A bootloader, is a computer program that is responsible for booting a computer. If it also provides an interactive menu with multiple boot choices then it's often called a boot manager. This page shows the basic steps you need to perform, if you for any reason want to switch bootloaders or apply some manual configuration.

The following bootloaders are available in Alpine Linux.

Syslinux is the default light-weight bootloader used in Alpine.
rEFInd is an easy to use EFI boot menu that allows booting different operating systems.
GRUB is a standard linux boot loader.
EFI Boot Stub allows booting linux directly from a motherboard supporting UEFI or another bootloader.
Gummiboot can be used for UEFI Secure Boot.

rEFInd

For (U)EFI systems, the refind package can provide a graphical EFI boot menu that allows to boot operating systems that are found on the available partitions.

As of now, refind is available in the testing repository. It can be safely installed in alpine by following the guidelines or it may be installed in other dual/multi-booted linux distribution.

The below command installs the alpine-linux refind package:

# apk add refind@testing

The below command installs refind to the EFI partition

# refind-install --alldrivers

The --alldrivers option includes all filesystem drivers instead of only the one needed to load the currently running kernel, to allow finding and booting operating systems from more partitions.

For a Debian based distribution, it can be installed to the EFI partition like this:

# apt install refind # installs the debian package # refind-install --alldrivers # installs refind to the EFI partition

And a first (default) boot menu line needs to be configured with Alpine's default boot parameters. Assuming the bootable partition is mounted at /media/sdXY it can be done like this (at time of writing):

# echo '"Alpine" "modules=loop,squashfs,sd-mod,usb-storage quiet initrd=\boot\intel-ucode.img initrd=\boot\amd-ucode.img initrd=\boot\initramfs-lts"' > /media/sdXY/boot/refind_linux.conf

Note: At the time of writing, it was still needed to use backslashes in the .conf file.
The path in the config file needs to be relative to the partition that the kernel resides on. If /boot resides on its own separate partition, then \boot needs to be removed from the paths.

There is also a second config file, refind.conf which is located inside the EFI partition. This file has to be edited manually, but it does not get affected by btrfs snapshot rollbacks. The config file is self-explanatory. The author's website is also extremely rich in information.

For btrfs filesystem, the necessary boot entry for Alpine Linux is as follows:

Contents of /boot/efi/EFI/refind/refind.conf

menuentry "Alpine Linux" {
    icon     /EFI/refind/icons/os_alpine.png
    volume   BTRFSVOL
    loader   @alpine/boot/vmlinuz-lts
    initrd   @alpine/boot/initramfs-lts
    options  "root=UUID=823a3283-30a7-4fef-b50b-8a2230c71b5b rw rootflags=subvol=@alpine rootfstype=btrfs"
    # PARTUUID not working for alpine
 }
  

For default alpine sys mode installation, the complete file content of refind.conf necessary to dual boot Alpine Linux and windows is as follows:

Contents of /boot/efi/EFI/refind/refind.conf

timeout 20
resolution 1920 1080
use_graphics_for windows,linux

menuentry "Windows" {
   loader /EFI/Microsoft/Boot/bootmgfw.efi
   icon /EFI/refind/icons/os_win.png
}
menuentry "Alpine Linux" {
    icon     /EFI/refind/icons/os_alpine.png
    volume   ROOT
    loader   /boot/vmlinuz-lts
    initrd   /boot/initramfs-lts
    options  "root=UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx rw"
}  
 

The refind utility currently does not have Alpine Linux icon file. You can place this icon in /boot/efi/EFI/refind/icons/ folder.

Tip: If the above configuration is working perfectly,there is no need for grub and can be safely removed.

Syslinux

If you want to switch from another bootloader back to Syslinux, or if you for some reason want to install Syslinux manually, the following steps are required.

Install the syslinux package:

# apk add syslinux

If you're using GPT partitions, install the GPT MBR onto the drive you want to install the bootloader on (in this case /dev/sda):

# dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/gptmbr.bin of=/dev/sda

Or if you're using DOS partitions, install the DOS MBR instead:

# dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/mbr.bin of=/dev/sda

Next install the required Syslinux binaries. Despite being called extlinux, Syslinux supports booting from FAT12/16/32, NTFS, ext2/3/4, btrfs, XFS, and UFS/FFS filesystems.

# extlinux --install /boot

The configuration file is located in /boot/extlinux.conf.

Alpine ships with a script update-extlinux which automatically (re)generates this file, for example on updates to Syslinux. The settings for this script can be found in /etc/update-extlinux.conf, including the option to disable automatic overwriting of /boot/extlinux.conf.

You can also place additional menu entries in the /etc/update-extlinux.d/ directory, e.g. for dual booting.

Using EFI with syslinux

Alpine Linux installer automatically uses Grub, if EFI mode is detected. The below section is specifically about using EFI with syslinux:

Assuming /mnt is a FAT32 partition of type EF00 and /boot belongs to the rootfs created after running setup-disk:

# mkdir -p /mnt/EFI/syslinux # cp /usr/share/syslinux/efi64/* /mnt/EFI/syslinux/ # cp /boot/extlinux.conf /mnt/EFI/syslinux/syslinux.cfg # cp /boot/vmlinuz* /mnt/EFI/syslinux/ # cp /boot/initramfs* /mnt/EFI/syslinux/

You may need to modify /mnt/EFI/syslinux/syslinux.cfg to change the paths to absolute paths (just add a / in front of the vmlinuz/initramfs entries), or copy the files to /mnt/EFI/syslinux instead (XXX: untested).

At the end, the file /mnt/EFI/syslinux/syslinux.cfg should look like this:

Contents of /mnt/EFI/syslinux/syslinux.cfg

...
DEFAULT menu.c32
PROMPT 0
MENU TITLE Alpine/Linux Boot Menu
MENU HIDDEN
MENU AUTOBOOT Alpine will be booted automatically in # seconds
TIMEOUT 10
LABEL lts
  MENU DEFAULT
  MENU LABEL Linux lts
  LINUX /vmlinuz-lts
  INITRD /initrd-lts
  APPEND root=/dev/sda3 modules=sd-load,usb-storage,ext4 quiet

And finally, add syslinux to EFI boot menu (assuming /dev/sda is your hard drive)

# apk add efibootmgr # efibootmgr -c -d /dev/sda -p 1 -l \\EFI\\syslinux\\syslinux.efi -L "ALPINE-SYSLINUX"

You can now verify that the boot entry has been added

efibootmgr

BootCurrent: 0001
Timeout: 0 seconds
BootOrder: 0001,0000,0002,...
Boot001* ALPINE-SYSLINUX HD(1,GPT,xxxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)/FILE(\EFI\syslinux\syslinux.efi)

GRUB

To install GRUB in BIOS mode, (optionally) remove the Syslinux package and install the required GRUB packages:

# apk del syslinux # apk add grub grub-bios

For EFI, install GRUB's EFI package instead. Note that /boot has to be an EFI compatible filesystem like FAT32.

# apk add grub-efi efibootmgr

Next install the MBR and GRUB binaries to disk for BIOS mode:

# grub-install /dev/vda

For EFI mode:

# grub-install --target=x86_64-efi --efi-directory=/boot

then add this line to /etc/default/grub:

Contents of /etc/default/grub

# GRUB_CMDLINE_LINUX_DEFAULT="quiet rootfstype=ext4 modules=sd-mod,usb-storage,ext4"

GRUB ships with an automatic config generator, including some automatic detection of other operating systems installed on the device:

# grub-mkconfig -o /boot/grub/grub.cfg

This script can be configured via the /etc/default/grub file. See [1] for a list of available options.

EFI Boot Stub

To boot directly from your motherboard's UEFI boot menu, a boot entry needs to be created with either a UEFI shell or efibootmgr.

efibootmgr

Install efibootmgr:

# apk add efibootmgr

Create a boot entry. It's recommended to do this in a script, as efibootmgr does not allow editing entries.

Contents of add-bootentry

#!/bin/sh

params="root=/dev/sdXZ rootfstype=ext4 rw \
  initrd=\intel-ucode.img \
  initrd=\initramfs-lts"

efibootmgr --create --label "Alpine Linux" \
  --disk /dev/sdX --part Y \
  --loader /vmlinuz-lts \
  --unicode "${params}" \
  --verbose

Where /dev/sdXY contains the EFI partition and /dev/sdXZ contains the root partition. If you are using linux-edge, replace lts with edge in the script

Note:

The kernel contains the exhaustive list of ways to specify the block device. For a more robust boot entry, it is recommended to use a persistent name such as the PARTUUID.

Optionally, set the newly created entry as the default:

# efibootmgr -n XXXX

Where XXXX is the boot number of the new entry.

Note: The loader and initrd file arguments are relative to the EFI partition. In a default installation, alpine places these files in /boot/, while EFI is mounted to /boot/efi/. You can either update fstab to mount EFI at /boot/, or manually copy them to /boot/efi/.

Using a UKI

Unified Kernel Image (UKI) is supported in UEFI only. It is possible to boot directly into a UKI. A UKI is a single file which contains the initfs, kernel and cmdline. While this is typically done in order to enable SecureBoot, it is perfectly feasible to skip enrolling the custom keys and leave SecureBoot off.

The page UEFI Secure Boot contains the instructions for setting an a UKI. Additionally, it is possible to install the UKI in the default fallback path used by most UEFI implementations. By installing the UKI into this path, the system will automatically boot into it if no other entries are defined. This can be automated as part of the kernel hook by adding the following to /etc/kernel-hooks.d/secureboot.conf

Contents of /etc/kernel-hooks.d/secureboot.conf

# For the edge kernel, install the UKI into the default UEFI path.
if [ "$1" == "edge" ]; then
  output_dir="/efi/EFI/Boot/"
  output_name="bootx64.efi"
fi

bootx64.efi is only correct for x86_64 systems. For other architectures the exact name will vary.

Bootloaders