Setting Wifi Raspberry Pi with the Command Line

Setting up WiFi in Occidentalis, is also pretty straight forward. You just need to add the name of your wireless network (its SSID) and your password to a configuration file.
Step 1.
Boot the Raspberry Pi without the WiFi adapter plugged in.
Step 2.
Open a Terminal session by clicking on the LXTerminal icon, and enter the following command into it:
  1. sudo nano /etc/network/interfaces
learn_raspberry_pi_occ_1.png
  1. auto lo
  2.  
  3. iface lo inet loopback
  4. iface eth0 inet dhcp
  5.  
  6. allow-hotplug wlan0
  7. auto wlan0
  8.  
  9.  
  10. iface wlan0 inet dhcp
  11. wpa-ssid "ssid"
  12. wpa-psk "password"
If you are using a 'hidden' SSID, try the following (hat-tip to http://www.dafinga.net/2013/01/how-to-setup-raspberry-pi-with-hidden.html)
  1. auto lo
  2.  
  3. iface lo inet loopback
  4. iface eth0 inet dhcp
  5.  
  6. auto wlan0
  7. allow-hotplug wlan0
  8. iface wlan0 inet dhcp
  9. wpa-scan-ssid 1
  10. wpa-ap-scan 1
  11. wpa-key-mgmt WPA-PSK
  12. wpa-proto RSN WPA
  13. wpa-pairwise CCMP TKIP
  14. wpa-group CCMP TKIP
  15. wpa-ssid "My Secret SSID"
  16. wpa-psk "My SSID PSK"
  17.  
  18. iface default inet dhcp
Step 3.
This opens an editor screen of the wifi configuration file you need to change.
learn_raspberry_pi_occ_2.png
The two places where you need to make a change are on the last two lines. Change the file so that it looks like this:
learn_raspberry_pi_occ_3.png
Of course, you should put in your network and password! Note that you need to keep the double-quote characters around your wireless network name and password.
This kind of editor does not let you use the mouse. Instead, use the cursor keys to move around the file.

Step 4.
When you have finished press [ctrl]x. This will ask if you want to save the modified files.
learn_raspberry_pi_occ_4.png
Press 'Y' and then Return to save the file with the same name.

Step 5.
Shut down your Raspberry Pi, plug the WiFi adapter in and start it up again. You should find that the Raspberry Pi connects using the WiFi adapter as it boots up.

Source : https://learn.adafruit.com/adafruits-raspberry-pi-lesson-3-network-setup/setting-up-wifi-with-occidentalis