grub-macbless

grub-macbless file

file
    The path to the file that will be blessed. This is usually the macOS boot loader itself, often located at /System/Library/CoreServices/boot.efi. The full path to the file *must* be specified, including the device where the macOS partition resides (e.g., /dev/sda2/System/Library/CoreServices/boot.efi)

The grub-macbless command is a utility, primarily used within the GRUB bootloader environment, designed to 'bless' a specified file on a macOS (formerly OS X) partition. 'Blessing' a file, in macOS parlance, essentially marks it as a potential boot candidate, informing the macOS boot loader (BootX or later) that this file can be used to initiate the operating system boot process. This is essential when GRUB is used to chainload the macOS bootloader.

It operates by modifying certain flags or attributes associated with the file on the filesystem. These flags are specific to the macOS filesystem (typically HFS+ or APFS). The command does *not* directly manipulate the macOS boot loader's configuration; rather, it modifies filesystem metadata to indicate boot eligibility. It's crucial for ensuring that macOS can be started when using GRUB as the primary boot manager, as GRUB itself needs to hand off control to the macOS boot loader to initiate the macOS boot sequence. Improper use can lead to an unbootable macOS installation.

Modern versions of macOS may employ more sophisticated boot procedures; thus, the direct use of grub-macbless might not always be sufficient, especially when dealing with newer macOS versions or advanced boot configurations like FileVault or SIP (System Integrity Protection).

This command relies on specific filesystem structures and macOS bootloader expectations. Using it incorrectly can prevent macOS from booting. It's less likely to be effective on modern macOS installations using APFS and complex boot processes. SIP can also interfere.

To bless the standard boot loader on the macOS partition located at /dev/sda2, the command would be: grub-macbless /dev/sda2/System/Library/CoreServices/boot.efi.

Important: Ensure that the filesystem is mounted before attempting to bless the file.

grub-macbless primarily works with HFS+ and potentially with APFS filesystems. However, the success rate on APFS is lower due to changes in the boot process in recent macOS versions.

grub-macbless was created to facilitate dual-booting scenarios, where GRUB is the primary bootloader and macOS needs to be booted as a secondary OS. As GRUB gained support for macOS filesystems, the need to mark files bootable on these filesystems arose. The command has evolved along with GRUB's capabilities.

bless(8) - macOS bless command