Kernel parameters

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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.

Configuration

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=/dev/sda3 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=/dev/sda3 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=/dev/sda2 quiet splash

Press Enter to boot with these parameters.

• 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=/dev/sda2 quiet splash

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

• Press e when the menu shows up and add them on the linux line:

linux /boot/vmlinuz-linux root=UUID=978e3e81-8048-4ae1-8a06-aa727458e8ff quiet splash

Press Ctrl+x to boot with these parameters.

• To make the change persistent after reboot, you could manually edit /boot/grub/grub.cfg with the exact line from above, but the best practice is to:

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=/dev/sda3 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.

• 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=PARTUUID=978e3e81-8048-4ae1-8a06-aa727458e8ff 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=PARTUUID=978e3e81-8048-4ae1-8a06-aa727458e8ff 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=PARTUUID=978e3e81-8048-4ae1-8a06-aa727458e8ff 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=/dev/disk/by-label/ROOT 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 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. ro Mount root device read-only on boot (default1). rw Mount root device read-write on boot. initrd= Specify the location of the initial ramdisk. For UEFI boot managers and EFISTUB, the path must be specified using backslashes (\) as path separators. 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. init=/bin/sh Boot to shell. systemd.unit= Boot to a specified target. resume= Specify a swap device to use when waking from hibernation. nomodeset Disable Kernel mode setting. zswap.enabled Enable Zswap. panic= Time before automatic reboot on kernel panic. debug Enable kernel debugging (events log level). mem= Force usage of a specific amount of memory to be used. maxcpus= Maximum number of processors that an SMP kernel will bring up during bootup. selinux= Disable or enable SELinux at boot time. netdev= Network devices parameters. video= Override framebuffer video defaults.

1mkinitcpio 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