Scan available wifi access points on Ubuntu

Turn on the interface and scan all available WiFi networks

sudo ifconfig wlan0 up
sudo iwlist wlan0 scan

Anther way to turn up and off wifi interface (ifdown will release the existing IP address to the DHCP server; ifup will get another IP address from the DHCP server)

sudo ifdown wlan0
sudo ifup wlan0

A few other useful command

wpa_cli status