David Mertz, Ph.D.
Welcome to "Understanding System Startup", the second of eight tutorials designed to prepare you for LPI exam 201. In this tutorial you will learn the steps a Linux system goes through during system initialization, and how to modify and customize those behaviors for you specific needs.
The Linux Professional Institute (LPI) certifies Linux system administrators at junior and intermediate levels. There are two exams at each certification level. This series of eight tutorials helps you prepare for the first of the two LPI intermediate level system administrator exams--LPI exam 201. A companion series of tutorials is available for the other intermediate level exam--LPI exam 202. Both exam 201 and exam 202 are required for intermediate level certification. Intermediate level certification is also known as certification level 2.
Each exam covers several or topics and each topic has a weight. The weight indicate the relative importance of each topic. Very roughly, expect more questions on the exam for topics with higher weight. The topics and their weights for LPI exam 201 are:
Topic 201: Linux Kernel (5) Topic 202: System Startup (5) Topic 203: Filesystems (10) Topic 204: Hardware (8) Topic 209: File Sharing Servers (8) Topic 211: System Maintenance (4) Topic 213: System Customization and Automation (3) Topic 214: Troubleshooting (6)
Welcome to "Understanding the System Startup", the second of eight tutorials designed to prepare you for LPI exam 201. In this tutorial you will learn the steps a Linux system goes through during system initialization, and how to modify and customize those behaviors for you specific needs.
To get the most from this tutorial, you should already have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this tutorial.
This tutorial is at the border of Linux, strictly speaking. Topic 201 addressed the kernel itself, which is the core of Linux. This tutorial moves on to the ancillary tools and scripts that are necessary both to get the kernel running, and to initialize a system to the point it does something meaningful. Hower the scripts and tools associated with initialization are maintained by the creators of Linux distributions, or individualized by system administrators, rather than developed as part of the Linux kernel per se. Still, every Linux system--even an embedded one--will require some basic initialization steps similar to those discussed here.
In later tutorials, we will look at a variety of tools for networking, system maintenance, manipulating files and data, and so on, which are important for a working Linux installation and part of almost every Linux distribution, but are even less part of Linux per se than are initialization scripts.
It is useful to break the Linux boot process into 9 steps that will occur in almost every Linux configuration. Steps 10 and beyond might involve launching additional services, logging into a graphical environment, restoring UI settings, or other more personalized details that are outside this tutorial.
1. Hardware/firmware: The BIOS or firmware system reads the master boot record on the harddisk or other boot device (e.g. CD, floppy, netboot, etc).
2. A boot loader runs. Linux systems on x86 systems typically use
GRUB. Some older systems might use
loadlinto boot via an intermediate DOS partition. On PowerPC systems, this might be
yaboot. In general, a boot loader is a simple program that knows where to look for the Linux kernel, perhaps choosing among several versions, or even selecting other operating systems on the same machine.
3. The kernel loads.
4. The root filesystem is mounted. In some cases, a temporary ramdisk image is loaded before the true root filesystem to enable special drivers or modules that might be necessary for the true root filesystem.
Keep reading boot steps on the next page
One we have a root filesystem in place (see last page), we are ready for initialization proper.
5. Start the process
init, the parent of all other Linux processes.
6. Read the contents of
/etc/inittab to configure the remaining boot
steps. Of special importance is the line in
controls the runlevel the system will boot to (and therefore, which
further steps will be taken during initialization).
Actually, everything after this point is completely controlled by the
content of the file
/etc/inittab. Specifically, the scripts and
tools that run generally follow some conventions, but in theory you
could completely change
/etc/inittab to run different scripts.
One specific setting in
/etc/inittab is particularly crucial. A
line similar to:
Generally occurs near the top of the file, and sets the
This runlevel controls what actions are taken in the remainder on the
Just what happens as an
/etc/inittab script is processed? And
specifically, what conventional files and directories are involved in
7. Runlevel-neutral system initialization. Generally there are some
initialization actions that are performed regardless of runlevel.
These steps are indicated in
/etc/inittab with a line like:
# System initialization. si::sysinit:/etc/rc.d/rc.sysinit On some Linux systems (mostly Debian based), you will see something more like: si::sysinit:/etc/init.d/rcS If the latter case, '/etc/init.d/rcS' is a script that simply runs each of the scripts matching '/etc/rcS.d/[Ss]??*'. On the other hand, if your system uses '/etc/rc.d/rc.sysinit', that file contains a single long script to perform *all* the initialization.
8. Runlevel-specific system initialization. You may actually define
as many actions as you like related to runlevel, and each action
may pertain to one or more runlevels. As a rule,
will contain some lines like:
l0:0:wait:/etc/rc.d/rc 0 # ... l5:5:wait:/etc/rc.d/rc 5 l6:6:wait:/etc/rc.d/rc 6 In turn, the script '/etc/rc.d/rc' will run all the files matched by the pattern '/etc/rc$1.d/[KkSs]??*'. For example, on the sample system described, that start at runlevel 5, we would run (in order): /etc/rc5.d/K15postgresql /etc/rc5.d/S01switchprofile /etc/rc5.d/S05harddrake ... /etc/rc5.d/S55sshd ... /etc/rc5.d/S99linuxconf /etc/rc5.d/S99local The files(s) starting with 'K' or 'k' are "kill scripts" that end processes or cleanup their actions. Those starting with 'S' or 's' are startup scripts that generally launch new processes or otherwise prepare the system to run at that runlevel. Most of these files ran will be shell scripts, and most will be links (often to files in '/etc/init.d/')
Most of the time, once a Linux system is running at a runlevel, you
wish to log into the system as a user. To let that happen, a program
getty runs to handle the login process. A number of
variations on the basic
getty are used by distribution creators,
mingetty. All do basically the same
9. Login at the prompt. Our good friend
/etc/inittab/ will usually
getty programs on one or more virtual terminals, and do so
in for several different runlevels. Those are configured with
# Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:2345:respawn:/sbin/mingetty tty3 4:2345:respawn:/sbin/mingetty tty4 5:2345:respawn:/sbin/mingetty tty5 6:2345:respawn:/sbin/mingetty tty6 The first number is reminds us of the virtual terminal where the getty runs, the next set of number are the several runlevels where this will happen. E.g. we launch 'mingetty' in all of the runlevels 2, 3, 4 and 5.
The concept of runlevel is somewhat arbitrary, or at least it is not
hardcoded into a Linux kernel. Valid runlevel numbers to set with the
initdefault option (or override otherwise) are 0-6. By convention,
the following meanings are given to each number:
# Default runlevel. The runlevels used by Mandrake Linux are: # 0 - halt (Do NOT set initdefault to this) # 1 - Single user mode # 2 - Multiuser, without NFS (The same as 3, if you do not have networking) # 3 - Full multiuser mode # 4 - unused # 5 - X11 # 6 - reboot (Do NOT set initdefault to this)
This convention, as you can see, is as used in the Mandrake Linux distribution, but most distributions obey the same convention. Text only or embedded distributions might omit usage of some of the levels, but will still reserve their hypothetical use at the given numbers.
We have seen a number of
/etc/inittab lines in examples, but it is
worth understanding explicitly what these lines do. Each one has the
id field is simply a short abbreviation naming the configuration
line (1-4 character in recent versions of
init, 1-2 in ancient
runlevels we have explained well enough. Next is the
action taken by the line. Some actions are "special", such as:
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
Which traps the ctrl-alt-delete key sequence (regardless of runlevel). But most actions simply relate to spawning. A non-exhaustive list of actions includes:
respawn: The process will be restarted whenever it terminates (e.g.
wait: The process will be started once when the specified runlevel
is entered and init will wait for its termination.
once: The process will be executed once when the specified
runlevel is entered.
boot: The process will be executed during system boot (but after
sysinit). The runlevels field is ignored.
A few years ago, a program called
lilo was pretty much universally
used to boot Linux on x86 systems. The name
lilo is short for "LInux
LOader." Nowadays, another program called
grub (GRand Unified
Bootloader) is more popular. On non-x86 Linux systems, other boot
loaders are used, but they are generally configured in the same manner
While there are differences in their configuration syntaxes,
grub perform largely the same task. Basically, either one
presents a choice of operating systems (including, perhaps, multiple
Linux kernels), and loads the selected OS kernel into memory. Both
programs let you pass arguments on to a Linux kernel along the way,
and both can be configured to boot non-Linux operating systems on the
grub (or other boot loaders) generally live in the
MBR (Master Boot Record) of the primary harddisk, which is
automatically loaded by system BIOS.
lilo was restricted to loading
specific raw sector from a harddisk.
grub is more sophisticated in
itself understanding a number of filesystem types such as ext2/3,
ReiserFS, VFAT, UFS. This means that
grub doesn't need to rewrite
the MBR every time a configuration file is changes, as
lilo boot loader is configured with the contents of the file
/etc/lilo.conf. For full details on configuration options, read the
man page on
lilo.conf. Several initial options control general
behavior. For example, you will often see
boot=/dev/hda or similar;
this means to install
lilo to the MBR of the first IDE harddisk.
You might also install
lilo within a particular partition, usually
because you use a different main boot loader. For example,
boot=/dev/sda3 installs lilo to the third partition of the first
SCSI disk. Other options control the appearance and wait time of
The thing to keep in mind is that after you have edited a
/etc/lilo.conf configuration, you need to run
lilo to actually
install a new boot sector used during initialization. It is easy to
forget to install new settings, but the boot loader itself cannot read
the configuration, except as encoded as raw sector offsets (which
lilo calculates when run).
In the main, when using
lilo you are interested in the one or more
image= lines, and perhaps in some
other= lines if you multiboot to
other operating systems. A sample
/etc/lilo.conf might contain:
image=/boot/bzImage-2.7.4 label="experimental" image=/boot/vmlinuz label="linux" initrd=/boot/initrd.img append="devfs=mount acpi=off quiet" vga=788 read-only other=/dev/hda3 label=dos
This would allow you to choose at runtime either a 2.7.4 development kernel, or a stable kernel (the latter happens to utilize an initial ramdrive during boot). You can also select a DOS installation installed to partition 3 on the first IDE drive.
A nice thing about
grub is that it does not need to be reinstalled
each time you change boot configuration. Of course, you do need to
install it once in the first place, usually using a command like
grub-install /dev/hda. Generally, distributions will do this for
you during installation, so you may never explicitly run this.
grub knows how to read many filesystems, normally you
can simply change the contents of
/boot/grub/menu.lst to change the
options for the next bootup. Let us look at a sample configuration:
timeout 5 color black/yellow yellow/black default 0 password secretword title linux kernel (hd0,1)/boot/vmlinuz root=/dev/hda2 quiet vga=788 acpi=off initrd (hd0,1)/boot/initrd.img title experimental kernel (hd0,1)/boot/bzImage-2.7.4 root=/dev/hda2 quiet title dos root (hd0,4) makeactive chainloader +1
grub allow you to pass special parameters to the
kernel you select. If you use
lilo you may pass boot prompt
arguments by appending them to your kernel selection. For example,
you might type:
LILO: linux ether=9,0x300,0xd0000 root=/dev/ha2 vga=791 acpi=on
For a custom boot setting (special options to the ethernet module, specify the root partition, choose video mode, etc). Of course, it is not all that friendly, since you need to know the exact options available, and type them exactly right.
Of particular importance is the option to change the runlevel from the boot loader. For example, for recovery purposes you may want to run in single user mode, which you can do with, e.g.:
LILO: experimental single
LILO: linux 1
Another rather special option is the
init= argument that lets you
use a program other than
init as the first process. An option for a
fallback situation might be
init=/bin/sh, which at least gets you an
Linux shell if
init fails catastrophically.
grub you have even more flexibility. In fact,
grub is a
whole basic shell that lets you change boot loader configurations and
even read filesystems. For custom boot options, simply press
grub shell, then add options (such as a numeric runlevel, or the
keyword "single", as with
lilo). All the other boot prompt
arguments you might type under
lilo can be edited in a
command, using simple
readlines style editing.
For some real sense of the power, you can open a
grub command line.
For example, suppose you think your
/etc/inittab might be
misconfigured, and you need to examine it before booting. You might
grub> cat (hd0,2)/etc/inittab
This would let you manually view your initialization without even launching any operating system. If there was a problem there, perhaps you would want to boot into single user mode and fix it.
In the main, once you understand the steps in a Linux post-kernel boot
process (i.e. the
init process, and everything it calls), you also
understand how to customize it. Basically, customization is just a
matter of editing
/etc/inittab and the various scripts in
For example, I recently needed to customize the video bios on a
Debian-based Linux laptop using a third-party tool. If this didn't run
before X11 ran, my XOrg driver would not detect the correct video
modes. Once I figured out what the issue actually was, the solution is
as simple as creating the script
other words, I run an extra script during every system startup.
Notably, I made sure this ran before
the simple convention that scripts run in alphabetical order (if I
wanted it to run later, I might have named it
instead. Arguably, I might have put this script only in the
/etc/rc5.d/ directory to run when X11 does--but my way lets me
startx from other runlevels.
Everthing in the initialization process is out in the open, right in the filesystem; and almost all of it is in editable text scripts.
The nicest thing about Linux from a maintenance perspective is that
everything is a file. Of course, it can be perplexing at times to
know which file something lives in. But as a rule, Linux recovery
amounts to using basic filesystem utilities like
rm, and a text editor like
vi. Of course, to automate these
activities, tools like
bash are helpful; or at a
python. This particular tutorial does not
address basic file manipulation.
Assuming you know how to manipulate and edit files, the only "gotcha" perhaps remaining for a broken system is not being able to use the filesystems at all.
Your best friend in repairing a broken filesystem is
203 in this series has more information, so we will just introduce the
The tool called
fsck is actually just a frontend for a number of
fsck.* tools, such as
fsck.reiser. You may specify the type explicitly using the
fsck will make an effort to figure it out on its own.
Read the manpage for
fsck.* for more details. The main
think you want to know is that the
-a option will try to fix
everything it can automatically.
You can check an unmounted filesystem by mentioning its raw device.
fsck /dev/hda8 to check a partition not in use. You
can also check a rooted filesystem, such as
fsck /home; but
generally you only want to do that if the filesystem is already
mounted as read-only, not as read-write.
A flexible feature of Linux systems is the fine tuned control you have
over mounting and unmounting filesystems. Unlike under Windows and
some other operating systems, partitions are not automatically
assigned locations by the Linux kernel, but are instead attached to
/ root hierarchy by the
mount command. Moreover,
different filesystems types (on different drives, even) may be mounted
within the same hierarchy. Unmounting a particular partition is done
umount command, specifying either the mount point (e.g.
/home) or the raw device (e.g.
For recovery purposes, the ability to control mount points lets you
perform forensic analysis on partitions--using
fsck or other
tools--without risking further damage to a damaged filesystem. You may
also custom mount a filesystem using various options; the most
important of these is mounting read-only using either of the synonyms
As a quick example, you might wish to substitute one user directory location for another, either because of damage to one, or simply to expand disk space or move to a faster disk. You might perform this switch using something like:
# umount /home # old /dev/hda7 home dir # mount -t xfs /dev/sda1 /home # new SCSI disk using XFS # mount -t ext3 /dev/sda2 /tmp # also put the /tmp on SCSI
For recovery, system upgrades, and special purposes, it is useful to
be able to mount and unmount filesystems at will. But for day-to-day
operation, you generally want a pretty fixed set of mounts to happen
at every system boot. You control the mounts that happen at bootup by
putting configuration lines in the file
/etc/fstab. A typical
configuration might look something like the below example. The Topic
203 tutorial contains more details.
/dev/hda7 / ext3 defaults 1 1 none /dev/pts devpts mode=0620 0 0 /dev/hda9 /home ext3 defaults 1 2 none /mnt/cdrom supermount dev=/dev/hdc,fs=auto,ro,--,iocharset=iso8859-1,codepage=850,umask=0 0 0 none /mnt/floppy supermount dev=/dev/fd0,fs=auto,--,iocharset=iso8859-1,sync,codepage=850,umask=0 0 0 none /proc proc defaults 0 0 /dev/hda8 swap swap defaults 0 0