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You may find that you want to change the order of the operating system options or set a default boot option. This requires a change of the Grub2 configuration. You should not directly edit /boot/grub/grub.cfg because when you run 'sudo update-grub' it will overwrite grub.cfg from the contents of /etc/grub.d therefore we will make our edits to /etc/grub.d and then run update-grub.You may find that you want to change the order of the operating system options or set a default boot option. ''technical point: GRUB 2 works like this: /etc/default/grub contains customization; /etc/grub.d/ scripts contain GRUB menu information and operating system boot scripts. When the update-grub command is run, it reads the contents of the grub file and the grub.d scripts and creates the grub.cfg file.''