Kernel parameters

From ArchWiki

There are three ways to pass options to the kernel and thus control its behaviour:

  1. When building the kernel—in the kernel's config file. See Kernel#Compilation for details.
  2. When starting the kernel—using command line parameters (usually through a boot loader).
  3. At runtime—through the files in /proc/sys/ (see sysctl) and /sys/.

Between the three methods, the configurable options differ in availability, their name and the method in which they are specified. This page only explains the second method (kernel command line parameters) and shows a list of the most used kernel parameters in Arch Linux.

Most parameters are associated with subsystems and work only if the kernel is configured with those subsystems built in. They also depend on the presence of the hardware they are associated with.

Kernel command line parameters either have the format parameter or parameter=value.

Note: All kernel parameters are case-sensitive.
Tip: The options of loadable modules can be set via .conf files in /etc/modprobe.d/. See Kernel module#Using files in /etc/modprobe.d/.

Configuration

Note:

Kernel parameters can be set either temporarily by editing the boot entry in the boot loader's boot selection menu, or permanently by modifying the boot loader's configuration file.

The following examples add the quiet and splash parameters to Syslinux, systemd-boot, GRUB, GRUB Legacy, LILO, and rEFInd.

Syslinux

  • Press Tab when the menu shows up and add them at the end of the string:
linux /boot/vmlinuz-linux root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw initrd=/boot/initramfs-linux.img quiet splash
Press Enter to boot with these parameters.
  • To make the change persistent after reboot, edit /boot/syslinux/syslinux.cfg and add them to the APPEND line:
APPEND root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash

For more information on configuring Syslinux, see the Syslinux article.

systemd-boot

  • Press e when the menu appears and add the parameters to the end of the string:
initrd=\initramfs-linux.img root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash
Press Enter to boot with these parameters.
Note:
  • If you have not set a value for menu timeout, you will need to hold Space while booting for the systemd-boot menu to appear.
  • If you cannot edit the parameters from the boot menu, you may need to edit /boot/loader/loader.conf and add editor 1 to enable editing.
  • To make the change persistent after reboot, edit /boot/loader/entries/arch.conf (assuming you set up your EFI system partition) and add them to the options line:
options root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash

For more information on configuring systemd-boot, see the systemd-boot article.

GRUB

  • Press e when the menu shows up and add them on the linux line:
linux /boot/vmlinuz-linux root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash
Press Ctrl+x to boot with these parameters.
Edit /etc/default/grub and append your kernel options between the quotes in the GRUB_CMDLINE_LINUX_DEFAULT line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
And then automatically re-generate the grub.cfg file with:
# grub-mkconfig -o /boot/grub/grub.cfg

For more information on configuring GRUB, see the GRUB article.

GRUB Legacy

  • Press e when the menu shows up and add them on the kernel line:
kernel /boot/vmlinuz-linux root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash
Press b to boot with these parameters.
  • To make the change persistent after reboot, edit /boot/grub/menu.lst and add them to the kernel line, exactly like above.

For more information on configuring GRUB Legacy, see the GRUB Legacy article.

LILO

  • Add them to /etc/lilo.conf:
image=/boot/vmlinuz-linux
        ...
        quiet splash

For more information on configuring LILO, see the LILO article.

rEFInd

  • Press +, F2, or Insert on the desired menu entry and press it again on the submenu entry. Add kernel parameters at the end of the string:
root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw initrd=\boot\initramfs-linux.img quiet splash
Press Enter to boot with these parameters.
  • To make the change persistent after reboot, edit /boot/refind_linux.conf and append them between the quotes in all required lines, for example
"Boot using default options"   "root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash"
  • If you have disabled auto-detection of OSes in rEFInd and are defining OS stanzas instead in esp/EFI/refind/refind.conf to load your OSes, you can edit it like:
menuentry "Arch Linux" {
	...
	options  "root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet splash"
	...
}

For more information on configuring rEFInd, see the rEFInd article.

EFISTUB

See EFISTUB#Using UEFI directly.

dracut

dracut is capable of embedding the kernel parameters in the initramfs, thus allowing to omit them from the boot loader configuration. See dracut#Kernel command line options.

Hijacking cmdline

Even without access to your bootloader it is possible to change your kernel parameters to enable debugging (if you have root access). This can be accomplished by overwriting /proc/cmdline which stores the kernel parameters. However /proc/cmdline is not writable even as root, so this hack is accomplished by using a bind mount to mask the path.

First create a file containing the desired kernel parameters 

/root/cmdline
root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 ro console=tty1 logo.nologo debug

Then use a bind mount to overwrite the parameters 

# mount -n --bind -o ro /root/cmdline /proc/cmdline

The -n option skips adding the mount to /etc/mtab, so it will work even if root is mounted read-only. You can cat /proc/cmdline to confirm that your change was successful.

Parameter list

This list is not comprehensive. For a complete list of all options, please see the kernel documentation.

parameter Description
init Run specified binary instead of /sbin/init as init process. The systemd-sysvcompat package symlinks /sbin/init to /usr/lib/systemd/systemd to use systemd. Set it to /bin/sh to boot to the shell.
initrd Specify the location of the initial ramdisk. For UEFI boot managers and EFISTUB, the path must be specified using backslashes (\) as path separators.
cryptdevice Specify the location of a dm-crypt-encrypted partition plus a device mapper name.
debug Enable kernel debugging (events log level).
lsm Set the initialisation order of the Linux security modules, used to enable AppArmor, SELinux or TOMOYO.
maxcpus Maximum number of processors that an SMP kernel will bring up during bootup.
mem Force usage of a specific amount of memory to be used.
netdev Network devices parameters.
nomodeset Disable Kernel mode setting.
panic Time before automatic reboot on kernel panic.
resume Specify a swap device to use when waking from hibernation.
ro Mount root device read-only on boot (default1).
root Root filesystem. See init/do_mounts.c for kernel's supported device name formats. Note that an initramfs with udev supports more name formats.
rootflags Root filesystem mount options. Useful for setting options that cannot be applied by remounting (i.e. by systemd-remount-fs.service(8)). For example, the discard option of an XFS root volume.
rw Mount root device read-write on boot.
systemd.unit Boot to a specified target.
video Override framebuffer video defaults.

1 mkinitcpio uses ro as default value when neither rw or ro is set by the boot loader. Boot loaders may set the value to use, for example GRUB uses rw by default (see FS#36275 as a reference).

See also