# # Dualbooting with Linux

## # Method A: OpenLinuxBoot

OpenCore now includes a driver OpenLinuxBoot.efi which should make dualbooting with Linux much easier.

The steps to using it are:

1. Install Linux as you normally would e.g. by booting from an .iso image burnt to a removable USB drive - OpenLinuxBoot.efi is not involved in this stage
2. Add OpenLinuxBoot.efi and ext4_x64.efi to your config.plist Drivers section
3. Make sure RequestBootVarRouting and LauncherOption are enabled in config.plist; it is also recommended to enable HideAuxiliary in order to hide older Linux kernels except when you need to see them (which you would do by pressing SPACE to show auxiliary entries in the OpenCore boot menu)
4. Reboot into OpenCore, installed Linux should now just appear
5. Never use Grub again

If you previously used method B or C below, you will need to remove those settings in order not to see your Linux distro twice in the OpenCore boot menu.

For further information on how OpenLinuxBoot works and additional trouble-shooting guidance, see the OpenLinuxBoot section in OpenCore’s Configuration.pdf document.

## # Method B: Chainloading a EFI Bootloader (GRUB2, Systemd-boot)

#### # Method 1: Using BlessOverride

If Linux is not picked up automagically, add the following to your config.plist:

Misc -> BlessOverride -> \EFI\arch\grubx64.efi

Some common Linux bootloader paths:

• \EFI\arch\grubx64.efi
• \EFI\ubuntu\grubx64.efi
• \EFI\systemd\systemd\systemd-bootx64.efi
• Check your distribution ¯\_(ツ)_/¯

efibootmgr is a program that manipulates the EFI Boot Manager in your UEFI Firmware (ex-BIOS). With it, you can create new entries and add your linux boot manager (GRUB2, systemd-boot,...) in a way that will make it appear on OpenCore and can be selected and set as default too by OC (using Ctrl + Enter when you highlight it). To do that:

1. Know which bootloader/manager you're using (GRUB2 or systemd-boot or anything else)

2. Boot to linux through OpenCore, you may want to use UEFI Shell to execute the EFI Application for your bootloader/manager

3. Find out your bootloader/manager's path, usually it's in the EFI (if you properly set it up)

1. In a terminal window on your linux install, run lsblk (available in most distributions)

$lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT ... sda 8:0 0 223.6G 0 disk ├─sda1 8:1 0 200M 0 part /boot/efi ├─sda2 8:2 0 116.3G 0 part └─sda3 8:3 0 107G 0 part / ... • Check for the mount point section to get your system root / partition (here sda3) • /boot/efi has my EFI partition mounted in it (if you properly added it in fstab, which you should) • the bootloader/manager is in /boot/efi • /boot/efi partition number is 1 in this case (it could be sda1 or nvme0nXp1 or anything else), if you have your EFI in another partition please remember which number it is 2. Change the directory to where your EFI partition is mounted by running cd /path/to/efi (for example cd /boot/efi) 3. Once you're in, you'll usually find a folder named EFI which contains BOOT and other folders, one of these folders may contain your bootloader/manager EFI Application binary, commonly found in • EFI/arch/grubx64.efi - for Arch with grub2 • EFI/ubuntu/grubx64.efi - for Ubuntu with grub2 • EFI/systemd/systemd-bootx64.efi - for systemd-boot (path used with Arch) • EFI/fedora/grubx64.efi - for Fedora with grub2 • or run find . -iname "grubx64.efi" or find . -iname "systemd-bootx64.efi" in your EFI folder (you can change the file name to whatever you're using) 4. Keep note of: • the binary path • the binary's partition number • the binary's disk path (/dev/sda or /dev/nvme0nX) 4. Install efibootmgr in your linux system (usually it comes built-in in ubuntu, but requires install on arch for example) 5. Once installed, run as sudoer/superuser (or use sudo) efibootmgr -c -L "Linux" -l "\EFI\pathto\filex64.efi" -d "/dev/sda" -p 1 • -c: Create • -L "Linux": Label the boot entry (you can change it to whatever you want) • -l "\EFI\pathto\filex64.efi": loader file path, must be in a format the UEFI Firmware can use, which means \ for pathing instead of / you find in unix • -d "/dev/sda": disk path so that efibootmgr know which disk the UEFI firmware should read the file from, it can be /dev/nvme0nX (with X as a number) if you're using NVMe • -p 1: point the partition number we found earlier, in case your EFI partition is the first one, this can be omitted 6. Reboot and check OpenCore, you will find a new entry named EFI, there can be many as it can also point to other boot entries, that's by design by OpenCore, not a bug. Note: This can be used for any EFI application you want to add to the UEFI Boot Manager. ## # Method C: Chainloading the kernel (must support EFISTUB) Some linux kernels are built with EFISTUB enabled in their configuration, which makes them loadable by the UEFI firmware like a regular UEFI application (neat, right?), we can use this feature with OpenCore and let it load the kernel as an EFI application while also passing boot arguments and other information. ### # 1. Identifying your root partition We first need to determine your root partition and its UUID/PARTUUID, this information will help us point to the proper partition for the kernel/system root. #### # 1. Your kernel and system root are in the same partition: (using Arch in this example) • In a terminal window on your linux install, run lsblk (available in most distributions)$ lsblk
NAME         MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
...
sda            8:0    0 223.6G  0 disk
├─sda1         8:1    0   200M  0 part /boot/efi
├─sda2         8:2    0 116.3G  0 part
└─sda3         8:3    0   107G  0 part /
...

• Check for the mount point section to get your system root / partition (here sda3)
• /boot/efi has my EFI partition mounted in it (if you properly added it in fstab, which you should)
• the kernel and initramfs are stored in /boot which is part of my main system root partition
• Now we need to know which UUID/PARTUUID, run blkid | grep -i <system_root_partition> , eg: blkid | grep -i sda3 (must be root user)

# blkid | grep -i sda3
/dev/sda3: UUID="3d4768d7-d33e-4f9f-a821-e80eba22ca62" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="a1073e53-c768-4ce5-89ad-b558669bdb89"

• You'll get both your UUID/PARTUUID, save these somewhere.
• Explore /boot and list your files, you should find your kernel and initramfs

$cd /boot$ ls -l
total 51698
drwxr-xr-x 7 root root    34304 Jan  1  1970 efi
drwxr-xr-x 6 root root     4096 Sep 18 19:42 grub
-rw-r--r-- 1 root root 10144696 Sep 22 23:31 initramfs-linux.img
-rw-r--r-- 1 root root 32047033 Sep 22 23:32 initramfs-linux-fallback.img
-rw-r--r-- 1 root root  3161088 Jun 16 17:50 intel-ucode.img
-rw-r--r-- 1 root root  7541344 Sep 22 23:31 vmlinuz-linux

• You see my kernel is named vmlinuz-linux with initramfs-linux.img as its initramfs, with a fallback img, but also intel-ucode.img for the trash intel mitigation
• In case you're using AMD, you might also find amd-ucode.img
• Some other distributions may have these images stored somewhere else, check your distribution and how it handles CPU ucode firmware
• Reboot to OpenCore, and press Space, it should show more options, one of them should be OpenShell.efi. In case you do not have it, download the OpenCore zip, and take it from OC/Tools, and add it to your config.plist, ProperTree can do that with OC Snapshot.

#### # 2. Your kernel is in your EFI partition

In case you're using systemd-boot, make sure you install the kernel in your EFI partition where systemd UEFI bootloader can detect and load your kernel: basically make sure it's in a FAT32 partition which your EFI partition should already be.

### # 2. Identifying your UEFI paths and devices

• In the shell

• run map -r -b to list all your devices (-b to break the scroll)

• BLKX: are all the partitions/disks scanned

• FSX: are all the explorable filesystems

• PciRoot(0x0)/Pci(AxB,CxD)/Sata or Nvme are your drives types (nvme or ahci)

• HD(X,GPT,<PARTUUID>,...) or HD(X,MBR) are your drive partitions where X is a number, GPT/MBR the disk partition scheme then followed with PARTUUID

• Example:

Mapping table
FS0: Alias(s):HD0a0a1:;BLK1:
PciRoot(0x0)/Pci(0x1F,0x2)/Sata(0x0,0x0,0x0)/HD(1,GPT,4C86153F-3A91-4328-878A-807A8FA944A1,0x28,0x64000)
FS1: Alias(s):HD0a0a2:;BLK3:
FS2: Alias(s):HD0a0a2:;BLK4:
FS4: Alias(s):HD0a0a2:;BLK6:
FS5: Alias(s):HD0a0a2:;BLK7:
FS6: Alias(s):HD0a0a2:;BLK8:
FS3: Alias(s):HD0a0a2:;BLK5:
BLK0: Alias(s):
PciRoot(0x0)/Pci(0x1F,0x2)/Sata(0x0,0x0,0x0)
BLK2: Alias(s):
BLK9: Alias(s):

• BLK0 seems to be the disk BLK as its path does not show any partition numbering or UUID
• This is logical as the first thing your firmware will look for is the disk, not its partitions
• I have BLK1/BLK2/BLK9 which are my 3 main partitions
• Because I have APFS driver loaded from OpenCore, there are new BLK devices shown because of that and thus added more partitions, which are actually just APFS container partitions, those can be ignored
• The reason why they can be ignored is because you can see that they have the same PARTUUID, also we're not here for those partitions
• FS0 seems to be BLK1, which is my EFI partition which is also FAT32 formatted, where OC lives
• In a multi-disk setup, FS0: can be whatever the UEFI firmware detects first, it does not mean that OC is always in FS0, in these cases FS0: could point to a USB device or SATA device. Usually, most firmware will follow this setup to read/find partitions: USB > SATA > NVME, this is not a norm as some other firmware could do something else (it can also depend on the boot order setting).
• Check if your linux kernel is there (in case of systemdboot)
• cd FSX:
• ls
• Use your eyes and a brain cell
• Just make sure you're properly reading the partition location before doing any actions
• FS0/BLK2/BLK9 all live in a SATA drive (which is my main boot drive for this example device)
• This matches linux reading the device as sdX and not nvmeXnX
• In the case of a NVMe drive, you would be seeing Nvme instead of Sata
• BLK9's PARTUUID matches my root filesystem a1073e53-c768-4ce5-89ad-b558669bdb89
• But remember that it's in capital letters!
• BLK1 and BLK2 have their explorable FSX, which means the UEFI firmware can explore and read files from them, however BLK9 which is an ext4 partition isn't, this means the UEFI requires a suitable driver to load its contents.
• Identify your EFI partition (in this example it's FS0:)

• run map -r > FSX:\drives_map.txt
• This will run map and save the output to FSX: in a file named drives_map.txt
• Note that UEFI Firmware use Windows-style path slash \ not /
• Reboot to macOS/Linux distribution with ProperTree in it

### # 3. Adding the boot entry to OpenCore's configuration file

• In case:

• Your kernel is in the system partition:
• Download a suitable UEFI driver for your filesystem in use:
• Extract the zip
• Explore the extracted zip > refind > drivers_x64
• You'll find these drivers:
• ext4
• ext2
• btrfs
• Pick the UEFI driver that you want and copy it to OC > Drivers
• Add it to your config.plist (you can use ProperTree > File > OC Snapshot)
• Your kernel is in your EFI partition (systemdboot), you don't need to set up any fs drivers
• (Optional) In case you're using GRUB2, make sure you get a copy of grub.cfg to get your kernel arguments

• Open config.plist in your plist editor (recommend ProperTree), under Misc > Entries, make a new child with type Dictionary:

Key Type Value Notes
Comment String Whatever you want to write here, it's just a comment I guess lol -
Enabled Boolean True/ON/1 -
Name String Name of the OS you want Can be anything you want
Path String Template:
PciRoot(your_ids)/Pci(your_ids)/.../Type(your_ids)/HD(X,SCH,PARTUUID,stuff,from_map)/\path\to\kernel
Example:
PciRoot(0x0)/Pci(0x1D,0x0)/Pci(0x0,0x0)/NVMe(0x1,9B-64-50-F1-5C-38-25-00)/HD(6,GPT,608DA4F8-DA9E-4792-829F-A4CF4E5B8B82,0x2C431800,0x3B10800)/\boot\vmlinuz-linux
check below
Arguments String Template:
initrd=\path\to\rd.img\if\used other_kernel_arguments
Example:
initrd=\boot\intel_ucode.img initrd=\boot\initramfs-linux.img root=PARTUUID=608da4f8-da9e-4792-829f-a4cf4e5b8b82 ro quiet intel_iommu=on iommu=pt
check below
Auxiliary Boolean False/NO/0 - False: Always shown
- True: Hidden until you press space
- Requires Misc\Boot\HideAuxiliary=True if this is set to True
• Path Notes:
• your_ids: from the map text file
• Type: NVMe/SATA
• X: partition number
• SCH: disk scheme (GPT/MBR)
• PARTUUID
• stuff,from_map: get them from the map
• \path\to\kernel: it's the path to the kernel binary
• TL;DR: Copy the path of the partition from the map text file.
• Note1: use backslash \ for kernel path, that's how UEFI firmware handles file paths
• Note2: use regular slash / for the PciRoot path, like shown in the example and template
• Arguments Notes:
• You can add as much initrd= as you want (if you're using linux, you should know why and how)
• Your other arguments are the same as in any other bootloader, you can add/remove/modify in the config directly
• Notes

• You can use the same scheme as above to add other efi files or uefi-loadable files (like systemdboot efi, Windows bootmgfw.efi...)
• You will NOT be able to set it as the default boot option with Ctrl+Enter, so you'll have to select it each time you want to boot it