How to add a new boot item to the Bootloader in Linux?

To add a new boot item to the bootloader in Linux, you typically use the grub-mkconfig command or manually edit the GRUB configuration files. Here’s a step-by-step guide:

Identify the Bootloader: Most Linux distributions use GRUB (GRand Unified Bootloader) as the default bootloader.
Locate GRUB Configuration Files:
- For GRUB 2, the main configuration file is usually located at /etc/default/grub.
- The menu entries are typically in /boot/grub/grub.cfg or generated from scripts in /etc/grub.d/.
Edit GRUB Configuration:
- Open the /etc/default/grub file with a text editor to modify default settings if needed.
- To add a new boot item manually, you can add an entry to /etc/grub.d/40_custom. For example, to add an entry for a Linux kernel, you might add:
```
menuentry "My Custom Linux" {
    set root=(hd0,gpt2)
    linux /boot/vmlinuz-mykernel root=/dev/sda2
    initrd /boot/initrd.img-mykernel
}
```
- Replace (hd0,gpt2), /boot/vmlinuz-mykernel, and /boot/initrd.img-mykernel with the appropriate values for your system.
Update GRUB:
- After making changes, update GRUB to regenerate the configuration file:
```
sudo grub-mkconfig -o /boot/grub/grub.cfg
```
- This command reads the scripts in /etc/grub.d/ and the /etc/default/grub file to create a new /boot/grub/grub.cfg.
Reboot and Select the New Entry:
- Reboot your system.
- During the boot process, press Shift (or Esc depending on your system) to enter the GRUB menu and select your new boot item.

Example: Suppose you have a custom Linux kernel compiled for a specific project. You would place this kernel and its initrd image in /boot, then add an entry in /etc/grub.d/40_custom as shown above, specifying the correct paths and partitions.

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