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Chapter 4. Configuring kernel command-line parameters

Kernel command-line parameters are a way to change the behavior of certain aspects of the Red Hat Enterprise Linux kernel at boot time. As a system administrator, you have full control over what options get set at boot. Certain kernel behaviors are only able to be set at boot time, so understanding how to make these changes is a key administration skill.

Important

Opting to change the behavior of the system by modifying kernel command-line parameters may have negative effects on your system. You should therefore test changes prior to deploying them in production. For further guidance, contact Red Hat Support.

4.1. Understanding kernel command-line parameters

Kernel command-line parameters are used for boot time configuration of:

  • The Red Hat Enterprise Linux kernel
  • The initial RAM disk
  • The user space features

Kernel boot time parameters are often used to overwrite default values and for setting specific hardware settings.

By default, the kernel command-line parameters for systems using the GRUB bootloader are defined in the kernelopts variable of the /boot/grub2/grubenv file for each kernel boot entry.

Note

For IBM Z, the kernel command-line parameters are stored in the boot entry configuration file because the zipl bootloader does not support environment variables. Thus, the kernelopts environment variable cannot be used.

Additional resources

4.2. What grubby is

grubby is a utility for manipulating boot loader configuration files.

You can also use grubby for changing the default boot entry, and for adding or removing arguments from a GRUB2 menu entry.

Additional resources

  • The grubby(8) manual page

4.3. What boot entries are

A boot entry is a collection of options which are stored in a configuration file and tied to a particular kernel version. In practice, you have at least as many boot entries as your system has installed kernels. The boot entry configuration file is located in the /boot/loader/entries/ directory and can look like this: 

6f9cc9cb7d7845d49698c9537337cedc-4.18.0-5.el8.x86_64.conf

The file name above consists of a machine ID stored in the /etc/machine-id file, and a kernel version.

The boot entry configuration file contains information about the kernel version, the initial ramdisk image, and the kernelopts environment variable, which contains the kernel command-line parameters. The example contents of a boot entry config can be seen below: 

title Red Hat Enterprise Linux (4.18.0-74.el8.x86_64) 8.0 (Ootpa)
version 4.18.0-74.el8.x86_64
linux /vmlinuz-4.18.0-74.el8.x86_64
initrd /initramfs-4.18.0-74.el8.x86_64.img $tuned_initrd
options $kernelopts $tuned_params
id rhel-20190227183418-4.18.0-74.el8.x86_64
grub_users $grub_users
grub_arg --unrestricted
grub_class kernel

The kernelopts environment variable is defined in the /boot/grub2/grubenv file.

Additional resources

4.4. Changing kernel command-line parameters for all boot entries

Change kernel command-line parameters for all boot entries on your system.

Prerequisites

  • Verify that the grubby utility is installed on your system.
  • Verify that the zipl utility is installed on your IBM Z system.

Procedure

  • To add a parameter: 

    # grubby --update-kernel=ALL --args="<NEW_PARAMETER>"

    For systems that use the GRUB bootloader, the command updates the /boot/grub2/grubenv file by adding a new kernel parameter to the kernelopts variable in that file.

    On IBM Z that use the zIPL bootloader, the command adds a new kernel parameter to each /boot/loader/entries/<ENTRY>.conf file.

    • On IBM Z, execute the zipl command with no options to update the boot menu.

  • To remove a parameter: 

    # grubby --update-kernel=ALL --remove-args="<PARAMETER_TO_REMOVE>"
    • On IBM Z, execute the zipl command with no options to update the boot menu.

  • After each update of your kernel package, propagate the configured kernel options to the new kernels: 

    # grub2-mkconfig -o /etc/grub2.cfg
    Important

    Newly installed kernels do not inherit the kernel command-line parameters from your previously configured kernels. You must run the grub2-mkconfig command on the newly installed kernel to propagate the needed parameters to your new kernel.

Additional resources

4.5. Changing kernel command-line parameters for a single boot entry

Make changes in kernel command-line parameters for a single boot entry on your system.

Prerequisites

  • Verify that the grubby and zipl utilities are installed on your system.

Procedure

  • To add a parameter: 

    #  grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="<NEW_PARAMETER>"
    • On IBM Z, execute the zipl command with no options to update the boot menu.

  • To remove a parameter use the following: 

    #  grubby --update-kernel=/boot/vmlinuz-$(uname -r) --remove-args="<PARAMETER_TO_REMOVE>"
    • On IBM Z, execute the zipl command with no options to update the boot menu.
Note

On systems that use the grub.cfg file, there is, by default, the options parameter for each kernel boot entry, which is set to the kernelopts variable. This variable is defined in the /boot/grub2/grubenv configuration file.

Important

On GRUB2 systems:

  • If the kernel command-line parameters are modified for all boot entries, the grubby utility updates the kernelopts variable in the /boot/grub2/grubenv file.
  • If kernel command-line parameters are modified for a single boot entry, the kernelopts variable is expanded, the kernel parameters are modified, and the resulting value is stored in the respective boot entry’s /boot/loader/entries/<RELEVANT_KERNEL_BOOT_ENTRY.conf> file.

On zIPL systems:

  • grubby modifies and stores the kernel command-line parameters of an individual kernel boot entry in the /boot/loader/entries/<ENTRY>.conf file.

Additional resources

4.6. Changing kernel command-line parameters temporarily at boot time

Make temporary changes to a Kernel Menu Entry by changing the kernel parameters only during a single boot process.

Procedure

  1. Select the kernel you want to start when the GRUB 2 boot menu appears and press the e key to edit the kernel parameters.
  2. Find the kernel command line by moving the cursor down. The kernel command line starts with linux on 64-Bit IBM Power Series and x86-64 BIOS-based systems, or linuxefi on UEFI systems.

  3. Move the cursor to the end of the line.

    Note

    Press Ctrl+a to jump to the start of the line and Ctrl+e to jump to the end of the line. On some systems, Home and End keys might also work.

  4. Edit the kernel parameters as required. For example, to run the system in emergency mode, add the emergency parameter at the end of the linux line: 

    linux   ($root)/vmlinuz-4.18.0-348.12.2.el8_5.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet emergency

    To enable the system messages, remove the rhgb and quiet parameters.

  5. Press Ctrl+x to boot with the selected kernel and the modified command line parameters.
Important

Press Esc key to leave command line editing and it will drop all the user made changes.

Note

This procedure applies only for a single boot and does not persistently make the changes.

4.7. Configuring GRUB settings to enable serial console connection

The serial console is beneficial when you need to connect to a headless server or an embedded system and the network is down. Or when you need to avoid security rules and obtain login access on a different system.

You need to configure some default GRUB settings to use the serial console connection.

Prerequisites

  • You have root permissions.

Procedure

  1. Add the following two lines to the /etc/default/grub file: 

    GRUB_TERMINAL="serial"
GRUB_SERIAL_COMMAND="serial --speed=9600 --unit=0 --word=8 --parity=no --stop=1"

    The first line disables the graphical terminal. The GRUB_TERMINAL key overrides values of GRUB_TERMINAL_INPUT and GRUB_TERMINAL_OUTPUT keys.

    The second line adjusts the baud rate (--speed), parity and other values to fit your environment and hardware. Note that a much higher baud rate, for example 115200, is preferable for tasks such as following log files.

  2. Update the GRUB configuration file.

    • On BIOS-based machines: 

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

    • On UEFI-based machines: 

      # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
  3. Reboot the system for the changes to take effect.