Learn GRUB2 (/etc/grub.d/*)

1. update-grub  . The complete command  is :  sudo grub-mkconfig -o /boot/grub/grub.cfg.

2. conf file : /boot/grub/grub.cfg, each section is clearly delineated with "(### BEGIN)" and references the script in the /etc/grub.d directory from which the information was generated.

3. /etc/grub.d/* (note: all the files in this directory must be executable)

     00_header Sets initial appearance items such as thegraphics mode,default selection,timeout, etc. These settings are normally importedfrom information from /etc/default/grub. Users normally do not need to make changes to this file.

     05_debian_theme The settings in this file set the GRUB 2background image,text colors, selection highlighting andthemes. In the absence of a splash image, this file sets a monochromatic theme for the initial menu display.

     10_linux Identifies kernels on theroot device for the operating system in use and creates menu entries.

     20_memtest86+ Searches for /boot/memtest86+.bin and includes it as an option on the GRUB 2 boot menu. 

     30_os-prober This script uses os-prober tosearch for Linux and other operating systems and place the results in the GRUB 2 menu.

     40_custom A template foradding custom menu entries which will be inserted into grub.cfg upon execution of theupdate-grub command. The contents of this file, below the "exec tail -n +3 $0" line and the default comments, are imported directly into /boot/grub/grub.cfg without any changes.