A bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel - Linux in the case of Fedora. The kernel, in turn, initializes the rest of the operating system. The grubby utility updates the bootloader-specific configuration files.
The utility is a recommended way for making routine changes to the kernel boot parameters and setting a default kernel. Use the following commands:. GRUB 2 also allows the user to pass arguments to the kernel. This file contains menu information. The GRUB 2 configuration file, grub. Edits of grub. Normal operations on grub. If you use grubby to modify the default kernel the changes will be inherited when new kernels are installed.
For more information on grubby , see Section In general, it is not recommended to replace the grub. Among various code snippets and directives, the grub.
These blocks always start with the menuentry keyword followed by a title, list of options, and an opening curly bracket, and end with a closing curly bracket. Anything between the opening and closing bracket should be indented. For example, the following is a sample menuentry block for Red Hat Enterprise Linux 7 with Linux kernel 3.
Then the initrd directives followed by the path to the kernel and the initramfs image respectively. This directive is called initrd because the previous tool which created initial RAM disk images, mkinitrd , created what were known as initrd files. The grub. Changes to the GRUB 2 menu can be made temporarily at boot time, made persistent for a single system while the system is running, or as part of making a new GRUB 2 configuration file.
On some systems, Home and End might also work. Edit the kernel parameters as required. For example, to run the system in emergency mode, add the emergency parameter at the end of the linux16 line:.
The rhgb and quiet parameters can be removed in order to enable system messages. These settings are not persistent and apply only for a single boot. To make persistent changes to a menu entry on a system, use the grubby tool. The grubby tool can be used to read information from, and make persistent changes to, the grub.
It enables, for example, changing GRUB 2 menu entries to specify what arguments to pass to a kernel on system start and changing the default kernel. If that file cannot be found it will search for an architecture dependent default. To make a persistent change in the kernel designated as the default kernel, use the grubby command as follows:. On UEFI systems, all grubby commands must be entered as root.
The --update-kernel option can be used to update a menu entry when used in combination with --args to add new arguments and --remove-arguments to remove existing arguments. These options accept a quoted space-separated list. The command to simultaneously add and remove arguments a from GRUB 2 menu entry has the follow format:. This command removes the Red Hat graphical boot argument, enables boot message to be seen, and adds a serial console.
As the console arguments will be added at the end of the line, the new console will take precedence over any other consoles configured. To review the changes, use the --info command option as follows:.
To add the same kernel boot arguments to all the kernel menu entries, enter a command as follows:. To change a value in an existing kernel argument, specify the argument again, changing the value as required. For example, to change the virtual console font size, use a command as follows:. To reflect the latest system boot options, the boot menu is rebuilt automatically when the kernel is updated or a new kernel is added. However, users may want to build a menu containing specific entries or to have the entries in a specific order.
GRUB 2 allows basic customization of the boot menu to give users control of what actually appears on the screen. The following files are included:. With such setting, the system always boots immediately on the default menu entry, and if the default kernel fails to boot, it is not possible to boot an older kernel.
The file can be viewed by the root user as follows:. To specify which operating system should be loaded first, pass its number to the grub2-set-default command. For example:. Note that the position of a menu entry in the list is denoted by a number starting with zero; therefore, in the example above, the third entry will be loaded. This value will be overwritten by the name of the next kernel to be installed.
To list the available menu entries, run the following command as root :. For reliability reasons, the symbolic link is not used in other examples in this chapter. It is better to use absolute paths when writing to a file, especially when repairing a system. On BIOS-based machines, issue the following command as root :. On UEFI-based machines, issue the following command as root :. Menu entries are also automatically added to the boot menu when updating the kernel. This file can be edited or copied.
Note that as a minimum, a valid menu entry must include at least the following:. Add a comment. Active Oldest Votes. Improve this answer. EvgEnZh 8 8 bronze badges.
For more CentOS 7-specific grub steps, check the official wiki: wiki. I see no point in doing readlink — poige. From the manual: new-kernel-package - tool to script kernel installation. HBruijn HBruijn 6, 21 21 silver badges 32 32 bronze badges.
Please edit your answer to include this information for future reference — pqnet. That is exactly why I started my answer with the steps needed to find out. I think querying the installation scripts is likely to be pretty universal in the RPM world in helping you understand how the kernel update is effected. Yeah HBruijn's right. I managed to do this on CentOS 7 with rpm -q --scripts kernel. The output wasn't that easy to decipher, but it's nice to have a definitive way to find the answer.
From the man page for readlink: -e, --canonicalize-existing canonicalize by following every symlink in every component of the given name recursively, all components must exist. Setaa Setaa 1 1 silver badge 4 4 bronze badges. GManNickG 4 4 bronze badges. Press any key during the time will interrupt the timeout. If the last boot failed, it will stuck at boot menu until you choose an entry and hit Enter. The value of the key is usually your Linux Distribution name, exported via a simple command.
Grub boot-loader supports themes! You can find a theme either in gnome-look. After editing the config file, or running grub-set-default command, you need to update the main config file to apply change.
Grub-Customizer is a free open-source tool to edit the Grub boot menu. It can do the most previous settings as easy as a few clicks. Including add, remove, edit, and rearrange the menu entries.
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