Dualbooting

It's assumed you have a box where you already run another operating system, and would like to be able to dual boot either the installed system or Alpine Linux.

Warning: This Dualboot manual is intended for experienced users. You will have to manually edit partitions on your harddisk. Take adequate backup of important data before proceeding further. If you want to avoid partitioing your existing hard disk, it may be better for you to buy an additional disk and use setup-alpine based Install, or use virtualization

Install Alpine Linux

To Dual boot Alpine Linux, you must follow the setup-disk based Installation, except for the following points:

To Dualboot, Alpine Linux needs a separate partition. If you don't have a partition refer manual partitioning to create and format it. If you are using btrfs filesystem, there is no need to create a partition and refer btrfs tutorial for more information.
Mount the / (root) partition i.e partition(here sdXY), where you'd like to install Alpine Linux on a mount point i.e say /mnt.
# mount /dev/sdXY /mnt

Note: To allow multiple linux installations to co-exist, it is better to keep the /boot folder (where kernel files reside) inside the "/" root partition. Following the below instructions will result in:
Contents of /etc/fstab
... UUID=ABCD-1234 /boot/efi vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=utf8,shortname=mixed,errors=remount-ro 0 2 ...
If you're using EFI, pay special attention to ESP partition. ESP partition is also used by other operating systems. Make sure you create a mount point /mnt/boot/efi.
# mkdir -p /mnt/boot/efi
Identify the correct partition name for ESP partition and mount it on the above mount point.
# mount /dev/sdXY /mnt/boot/efi
Install Alpine Linux on the chosen partition.
# setup-disk -m sys /mnt
Your dual boot installation can be considered complete. You be should be able to dual boot once you reboot.

Refer configure bootloader section, if you would like to change your default bootloader(Optional).

Configure bootloader

There are different bootloaders to get a boot menu that allows selecting the operating system to boot. It is easiest to simply install and use rEFInd boot manager for dual boot between Alpine Linux and Windows.

Otherwise, one may adjust the bootloader that has already been installed by the other operating system.

Dual or Multiboot with Linux

The following example is given to give you some ideas for adjusting grub to boot Alpine.

Reboot your system (start Ubuntu).

Start a 'terminal' (ALT-F2 + "terminal" + [Run])

Take notes of the UUID of the partition you are planning to use:

# blkid /dev/sdXY

Start editing the grub2 configuration

# gedit /etc/grub.d/40_custom

I edited the file to look something like this:

Contents of /etc/grub.d/40_custom

#!/bin/sh
echo "Adding Alpine" >&2
cat << EOF
menuentry "Alpine Linux" {
 set root=(hd0,3)
 linux /boot/vmlinuz-lts root=UUID=8de6973a-4a8c-40ed-b710-c4e2b42d6b7a modules=sd-mod,usb-storage,ext4 quiet
 initrd /boot/initramfs-lts
}
EOF

Note: The root, linux, initrd, and UUID values mentioned above should be replaced with those of your system, e.g. the UUID you got when running the 'blkid' command.

Finally the configuration changes need to be applied to the grub2 bootloader:

update-grub

Now it's time to test. Reboot your box.

Note: You might need to press SHIFT when booting your box in order to see the grub-menu.

Dualboot with Windows

For Windows partitions to be detected with grub, you need `os-prober` and `grub-mount` installed at the time grub-mkconfig runs.

Note: The following needs to be tested

To upgrade the grub installation, the efi directory must be mounted in /boot/efi/. To know in which partition the efi is, we must examine the partition table, so if your main disk is /dev/sda then.

# fdisk -l /dev/sda

Device             Start        End   Sectors   Size Type
/dev/sda1           2048     309247    307200   150M EFI System

# mkdir /boot/efi

# mount /dev/sda1 /boot/efi/

# apk add efibootmgr

(If your system supports UEFI. Most PC's from 2010, support UEFI)

Install grub

# grub-install –root-directory=/mnt

After grub re-installation, unmount efi partition:

# umount /boot/efi/

Troubleshooting

Check Mounting on /dev/sdXY sysroot failed if encountered.

References