Kernel parameters
There are three ways to pass options to the kernel and thus control its behaviour:
- When building the kernel. See Kernel Compilation for details.
- When starting the kernel (usually, when invoked from a boot loader).
- At runtime (through the files in
/proc
and/sys
). See sysctl for details.
This page now explains in more detail the second method and shows a list of most used kernel parameters in Arch Linux.
Contents
Configuration
Kernel parameters can be set either temporarily by editing the boot menu when it shows up, or 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 theAPPEND
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 theoptions
line:
-
options root=/dev/sda2 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 thelinux
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, while you could manually edit
/boot/grub/grub.cfg
with the exact line from above, the best practice is to:
- Edit
/etc/default/grub
and append your kernel options to theGRUB_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 thekernel
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 thekernel
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
- To make the change persistent after reboot, edit
/boot/refind_linux.conf
and append them to 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/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 kernel parameters in rEFInd, see Configuring the rEFInd Bootmanager
EFISTUB
See EFISTUB#Using UEFI directly.
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
Parameters always come in parameter
or parameter=value
. All of these parameters are case-sensitive.
parameter | Description |
---|---|
root= | Root filesystem. |
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. |
init= | Run specified binary instead of /sbin/init (symlinked to systemd in Arch) as init process.
|
init=/bin/sh | Boot to shell. |
systemd.unit= | Boot to a specified target. |
nomodeset | Disable Kernel mode setting. |
zswap.enabled | Enable Zswap. |
video=<videosetting> | 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).
For a complete list of all options, please see the kernel documentation.