the abbreviation to the path to the user's home directory.

Example (not using ~)

/home4/susanQ/public_html/111/ is the Unix path to Susan Queue's 111 folder on shell.uoregon.edu.

Example (using ~)

~/public_html/111/ is the Unix path to Suzie's folder on shell.uoregon.edu

Example

$ pwd
/tmp

$ cd ~

$ pwd
/home4/susanQ/

$ echo $HOME
/home4/susanQ/

(~ is a synonym for the value of the shell's HOME variable)

URLs

In a URL, the tilde (~) has a related but different meaning: it stands for the path to your public_html folder on the web server.

Example: 

URL:
http://uoregon.edu/~susanQ/

Corresponding Unix path on the server:
/home4/susanQ/public_html/

Example: 

URL:
http://uoregon.edu/~susanQ/111/
Corresponding Unix path on the server:
/home4/susanQ/public_html/111/

1-Armed Bandits && Demorgan's Law

When simulating the spin of a three-wheel slot machine, how do we detect three of a kind?





The following condition recognizes three of a kind:

var s1 = 'lemon', s2 = 'lemon', s3 = 'lemon';
console.log((s1 == s2) && (s1 == s3) && (s2 == s3));

We can modify it to control a loop that spins the wheels:

       s1 = RandomOneOf(['lemon', 'cherry', 'bar', 'donut']);
       s2 = RandomOneOf(['lemon', 'cherry', 'bar', 'donut']);
       s3 = RandomOneOf(['lemon', 'cherry', 'bar', 'donut']);

       while (!((s1 == s2) && (s1 == s3) && (s2 == s3))) {
           //spin again...
       }

Applying Demorgan's Law we get:

       while (!(s1 == s2) || !(s1 == s3) || !(s2 == s3)) {
           //spin again...
       }

Equivalently:

       while ((s1 != s2) || (s1 != s3) || (s2 != s3)) {
           //spin again...
       }

To hit the jackpot, we must spin the wheels at least once. This leads to a final, simpler version using a do-while:

do {
       s1 = RandomOneOf(['lemon', 'cherry', 'bar', 'donut']);
       s2 = RandomOneOf(['lemon', 'cherry', 'bar', 'donut']);
       s3 = RandomOneOf(['lemon', 'cherry', 'bar', 'donut']);
} while ((s1 != s2) || (s1 != s3) || (s2 != s3));

The do...while loop will always be executed at least once, even if the condition is false, because the statements are executed before the condition is tested.

It is a post-test loop; the while loop and the for loop are pre-test loops.

=> Use a do-while when an action must be performed at least once.

Web Apps w/o Programming: If this, then that (IFTTT)

Image by alex drennan via Flickr
IFTTT. Put the internet to work for you by creating tasks that fit this simple structure:
if this then that.

 A Web Tool That Lets You Automate the Internet. (NYT)

7 Ways to Automate Your Life With ifttt. (NYT)

CHART OF THE DAY: Why The Newspaper Industry Collapsed

SSH and the Unix Shell's Erase Key

SSH sometime uses the Delete key to erase characters, but I prefer to use the Backspace key. How do I set it up?

You have several options, as follows:

• Windows: Start SSH, select Edit > Settings > keyboard, then select Backspace sends Delete (or whichever key you want to use), then click OK.
  
  
• OS X: Start Applications > Utilities > Terminal, the select Terminal > Window Settings..., then select Keyboardin the drop-down list, then check Delete key sends backspace.
  
  
• Set your Unix shell's Erase key--
  
  Use Pico or Emacs to edit your login shell's startup file, .bash_profile:
  
  $ pico .bash_profile
  
  Add this line to the .bash_profile:
  
  stty erase '^h'
  
  save the file and exit pico.

Customizing your Unix Command Line Prompt

Image by p373 via FlickrHow can I get the path for the current directory displayed in my Unix prompt?

Read Generic UNIX Interactive Prompts