72 lines
3.4 KiB
Text
72 lines
3.4 KiB
Text
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22 Error messages produced by GRUB
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**********************************
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22.1 GRUB only offers a rescue shell
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====================================
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GRUB's normal start-up procedure involves setting the 'prefix'
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environment variable to a value set in the core image by 'grub-install',
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setting the 'root' variable to match, loading the 'normal' module from
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the prefix, and running the 'normal' command (*note normal::). This
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command is responsible for reading '/boot/grub/grub.cfg', running the
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menu, and doing all the useful things GRUB is supposed to do.
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If, instead, you only get a rescue shell, this usually means that
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GRUB failed to load the 'normal' module for some reason. It may be
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possible to work around this temporarily: for instance, if the reason
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for the failure is that 'prefix' is wrong (perhaps it refers to the
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wrong device, or perhaps the path to '/boot/grub' was not correctly made
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relative to the device), then you can correct this and enter normal mode
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manually:
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# Inspect the current prefix (and other preset variables):
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set
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# Find out which devices are available:
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ls
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# Set to the correct value, which might be something like this:
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set prefix=(hd0,1)/grub
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set root=(hd0,1)
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insmod normal
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normal
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However, any problem that leaves you in the rescue shell probably
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means that GRUB was not correctly installed. It may be more useful to
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try to reinstall it properly using 'grub-install DEVICE' (*note Invoking
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grub-install::). When doing this, there are a few things to remember:
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* Drive ordering in your operating system may not be the same as the
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boot drive ordering used by your firmware. Do not assume that your
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first hard drive (e.g. '/dev/sda') is the one that your firmware
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will boot from. 'device.map' (*note Device map::) can be used to
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override this, but it is usually better to use UUIDs or file system
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labels and avoid depending on drive ordering entirely.
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* At least on BIOS systems, if you tell 'grub-install' to install
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GRUB to a partition but GRUB has already been installed in the
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master boot record, then the GRUB installation in the partition
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will be ignored.
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* If possible, it is generally best to avoid installing GRUB to a
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partition (unless it is a special partition for the use of GRUB
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alone, such as the BIOS Boot Partition used on GPT). Doing this
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means that GRUB may stop being able to read its core image due to a
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file system moving blocks around, such as while defragmenting,
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running checks, or even during normal operation. Installing to the
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whole disk device is normally more robust.
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* Check that GRUB actually knows how to read from the device and file
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system containing '/boot/grub'. It will not be able to read from
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encrypted devices with unsupported encryption scheme, nor from file
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systems for which support has not yet been added to GRUB.
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22.2 Firmware stalls instead of booting GRUB
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============================================
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The EFI implementation of some older MacBook laptops stalls when it gets
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presented a grub-mkrescue ISO image for x86_64-efi target on an USB
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stick. Affected are models of year 2010 or earlier. Workaround is to
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zeroize the bytes 446 to 461 of the EFI partition, where mformat has put
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a partition table entry which claims partition start at block 0. This
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change will not hamper bootability on other machines.
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