what you entered is not a command, it's a filepath.
what you wanted to enter was echo $HOSTNAME, or simply `hostname`. just because you're root doesn't mean that the tiny world on your usb stick belongs to you. there are methods to access files, and there are file permissions in place that will prevent you from accessing all files, unless you know how to change them properly.
edit: thorin is correct, cat or more will also spill the contents of /etc/hostname
just be sure not to cat or more a binary.