Difference between revisions of "Kernel parameters"

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[[Category:Kernel]]
 
[[Category:Kernel]]
 
[[es:Kernel parameters]]
 
[[es:Kernel parameters]]
[[ja:Kernel parameters]]
+
[[ja:カーネルパラメータ]]
 +
[[ru:Kernel parameters]]
 
[[zh-CN:Kernel parameters]]
 
[[zh-CN:Kernel parameters]]
 
There are three ways to pass options to the kernel and thus control its behaviour:
 
There are three ways to pass options to the kernel and thus control its behaviour:
  
# When building the kernel.
+
# When building the kernel. See [[Kernel Compilation]] for details.
 
# When starting the kernel (usually, when invoked from a boot loader).
 
# When starting the kernel (usually, when invoked from a boot loader).
# At runtime (through the files in {{ic|/proc}} and {{ic|/sys}}).
+
# At runtime (through the files in {{ic|/proc}} and {{ic|/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.
 
This page now explains in more detail the second method and shows a list of most used kernel parameters in Arch Linux.
  
 
== Configuration ==
 
== Configuration ==
 +
 +
{{Note|
 +
* You can check the parameters your system was booted up with by running {{ic|$ cat /proc/cmdline}} and see if it includes your changes.
 +
* The Arch Linux [https://www.archlinux.org/download/ installation medium] uses [[Syslinux]] for [[Wikipedia:BIOS|BIOS]] systems, and [[systemd-boot]] for [[UEFI]] systems.}}
  
 
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.
 
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.
  
Here we are adding the parameters {{ic|quiet}} and {{ic|splash}} to [[Syslinux]], [[GRUB]], [[GRUB Legacy]] and [[LILO]].
+
The following examples add the {{ic|quiet}} and {{ic|splash}} parameters to [[Syslinux]], [[systemd-boot]], [[GRUB]], [[GRUB Legacy]], [[LILO]], and [[rEFInd]].
  
==== Syslinux ====
+
=== Syslinux ===
  
* Press {{Keypress|Tab}} when the menu shows up and add them at the end of the string:
+
* Press {{ic|Tab}} when the menu shows up and add them at the end of the string:
  
 
: {{bc|1=linux /boot/vmlinuz-linux root=/dev/sda3 initrd=/boot/initramfs-linux.img ''quiet splash''}}
 
: {{bc|1=linux /boot/vmlinuz-linux root=/dev/sda3 initrd=/boot/initramfs-linux.img ''quiet splash''}}
  
: Press {{Keypress|Enter}} to boot with these parameters.
+
: Press {{ic|Enter}} to boot with these parameters.
  
 
* To make the change persistent after reboot, edit {{ic|/boot/syslinux/syslinux.cfg}} and add them to the {{ic|APPEND}} line:
 
* To make the change persistent after reboot, edit {{ic|/boot/syslinux/syslinux.cfg}} and add them to the {{ic|APPEND}} line:
Line 31: Line 36:
 
For more information on configuring Syslinux, see the [[Syslinux]] article.
 
For more information on configuring Syslinux, see the [[Syslinux]] article.
  
==== GRUB ====
+
=== systemd-boot ===
  
* Press {{Keypress|e}} when the menu shows up and add them on the {{ic|linux}} line:
+
* Press {{ic|e}} when the menu appears and add the parameters to the end of the string:
 +
 
 +
: {{bc|1=initrd=\initramfs-linux.img root=/dev/sda2 ''quiet splash''}}
 +
 
 +
: Press {{ic|Enter}} to boot with these parameters.
 +
 
 +
{{Note|If you have not set a value for menu timeout, you will need to hold {{ic|Space}} while booting for the systemd-boot menu to appear.}}
 +
 
 +
* To make the change persistent after reboot, edit {{ic|/boot/loader/entries/arch.conf}} (assuming you set up your [[EFI System Partition]]) and add them to the {{ic|options}} line:
 +
 
 +
: {{bc|1=options root=/dev/sda2 ''quiet splash''}}
 +
 
 +
For more information on configuring systemd-boot, see the [[systemd-boot]] article.
 +
 
 +
=== GRUB ===
 +
 
 +
* Press {{ic|e}} when the menu shows up and add them on the {{ic|linux}} line:
  
 
: {{bc|1=linux /boot/vmlinuz-linux root=UUID=978e3e81-8048-4ae1-8a06-aa727458e8ff ''quiet splash''}}
 
: {{bc|1=linux /boot/vmlinuz-linux root=UUID=978e3e81-8048-4ae1-8a06-aa727458e8ff ''quiet splash''}}
  
: Press {{Keypress|b}} to boot with these parameters.
+
: Press {{ic|Ctrl+x}} to boot with these parameters.
  
* To make the change persistent after reboot, while you ''could'' manually edit {{ic|/boot/grub/grub.cfg}} with the exact line from above, for beginners it's recommended to:
+
* To make the change persistent after reboot, while you ''could'' manually edit {{ic|/boot/grub/grub.cfg}} with the exact line from above, the best practice is to:
  
 
:Edit {{ic|/etc/default/grub}} and append your kernel options to the {{ic|GRUB_CMDLINE_LINUX_DEFAULT}} line:
 
:Edit {{ic|/etc/default/grub}} and append your kernel options to the {{ic|GRUB_CMDLINE_LINUX_DEFAULT}} line:
Line 51: Line 72:
 
For more information on configuring GRUB, see the [[GRUB]] article.
 
For more information on configuring GRUB, see the [[GRUB]] article.
  
==== GRUB Legacy ====
+
=== GRUB Legacy ===
  
* Press {{Keypress|e}} when the menu shows up and add them on the {{ic|kernel}} line:
+
* Press {{ic|e}} when the menu shows up and add them on the {{ic|kernel}} line:
  
 
: {{bc|1=kernel /boot/vmlinuz-linux root=/dev/sda3 ''quiet splash''}}
 
: {{bc|1=kernel /boot/vmlinuz-linux root=/dev/sda3 ''quiet splash''}}
  
: Press {{Keypress|b}} to boot with these parameters.
+
: Press {{ic|b}} to boot with these parameters.
  
 
* To make the change persistent after reboot, edit {{ic|/boot/grub/menu.lst}} and add them to the {{ic|kernel}} line, exactly like above.
 
* To make the change persistent after reboot, edit {{ic|/boot/grub/menu.lst}} and add them to the {{ic|kernel}} line, exactly like above.
Line 63: Line 84:
 
For more information on configuring GRUB Legacy, see the [[GRUB Legacy]] article.
 
For more information on configuring GRUB Legacy, see the [[GRUB Legacy]] article.
  
==== LILO ====
+
=== LILO ===
  
 
* Add them to {{ic|/etc/lilo.conf}}:
 
* Add them to {{ic|/etc/lilo.conf}}:
Line 73: Line 94:
  
 
For more information on configuring LILO, see the [[LILO]] article.
 
For more information on configuring LILO, see the [[LILO]] article.
 +
 +
=== rEFInd ===
 +
 +
* To make the change persistent after reboot, edit {{ic|/boot/refind_linux.conf}} and append them to all/required lines, for example
 +
 +
: {{bc|1="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 {{ic|''esp''/refind/refind.conf}} to load your OSes, you can edit it like:
 +
 +
: {{bc|<nowiki>
 +
menuentry "Arch Linux" {
 +
...
 +
options  "root=PARTUUID=978e3e81-8048-4ae1-8a06-aa727458e8ff rw quiet splash"
 +
...
 +
}
 +
</nowiki>}}
 +
 +
For more information on configuring kernel parameters in rEFInd, see [http://www.rodsbooks.com/refind/linux.html 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 {{ic|/proc/cmdline}} which stores the kernel parameters. However {{ic|/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
 +
 +
{{hc|/root/cmdline|2=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 {{ic|-n}} option skips adding the mount to {{ic|/etc/mtab}}, so it will work even if root is mounted read-only. You can {{ic|cat /proc/cmdline}} to confirm that your change was successful.
  
 
== Parameter list ==
 
== Parameter list ==
Line 80: Line 137:
 
{{Note|Not all of the listed options are always available. Most 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.}}
 
{{Note|Not all of the listed options are always available. Most 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.}}
  
{| border="1"
+
{| class="wikitable"
 
!parameter!!Description
 
!parameter!!Description
 
|-
 
|-
|{{ic|1=root=}}|| Root filesystem.
+
| root= || Root filesystem.
 
|-
 
|-
|{{ic|1=ro}}|| Mount root device read-only on boot.
+
| rootflags= || Root filesystem mount options.
 
|-
 
|-
|{{ic|1=rw}}|| Mount root device read-write on boot (default).
+
| ro || Mount root device read-only on boot (default<sup>1</sup>).
 
|-
 
|-
|{{ic|1=initrd=}}|| Specify the location of the initial ramdisk.
+
| rw || Mount root device read-write on boot.
 
|-
 
|-
|{{ic|1=init=}}|| Run specified binary instead of {{ic|/sbin/init}} (symlinked to [[systemd]] in Arch) as init process.
+
| initrd=|| Specify the location of the initial ramdisk.
 
|-
 
|-
|{{ic|1=init=/bin/sh}}|| Boot to shell.
+
| init= || Run specified binary instead of {{ic|/sbin/init}} (symlinked to [[systemd]] in Arch) as init process.
 
|-
 
|-
|{{ic|1=systemd.unit=}}||
+
| init=/bin/sh || Boot to shell.
 
|-
 
|-
|{{ic|1=systemd.unit=multi-user}}|| Boot to a specified runlevel.
+
| systemd.unit= || Boot to a [[systemd#Targets table|specified target]].
 
|-
 
|-
|{{ic|1=systemd.unit=rescue}}|| Boot to single-user mode (root).
+
| nomodeset || Disable [[Kernel mode setting]].
 
|-
 
|-
|{{ic|nomodeset}}|| Disable [[Kernel Mode Setting]].
+
| zswap.enabled || Enable [[Zswap]].
 
|-
 
|-
 +
| video=<videosetting> || Override framebuffer video defaults.
 
|}
 
|}
 +
 +
<sup>1</sup> [[mkinitcpio]] uses {{ic|ro}} as default value when neither {{ic|rw}} or {{ic|ro}} is set by the [[boot loader]]. Boot loaders may set the value to use, for example GRUB uses {{ic|rw}} by default (see {{Bug|36275}} as a reference).
  
 
For a complete list of all options, please see the [https://www.kernel.org/doc/Documentation/kernel-parameters.txt kernel documentation].
 
For a complete list of all options, please see the [https://www.kernel.org/doc/Documentation/kernel-parameters.txt kernel documentation].
Line 109: Line 169:
 
== See also ==
 
== See also ==
  
* [[sysctl]]
+
* [https://www.kernel.org/doc/Documentation/kernel-parameters.txt Linux "Kernel Parameters" documentation]
 
* [[Power saving#Kernel parameters]]
 
* [[Power saving#Kernel parameters]]
 
* [http://files.kroah.com/lkn/lkn_pdf/ch09.pdf List of kernel parameters with further explanation and grouped by similar options]
 
* [http://files.kroah.com/lkn/lkn_pdf/ch09.pdf List of kernel parameters with further explanation and grouped by similar options]

Latest revision as of 08:50, 7 September 2016

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

  1. When building the kernel. See Kernel Compilation for details.
  2. When starting the kernel (usually, when invoked from a boot loader).
  3. 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.

Configuration

Note:

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 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.
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.
  • 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.

GRUB

  • 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, 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 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.

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.

Note: Not all of the listed options are always available. Most 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.
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.

See also