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en-US/grub2.adoc
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= GRUB 2
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'''
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[IMPORTANT]
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======
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This page was automatically converted from https://fedoraproject.org/wiki/GRUB_2
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It is probably
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* Badly formatted
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* Missing graphics and tables that do not convert well from mediawiki
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* Out-of-date
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* In need of other love
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Please fix it, remove this notice, and then add to `_topic_map.yml`
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Pull requests accepted at https://pagure.io/fedora-docs/quick-docs
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Once that is live, go to the original wiki page and add an `{{old}}`
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tag, followed by a note like
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....
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{{admon/note|This page has a new home!|
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This wiki page is no longer maintained. Please find the up-to-date
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version at: https://docs.fedoraproject.org/whatever-the-url
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}}
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....
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======
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'''
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[[introduction]]
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Introduction
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------------
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GRUB 2 is the latest version of GNU GRUB, the GRand Unified Bootloader.
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A bootloader is the first software program that runs when a computer
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starts. It is responsible for loading and transferring control to the
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operating system kernel, (Linux, in the case of Fedora). The kernel, in
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turn, initializes the rest of the operating system.
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GRUB 2 has replaced what was formerly known as GRUB (i.e. version 0.9x),
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which has, in turn, become GRUB Legacy.
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Starting with Fedora 16, GRUB 2 is the default bootloader on x86 BIOS
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systems. For upgrades of BIOS systems the default is also to install
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GRUB 2, but you can opt to skip bootloader configuration entirely.
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[[tasks-common-issues]]
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Tasks / Common issues
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---------------------
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[[updating-grub-2-configuration-on-bios-systems]]
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Updating GRUB 2 configuration on BIOS systems
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The grub2 packages contain commands for installing a bootloader and for
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creating a bootloader configuration file.
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grub2-install will install the bootloader - usually in the MBR, in free
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unpartioned space, and as files in /boot. The bootloader is installed
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with something like:
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....
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grub2-install /dev/sda
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....
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grub2-mkconfig will create a new configuration based on the currently
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running system, what is found in /boot, what is set in
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/etc/default/grub, and the customizable scripts in /etc/grub.d/ . A new
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configuration file is created with:
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....
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grub2-mkconfig -o /boot/grub2/grub.cfg
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....
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The configuration format has evolved over time, and a new configuration
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file might be slightly incompatible with the old bootloader. It is
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therefore a good idea to first run grub2-install whenever you would need
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to run grub2-mkconfig.
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The Fedora installer, anaconda, will run these grub2 commands and there
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is usually no reason to run them manually.
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It is generally safe to directly edit /boot/grub2/grub.cfg in Fedora.
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Grubby in Fedora patches the configuration when a kernel update is
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performed and will try to not make any other changes than what is
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necessary. (Other distributions, in particular Debian and Debian-derived
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distributions provide a software patch that adds an command which is
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neither included nor needed in Fedora.) Manual changes might however be
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overwritten with grub2-mkconfig next time the system is upgraded with
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anaconda. Some customizations can be placed in /etc/grub.d/40_custom or
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/boot/grub2/custom.cfg and will survive running grub2-mkconfig.
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[[updating-grub-2-configuration-on-uefi-systems]]
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Updating GRUB 2 configuration on UEFI systems
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To install or fix GRUB 2 on a UEFI system on Fedora 18 or newer, you
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need to do four things:
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[[create-an-esp]]
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Create an ESP
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^^^^^^^^^^^^^
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UEFI firmware, in general, likes to boot from an EFI System Partition on
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a disk with a GPT label. In `gdisk`, it looks something like this:
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....
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Number Start (sector) End (sector) Size Code Name
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1 2048 264191 128.0 MiB EF00 EFI System
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....
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That partition should be formatted as FAT. If in doubt, FAT32 is a good
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dialect of FAT to choose.
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Fedora expects this partition to be mounted at `/boot/efi`.
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[[install-the-bootloader-files]]
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Install the bootloader files
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If you don't already have the relevant packages installed, do for Fedora
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22 and later versions with link:dnf[DNF] or with YUM for older Fedora
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releases:
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....
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dnf install grub2-efi grub2-efi-modules shim
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yum install grub2-efi grub2-efi-modules shim
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....
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If you do, then try:
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....
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dnf reinstall grub2-efi grub2-efi-modules shim
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yum reinstall grub2-efi grub2-efi-modules shim
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....
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instead.
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Make sure that /boot/efi is mounted when you do this.
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This installs the signed shim and the GRUB 2 binary.
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[[create-a-grub-2-configuration]]
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Create a GRUB 2 configuration
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Under EFI, GRUB 2 looks for its configuration in
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`/boot/efi/EFI/fedora/grub.cfg`. For newly installed kernels to work,
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`grubby` expects `/etc/grub2-efi.cfg` to be a symlink to the real
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grub.cfg (i.e. `/boot/efi/EFI/fedora/grub.cfg`).
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If you already have a grub 2 EFI config file, you should be okay. If
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not, grub2-mkconfig can help, but your mileage may vary.
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` grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg`
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[[create-a-boot-menu-entry]]
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Create a boot menu entry
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^^^^^^^^^^^^^^^^^^^^^^^^
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TL;DR: This should happen automatically. If it doesn't, read on.
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When you power on your system, your firmware will look for EFI variables
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that tell it how to boot. If you're already booted in EFI mode and EFI
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runtime services are working correctly, you can configure your boot menu
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with `efibootmgr`. If not, you'll have to bootstrap the process.
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Fortunately, `shim` can help you bootstrap. The EFI program
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`/boot/efi/EFI/BOOT/fallback.efi` will look for files called `BOOT.CSV`
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in your ESP and will add boot entries corresponding to them, *if such
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entries do not already appear to exist*. `shim` provides a `BOOT.CSV`
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file that will add an entry for `grub2-efi` for you. So just using the
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EFI Shell to invoke `fallback.efi` should do the trick. You can do this
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with commands like:
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....
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> fs0:
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> cd EFI\BOOT
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> fallback.efi
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....
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If you have no boot entries at all, then just booting off your disk in
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UEFI mode should automatically invoke `/boot/efi/EFI/BOOT/BOOTX64.EFI`,
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which will, in turn, invoke `fallback.efi`.
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If you already have incorrect boot entries, you'll either need to delete
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them or to modify `BOOT.CSV` to create new entries with different names.
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[[adding-other-operating-systems-to-the-grub-2-menu]]
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Adding Other operating systems to the GRUB 2 menu
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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grub2-mkconfig will add entries for other operating systems it can find.
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That will be done based on the output of the os-prober tool.
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That might however not work so well, especially not for booting other
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Linux operating systems, and especially not on UEFI systems. See
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http://www.gnu.org/software/grub/manual/grub.html#Multi_002dboot-manual-config
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.
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[[setting-default-entry]]
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Setting default entry
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~~~~~~~~~~~~~~~~~~~~~
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Due to `grub2-mkconfig` (and os-prober) we cannot predict the order of
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the entries in `/boot/grub2/grub.cfg`, so we set the default by
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name/title instead.
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Open `/etc/default/grub` and ensure this line exists:
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....
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GRUB_DEFAULT=saved
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....
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and ensure this line not exists:
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....
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GRUB_SAVEDEFAULT=true
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....
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or ensure this line exists:
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....
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GRUB_SAVEDEFAULT=false
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....
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Apply the change to `grub.cfg` by running:
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....
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grub2-mkconfig -o /boot/grub2/grub.cfg
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....
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Now list all possible menu entries
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....
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grep -P "submenu|^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2
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....
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Now set the desired default menu entry
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....
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grub2-set-default "<submenu title><menu entry title>"
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....
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Verify the default menu entry
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....
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grub2-editenv list
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....
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If you understand the risks involved and still want to directly modify
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/boot/grub2/grub.cfg, here's how you can do it:
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Edit /boot/grub2/grub.cfg, and change the line
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....
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set default="0"
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....
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to
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....
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set default="5"
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....
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[[encountering-the-dreaded-grub-2-boot-prompt]]
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Encountering the dreaded GRUB 2 boot prompt
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If improperly configured, GRUB 2 may fail to load and subsequently drop
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to a boot prompt. To address this issue, proceed as follows:
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\0. Load the XFS and LVM modules
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....
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insmod xfs
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insmod lvm
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....
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\1. List the drives which GRUB 2 sees:
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....
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grub2> ls
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....
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\2. The output for a dos partition table /dev/sda with three partitons
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will look something like this:
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....
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(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)
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....
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\3. While the output for a gpt partition table /dev/sda with four
|
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partitions will look something like this:
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....
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(hd0) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)
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....
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\4. With this information you can now probe each partition of the drive
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and locate your vmlinuz and initramfs files:
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|
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....
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ls (hd0,1)/
|
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....
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Will list the files on /dev/sda1. If this partition contains /boot, the
|
||||
output will show the full name of vmlinuz and initramfs.
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\5. Armed with the location and full name of vmlinuz and initramfs you
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can now boot your system.
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5a. Declare your root partition:
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|
||||
....
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grub> set root=(hd0,3)
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....
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5b. Declare the kernel you wish to use:
|
||||
|
||||
....
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grub> linux (hd0,1)/vmlinuz-3.0.0-1.fc16.i686 root=/dev/sda3 rhgb quiet selinux=0
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# NOTE : add other kernel args if you have need of them
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||||
# NOTE : change the numbers to match your system
|
||||
....
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||||
5c. Declare the initrd to use:
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||||
|
||||
....
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grub> initrd (hd0,1)/initramfs-3.0.0-1.fc16.i686.img
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# NOTE : change the numbers to match your system
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....
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||||
5d. Instruct GRUB 2 to boot the chosen files:
|
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|
||||
....
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grub> boot
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....
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\6. After boot, open a terminal.
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||||
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||||
\7. Issue the grub2-mkconfig command to re-create the grub.cfg file
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||||
grub2 needed to boot your system:
|
||||
|
||||
....
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||||
grub2-mkconfig -o /boot/grub2/grub.cfg
|
||||
....
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||||
|
||||
\8. Issue the grub2-install command to install grub2 to your hard drive
|
||||
and make use of your config:
|
||||
|
||||
....
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||||
grub2-install --boot-directory=/boot /dev/sda
|
||||
# Note: your drive may have another device name. Check for it with mount command output.
|
||||
....
|
||||
|
||||
[[additional-scenario]]
|
||||
Additional Scenario
|
||||
~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
It's also possible to boot into a _configfile_ that's located on another
|
||||
partition. If the user is faced with such a scenario, as is often the
|
||||
case with multi-boot systems containing Ubuntu and Fedora, the following
|
||||
steps in the grub rescue shell might become useful to know:
|
||||
|
||||
....
|
||||
insmod part_msdos
|
||||
insmod xfs
|
||||
insmod lvm
|
||||
set root='hd0,msdos1'
|
||||
configfile /grub2/grub.cfg
|
||||
....
|
||||
|
||||
Where, *hd0,msdos1* is the pertinent _boot_ partition, which holds the
|
||||
grub.cfg file.
|
||||
|
||||
[[other-grub-2-issues]]
|
||||
Other GRUB 2 issues
|
||||
~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
''' Absent Floppy Disk ''': It has been reported by some users that GRUB
|
||||
2 may fail to install on a partition's boot sector if the computer
|
||||
floppy controller is activated in BIOS without an actual floppy disk
|
||||
drive being present. A possible workaround is to run (post OS install)
|
||||
from rescue mode:
|
||||
|
||||
....
|
||||
grub2-install <target device> --no-floppy
|
||||
....
|
||||
|
||||
[[setting-a-password-for-interactive-edit-mode]]
|
||||
Setting a password for interactive edit mode
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
If you wish to password-protect GRUB2's interactive edit mode *but* you
|
||||
do not want to require users to enter a password to do a plain, simple,
|
||||
ordinary boot, create /etc/grub.d/01_users with the following lines:
|
||||
|
||||
....
|
||||
cat << EOF
|
||||
set superusers="root"
|
||||
export superusers
|
||||
password root secret
|
||||
EOF
|
||||
....
|
||||
|
||||
To apply your changes run:
|
||||
|
||||
....
|
||||
grub2-mkconfig -o /boot/grub2/grub.cfg
|
||||
....
|
||||
|
||||
You can encrypt the password by using pbkdf2. Use grub2-mkpasswd-pbkdf2
|
||||
to encrypt the password, then replace the password line with:
|
||||
|
||||
....
|
||||
password_pbkdf2 root grub.pbkdf2.sha512.10000.1B4BD9B60DE889A4C50AA9458C4044CBE129C9607B6231783F7E4E7191D8254C0732F4255178E2677BBE27D03186E44815EEFBAD82737D81C87F5D24313DDDE7.E9AEB53A46A16F30735E2558100D8340049A719474AEEE7E3F44C9C5201E2CA82221DCF2A12C39112A701292BF4AA071EB13E5EC8C8C84CC4B1A83304EA10F74
|
||||
....
|
||||
|
||||
More details can be found at
|
||||
https://help.ubuntu.com/community/Grub2/Passwords[Ubuntu Help: GRUB2
|
||||
Passwords].
|
||||
|
||||
Starting from atleast Fedora 21, the `--md5pass` kickstart option must
|
||||
be set using output from grub2-mkpasswd-pbkdf2.
|
||||
|
||||
[[using-old-graphics-modes-in-bootloader]]
|
||||
Using old graphics modes in bootloader
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
Terminal device is chosen with GRUB_TERMINAL; additional quote from
|
||||
http://www.gnu.org/software/grub/manual/grub.html#Simple-configuration
|
||||
|
||||
Valid terminal output names depend on the platform, but may include
|
||||
‘console’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal),
|
||||
‘gfxterm’ (graphics-mode output), ‘ofconsole’ (Open Firmware console),
|
||||
or ‘vga_text’ (VGA text output, mainly useful with Coreboot).
|
||||
|
||||
The default is to use the platform's native terminal output.
|
||||
|
||||
The default in Fedora is gfxterm and to get the legacy graphics modes
|
||||
you need to set GRUB_TERMINAL to right variable from the description
|
||||
above in /etc/default/grub
|
||||
|
||||
[[enable-serial-console-in-grub]]
|
||||
Enable Serial Console in Grub
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
To enable Serial console in grub add the following entry's to
|
||||
/etc/default/grub
|
||||
|
||||
( Adjust baudrate/parity/bits/flow control to fit your environment and
|
||||
cables)
|
||||
|
||||
....
|
||||
GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,115200n8'
|
||||
GRUB_TERMINAL=serial
|
||||
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"
|
||||
....
|
||||
|
||||
And re-generate grub
|
||||
|
||||
`grub2-mkconfig -o /boot/grub2/grub.cfg`
|
||||
|
||||
[[further-reading]]
|
||||
Further Reading
|
||||
---------------
|
||||
|
||||
* http://www.gnu.org/software/grub/manual/grub.html
|
||||
* Features/Grub2
|
||||
* Anaconda/Features/Grub2Migration
|
||||
'''
|
||||
|
||||
See a typo, something missing or out of date, or anything else which can be
|
||||
improved? Edit this document at https://pagure.io/fedora-docs/quick-docs.
|
||||
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Reference in a new issue