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 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.

4.1. What are 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 config file because the zipl bootloader does not support environment variables. Thus, the kernelopts environment variable cannot be used.

Additional resources

  • For more information about what kernel command line parameters you can modify, see kernel-command-line(7), bootparam(7) and dracut.cmdline(7) manual pages.

4.2. What is grubby

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.

4.3. What are boot entries

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.

4.4. Setting kernel command line parameters

This section explains how to change kernel command line parameters on the AMD64 and Intel 64 architectures, the 64-bit ARM architectures, and the little-endian variant of IBM Power Systems.

4.4.1. 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

Procedure

  1. Open the /etc/default/grub file with the vim editor: 

    # vim /etc/default/grub
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="crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true
  2. Add, edit, or remove a parameter on the GRUB_CMDLINE_LINUX line.

  3. Update the GRUB2 configuration file: 

    # grub2-mkconfig -o /boot/grub2/grub.cfg
  4. Reboot your system for the changes to take effect.

As a result, the boot loader is reconfigured, and the kernel command line parameters that you specified are applied.

Additional resources

  • For more information on how to modify the kernel parameters using the GRUB2 configuration file, see Editing a Menu Entry.

4.4.2. 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

Procedure

  • To add a parameter execute the following: 

    # grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="<NEW_PARAMETER>"

  • To remove a parameter use the following: 

    # grubby --update-kernel=/boot/vmlinuz-$(uname -r) --remove-args="<PARAMETER_TO_REMOVE>"
Note

By default, there is 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

When you use the grubby utility to modify a specific boot entry, the contents of the edited kernelopts are stored in the relevant kernel boot entry in /boot/loader/entries/<RELEVANT_KERNEL_BOOT_ENTRY.conf> and will override the value of kernelopts set in /boot/grub2/grubenv.

Additional resources

  • For further examples on how to use grubby see grubby tool.