Using LILO to Boot

LILO is the most common user method of booting a Linux system because it doesn't involve using a boot floppy disk. Chapter 4, "LILO," examined LILO in detail. LILO is a program that sits in the boot sector of a disk partition or the master boot record of the entire hard disk and points to the partition and location of the Linux kernel image.

If LILO is installed as a first-stage boot loader (meaning it boots Linux automatically), Linux starts to boot whenever the power is turned on. If you want to halt the boot process, you can use the Ctrl+Alt+Del sequence when the machine starts the boot sequence. (You must be careful when you hit Ctrl+Alt+Del, as you may reboot the machine by accident. Wait until you see the loader start its actions.) The Ctrl+Alt+Del sequence instructs LILO to pause and display the following prompt:


From this boot prompt, you can tell LILO which operating system to load (DOS, Linux, OS/2, and so on). If you press the Tab key when the boot prompt is displayed, LILO displays a list of all partitions and operating systems it knows about. The operating system partitions must have their configuration information included in the LILO information. Providing this information is simply a matter of identifying the partition device name and a name for the operating system when you are creating the LILO configuration file. Chapter 4, "LILO," covered these steps.

Because LILO writes data to the disk drive that other operating systems cannot read, it is not always the best solution if you install and remove operating systems frequently from your hard disks. Whenever you make changes to the configuration of your Linux system or other partitions on the hard disk, update the LILO information by rerunning LILO.

Using a Boot Floppy Disk

If you don't want to rely on LILO (which modifies disk sectors and may cause problems when you use several operating systems or change operating systems frequently), you can use a boot floppy disk to start up Linux. A boot floppy disk is a single floppy disk that contains a complete copy of the Linux kernel and instructions for accessing the root partition on your hard drive. The boot floppy disk must be of the proper format to run on the first disk drive on your system (drive A in DOS terms). Linux cannot boot from a second (drive B) floppy disk drive.

In many ways, a boot floppy disk is the easiest and most versatile method of starting Linux. If, for example, you have your hard disk partitioned to contain both DOS and Linux, with DOS the normal boot partition, simply turning on your PC boots DOS without a hitch. If you want Linux to boot, you insert the boot floppy disk and start the machine. Linux boots from the floppy disk, and then accesses the hard drives as if it had booted from them.

If you are using a boot floppy disk to start Linux, be sure to update the kernel image on the floppy disk every time you make a change to the system that involves rebuilding the kernel. Keep in mind that you must rebuild the kernel almost every time you add devices or device drivers. Use the procedure outlined in the following paragraphs to update your existing floppy disk, or, even better, create a new floppy disk and save your current floppy disk for emergencies. You are not prompted to create a new boot floppy disk when you make changes to the kernel, so you must remember to perform this step.

To create a boot floppy disk, you need a blank, formatted, high-density floppy disk (l.44M or l.2M, depending on the A drive on your system). Format the floppy disk under DOS to lay down the sector and track information properly for Linux to read. Some small kernels can fit on low-density floppy disks, but high-density drives are most likely to be used on your system because they are the standard. The high-density floppy disk can have information on it as well as the Linux kernel, but make sure you have enough disk space for the kernel image.

Some versions of Linux (such as Slackware) can create the boot floppy disk as part of the normal setup routine. If you are using a distribution that has a setup routine, try choosing the Configure option on the menu, follow through the prompts, and see whether you are prompted to create a boot floppy disk at the end. Alternatively, some distributions have a separate menu option for creating the boot floppy disk.

If you want to create the boot floppy disk manually, locate the Linux kernel on your system. Usually it is in the root directory. The kernel name changes depending on the version of Linux, but it is often called Image or vmlinux. Some versions of Linux store the kernel image in the /etc directory.

Some versions of Linux also store the kernel in a compressed format. The names of compressed kernels end in z, as in vmlinuz or vmlinux.z. A compressed kernel takes up less space on the hard disk or floppy disk, and it is decompressed when the Linux kernel boots. A compressed kernel takes a little longer to load than a kernel that isn't compressed, but because it is decompressed only when the system boots, the trade-off is usually beneficial (unless you have tons of empty disk space).

You should be able to find the kernel quite easily by watching the startup messages when you boot the system and noting the kernel name, and then using find or whereis to locate it. Much easier is to change to the root directory and look for a large file called Image or vmlinux. The file is owned by root and has only read permission in many distributions of Linux. For example, when you do a listing command (such as ls-l), you see an entry like the following:

This entry shows a kernel image file of almost half a megabyte that is compressed. The date and time of the kernel match the last time you rebuilt the kernel or the time and date you installed Linux.

Once you have identified the kernel file, instruct Linux that the file is the root device and indicate which partition it is on by using the rdev command. For example, to set the root device to the kernel vmlinuz in the root directory of the partition /dev/sda3, you would issue the command rdev /vmlinuz /dev/sda3

Because you must specify the path to the kernel completely, the leading slash is included to show the root directory.

If you issue the rdev command by itself, it displays the current partition of the root filesystem:


You can use this command to check the current settings if you are not sure which partition is your root filesystem. (This chapter looks at the rdev command in a little more detail in a later section.)

After you set the root device, you can copy the kernel to your formatted floppy disk. Use the cp command and the device name of the floppy disk:

cp /vmlinuz /dev/fd0

Once the image file has been transferred, the floppy disk should be able to boot Linux. If it doesn't, either the image didn't transfer properly due to a lack of disk space or a corrupted disk sector, or there is a problem with the kernel image.

Creating and Using a Maintenance Disk

Every system should have a maintenance disk (also called an emergency boot disk) that you can use to boot the Linux system in case anything happens to the boot system (such as LILO). A maintenance disk is a combination boot and root disk that boots a complete Linux kernel independent of your hard disk installation. After you load the maintenance disk, you can use it to mount the hard disk and check for problems, or use one of the hard disk utilities to rebuild LILO or the kernel, depending on the problem with the drive.

To create a maintenance disk, you create a root filesystem on a floppy disk, copy essential tools to it, install LILO, and then make the disk bootable by copying the kernel. Perform this process every time you make a change to the Linux kernel so that your maintenance disk has the same kernel build. Keeping your maintenance disk up-to-date prevents hassles with utilities and devices.

You probably already have a set of maintenance disks in the pair of floppy disks you used to install Linux in the first place. Although these disks are not configured for your system, you can use them to load Linux and mount your hard drive. Many Linux setup procedures have a built-in routine to create boot floppy disks. You can use this routine to create the maintenance floppy disk.

If you have to boot off the maintenance floppy disk for any reason, mount the existing hard drive with the mount command. For example, if you are booting off your floppy disk and want to mount the partition /dev/sda2 (which wouldn't boot Linux for some reason), issue the command mount -t ext2 /dev/sda2 /mnt which mounts the hard drive partition under the directory /mnt. The directory must exist before you perform this operation, and it should be empty. The -t option specifies the file type. If your filesystem is not an extended filesystem, change the type.

Shutting Down Linux

The temptation to treat Linux like DOS can be overwhelming when you are ready to finish with your session. Simply turning off the power should shut down everything, right? Well, it does, but it also can completely corrupt all the contents of your hard disk partition, as well as lose any information you were just working on. Granted, that's a very rare and extreme case, but arbitrarily turning off the power to a Linux session is still a bad idea.

Linux manages the hard disk and user spaces in RAM, using i-node tables to maintain the disk information and a memory manager for user information. Linux writes any changes to the i-node tables to the disk drive every so often, but it maintains the RAM copies as the most recent because of RAM's greater speed. If you shut down the power before Linux writes any changes to the disk, the disk contents and the i-node tables written on the disk may not match, causing lost files and an incorrect list of what disk space is available. Even worse, if Linux was in the process of writing the i-node table or any other information at the moment the power is turned off, the write process is interrupted, and disk head crashes or bad sector information can result. The same principle applies to any processes that are running. If, for example, you were running a database reindex when you killed the power, the indexes and databases may be corrupted. Shutting down the Linux system properly makes sure that all processes write and close all open files and terminate cleanly.

There are two easy ways to shut down the Linux system properly. The easiest is to use the Ctrl+Alt+Del sequence. On many Linux versions, this keyboard combination issues a shutdown command that closes all the processes properly, and then reboots the machine. Linux essentially traps the Ctrl+Alt+Del sequence and uses it to shut down the machine. Not all versions of Linux support this sequence, though, so check your documentation carefully.

If your system doesn't trap Ctrl+Alt+Del and reboots the machine when you issue it without shutting down Linux properly, it's the same effect as turning off the power. Make sure your Linux version supports this command before you use it!

The other method of shutting down Linux is with the UNIX command shutdown

When you issue the shutdown command, Linux starts to terminate all processes and then shuts down the kernel. The shutdown command displays several different messages, depending on the version of Linux, but all inform you of the process or check that you really want to shut down the system.

The shutdown command allows you to specify a time until shutdown, as well as an optional warning message to be displayed to all users logged in. The format of the command is shutdown time message As an example, this command shutdown 15 'Backup Time!1

shuts down the system after 15 minutes and display the message "Backup Time!" to all users on the system, prompting them to log off. This command is handy when you enforce a policy of shutting down at specific intervals, either for maintenance or backups.

In most versions of Linux, the shutdown command accepts the -r option. This option causes the PC to reboot after the shutdown has occurred. You can use this option to reboot to another operating system or to restart Linux after making changes to the kernel or devices. You can use the -r option with a time or message, if you want. The command shutdown -r 5

reboots the system after five minutes.

In most cases, using Ctrl+Alt+Del or the shutdown command results in the display of a number of status messages on the main console. When Linux has finished shutting down the system, you see the message

The system is halted

When this message appears on-screen, it is safe to shut off the system power or reboot the machine. Although it may seem a little strange to have to follow these extra steps, you will find that many high-end operating systems such as UNIX (and even Windows NT and Windows 95) require you to follow a specific shutdown procedure to prevent loss of information. Get in the habit!

Understanding the init Daemon

The init daemon is usually invoked as the last step in the booting of the Linux kernel. The init daemon is one of the most important Linux daemons because it creates processes for the rest of the system. The init daemon is executed when Linux starts and stays active until Linux is shut down. Understanding what init (and its linked utility telinit) does and how it controls the operating system is important to better administering the Linux system.

Both init and telinit use several configuration files to perform their tasks, so the following sections look at those files in detail, too. These files are closely involved in the starting and stopping of terminals and console sessions. The init program is usually kept in the /bin directory, although some versions of Linux keep it in /sbin. The same directories apply to the telinit utility. The configuration files are always kept in /etc, though.

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