Kernel parameters

Related articles
Kernel
There are three ways to pass options to the kernel and thus control its behaviour:
When building the kernel—in the kernel's
config
file. See
Kernel#Compilation
for details.
When starting the kernel—using command line parameters (usually through a
boot loader
, or as well in
unified kernel image
).
At runtime—through the files in
/proc/sys/
(see
sysctl
) and
/sys/
.
Note
The options of loadable modules can be set via
.conf
files in
/etc/modprobe.d/
. See
Kernel module#Using modprobe.d
.
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
, or
module
.
parameter
=
value
.
Note
You can check the parameters your system was booted up with by running
cat /proc/cmdline
and see if it includes your changes.
All kernel parameters are case-sensitive.
Boot loader configuration
Note
The Arch Linux
installation medium
uses
systemd-boot
for
UEFI
systems, and
Syslinux
for
BIOS
ones.
Kernel parameters can be set either temporarily by editing the boot entry in the boot loader boot selection menu, or permanently by modifying the boot loader configuration file.
The following examples add the
quiet
and
splash
parameters to the
Clover
,
GRUB
,
GRUB Legacy
,
LILO
,
Limine
,
rEFInd
,
Syslinux
and
systemd-boot
boot loaders.
Clover
Add them to
esp
/EFI/CLOVER/config.plist
:
<string>initrd=\initramfs-linux.img root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw
quiet splash
</string>
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.
To make the change persistent after reboot, you could
manually edit /boot/grub/grub.cfg
with the exact line from above, or if using
grub-mkconfig
:
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
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.
LILO
Add them to
/etc/lilo.conf
using
append
or
addappend
:
image=/boot/vmlinuz-linux
        ...
        append="
quiet splash
"
Limine
To temporarily add kernel parameters, press
e
when the boot entry selection screen appears and modify the
cmdline
line:
cmdline: root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw
quiet splash
To apply changes permanently, edit the
kernel_cmdline
line in the Limine configuration file:
/+Arch Linux
    ...
    kernel_cmdline: root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw
quiet splash
kernel_cmdline
is alias of
cmdline
rEFInd
Press
Insert
,
F2
,
Tab
, or
+
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
"
    ...
}
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
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
If you are using a
unified kernel image
to boot, edit
/etc/kernel/cmdline
.
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
. Note that this only works for parameters understood by dracut, like
root=
and
rd.*
. They do not become real kernel parameters.
EFI boot stub
See
EFI boot stub#Using UEFI directly
.
Hijacking cmdline
Even without access to your boot loader 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.
Note
This trick only hijacks userspace processes which read
/proc/cmdline
. It does not work for parameters read by the kernel itself. For example, adding
debug
this way will not enable kernel debugging.
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 --bind -o ro /root/cmdline /proc/cmdline
You can
cat /proc/cmdline
to confirm that your change was successful.
Parameter list
This list is not comprehensive. In addition to the kernel itself, other programs can also read parameters from
/proc/cmdline
and change their behavior.
For a mostly complete list of options understood by the kernel and
boot loaders
, see
The kernel's command-line parameters
. A basic list is in
bootparam(7)
.
For options understood by
systemd
, see
kernel-command-line(7)
.
For options understood by
mkinitcpio
with a busybox based initial ramdisk, see
mkinitcpio(8) § EARLY INIT ENVIRONMENT
and
Mkinitcpio#Runtime customization
.
For options understood by
dracut
, see
dracut.cmdline(7)
.
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 an
EFI boot stub
, 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. This is
mkinitcpio
's default
1
.
root
Root file system. See
init/do_mounts.c
for kernel's supported device name formats. Note that an
initramfs
with
udev
supports
more name formats
. A setup compatible with
systemd#GPT partition automounting
allows to omit the parameter entirely or to alternatively use
root=dissect
.
rootflags
Root file system 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 or
subvol=
option of
Btrfs when using a subvolume as root
.
rw
Mount root device read-write on boot. This is the kernel's default
1
.
systemd.unit
Boot to a
specified target
.
video
Override framebuffer video defaults.
The kernel uses
rw
if neither
ro
or
rw
are explicitly set on kernel command line (see
bootparam(7) § General non-device-specific boot arguments
). However,
mkinitcpio
uses
ro
as the default value overriding the kernel's default (see
mkinitcpio(8) § EARLY INIT ENVIRONMENT
). Boot loaders may also have their own configured default, for example,
grub-mkconfig
uses
rw
(see
FS#36275
as a reference).
Note
rw
is required when using mkinitcpio's
fsck hook
(see
[1]
) or
when using F2FS as the root file system
.
See also
bootparam(7)
kernel-command-line(7)
Power saving#Kernel parameters
Kernel Boot Command-Line Parameter Reference
—the chapter 9 of the
Linux Kernel in a Nutshell
by
Greg Kroah-Hartman