Changes in grub.cfg are ignored

Question

I'm trying to change the default OS in grub, but when I do

sudo gedit /boot/grub/grub.cfg and change it to option 3 instead of 0 and run update-grub, nothing changes. The default selected OS is still the first one and when I look in grub.cfg it's back to 0. What am I doing wrong?

Answer 1

As stated on the top of /boot/grub/grub.cfg:

# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

Edit /etc/default/grub instead.

Here are details on how to configure the grub file; see the other answers there for a GUI-based method.