Ubuntu - How To Edit Grub Boot Parameters

The reason that you will usually want to change boot parameters is to configure the default kernel to something specific to your hardware. Grub allows you to change a boot parameter temporarily , in which case it won't survive a system reboot, or permanently, which means that it will. When trying to fix something on your machine using kernel parameters, a reasonably safe way to proceed is to first make the changes temporarily, check that you get what you expect, and then make the change permanent.

If changing kernel boot parameters sound scary, it isn't. None of the boot parameters that I have come across are destructive in any way. The worst result could be that your machine hangs until you restart it. Even if you make the change permanently, you always still have the opportunity of intercepting the Grub boot and changing the parameter back.

1. Changing Boot Parameters Temporarily

Generally, this is used when you want to try a parameter change to see it is beneficial to your system. When the PC boots up, you will see the Grub countdown, which is set to 3 seconds by default. Press "Esc" to intercept this countdown and go enter a Grub menu. Then

• Press 'e' to start editing.
• Scroll down to the "kernel..." line. The is the line that tells Grub which kernel to boot with and the parameters to be passed to the kernel when it boots are placed at the end of this line.
  
• Press 'e' again to edit this line.
• Move to the end of the line. You will see any existing parameters and can add other new parameters to the end.
• Parameters are separated by spaces and are mostly either a single word (e.g. nolapic), or an equation (e.g. acpi=off).
• Once you have added the parameter to the end of the line, press Enter to accept the editing.
• Then press 'b' to boot using that kernel and those parameters.

Remember, this change is not permanent. At next boot, it will be undone.

2. Making Boot Parameter Changes Permanent

Assuming that you booted with a new or changed parameter and would now like to make this a more permanent change, you need to edit the file that contains these settings. This file (/boot/grub/menu.lst) is owned by root and, therefore, requires root privileges to edit. There are a number of ways to do this depending on your editor of choice. For me

sudo vi /boot/grub/menu.lst

works. If you are not familiar with vi, then it might by something along the lines of

gksudo gedit /boot/grub/menu.lst

will edit this file in the Ubuntu gedit graphical text editor.

Now, there are two flavours of permanent to consider. The first is to make the change survive reboots, the second is to survive an upgrade.

2.1 Making a change survive reboots

Find the section shown below. The details will vary depending on which kernel(s) are installed on your machine.

## ## End Default Options ##

title Ubuntu, kernel 2.6.20-15-generic
root (hd0,0)
kernel /boot/vmlinuz-2.6.20-15-generic root=UUID=fccafcc7-d7cc-4594-9459-a8f0db7b9f7f ro quiet splash
initrd /boot/initrd.img-2.6.20-15-generic
quiet
savedefault

Add your parameters to the end of the line starting with "kernel...". This should be the same line that you would edit in the temporary change scenario above. Save the file and the change will survive a reboot.

2.2 Making a change survive upgrades

Depending on how frequently you upgrade your system, you might find that when a new kernel is installed, or grub is upgraded (there may be other scenarios), the /boot/grub/menu.lst file may be replaced by a newer version, or a new line might be added for the new kernel. In this case, your "permanent" changes might be as permanent as you thought.

The best way to address this is to change the "#kopt..." line in /boot/grub/menu.lst. Note that the line is not commented out, even though it starts with a "#". See the excerpt below.

### BEGIN AUTOMAGIC KERNELS LIST
## lines between the AUTOMAGIC KERNELS LIST markers will be modified
## by the debian update-grub script except for the default options below

## DO NOT UNCOMMENT THEM, Just edit them to your needs

## ## Start Default Options ##
## default kernel options
## default kernel options for automagic boot options
## If you want special options for specific kernels use kopt_x_y_z
## where x.y.z is kernel version. Minor versions can be omitted.
## e.g. kopt=root=/dev/hda1 ro
## kopt_2_6_8=root=/dev/hdc1 ro
## kopt_2_6_8_2_686=root=/dev/hdc2 ro
# kopt=root=UUID=fccafcc7-d7cc-4594-9459-a8f0db7b9f7f ro

Add the parameter changes to the end of the line "#kopt...". These parameters will then be automatically added to any new kernel entries and should also survive an upgrade of grub.

One last point: When installing Ubuntu, the very first screen you get to when booting with the install CD has an option to set boot parameters for your system. If you know ahead of time of boot parameters that need to be set to make your graphics card work, for example, then setting them here will not only make your installation much smoother, but they will also get added as default options, as in (2.2) above.

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