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This article mainly introduces how to configure and troubleshoot the GNU boot loader in linux, which has a certain reference value, and interested friends can refer to it. I hope you can learn a lot after reading this article.

Introduction to GRUB Bootloader

In modern systems, you will find that there are two main GRUB versions (one is v1, sometimes called GRUB Legacy, and the other is v2), although most * * release systems use v2 by default. Today, only Red Hat Enterprise Linux 6 and its derivative systems still use v1.

Therefore, in this guide, we will focus on the features of the v2 version.

Regardless of the version of GRUB, a boot loader allows the user to:

Modify the behavior of the system by specifying the use of a different kernel

Select one boot from multiple operating systems

Add or edit configuration blocks to change startup options, etc.

Today, GNU projects are responsible for maintaining GRUB and provide rich documentation on their websites. When you are reading this guide, we strongly recommend that you take a look at the GNU official documentation.

When the system boots, you will see the following GRUB screen in the main console. At first, you can be prompted to select a kernel among multiple kernel versions (by default, the system will boot with a * kernel), and you can enter GRUB command-line mode (using c), or edit the startup item (press e).

GRUB Startup screen

One of the reasons you might consider booting with an older kernel is that a previously working hardware device has a "acting up" after an upgrade (for example, you can refer to this link in the AskUbuntu forum).

The configuration file for GRUB v2 is read from the / boot/grub/grub.cfg or / boot/grub2/grub.cfg file at startup, while the configuration file used by GRUB v1 is from / boot/grub/grub.conf or / boot/grub/menu.lst. These files should not be manually edited directly, but should be updated with the contents of / etc/default/grub and the files in the / etc/grub.d directory.

On CentOS 7, when the system is initially installed, the following configuration file is generated:

GRUB_TIMEOUT=5 GRUB_DISTRIBUTOR= "$(sed's, release. * $, g' / etc/system-release)" GRUB_DEFAULT=saved GRUB_DISABLE_SUBMENU=true GRUB_TERMINAL_OUTPUT= "console" GRUB_CMDLINE_LINUX= "vconsole.keymap=la-latin1 rd.lvm.lv=centos_centos7-2/swap crashkernel=auto vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos_centos7-2/root rhgb quiet" GRUB_DISABLE_RECOVERY= "true"

In addition to online documentation, you can also consult the GNU GRUB manual using the following command:

# info grub

If you are particularly interested in the options available in the / etc/default/grub file, you can refer directly to the help documentation in the configuration section:

# info-f grub-n 'Simple configuration'

Using the above command, you will find that GRUB_TIMEOUT is used to set the time between the startup screen appears and the system starts automatically (unless interrupted by the user). When the value of the variable is-1, it will not start unless the user actively makes a choice.

When multiple operating systems or kernels are installed on the same machine, GRUB_DEFAULT needs to use an integer to specify which operating system or kernel entry the GRUB boot screen chooses to boot by default. We can either view the list of startup entries through the above startup screen, or we can use the following command:

On CentOS and openSUSE systems

# awk-F\'$1 boot/grub2/grub.cfg = "menuentry" {print $2}'/ boot/grub2/grub.cfg

On the Ubuntu system

# awk-F\'$1 boot/grub/grub.cfg = "menuentry" {print $2}'/ boot/grub/grub.cfg

In the example shown in the figure below, if we want to use the kernel with version 3.10.0-123.el7.x86_64 (the fourth entry), we need to set GRUB_DEFAULT to 3 (entries are numbered from zero), as shown below:

GRUB_DEFAULT=3

Use the old kernel to boot the system

* * one GRUB configuration variable that requires special attention is GRUB_CMDLINE_LINUX, which is used to pass options to the kernel. You can find detailed documentation of the options that can be passed to the kernel through GRUB in the kernel variable file and in the man 7 bootparam.

The current options on my CentOS 7 server are:

GRUB_CMDLINE_LINUX= "vconsole.keymap=la-latin1 rd.lvm.lv=centos_centos7-2/swap crashkernel=auto vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos_centos7-2/root rhgb quiet"

Why do you want to change the default kernel parameters or pass additional options? To put it simply, in many cases, you need to tell the kernel some hardware parameters that cannot be determined by the kernel itself, or override some kernel-detected values.

This happened to me not long ago, when I tried Vector Linux derived from Slackware on my 10-year-old notebook. After the installation, the kernel did not detect the correct configuration of my graphics card, so I had to pass the modified kernel options through GRUB to make it work.

Another example is when you need to switch the system to single-user mode to perform maintenance work. To do this, you can directly append single to the GRUB_CMDLINE_LINUX variable and restart:

GRUB_CMDLINE_LINUX= "vconsole.keymap=la-latin1 rd.lvm.lv=centos_centos7-2/swap crashkernel=auto vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos_centos7-2/root rhgb quiet single"

After editing / etc/default/grub, you need to run the update-grub (on Ubuntu) or grub2-mkconfig-o / boot/grub2/grub.cfg (on CentOS and openSUSE) commands to update the grub.cfg file (otherwise, the changes will be lost at system startup).

This command processes the startup configuration files mentioned earlier to update the grub.cfg file. This approach ensures that changes are persisted, while options passed through GRUB at startup time are valid only during the current session.

Fix Linux GRUB issu

If you install a second operating system, or if your GRUB configuration file is corrupted due to human error, there are still some ways to recover and boot the system again.

Press c in the splash screen to enter GRUB command line mode (remember, you can also press e to edit the default startup options), and you can enter the help command at the GRUB prompt to get available commands:

Fix Grub configuration problems with Linux

We will focus on the ls command, which lists installed devices and file systems, and we will look at what it finds. In the picture below, we can see that there are 4 hard drives (hd0 to hd3).

It seems that only hd0 has been partitioned (msdos1 and msdos2 can prove that 1 and 2 here are partition numbers, and msdos is the partition scheme).

Now let's see if we can find GRUB on the * partition hd0 (msdos1). This approach allows us to start Linux and use advanced tools to repair the configuration file, or simply reinstall GRUB if necessary:

# ls (hd0,msdos1) /

From the highlighted area, you can see that the grub2 directory is in this partition:

Find Grub configuration

Once we are sure that GRUB is at (hd0, msdos1), let's tell GRUB where to find its configuration file and instruct it to try to launch its menu:

Set prefix= (hd0,msdos1) / grub2 set root= (hd0,msdos1) insmod normal normal

Find and launch the Grub menu

Then, in the GRUB menu, select an entry and press enter to start using it. Once the system boots successfully, you can run the grub2-install / dev/sdX command to fix the problem (change sdX to the device on which you want to install GRUB). The startup information will then be updated and all related files will be restored.

# grub2-install / dev/sdX

Other more complex scenarios and their fix recommendations are documented in the Ubuntu GRUB2 troubleshooting guide. The concepts described in this guide are also valid for other distributions.

Thank you for reading this article carefully. I hope the article "how to configure and troubleshoot the GNU boot loader in linux" shared by the editor will be helpful to everyone. At the same time, I also hope you will support us and pay attention to the industry information channel. More related knowledge is waiting for you to learn!

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