Ubuntu 14.04 Server - WiFi WPA2 Personal

I suggest you set up /etc/network/interfaces something like:

auto lo
iface lo inet loopback

auto wlan0
iface wlan0 inet static
address 192.168.1.150
netmask 255.255.255.0
gateway 192.168.1.1
wpa-ssid <your_router>
wpa-psk <your_wpa_key>
dns-nameservers 8.8.8.8 192.168.1.1

Be sure to select a static address outside the range used by the DHCP server in the router, switch or other access point. Of course, substitute your details here.

Get the system to read and use the changes:

sudo ifdown wlan0 && sudo ifup -v wlan0

Did you connect?

ping -c3 192.168.1.1
ping -c3 www.google.com

I managed to connect to my WPA2 access point by putting the following in /etc/network/interfaces. Slightly modified from the accepted answer, and using DHCP.

auto wlan0
iface wlan0 inet dhcp
wpa-ssid <your_router>
wpa-psk <your_wpa_key>

Then a simple sudo ifup -v wlan0 and it connected. All good.

Using either DHCP or a static config (doesn't matter which)--AND assuming your wifi worked during install--make your /etc/network/interfaces look something like below (for wlan0 should match the name of your wifi card listed under ifconfig -a e.g. your detected wifi card could be nicknamed eth1 by the OS for all I know.):

 auto lo iface lo inet loopback     
 auto wlan0 iface wlan0 inet dhcp    
 wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

To configure wpa_supplicant use the command (Referenced in the config above)

wpa_passphrase "YOUR_SSID" SSID_PASSWORD | sudo tee /etc/wpa_supplicant/wpa_supplicant.conf

Next, create a new executable script named iwconfig (you can name this script anything really, "iwconfig-default-ssid", perhaps?--I just made it short for the example):

sudo touch /etc/network/if-up.d/iwconfig && sudo chmod 700
/etc/network/if-up.d/iwconfig && sudo ln -s
/etc/network/if-up.d/iwconfig /etc/network/if-pre-up.d/iwconfig

Now edit /etc/network/if-up.d/iwconfig and add the SSID you want Ubuntu Server to connect to on startup:

#!/bin/sh
iwconfig wlan0 essid "YOUR_DEFAULT_SSID" mode managed

Now bring ifdown (if you haven't already), then ifup, and you should be golden now and when you reboot (as long as you're near your SSID.)

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If you're out in public with your laptop with this config, you'll have to use: iwlist wlan0 scan, then sudo iwconfig essid "PUBLIC_ESSID" mode managed to connect with anything (and/or make a unique script for each place(s) you visit--just don't put any of these scripts under the 'if-up.rc.d' folder. /etc/network/interfaces can also handle location alias, so check the man/forums for help on this.)

Or you can try your luck with the CLI frontend for wicd when roaming about town:

sudo apt-get install wicd-curses