Hi Jing, Busybox is a standalone statically linked bash env. You can download the source from here: https://busybox.net/downloads/
Or you can just download and copy it to your /usr/sbin : https://busybox.net/downloads/binaries/ The script is creating a runtime filesystem for your guest. Its linking all the standard shell commands to busybox, so in the guest runtime those are executed by busybox. Arindam
