How to setup wlan via command line in linux

Debian and other distros have wpa_suplicant running as a service by default in order to manage the wifi networks. wpa_suplicant can be handled by different clients/front-ends such as the network manager GUI. This is better explained in this debian wiki.

wpa_cli is the command line wpa_suplicant client to manage the wifi networks.

Edit: I have just found this post explaining how to use nmcli and it is much better than wpa_cli since it is compatible with the GUI Network Manager and their settings and saved wifi networks.

Example of use of wpa_cli:

Check that I already have a wifi enabled network interface:

# iwconfig
wlan0     IEEE 802.11bgn  ESSID:off/any  
          Mode:Managed  Access Point: Not-Associated   Tx-Power=22 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Encryption key:off
          Power Management:on

Check if wpa_suplicant process is running:

# ps -e | grep wpa
1881 ?        00:00:07 wpa_supplicant

Enter in wpa client interactive mode:

# wpa_cli

List available access points:

> scan
> scan_results

… and you get something like this:

bssid / frequency / signal level / flags / ssid
e0:60:66:7c:81:7f       2417    -66     [WPA2-PSK-CCMP][ESS]    vodafone817E
e0:60:66:61:83:4b       2452    -76     [WPA2-PSK-CCMP][WPS][ESS]       vodafone834A
f8:8e:85:c5:65:c2       2462    -76     [WPA-PSK-CCMP+TKIP][WPS][ESS]   MOVISTAR_65C1
a8:d3:f7:46:0c:be       2472    -83     [WPA-PSK-CCMP+TKIP][WPA2-PSK-CCMP+TKIP][WPS][ESS]       Orange-0CBC
...

Add your AP:

> add_network
> set_network 0 ssid "vodafone817E"
> set_network 0 psk "my-pass-phrase"

Select it as current:

> enable_network 0

Connect to it:

> reconnect

Check the status:

> status

Exit wpa_cli:

> quit

From the shell, request DHCP for an IP and net settings:

# dhclient -r
# dhclient wlan0