How to restart Samba

sudo killall -HUP smbd

This will only send the HUP signal to all running smbd processes. The processes will trap that signal and do whatever their signal handlers tell them to do (I think a HUP will make them reread their configuration files). You will not get any output when that command completes successfully, so what you're seeing is expected.

Hello, how can I restart Samba via the Terminal under OS X 10.5.4?

smbd is managed by launchd, which is the daemon responsible for managing user and system processes. This piece of infrastructure is what runs programs on demand, for example, and is ultimately the parent process of everything in the system.

The launchctl command is how you interface with launchd. There's a man page that describes how to use launchctl, but in general you could do something like this to stop/start smbd:

sudo launchctl stop org.samba.smbd

sudo launchctl start org.samba.smbd