Kernel parameters - ArchWiki

Jump to content

# 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:

- You can check the parameters your system was booted up with by running `cat /proc/cmdline` and see if it includes your changes.
- The Arch Linux installation medium uses Syslinux for BIOS systems, and systemd-boot for UEFI systems.

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.

- 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

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

- "The kernel’s command-line parameters" documentation
- Power saving#Kernel parameters
- List of kernel parameters with further explanation and grouped by similar options

Category:

- Kernel