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翻译中。。。。。 Grub 2: Heal your bootloader
There are few things as irritating as a broken bootloader. Get the best out of Grub 2 and keep it shipshape.
Why do this?
- Grub 2 is the most popular bootloader that’s used by almost every Linux distribution.
- A bootloader is a vital piece of software, but they are susceptible to damage.
- Grub 2 is an expansive and flexible boot loader that offers various customisable options.
The Grub 2 Linux bootloader is a wonderful and versatile piece of software. While it isn’t the only bootloader out there, it’s the most popular and almost all the leading desktop distros use it. The job of the Grub bootloader is twofold. First, it displays a menu of all installed operating systems on a computer and invites you to pick one. Second, Grub loads the Linux kernel if you choose a Linux operating system from the boot menu.
As you can see, if you use Linux, you can’t escape the bootloader. Yet it’s one the least understood components inside a Linux distro. In this tutorial we’ll familiarise you with some of Grub 2’s famed versatility and equip you with the skills to help yourself when you have a misbehaving bootloader.
The most important parts of Grub 2 are a bunch of text files and a couple of scripts. The first piece to know is /etc/default/grub. This is the text file in which you can set the general configuration variables and other characteristics of the Grub 2 menu (see box titled “Common user settings”).
The other important aspect of Grub 2 is the /etc/grub.d folder. All the scripts that define each menu entry are housed there. The names of these scripts must have a two-digit numeric prefix. Its purpose is to define the order in which the scripts are executed and the order of the corresponding entries when the Grub 2 menu is built. The 00_header file is read first, which parses the /etc/default/grub configuration file. Then come the entries for the Linux kernels in the 10_linux file. This script creates one regular and one recovery menu entry for each kernel in the default /boot partition.
This script is followed by others for third-party apps such as 30_os-prober and 40_custom. The os-prober script creates entries for kernels and other operating systems found on other partitions. It can recognise Linux, Windows, BSD and Mac OS X installations. If your hard disk layout is too exotic for the os-prober script to pick up an installed distro, you can add it to the 40_custom file (see the “Add custom entries” box).
Grub 2 does not require you to manually maintain your boot options’ configuration file: instead it generates the /boot/grub/grub.cfg file with the grub2-mkconfig command. This utility will parse the scripts in the /etc/grub.d directory and the /etc/default/grub settings file to define your setup.
Graphical boot repair
A vast majority of Grub 2 issues can easily be resolved with the touch of a button thanks to the Boot Repair app. This nifty little application has an intuitive user interface and can scan and comprehend various kinds of disk layouts and partitioning schemes, and can sniff out and correctly identify operating system installations inside them. The utility works on traditional computers with a Master Boot Record (MBR) as well as the newer UEFI computers with the UID Partition Table (GPT) layout.
The easiest way to use Boot Repair is to install it inside a Live Ubuntu session. Fire up an Ubuntu Live distro on a machine with a broken bootloader and install Boot Repair by first adding its PPA repository with
sudo add-apt-repository ppa:yannubuntu/Boot Repair
Then refresh the list of repositories with
sudo apt-get update
before installing the app with
sudo apt-get install -y Boot Repair
Fire up the tool once it’s installed. The app will scan your hard disk before displaying its interface, which is made up of a couple of buttons. To follow the advice of the tool, simply press the Recommended Repair button, which should fix most broken bootloaders. After it’s restored your bootloader, the tool also spits out a small URL which you should note. The URL contains a detailed summary of your disks, including your partitions along with the contents of important Grub 2 files including /etc/default/grub and boot/grub/grub.cfg. If the tool hasn’t been able to fix your bootloader, you can share the URL on your distro’s forum boards to allow others to understand your disk layout and offer suggestions.
Boot Repair also lets you customise Grub 2’s options.
Bootloader bailout
Grub 2 boot problems can leave the system in several states. The text on the display where you’d expect the bootloader menu gives an indication of the current state of the system. If the system stops booting at the grub> prompt, it means the Grub 2 modules were loaded but it couldn’t find the grub.cfg file. This is the full Grub 2 command shell and you can do quite a bit here to help yourself. If you see the grub rescue> prompt, it means that the bootloader couldn’t find the Grub 2 modules nor could it find any of your boot files. However, if your screen just displays the word ‘GRUB’, it means the bootloader has failed to find even the most basic information that’s usually contained in the Master Boot Record.
You can correct these Grub failures either by using a live CD or from Grub 2’s command shell. If you’re lucky and your bootloader drops you at the grub> prompt, you have the power of the Grub 2 shell at your disposal to correct any errors.
The next few commands work with both grub> and grub rescue>. The set pager=1 command invokes the pager, which prevents text from scrolling off the screen. You can also use the ls command which lists all partitions that Grub sees, like this:
grub> ls
(hd0) (hd0,msdos5) (hd0,msdos6) (hd1,msdos1)
As you can see, the command also lists the partition table scheme along with the partitions.
You can also use the ls command on each partition to find your root filesystem:
grub> ls (hd0,5)/
lost+found/ var/ etc/ media/ bin/ initrd.gz
boot/ dev/ home/ selinux/ srv/ tmp/ vmlinuz
You can drop the msdos bit from the name of the partition. Also, if you miss the trailing slash and instead say ls (hd0,5) you’ll get information about the partition including its filesystem type, total size, and last modification time. If you have multiple partitions, read the contents of the /etc/issue file with the cat command to identify the distro, such as cat (hd0,5)/etc/issue.
Assuming you find the root filesystem you’re looking for inside (hd0,5), make sure that it contains the /boot/grub directory and the Linux kernel image you wish to boot into, such as vmlinuz-3.13.0-24-generic. Now type the following:
grub> set root=(hd0,5)
grub> linux /boot/vmlinuz-3.13.0-24-generic root=/dev/sda5
grub> initrd /boot/initrd.img-3.13.0-24-generic
The first command points Grub to the partition housing the distro we wish to boot into. The second command then tells Grub the location of the kernel image inside the partition as well as the location of the root filesystem. The final line sets the location of the initial ramdisk file. You can use tab autocompletion to fill in the name of the kernel and the initrd, which will save you a lot of time and effort.
Once you’ve keyed these in, type boot at the next grub> prompt and Grub will boot into the specified operating system.
Things are a little different if you’re at the grub rescue> prompt. Since the bootloader hasn’t been able to find and load any of the required modules, you’ll have to insert them manually:
grub rescue> set root=(hd0,5)
grub rescue> insmod (hd0,5)/boot/grub/normal.mod
grub rescue> normal
grub> insmod linux
As you can see, just like before, after we use the ls command to hunt down the Linux partition, we mark it with the set command. We then insert the normal module, which when activated will return us to the standard grub> mode. The next command then inserts the linux module in case it hasn’t been loaded. Once this module has been loaded you can proceed to point the boot loader to the kernel image and initrd files just as before and round off the procedure with the boot command to bring up the distro.
Once you’ve successfully booted into the distro, don’t forget to regenerate a new configuration file for Grub with the
grub-mkconfig -o /boot/grub/grub.cfg
command. You’ll also have to install a copy of the bootloader into the MBR with the
sudo grub2-install /dev/sda
command.
To disable a script under the /etc/grub.d, all you need to do is remove the executable bit, for example with chmod -x /etc/grub.d/20_memtest86+ which will remove the ‘Memory Test’ option from the menu.
Grub 2 and UEFI
UEFI-enabled machines (more or less, any machine sold in the last couple of years) have added another layer of complexity to debugging a broken Grub 2 bootloader. While the procedure for restoring a Grub 2 install on a UEFI machine isn’t much different than it is on a non-UEFI machine, the newer firmware handles things differently, which results in mixed restoration results.
On a UEFI-based system, you do not install anything in the MBR. Instead you install a Linux EFI bootloader in the EFI System Partition (ESP) and set it as the EFI’s default boot program using a tool such as efibootmgr for Linux, or bcdedit for Windows.
As things stand now, the Grub 2 bootloader should be installed properly when installing any major desktop Linux distro, which will happily coexist with Windows 8. However, if you end up with a broken bootloader, you can restore the machine with a live distro. When you boot the live medium, make sure you boot it in the UEFI mode. The computer’s boot menu will have two boot options for each removable drive – a vanilla option and an option tagged with UEFI. Use the latter to expose the EFI variables in /sys/firmware/efi/.
From the live environment, mount the root filesystem of the broken installation as mentioned in the tutorial. You’ll also have to mount the ESP partition. Assuming it’s /dev/sda1, you can mount it with
sudo mount /dev/sda1 /mnt/boot/efi
Then load the efivars module with modprobe efivars before chrooting into the installed distribution as shown in the tutorial.
Here on, if you’re using Fedora, reinstall the bootloader with the
yum reinstall grub2-efi shim
command followed by
grub2-mkconfig -o /boot/grub2/grub.cfg
to generate the new configuration file. Ubuntu users can do this with
apt-get install --reinstall grub-efi-amd64
With the bootloader in place, exit chroot, unmount all partitions and reboot to the Grub 2 menu.
Dude, where’s my Grub?
The best thing about Grub 2 is that you can reinstall it whenever you want. So if you lose the Grub 2 bootloader, say when another OS like Windows replaces it with its own bootloader, you can restore Grub within a few steps with the help of a live distro. Assuming you’ve installed a distro on /dev/sda5, you can reinstall Grub by first creating a mount directory for the distro with
sudo mkdir -p /mnt/distro
and then mounting the partition with
mount /dev/sda5 /mnt/distro
You can then reinstall Grub with
grub2-install --root-directory=/mnt/distro /dev/sda
This command will rewrite the MBR information on the /dev/sda device, point to the current Linux installation and rewrite some Grub 2 files such as grubenv and device.map.
Another common issue pops up on computers with multiple distros. When you install a new Linux distro, its bootloader should pick up the already installed distros. In case it doesn’t, just boot into the newly installed distro and run
grub2-mkconfig
Before running the command, make sure that the root partitions of the distros missing from the boot menu are mounted. If the distro you wish to add has /root and /home on separate partitions, only mount the partition that contains /root, before running the grub2-mkconfig command.
While Grub 2 will be able to pick most distros, trying to add a Fedora installation from within Ubuntu requires one extra step. If you’ve installed Fedora with its default settings, the distro’s installer would have created LVM partitions. In this case, you’ll first have to install the lvm2 driver using the distro’s package management system, such as with
sudo apt-get install lvm2
before Grub 2’s os-prober script can find and add Fedora to the boot menu.
Common user settings
Grub 2 has lots of configuration variables. Here are some of the common ones that you’re most likely to modify in the /etc/default/grub file. The GRUB_DEFAULT variable specifies the default boot entry. It will accept a numeric value such as 0, which denotes the first entry, or “saved” which will point it to the selected option from the previous boot. The GRUB_TIMEOUT variable specifies the delay before booting the default menu entry and the GRUB_CMDLINE_LINUX variable lists the parameters that are passed on the kernel command line for all Linux menu entries.
If the GRUB_DISABLE_RECOVERY variable is set to true, the recovery mode menu entries will not be generated. These entries boot the distro into single-user mode from where you can repair your system with command line tools. Also useful is the GRUB_GFXMODE variable, which specifies the resolution of the text shown in the menu. The variable can take any value supported by your graphics card.
Grub 2 has a command line, which you can invoke by pressing C at the bootloader menu.
Thorough fix
If the grub2-install command didn’t work for you, and you still can’t boot into Linux, you’ll need to completely reinstall and reconfigure the bootloader. For this task, we’ll use the venerable chroot utility to change the run environment from that of the live CD to the Linux install we want to recover. You can use any Linux live CD for this purpose as long as it has the chroot tool. However, make sure the live medium is for the same architecture as the architecture of the installation on the hard disk. So if you wish to chroot to a 64-bit installation you must use an amd64 live distro.
After you’ve booted the live distro, the first order of business is to check the partitions on the machine. Use fdisk -l to list all the partitions on the disk and make note of the partition that holds the Grub 2 installation that you want to fix.
Let’s assume we wish to restore the bootloader from the distro installed in /dev/sda5. Fire up a terminal and mount it with:
sudo mount /dev/sda5 /mnt
Now you’ll have to bind the directories that the Grub 2 bootloader needs access to in order to detect other operating systems:
$ sudo mount --bind /dev /mnt/dev
$ sudo mount --bind /dev/pts /mnt/dev/pts
$ sudo mount --bind /proc /mnt/proc
$ sudo mount --bind /sys /mnt/sys
We’re now all set to leave the live environment and enter into the distro installed inside the /dev/sda5 partition via chroot:
$ sudo chroot /mnt /bin/bash
You’re now all set to install, check, and update Grub. Just like before, use the
sudo grub2-install /dev/sda
command to reinstall the bootloader. Since the grub2-install command doesn’t touch the grub.cfg file, we’ll have to create it manually with
sudo grub-mkconfig -o /boot/grub/grub.cfg
That should do the trick. You now have a fresh copy of Grub 2 with a list of all the operating systems and distros installed on your machine. Before you can restart the computer, you’ll have to exit the chrooted system and unmount all the partitions in the following order:
$ exit
$ sudo umount /mnt/sys
$ sudo umount /mnt/proc
$ sudo umount /mnt/dev/pts
$ sudo umount /mnt/dev
$ sudo umount /mnt
You can now safely reboot the machine, which should be back under Grub 2’s control, and the bootloader under yours!
Add custom entries
If you wish to add an entry to the bootloader menu, you should add a boot stanza to the 40_custom script. You can, for example, use it to display an entry to boot a Linux distro installed on a removable USB drive. Assuming your USB drive is sdb1, and the vmlinuz kernel image and the initrd files are under the root (/) directory, add the following to the 40_custom file:
menuentry “Linux on USB” {
set root=(hd1,1)
linux /vmlinuz root=/dev/sdb1 ro quiet splash
initrd /initrd.img
}
For more accurate results, instead of device and partition names you can use their UUIDs, such as
set root=UUID=54f22dd7-eabe
Use
sudo blkid
to find the UUIDs of all the connected drives and partitions. You can also add entries for any distros on your disk that weren’t picked up by the os-prober script, as long as you know where the distro’s installed and the location of its kernel and initrd image files.
via: https://www.linuxvoice.com/grub-2-heal-your-bootloader/
作者：Mayank Sharma 译者：译者ID 校对：校对者ID