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kernel

Nov 28, 20244 min read

  • these vimlinux* ones are all you kernals

  • kernal parameters are stored /etc/default/grub In Linux, the GRUB parameters are typically stored in the file:

/etc/default/grub

This file contains the GRUB configuration options that control the behavior of the GRUB bootloader. For example, you can set kernel boot parameters, timeout settings, and default boot entries here.

Common Parameters in /etc/default/grub

  1. GRUB_CMDLINE_LINUX: Specifies kernel parameters.
  2. GRUB_TIMEOUT: Defines the timeout for the boot menu.
  3. GRUB_DEFAULT: Sets the default boot entry.

Regenerating GRUB Configuration

After making changes to /etc/default/grub, you need to regenerate the GRUB configuration file used at boot, usually located in /boot/grub2/grub.cfg (on Fedora) or /boot/grub/grub.cfg.

To regenerate the GRUB configuration, run:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

On systems using EFI, the path may differ:

sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

Kernal Parameters

Kernel parameters are options passed to the Linux kernel at boot time to control its behavior or configure system settings. These parameters can influence how the kernel initializes hardware, manages memory, interacts with devices, and other system-level operations.

How Kernel Parameters Work

  1. Passing Parameters: Kernel parameters are typically passed to the kernel through the bootloader (e.g., GRUB) configuration.
  2. Usage: They are used to enable or disable specific features, set system-wide limits, troubleshoot issues, or optimize performance.

Kernel parameters are options passed to the Linux kernel at boot time to control its behavior or configure system settings. These parameters can influence how the kernel initializes hardware, manages memory, interacts with devices, and other system-level operations.

How Kernel Parameters Work

  1. Passing Parameters: Kernel parameters are typically passed to the kernel through the bootloader (e.g., GRUB) configuration.
  2. Usage: They are used to enable or disable specific features, set system-wide limits, troubleshoot issues, or optimize performance.

Examples of Common Kernel Parameters

1. Hardware Configuration

  • acpi=off: Disables ACPI (Advanced Configuration and Power Interface) support, often used to troubleshoot hardware-related issues.
  • nomodeset: Disables kernel mode-setting for graphics, useful for resolving display problems.

2. Performance and Memory

  • isolcpus=2,3: Isolates specific CPU cores (e.g., cores 2 and 3) from the scheduler.
  • mem=2G: Limits the system to use only 2 GB of RAM.

3. Debugging and Troubleshooting

  • quiet: Suppresses kernel boot messages for a cleaner boot screen.
  • debug: Enables detailed kernel debug messages.
  • single: Boots the system into single-user mode for recovery.

4. Device Drivers

  • rdblacklist=modulename: Blacklists a specific kernel module from loading (e.g., rdblacklist=nouveau to prevent loading the Nouveau graphics driver).
  • snd_hda_intel.enable=1: Configures specific settings for the Intel HD Audio driver.

5. Filesystem

  • root=/dev/sda1: Specifies the root filesystem device.
  • ro: Mounts the root filesystem as read-only (used during the initial boot phase).

How to Add Kernel Parameters

  1. Edit GRUB Config:

    • Open /etc/default/grub for editing: 
      sudo nano /etc/default/grub
    • Modify the GRUB_CMDLINE_LINUX line, e.g.: 
      GRUB_CMDLINE_LINUX="quiet splash nomodeset"

  2. Update GRUB:

    • Regenerate the GRUB configuration: 
      sudo grub2-mkconfig -o /boot/grub2/grub.cfg
    • On EFI systems: 
      sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

  3. Reboot:

    • Restart the system for changes to take effect: 
      sudo reboot

Viewing Current Kernel Parameters

To see the kernel parameters currently in use:

cat /proc/cmdline

This command displays the parameters that were passed to the kernel during the boot process.

The rhgb parameter stands for Red Hat Graphical Boot. It is used in Red Hat-based Linux distributions, like Fedora, CentOS, and RHEL, to enable a graphical boot screen during the system’s startup process. Instead of displaying detailed kernel and system messages (known as the “boot log”), it shows a graphical splash screen.

Key Features of rhgb

  1. Hides Boot Messages:

    • It suppresses detailed system messages during the boot process.
    • Replaces text with a visually appealing progress bar or logo.

  2. Enhances User Experience:

    • Provides a cleaner, user-friendly boot experience for non-technical users.

  3. Switching to Verbose Mode:

    • If an error occurs during boot, pressing Esc (or a similar key) often switches to a detailed boot log view.

Disabling rhgb

Removing the rhgb parameter from the GRUB configuration allows you to see the verbose boot messages, which can be useful for troubleshooting.

  1. Edit GRUB Configuration:

    • Open the GRUB configuration file: 
      sudo nano /etc/default/grub
    • Remove rhgb from the GRUB_CMDLINE_LINUX line, e.g.: 
      GRUB_CMDLINE_LINUX="quiet"

  2. Update GRUB:

    • Regenerate the GRUB configuration: 
      sudo grub2-mkconfig -o /boot/grub2/grub.cfg
    • On EFI systems: 
      sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

  3. Reboot:

    • Restart your system: 
      sudo reboot

After rebooting, you’ll see detailed messages during startup instead of the graphical splash screen.

Common Use of rhgb with quiet

The quiet parameter is often used alongside rhgb to further reduce the visibility of kernel messages during boot. While rhgb manages the graphical splash, quiet minimizes textual output.

Example:

GRUB_CMDLINE_LINUX="quiet rhgb"

By removing one or both parameters, you can customize the boot experience to show more (or less) information.

Graph View

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