Linux basics 1/2

Linux Basics - 1/2
claudio@enealcubo.com
Jul 2013
Saturday, July 27, 13

Linux
Is a Unix-like operating system (OS) created
by Linus Torvalds. He developed Linux
because he wasn't happy with the currently
available options in Unix and felt he could
improve it. So he did what anybody else
would do, and created his own operating
system.

Linux
When Linus ﬁnished building a working
version of Linux, he freely distributed the
OS, which helped it gain popularity.Today,
Linux is used by millions of people around
the world. Many computer hobbyists (a.k.a.
nerds) like the operating system because it is
highly customizable. Programmers can even
modify the source code and create their own
unique version of the Linux operating system.

Kernel
In computing, the kernel is a computer program that
manages input/output requests from software, and
translates them into data processing instructions for the
central processing unit and other electronic components
of a computer.The kernel is a fundamental part of a
modern computer's operating system.

Linux Kernel
The Linux kernel is the operating system kernel used by
the Linux family of Unix-like operating systems. It is a
prominent example of free and open source software.
The Linux kernel is released under the GNU General
Public License version 2 (GPLv2)(plus some ﬁrmware
images with various non-free licenses), and is developed by
contributors worldwide.
https://www.kernel.org/
3.10.3 stable version

Linux Distros
A Linux distribution (often called distro for
short) is a member of the family of Unix-like
operating systems built on top of the Linux
kernel. Such distributions are operating
systems including a large collection of
software applications such as word
processors, spreadsheets, media players, and
database applications.

Linux Distros
Popular main distros
• Debian
• RedHat
• Suse
• Gentoo
• Slackware*

Linux Distros
Popular child distros
• Ubuntu (Debian based)
• Fedora (RedHat)
• Centos (RedHat)
http://upload.wikimedia.org/wikipedia/commons/1/1b/Linux_Distribution_Timeline.svg

Which Distro?
It depends on
• Software requirements
• Team or staff knowledge
• The one that makes you happy

Uses
• Desktop
• Phones
• Tablets
• TV’s
• Servers

How it works?
1. System startup - BIOS / BootMonitor
2. Stage #1 bootloader (master boot record)
3. Stage #2 bootloader (LILO GRUB)
4. Kernel
5. Init
6. Runlevel

How it works?
1. BIOS
• BIOS stands for Basic Input/Output System
• Performs some system integrity checks
• Searches, loads, and executes the boot loader program.
• It looks for boot loader in ﬂoppy, cd-rom, or hard drive.You can
press a key (typically F12 of F2, but it depends on your system)
during the BIOS startup to change the boot sequence.
• Once the boot loader program is detected and loaded into the
memory, BIOS gives the control to it.
• So, in simple terms BIOS loads and executes the MBR boot loader

How it works?
2. MBR
• MBR stands for Master Boot Record.
• It is located in the 1st sector of the bootable disk.Typically /
dev/hda, or /dev/sda
• MBR is less than 512 bytes in size.This has three components
1) primary boot loader info in 1st 446 bytes 2) partition table
info in next 64 bytes 3) mbr validation check in last 2 bytes.
• It contains information about GRUB (or LILO in old systems).
• So, in simple terms MBR loads and executes the GRUB boot
loader.

How it works?
3. GRUB
• GRUB stands for Grand Unified Bootloader.
• If you have multiple kernel images installed on your system, you can choose
which one to be executed.
• GRUB displays a splash screen, waits for few seconds, if you don’t enter
anything, it loads the default kernel image as specified in the grub
configuration file.
• GRUB has the knowledge of the filesystem (the older Linux loader LILO
didn’t understand filesystem).
• Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to
this).
• So, in simple terms GRUB just loads and executes Kernel and initrd images.

How it works?
4. Kernel
• Mounts the root file system as specified in the “root=” in grub.conf
• Kernel executes the /sbin/init program
• Since init was the 1st program to be executed by Linux Kernel, it
has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the
pid.
• initrd stands for Initial RAM Disk.
• initrd is used by kernel as temporary root file system until kernel is
booted and the real root file system is mounted. It also contains
necessary drivers compiled inside, which helps it to access the hard
drive partitions, and other hardware.

How it works?
5. Init
• Looks at the /etc/inittab ﬁle to decide the Linux run level.
• Following are the available run levels
• 0 – halt
• 1 – Single user mode
• 2 – Multiuser, without NFS
• 3 – Full multiuser mode
• 4 – unused
• 5 – X11
• 6 – reboot
• Init identiﬁes the default initlevel from /etc/inittab and uses that to load all appropriate
program.
• Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level
• If you want to get into trouble, you can set the default run level to 0 or 6. Since you know
what 0 and 6 means, probably you might not do that.
• Typically you would set the default run level to either 3 or 5.

How it works?
6. Runlevel
• When the Linux system is booting up, you might see various services
getting started. For example, it might say “starting sendmail …. OK”.Those
are the runlevel programs, executed from the run level directory as
deﬁned by your run level.
• Depending on your default init level setting, the system will execute the
programs from one of the following directories.
• Run level 0 – /etc/rc.d/rc0.d/

How it works?
6. Runlevel
• Please note that there are also symbolic links available for these
directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/
rc0.d.
• Under the /etc/rc.d/rc*.d/ directories, you would see programs that
start with S and K.
• Programs starts with S are used during startup. S for startup.
• Programs starts with K are used during shutdown. K for kill.
• There are numbers right next to S and K in the program names.
Those are the sequence number in which the programs should be
started or killed.
• For example, S12syslog is to start the syslog deamon, which has the
sequence number of 12. S80sendmail is to start the
sendmail daemon, which has the sequence number of 80. So, syslog
program will be started before sendmail.

How it works?

Installation Methods
• CD or DVD
• Normal
• Live
• USB installer
• Network installation
• Virtual installation (iso ﬁle)

System Requirements
Desktop
• 700Mhz processor
• 512Mb RAM
• 5Gb hard-drive space
• VGA 1024x768
• CD/DVD or USB port for installer media
• Internet access (not required but helpful)

System Requirements
Server CLI
• 300Mhz processor
• 128Mb RAM
• 1Gb hard-drive space
• VGA 640x480
• CD

Workshop
1. Internet access required.
2. Create a new virtual machine on Oracle
VirtualBox with the following settings.
1. 512Mb RAM
2. 5Gb hard disk
3. Use debian iso image
3. Star the virtual machine

Workshop
4. Choose the installation language
5. Choose the country or territory
6. Choose the locale settings
7. Choose the keymap to use
8. Type the hostname (linux-#x)
9. Type the domain name

Workshop
10.Type the root password (twice)
11.Create a new user
• Name, username and password
12.Choose the timezone
13.Chose the partition method (Guided -
entire disk)
14.Select the hard disk to use

Workshop
15.Select partition scheme (all ﬁles in one partition)
16.Finish partitioning
17.Write changes to disk
18.Choose debian mirror (Mexico)
19.Choose archive mirror (ftp.mx.debian.org)
20.Use proxy? No
21.

Installation facts
• Language?
• Locale settings?
• Territory?
• Hostname and domainame?
• Root user?
• Hard disk to use?
• Partition scheme?
• Mirror?
• Packages?

root or superuser
In computing, the superuser is a special user account used for
system administration. Depending on the operating system,
the actual name of this account might be: root, administrator,
admin or supervisor. In some cases the actual name is not
significant, rather an authorization flag in the user's profile
determines if administrative functions can be performed.
In operating systems which have the concept of a superuser, it
is generally recommended that most application work be
done using an ordinary account which does not have the
ability to make system-wide changes.

hostname
A hostname is a label that is assigned to a device
connected to a computer network and that is used
to identify the device in various forms of electronic
communication such as the World Wide Web, e-
mail or Usenet. Hostnames may be simple names
consisting of a single word or phrase.

domainame
A domain name is an identiﬁcation string that
deﬁnes a realm of administrative autonomy,
authority, or control on the Internet. Domain
names are formed by the rules and procedures of
the Domain Name System (DNS).

Hard disk
• Linux identiﬁed the hard disk with a OS
resource.Typically this are the resources.
• IDE
• SCCI

Partitions
Partitions are divisions in the formatting of the hard
disk. It’s a logical – as opposed to a physical –
division, so you can edit and manipulate them for
various purposes. Think breaking a disk into two
conﬁguration parts. Partitions are really handy
because they act as a sandbox. f you have a 1 TB hard
drive partitioned into a 250 GB partition and a 750
GB partition, what you have on the latter will not
affect the other, and vice versa. You can share one of
those partitions on the network and never worry
about people accessing information on the other.

Partition Schemes
It depends on the using of the SO and the size
of the hard disk
• Desktop
• All ﬁles in one partition
• Server
• Split the data in multiple partitions

Partition Schemes
Server partition scheme

Partition types
• Foreign partitions (other SO)
• Primary partitions
• The number of partitions on an Intel-based system was limited from the very
beginning:The original partition table was installed as part of the boot sector and
held space for only four partition entries.These partitions are now called primary
partitions.
• Logical partitions
• One primary partition of a hard drive may be subpartitioned.These are logical
partitions.This effectively allows us to skirt the historical four partition limitation.
• Swap partitions
• If you have too many processes running on a machine, the kernel will try to free up
RAM by writing pages to disk.This is what swap space is for. It effectively increases
the amount of memory you have available. However, disk I/O is about a hundred
times slower than reading from and writing to RAM. Consider this emergency
memory and not extra memory.

Partition - File Systems
/ - root partition
/var - This fs contains spool directories such as those for mail and printing. In
addition, it contains the error log directory. If your machine is a server and develops
a chronic error, those msgs can ﬁll the partition. Server computers ought to have /var
in a different partition than /.
/usr - This is where most executable binaries go. In addition, the kernel source tree
goes here, and much documentation.
/tmp - Some programs write temporary data ﬁles here. Usually, they are quite small.
However, if you run computationally intensive jobs, like science or engineering
applications, hundreds of megabytes could be required for brief periods of time. In
this case, keep /tmp in a different partition than /.
/home - This is where users home directories go. If you do not impose quotas on
your users, this ought to be in its own partition.
/boot - This is where your kernel images go. See discussion above for placement on
old systems.

Linux basics 1/2

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