472 lines
18 KiB
HTML
472 lines
18 KiB
HTML
<title>Components</title>
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<h1>3 <a name="s3"> Components </h1>
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<p> <a href="Bootdisk-HOWTO.html#toc3"> Contents of this section</a></p>
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<p></p>
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<h2>3.1 <A Name="ss3.1"> File Systems </h2>
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<p>The Linux kernel now supports two file system types for root
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disks to be automatically copied to ramdisk. These are minix
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and ext2, of which ext2 is the preferred file system.
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The ext2 support was added sometime between 1.1.17 and 1.1.57,
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I'm not sure exactly which. If you have a kernel within this range
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then edit /usr/src/linux/drivers/block/ramdisk.c and look for the
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word "ext2". If it is not found, then you will have to use a minix
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file system, and therefore the "mkfs" command to create it.</p>
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<p>To create an ext2 file system on a diskette on my system, I issue the
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following command:
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<blockquote><code>
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<pre>
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mke2fs /dev/fd0
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</pre>
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</code></blockquote>
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</p>
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<p>The mke2fs command will automatically detect the space available and
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configure itself accordingly. It does not therefore require any
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parameters.</p>
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<p>An easy way to test the result is to create a system using the above
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command or similar, and then attempt to mount the diskette. If it is
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an ext2 system, then the command:
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<blockquote><code>
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<pre>
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mount -t ext2 /dev/fd0 /<mount point>
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</pre>
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</code></blockquote>
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should work.</p>
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<p></p>
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<h2>3.2 <A Name="ss3.2"> Kernel </h2>
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<p></p>
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<h3> Building a Custom Kernel </h3>
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<p>In most cases it would be possible to copy your current kernel and
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boot the diskette from that. However there may be cases where you
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wish to build a separate one.</p>
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<p>One reason is size. The kernel is one of the
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largest files in a minimum system, so if you want to build a
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boot/root diskette, then you will have to reduce the size of the kernel
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as much as possible. The kernel now supports changing
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the diskette after booting and before mounting root, so it is not
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necessary any more to squeeze the kernel into the same disk as
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everything else, therefore these comments apply only if you choose
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to build a boot/root diskette.</p>
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<p>There are two ways of reducing kernel size:
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<ul>
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<li>Building it with the minumum set of facilities necessary
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to support the desired system. This means leaving out
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everything you don't need. Networking is a good thing to
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leave out, as well as support for any disk drives and
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other devices which you don't need when running your
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boot/root system.</li>
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<li>Compressing it, using the standard compressed-kernel
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option included in the makefile:
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<blockquote><code>
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<pre>
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make zImage
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</pre>
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</code></blockquote>
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Refer to the documentation included with the kernel source
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for up-to-date information on building compressed kernels.
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Note that the kernel source is usually in /usr/src/linux.</li>
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</ul>
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</p>
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<p>Having worked out a minimum set of facilities to include in a kernel,
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you then need to work out what to add back in. Probably the most
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common uses for a boot/root diskette system would be to examine
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and restore a corrupted root file system, and to do this you may
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need kernel support.</p>
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<p>For example, if your backups are all held on tape using Ftape to
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access your tape drive, then, if you lose your current root drive
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and drives containing Ftape, then you will not be able to restore
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from your backup tapes. You will have to reinstall Linux, download
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and reinstall Ftape, and then try and read your backups.</p>
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<p>It is probably desirable to maintain a copy of the same version
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of backup utilities used to write the backups, so that you don't
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waste time trying to install versions that cannot read your
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backup tapes.</p>
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<p>The point here is that, whatever I/O support you have added to
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your kernel to support backups should also be added into your
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boot/root kernel. Note, though, that the Ftape module (or at least
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the one I have) is quite large and will not fit on your boot/root
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diskette. You will need to put it on a utility diskette - this
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is described below in the section titled "ADDING UTILITY DISKETTES".</p>
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<p>The procedure for actually building the kernel is described in
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the documentation that comes with the kernel. It is quite easy to
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follow, so start by looking in /usr/src/linux. Note that if you
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have trouble building a kernel, then you should probably not
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attempt to build boot/root systems anyway.</p>
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<p></p>
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<h2>3.3 <A Name="ss3.3"> Devices </h2>
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<p>A /dev directory containing a special file for all devices to be
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used by the system is mandatory for any Linux system. The
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directory itself is a normal directory, and can be created with
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the mkdir command in the normal way. The device special files,
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however, must be created in a special way, using the mknod command.</p>
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<p>There is a shortcut, though - copy your existing /dev directory
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contents, and delete the ones you don't want. The only requirement
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is that you copy the device special files using the -R option. This
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will copy the directory without attempting to copy the contents of the
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files. Note that if you use lower caser, as in "-r", there will be
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a vast difference, because you will probably end up copying the
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entire contents of all of your hard disks - or at least as much
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of them as will fit on a diskette! Therefore, take care, and use
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the command:
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<blockquote><code>
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<pre>
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cp -dpR /dev /mnt
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</pre>
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</code></blockquote>
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assuming that the diskette is mounted at /mnt. The dp switches
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ensure that symbolic links are copied as links (rather than
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the target file being copied) and that the original file attributes
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are preserved, thus preserving ownership information.</p>
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<p>If you want to do it the hard way, use ls -l to display the major
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and minor device numbers for the devices you want, and create
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them on the diskette using mknod.</p>
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<p>Many distributions include a shell script called MAKEDEV in the
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/dev directory. This shell script could be used to create the devices,
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but it is probably easier to just copy your existing ones, especially
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for rescue disk purposes.</p>
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<p></p>
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<h2>3.4 <A Name="ss3.4"> Directories </h2>
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<p>It might be possible to get away with just /dev, /proc and /etc to run
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a Linux system. I don't know - I've never tested it. However a
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reasonable minimum set of directories consists of the following:
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<dl>
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<dt><b>/dev</b><dd><p>Required to perform I/O with devices</p>
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<dt><b>/proc</b><dd><p>Required by the ps command</p>
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<dt><b>/etc</b><dd><p>System configuration files</p>
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<dt><b>/bin</b><dd><p>Utility executables considered part of the system</p>
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<dt><b>/lib</b><dd><p>Shared libraries to provide run-time support</p>
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<dt><b>/mnt</b><dd><p>A mount point for maintenance on other disks</p>
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<dt><b>/usr</b><dd><p>Additional utilities and applications</p>
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</dl>
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</p>
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<p>Note that the directory tree presented here is for root diskette use only.
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Refer to the Linux File System Standard for much better information
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on how file systems should be structured in "standard" Linux
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systems.</p>
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<p>Four of these directories can be created very easily:
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<ul>
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<li>/dev is described above in the section titled DEVICES.</li>
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<li>/proc only needs to exist. Once the directory is created using
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mkdir, nothing more is required.</li>
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<li>Of the others, /mnt and /usr are included in this list only as
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mount points for use after the boot/root system is running.
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Hence again, these directories only need to be created.</li>
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</ul>
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</p>
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<p>The remaining 3 directories are described in the following sections.</p>
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<p></p>
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<h3> /etc </h3>
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<p>This directory must contain a number of configuration files. On most
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systems, these can be divided into 3 groups:
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<ul>
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<li>Required at all times, e.g. rc, fstab, passwd.</li>
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<li>May be required, but no-one is too sure.</li>
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<li>Junk that crept in. </li>
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</ul>
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</p>
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<p>Files which are not essential can be identified with the command:
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<blockquote><code>
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<pre>
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ls -ltru
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</pre>
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</code></blockquote>
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This lists files in reverse order of date last accessed, so if any
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files are not being accessed, then they can be omitted from a root
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diskette.</p>
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<p>On my root diskettes, I have the number of config files down to
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15. This reduces my work to dealing with three sets of files:
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<ul>
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<li>The ones I must configure for a boot/root system:
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<blockquote><code>
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<pre>
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rc system startup script
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fstab list of file systems to be mounted
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inittab parameters for the init process - the
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first process started at boot time.
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</pre>
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</code></blockquote>
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</li>
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<li>the ones I should tidy up for a boot/root system:
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<blockquote><code>
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<pre>
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passwd list of logins
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shadow contains passwords
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</pre>
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</code></blockquote>
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These should be pruned on secure systems to avoid copying
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user's passwords off the system, and so that when you boot
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from diskette, unwanted logins are rejected.</li>
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<li>The rest. They work at the moment, so I leave them alone.</li>
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</ul>
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</p>
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<p>Out of this, I only really have to configure two files, and what they
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should contain is suprisingly small.
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<ul>
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<li>rc should contain:
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<blockquote><code>
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<pre>
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#!/bin/sh
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/etc/mount -av
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/bin/hostname boot_root
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</pre>
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</code></blockquote>
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and I don't really need to run hostname - it just looks nicer
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if I do. Even mount is actually only needed to mount /proc to
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support the ps command - Linux will run without it.</li>
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<li>fstab should contain:
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<blockquote><code>
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<pre>
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/dev/fd0 / ext2 defaults
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/proc /proc proc defaults
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</pre>
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</code></blockquote>
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I don't think that the first entry is really needed, but I
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find that if I leave it out, mount won't mount /proc.</li>
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</ul>
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</p>
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<p>Inittab should be ok as is, unless you want to ensure that users on
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serial ports cannot login. To prevent this, comment out all the entries
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for /etc/getty which include a ttys or ttyS device at the end of the line.
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Leave in the tty ports so that you can login at the console.</p>
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<p>For the rest, just copy all the text files in your /etc directory, plus
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all the executables in your /etc directory that you cannot be sure you
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do not need. As a guide, consult the sample ls listing in
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"Sample Boot/Root ls-lR Directory Listing" -
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this is what I have, so probably it will be sufficient for you if
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you copy only those files.</p>
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<p></p>
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<h3> /bin </h3>
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<p>Here is a convenient point to place the extra utilities you need to
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perform basic operations, utilities such as ls, mv, cat, dd etc.</p>
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<p>See the section titled "Sample Boot/Root ls-lR Directory Listing"
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for the list of files that I place in my boot/root /bin
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directory. You may notice that it does not include any of the utilities
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required to restore from backup, such as
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cpio, tar, gzip etc. That is because I place these on a separate
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utility diskette, to save space on the boot/root diskette. Once I
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have booted my boot/root diskette, it then copies itself to the ramdisk
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leaving the diskette drive free to mount another diskette, the utility
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diskette. I usually mount this as /usr.</p>
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<p>Creation of a utility diskette is described below in the section
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titled "Adding Utility Diskettes".</p>
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<p></p>
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<h3> /lib </h3>
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<p>Two libraries are required to run many facilities under Linux:
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<ul>
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<li>ld.so</li>
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<li>libc.so.4</li>
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</ul>
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</p>
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<p>If they are not found in your /lib directory then the system will
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be unable to boot. If you're lucky you may see an error message
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telling you why.</p>
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<p>These should be present in you existing /lib directory. Note that
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libc.so.4 may be a symlink to a libc library with version number
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in the filename. If you issue the command:
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<blockquote><code>
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<pre>
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ls -l /lib
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</pre>
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</code></blockquote>
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you will see something like:
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<blockquote><code>
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<pre>
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libc.so.4 -> libc.so.4.5.21
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</pre>
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</code></blockquote>
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</p>
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<p>In this case, the libc library you want is libc.so.4.5.21.</p>
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<p></p>
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<h2>3.5 <A Name="ss3.5"> LILO </h2>
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<p></p>
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<h3> Overview </h3>
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<p>For the boot/root to be any use, it must be bootable. To achieve this,
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the easiest way (possibly the only way?) is to install a boot loader,
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which is a piece of executable code stored at sector 0, cylinder 0 of
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the diskette. See the section above titled "BOOT DISKETTE" for an
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overview of the boot process.</p>
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<p>LILO is a tried and trusted boot loader available from any Linux
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mirror site. It allows you to configure the boot loader, including:
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<ul>
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<li>Which device is to be mounted as the root drive.</li>
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<li>Whether to use a ramdisk.</li>
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</ul>
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</p>
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<p></p>
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<h3> Sample LILO Configuration </h3>
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<p>This provides a very convenient place to specify to the kernel how
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it should boot. My root/boot LILO configuration file, used with
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LILO 0.15, is:
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<blockquote><code>
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<hr>
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<pre>
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boot = /dev/fd0
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install = ./mnt/boot.b
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map = ./mnt/lilo.map
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delay = 50
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message = ./mnt/lilo.msg
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timeout = 150
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compact
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image = ./mnt/vmlinux
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ramdisk = 1440
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root = /dev/fd0
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</pre>
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<hr>
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</code></blockquote>
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</p>
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<p>Note that boot.b, lilo.msg and the kernel must first have been copied to
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the diskette using a command similar to:
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<blockquote><code>
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<hr>
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<pre>
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cp /boot/boot.b ./mnt
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</pre>
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<hr>
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</code></blockquote>
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</p>
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<p>If this is not done, then LILO will not run correctly at boot time if
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the hard disk is not available, and there is little point setting up
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a rescue disk which requires a hard disk in order to boot.</p>
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<p>I run lilo using the command:
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<blockquote><code>
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<pre>
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/sbin/lilo -C <configfile>
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</pre>
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</code></blockquote>
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</p>
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<p>I run it from the directory containing the mnt directory where I have
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mounted the diskette. This means that I am telling LILO to install a
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boot loader on the boot device (/dev/fd0 in this case), to boot a
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kernel in the root directory of the diskette.</p>
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<p>I have also specified that I want the root device to be the diskette,
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and I want a RAM disk created of 1440 1K blocks, the same size as the
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diskette. Since I have created an ext2 file system on the diskette,
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this completes all the conditions required for Linux to automatically
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switch the root device to the ramdisk, and copy the diskette contents
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there as well.</p>
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<p>The ramdisk features of Linux are described further in the section
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above titled "RAM DRIVES AND BOOT/ROOT SYSTEMS".</p>
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<p>It is also worth considering using the "single" parameter to cause
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Linux to boot in single-user mode. This could be useful to prevent
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users logging in on serial ports.</p>
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<p>I also use the "DELAY" "MESSAGE" and "TIMEOUT" statements so
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that when I boot the disk, LILO will give me the opportunity to
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enter command line options if I wish. I don't need them at present,
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but I never know when I might want to set a different root device
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or mount a filesystem read-only.</p>
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<p>The message file I use contains the message:</p>
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<p>
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<blockquote><code>
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<pre>
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Linux Boot/Root Diskette
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========================
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Enter a command line of the form:
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vmlinux [ command-line options]
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If nothing is entered, linux will be loaded with
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defaults after 15 seconds.
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</pre>
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</code></blockquote>
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</p>
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<p>This is simply a reminder to myself what my choices are.</p>
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<p>Readers are urged to read the LILO documentation carefully before
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atttempting to install anything. It is relatively easy to destroy
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partitions if you use the wrong "boot = " parameter. If you are
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inexperienced, do NOT run LILO until you are sure you understand it
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and you have triple-checked your parameters.</p>
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<p></p>
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<h3> Removing LILO </h3>
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<p>One other thing I might as well add here while I'm on the LILO topic:
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if you mess up lilo on a drive containing DOS, you can always replace
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the boot sector with the DOS boot loader by issuing the DOS command:
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<blockquote><code>
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<pre>
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FDISK /MBR
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</pre>
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</code></blockquote>
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</p>
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<p>where MBR stands for "Master Boot Record". Note that some purists
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disagree with this, and they may have grounds, but it works.</p>
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<p></p>
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<h3> Useful LILO Options </h3>
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<p></p>
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<p>LILO has several useful options which are worth keeping in mind when
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building boot disks:
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<ul>
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<li>Command line options - you can enter command line options
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to set the root device, ramdrive size, special device parameters, or
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other things. If you include the DELAY = nn statement in your LILO
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configuration file, then LILO will pause to allow you to select
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a kernel image to boot, and to enter, on the same line, any options.
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For example:
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<blockquote><code>
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<pre>
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vmlinux aha152x=0x340,11,3,1 ro
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</pre>
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</code></blockquote>
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will pass the aha152x parameters through to the aha152x scsi disk
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driver (provided that driver has been included when the kernel was
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built) and will ask for the root filesystem to be mounted read-only.</li>
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<li>Command line "lock" option - this option asks LILO to store
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the command line entered as the default command line to be used for
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all future boots. This is particularly useful where you have a device
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which cannot be autoselected. By using "lock" you can avoid having
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to type in the device parameter string every time you boot.
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For example:
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<blockquote><code>
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<pre>
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vmlinux aha152x=0x340,11,3,1 root=/dev/sda8 ro lock
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</pre>
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</code></blockquote>
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</li>
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<li>APPEND configuration statement - this allows device parameter
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strings to be stored in the configuration, as an alternative to
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using the "lock" command line option. Note that any keywords of
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the form word=value MUST be enclosed in quotes. For example:
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<blockquote><code>
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<pre>
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APPEND = "aha152x=0x340,11,3,1"
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</pre>
|
|
</code></blockquote>
|
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</li>
|
|
<li>DELAY configuration statement - this pauses for DELAY tenths
|
|
of seconds and allows the user to interrupt the automatic boot of
|
|
the default command line, so that the user can enter an alternate
|
|
command line.</li>
|
|
</ul>
|
|
</p>
|
|
<p></p>
|
|
|
|
<p><a href="Bootdisk-HOWTO-4.html"> Next </a> Chapter, <a href="Bootdisk-HOWTO-2.html"> Previous </a> Chapter</p><p>Table of contents of <a href="Bootdisk-HOWTO.html#toc3">this chapter</a>,
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|
General <a href="Bootdisk-HOWTO.html#toc">table of contents</a></p>
|
|
<p><a href="Bootdisk-HOWTO.html"> Top </a> of the document,
|
|
<a href="#0"> Beginning of this Chapter</a></p> |