In GRUB, the default boot menu entry was determined by the order of entries in /boot/grub/menu.lst, the default one being the n-th specified by the default=n parameter. In GRUB2 the main configuration file /boot/grub2/grub.cfg isn’t usually altered manually any more but automatically generated by invoking grub2-mkconfig. You can change the default boot target by changing the GRUB_DEFAULT paramater in /etc/default/grub…. Read more »
Getting started with ngspice can be quite tricky. It’s a very powerful piece of software and although the user manual is quite comprehensive, its complexity can easily scare off beginners. Here’s a simple step-by-step tutorial on how to simulate a very simple electronic circuit. It should enable you to run your own simulations of circuits you designed.