As we learn the Linux operating system, our confidence will build.  With this confidence, we will be apt to try things just for the thrill of not knowing what will happen.  When this starts we must be ready for an inevitable disaster.  Don't fret or worry!  This lab will present a couple of possible situations that you could encounter as you experiment and tinker with your system.

No working kernel

The first simulated situation is one most likely to be encountered by those among us that will tinker with building our own kernels.  Once a kernel is built, it is placed in the /boot directory with any other kernels used by your system.  Don't destroy the working kernels because you still need them until yours is confidently debugged.  Let's  create the 'no working kernel' situation.

• Make the /boot directory your present working directory,

• list the contents of this directory,

• change the name of the kernel (vmlinuz-version_number) by appending  '.save' to it,

• reboot your system,

• make notes of what occurs when your system boots.

• When Grub Boot Loader displays the kernel it is trying to locate, press the 'e' key.

• This places Grub in the edit mode.  Tab to the line needing edited.  Press the 'e' key again.

• Correct the kernel name in the line to include the '.save'.  Press ENTER.

• Press 'b' to boot the system.  This signals Grub to boot the system using the modified kernel.

• Providing that you've entered the correct name for the new kernel name, the system should boot.

• Once the system has rebooted change the name of the kernel back to its original name.
  

Bad conf files prevent a boot

Our second simulated problem will be a situation that can be encountered because any one of the several configuration and initialization files have errors.  Our choice will be the 'grub.conf' file.

• Copy the grub.conf file to grub.conf.save.
  

• Using your favorite editor, open the grub.conf file as the superuser.

• Find the line that contains 'kernel /vmlinuz-kernel_version_number ro root=/dev/hda2' and change the line to a comment.

• Save the 'grub.conf' file and exit your editor.

• Shutdown all open applications and reboot the system.

• What happens when the system tries to boot?

• This situation could be serious if we didn't know the strategy to overcome it.  One popular method of overcoming this situation is to reinstall the Linux system.  This is an overkill and we need to learn a more efficient way.  Remember we are working with Linux not Windows!

• You're going to have to rescue the system.  Pull out your CDROM and insert the first CDROM in the drive.  Reboot the system.

• When the CDROM is accessed, you will need to enter the rescue mode.  Follow the process outlined by the CDROM.

• The system will be booted into the rescue mode with the filesystem mounted under /mnt.

• At the command line prompt you need to change to the grub directory.  Now we must return the 'grub.conf' file to its original form.  This is accomplished by moving the grub.conf.save to grub.conf.

• Reboot the system and verify that your system is operating correctly.

A common situation that can occur to everyone of us, is a forgotten password.  Oh Dear!  Do I reinstall the system?  No!  Stop thinking that way!  We can work through this very easily.  During the boot process you must enter the system at runlevel 1.  We used this runlevel in the previous method by entering 'linux 1' at the boot: prompt when using the emergency boot disk.  If you permit the Grub Boot Loader to be used,

• When the Grub Boot Loader requests your input, press 'e'.  This places Grub in the edit mode.

• Three lines will appear.  Select the line 'kernel vmlinuz-kernel_version_number ro root=/dev/hda2', press 'e' again.

• This permits you to add text to the line.  Add linux 1 and press ENTER.

• Once you've returned to the original display with the changed kernel line, press 'b'.  The boot process will be initiated and the system will be put at runlevel 1.

• At the prompt type 'passwd' and press ENTER.  You'll be prompted to enter the new superuser password.  Type the password and press ENTER.

• When prompted, retype the same password and press ENTER.

• To change the password on a user account type 'password username' and press ENTER.  You'll be prompted for the new user account password.

• Type the new password and press ENTER.  You'll be prompted to retype the user account password.  Retype the user account password and press ENTER.

• Now we're ready to go to another runlevel.  Stop you don't need to reboot.  Type 'init 3' to go to runlevel 3, type 'init 5' to go to runlevel 5.  Take your pick.  Remember your passwords or you'll need to conduct this process again.