android fastboot error: waiting for device

I'm going to unlock my galaxy nexus today but when I enter the command in bash:
$fastboot oem unlock
The result is
< waiting for device>

The solution is quite simple but costed me several hours to find it:
use the command under android-sdk/platform-tools/:
sudo ./fastboot oem unlock

Hope it helpful!

Also, share a very useful link about how to get rid of red triangle Dead Android on Recovery Galaxy Nexuse: