mjhjmtu

Option 1

Just edit /etc/network/interfaces and write:

auto wlan0
iface wlan0 inet dhcp 
 wpa-ssid ssid
 wpa-psk password

After that write and close file and use command:

sudo dhclient wlan0

Replace ssid and password with your respective WiFi SSID and password.

Option 2

Provided you replace your Wireless network card, Wi-Fi Network name, and Wi-FI Password this should also work.

I am using:
- Wireless network card is wlan0
- Wireless network is "Wifi2Home"
- Wireless network key is ASCII code ABCDE12345

First, get your WiFi card up and running:

sudo ifconfig wlan0 up

Now scan for a list of WiFi networks in range:

sudo iwlist wlan0 scan

This will show you a list of wireless networks, pick yours from the list:

sudo iwconfig wlan0 essid Wifi2Home key s:ABCDE12345

To obtain the IP address, now request it with the Dynamic Host Client:

sudo dhclient wlan0

You should then be connected to the WiFi network. The first option is better, because it will be able to run as a cron job to start up the wifi whenever you need it going. If you need to turn off your WiFi for whatever reason, just type:

sudo ifconfig wlan0 down

FYI

I have also seen people using alternative commands. I use Debian, Solaris and OSX, so I'm not 100% sure if they are the same on Ubuntu. But here they are:

sudo ifup wlan0 is the same as sudo ifconfig wlan0 up
sudo ifdown wlan0 is the same as sudo ifconfig wlan down

There is Danijel J's two options are good, but there is also a 3rd option if you have this working via the 'standard Ubuntu tool' using nmcli, which should already be installed at /usr/bin/nmcli.

First, run

nmcli c

This will list your connections, with the first column being the SSID, and the second column being the UUID of the connection.

Copy the UUID of the SSID you want to connect to so you can paste it into the next command.

Next, run

nmcli c up uuid <paste uuid here>

and this will, using the same stuff as the 'standard Ubuntu tool' connect to your wifi!

If you have nmcli installed, I think this is the simplest solution.

For a new connection:

nmcli dev wifi connect <mySSID> password <myPassword>

Or if a connection was already set up:

nmcli con up <mySSID>

(or if that does not work, try nmcli con up id <mySSID>)

Install wpa_supplicant and you have a nice software for all of your Wireless needs.
You can then use wpa_cli command to access and set your network interactively.
there is also a number of 3rd party software available acting as a GTK+ or QT GUI for wpa_supplicant if you want to go gui at anytime.

Also you could set the connection configuration in /etc/wpa_supplicant.conf or /etc/wpa_supplicant/wpa_supplicant.conf depend on you linux OS.
for more information on interactive command type h inside wpa_cli.

Use nmtui if you are just looking to connect.

You'll be prompted to activate a connection, with the list of SSIDs and you can just enter your password.

Fixed issue.

Laptop with no WiFi switch. No bios always on or disable switch etc.

Keyboard had FN + F2 which wouldn't recognize

I attempted to bring up the interface got the

Not permitted due to RF-KILL blah blah

Installed rfkill

Sudo apt-get install rfkill

Did rfkill list , like othersmentioned and I saw in otherforums

The saw a soft block on the wriless cards
Did
Rfkill unblock (index #) of device

once that was done my WiFi led turned on.

Then did the "ip Link set dev xx up" or the "ifconfig xxx up" and the ifup or w.e to get the interface to be up not just the hardware for ip address and boom. Wifi
May want to edit etc/xxx/xxx/interface file to dhcp or statis it and onboot yes or auto . El/deb devices do the config of the interfaces differently ![enter image description
http://i66.tinypic.com/104s37t.jpg

Besides from above answers, you can also use wifi-menu on Arch Linux. It will show a CLI-Based GUI and you can choose a WiFi from WiFi list that is shown. wifi-menu depends on dialog though. You must have it installed first.

I know that this was asked 3 years ago. I am answering just in case someone else runs into this situation as I've just did.

The thing is that everything is correct until the dhclient part, where it gets stuck at "DHCPDISCOVER on wlan0..." forever.

Solution:
Go to your network manager (Wicd, Network Manager, whatever) and unmark the "Enable Networking" option. It seems that it interferes with the dhclient's normal functioning.

And dhclient worked like a charm.