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Chapter 5. 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 this 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.

5.1. 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 GRUB2 bootloader are defined in the kernelopts variable of the /boot/grub2/grubenv file for all kernel boot entries.

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

5.2. What grubby is

grubby is a utility for manipulating bootloader-specific configuration files.

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

For more details see the grubby(8) manual page.

5.3. 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 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

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

This procedure describes how to change kernel command-line parameters for all boot entries on your system.

Prerequisites

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

Procedure

  • To add a parameter: 

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

    For systems that use the GRUB2 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.

Additional resources

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

This procedure describes how to change 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

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

The following procedure allows you to 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 linux16 line: 

    linux16   /vmlinuz-3.10.0-0.rc4.59.el7.x86_64 root=/dev/mapper/rhel-root ro rd.md=0 rd.dm=0 rd.lvm.lv=rhel/swap crashkernel=auto rd.luks=0 vconsole.keymap=us rd.lvm.lv=rhel/root 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.