Linux Internals - Part I

Team Emertxe
Linux Internals
Day 1

Introduction
Let us ponder ...
● What exactly is an Operating System (OS)?
● Why do we need OS?
● How would the OS would look like?
● Is it possible for a team of us (in the room) to create an
OS of our own?
● Is it necessary to have an OS running in a Embedded
System?
● Will the OS ever stop at all?

Introduction
Operating System
Compiler Assembler Text Editor Database
System and Application Programs
Operating System
Humans
Program
Interface
User
Programs
Operating
System
OS
Interface
HW
Interface/
Privileged
Instr
Hardware

Introduction
What is Linux?
● Linux is a free and open source operating system that is
causing a revolution in the computer world
● Originally created by Linus Torvalds with the assistance of
developers called community
● This operating system in only a few short years is
beginning to dominate markets worldwide

Introduction
Why use Linux?
● Free & Open Source –GPL license, no cost
● Reliability –Build systems with 99.999% upstream
● Secure –Monolithic kernel offering high security
● Scalability –From mobile phone to stock market servers

Introduction
What is Open Source?
Freedom of
software
Freedom of
redistribute
Freedom of
copy
Freedom of
modify
Freedom of
use

Introduction
Open Source - How it all started?
● With GNU (GNU is not UNIX)
● Richard Stallman made the initial announcement in 1983, Free
Software Foundation (FSF) got formed during 1984
● Volunteer driven GNU started developing multiple projects,
but making it as an operating system was always a challenge
● During 1991 a Finnish Engineer Linus Torvalds developed core
OS functionality, called it as “Linux Kernel”
● Linux Kernel got licensed under GPL, which laid strong
platform for the success of Open Source
● Rest is history!

Introduction
Open Source - How it evolved?
● Multiple Linux distributions started emerging around the Kernel
● Some applications became platform independent
● Community driven software development started picking up
● Initially seen as a “geek-phenomenon”, eventually turned out to be
an engineering marvel
●
Centered around Internet
● Building a business around open source started becoming viable
● Redhat set the initial trend in the OS business
Customization
Applications
Kernel

Introduction
Open Source - Where it stands now?
OS Databases Server/Cloud Enterprise
Consumer Education CMS eCommerce

Introduction
Open Source vs Freeware
OSS Freeware
 Users have the right to access &
modify the source codes
 In case original programmer
disappeared, users & developer
group of the S/W usually keep its
support to the S/W.
 OSS usually has the strong users &
developers group that manage and
maintain the project
 Freeware is usually distributed in a
form of binary at ‘Free of Charge’,
but does not open source codes
itself.
 Developer of freeware could
abandon development at any time
and then final version will be the
last version of the freeware. No
enhancements will be made by
others.
 Possibility of changing its licensing
policy

Introduction
GPL
● Basic rights under the GPL – access to source code, right
to make derivative works
● Reciprocity/Copy-left
● Purpose is to increase amount of publicly available
software and ensure compatibility
● Licensees have right to modify, use or distribute
software, and to access the source code

Introduction
GPL - Issues
● Linking to GPL programs
● No explicit patent grant
● Does no discuss trademark rights
● Does not discuss duration
● Silent on sub-licensing
● Relies exclusively on license law, not contract

Introduction
Linux Properties
● Multitasking
● Multi-user
● Multiprocessing
● Protected Memory
● Hierarchical File System

Introduction
Linux Components
● Hardware Controllers: This subsystem is
comprised of all the possible physical devices
in a Linux installation - CPU, memory
hardware, hard disks
● Linux Kernel: The kernel abstracts and
mediates access to the hardware resources,
including the CPU. A kernel is the core of the
operating system
● O/S Services: These are services that are
typically considered part of the operating
system (e.g. windowing system, command
shell)
● User Applications: The set of applications in
use on a particular Linux system (e.g. web
browser)
User
Application
GNU
C
Library
System Call Interface
Kernel
Architecture Dependent
Kernel Code
Hardware Platform
Linux
UserSpaceKernelSpace

Introduction
Linux Directory Structure
/
/boot
/dev
/bin
/etc
/lib
/media
/mnt
/opt
/sbin
/tmp
/usr
/root
/home
/var
/proc
User home directories
Root user's home directory
Essential user command binaries
Essential super user command binaries
Essential shared libraries and kernel modules
Multi user utilities and application
Add-on application software packages
Host specific configuration
Device files
Mount point for removable media
Mount point for temporarily mounted file systems
Static boot-able images
Temporary files
Variable files (Logs)
Virtual FS documenting kernel and process status

User Interfaces
GUI
● Can use Mouse, Keyboard, Touch Screens

User Interfaces
CLI
● Textual mode used to execute requested commands

User Interfaces
The Shell
● What is a Shell
● Types of shells
– Login
– Non Login
● sh
● bash
● ksh
● csh
● Hands on:
– cat /etc/shells
– echo $0

User Interfaces
The Shell - Invocation
● The main task of a shell is providing a user environment
shell
Input
(ls)
List all
the
files
List all
the
files

User Interfaces
The Shell - bash
● Bash – The command interpreter
– .bash_profile (During login)
– .bashrc (New instance)
– .bash_logout (Logout)
– .bash_history (Command history)
● Hands on:
– Enter ls -a in your home directory
– Display contents of all files mentioned above

User Interfaces
The Shell – Environmental Variables
● Login-shell's responsibility is to set the non-login shell and
it will set the environment variables
● Environment variables are set for every shell and generally
at login time
● Environmental variables are set by the system.
● Environmental variables hold special values. For instance
$ echo $SHELL
● Environmental variables are defined in /etc/profile,
/etc/profile.d/ and ~/.bash_profile.
● When a login shell exits, bash reads ~/.bash_logout

User Interfaces
The Shell – Environmental Variables
● env - lists shell environment variable/value pairs
● export [var_name] - exports/sets a shell variable
– HOME - path to user’s home directory
– PATH - executable search path
– PWD - present working directory
– PS1 - command prompt
● N=10 - Assigning the variable. This a temporary variable
effective only inside the current shell)
● unset N - Unset the environment variable N

Basic Shell Commands
Types
●
An executable program like all those files can have in
/usr/bin.
●
A command built into the shell itself. bash provides a number
of commands internally called shell built-ins The cd
command, for example, is a shell built-in
●
A shell function. These are miniature shell scripts
incorporated into the environment.
●
An alias. Commands that you can define yourselves, built
from other commands.
●
To know the type, try
$ type <command>

Information
●
ls - list's all the files
●
pwd - gives present working directory
●
man - gives information about command
●
info <topic> - information pages on <topic>
●
which - shows full path of command
●
df - disk free
●
du - disk usage
●
stat - File and Inode information
●
uname - print system information

User Specific
● All Accesses into a Linux System are through a User
● User related Shell Command Set
– useradd - create user
– userdel - delete user
– su - [username] - start new shell as different user
– finger - user information lookup
– passwd - change or create user password
– who, w - to find out who is logged in
– whoami - who are you

Remote Access
● ssh - (secured login) is a program for logging into a
remote machine and for executing commands on a
remote machine.
Example: ssh username@ipaddress
● scp - (secured copy) copies files between hosts on a
network.
Example: scp filename username@ipaddress:/path/

File System related
● Every thing is viewed as a file in Linux. Even a directory
is a file.
– mount - Mounting filesystem
– find, locate - Search for files
– cd - change directory.
– cp - copy
– mv - move, rename
– rm – remove

File System related
– mkdir - make directory
– rmdir - remove directory
– cat, less, head, tail - used to view text files
– touch - create and update files
– wc - counts the number of lines in a file

Archiving
● gzip - This will compress folder or file
● gunzip - This will uncompress
● tar - Archiving files

Filters
● Filters are the programs, which read some input, perform
the transformation on it and gives the output. Some
commonly used filters are as follow
– tail - Print the last 10 lines of each FILE to standard
output.
– sort - Sort lines of text files
– tr - Translate, squeeze, and/or delete characters from
standard input, writing to standard output.
– wc - Print newline, word, and byte counts for each file

Pattern Matching
● Grep is pattern matching tool used to search the name
input file. Basically its used for lines matching a pattern
Example:
$ ls | grep *.c
This will list the files from the current directory with .c
extension

Files
Listing
user@user:~] ls -l
total 12
drwxrwxr-x 2 user user 4096 Jun 23 16:48 A-Direcory
brw-r--r-- 1 root root 7, 0 Jun 23 16:55 block_file
crw-r--r-- 1 root root 108, 0 Jun 23 16:49 character_file
lrwxrwxrwx 1 user user 12 Jun 23 16:50 link_to_regular_file -> regular_file
prw-rw-r-- 1 user user 0 Jun 23 16:50 named_pipe
-rw-rw-r-- 1 user user 0 Jun 23 16:48 regular_file
-rwxrwxr-x 1 user user 7639 Jun 23 16:54 server
srwxrwxr-x 1 user user 0 Jun 23 16:55 server_socket
permissions
owner
& group
file
size
created date
& time
file name

Files
Types
user@user:~] ls -l
total 12
drwxrwxr-x 2 user user
brw-r--r-- 1 root root
crw-r--r-- 1 root root
lrwxrwxrwx 1 user user
prw-rw-r-- 1 user user
-rwxrwxr-x 1 user user
srwxrwxr-x 1 user user
Directory
Block
Character
Soft Link
FIFO (sometimes =)
Plain Text
Socket

Files
Permissions
user@user:~] ls -l
total 1
-r w x r w x r - x 2 user user
Read 100 - 4
Write 010 - 2
Execute 001 - 1
Value used to Set

Files
Permissions
user@user:~] ls -l
total 1
-r w x r w x r w x 2 user user
user group others

Files
Permission Modification
● chmod – Change file permissions
● chown – Change file owner
● chmod [ ug+r, 746 ] file.txt
● chown -R user:group [ filename | dir ]

I/O Redirection
Output
●
Output redirection ( > )
●
Redirecting to append ( >> )
●
Redirecting the error (2>)
Examples:
$ ls > /tmp/output_file
Redirects the output of ls to output_file
$ ls -l >> /tmp/output_file
Appends the output of ls -l to output_file
$ ls 2> /tmp/output_file
Redirects the error output of ls to output_file

I/O Redirection
Pipe
● A pipe is a form of redirection that is used in Linux
operating systems to send the output of one program to
another program for further processing.
● A pipe is designated in commands by the vertical bar
character
Example:
$ ls -al /bin | less

Visual Editor - vi
● Screen-oriented text editor originally created for the
Unix operating system
● The name vi is derived from the shortest unambiguous
abbreviation for the ex command visual
● Improved version is called as vim
● To open a file
$ vi <filename>
or
$ vim <filename>

Visual Editor - vi
● vi opens a file in command mode to start mode.
● The power of vi comes from its 3 modes
– Escape mode (Command mode)
●
Search mode
● File mode
– Editing mode
● Insert mode
● Append mode
● Open mode
●
Replace mode
– Visual mode

Visual Editor – vi
Cursor Movement
● You will clearly need to move the cursor around your file.
You can move the cursor in command mode.
● vi has many different cursor movement commands. The
four basic keys appear below
– k move up one line
– h line move one character to the left
– l line move one character to the right
– j move down one line
● Yes! Arrow keys also do work. But these makes typing
faster

Basic Commands
● Open a file
$ vi <file_name>
● How to exit
:q -> Close with out saving.
:wq -> Close the file with saving.
:q! -> Close the file forcefully with out saving
● Already looks too complicated?
● Try by yourself, let us write a C program

Escape / Command Mode
● In command mode, characters you perform actions like moving the
cursor, cutting or copying text, or searching for some particular text
– Search mode
● vi can search the entire file for a given string of text. A string is a
sequence of characters. vi searches forward with the slash (/) key
and string to search. To cancel the search, press ESC .You can
search again by typing n (forward) or N (backward). Also, when vi
reaches the end of the text, it continues searching from the
beginning. This feature is called wrap scan
● Instead of (/), you may also use question (?). That would have
direction reversed
● Now, try out. Start vi as usual and try a simple search. Type
/<string> and press n and N a few times to see where the cursor
goes.

Escape / Command Mode
– File mode
●
Changing (Replacing) Text
:%s/first/sec - Replaces the first by second every where in
the file
:%s/orange/apple/gc - For all lines in a file, find string
"orange" and replace with string "apple" for each instance on
a line
●
File Interactions (edit and read)
:e filename - open another file without closing the current
:r filename - reads file named filename in place
●
Editor Settings
:set all - display all settings of your session

Escape / Command Mode – Useful Shortcuts
Command Operation
G Go to last line of the file
gg Go to first line of the file
. Repeat the previous command
Ctrl a Increment number under the cursor by 1
Ctrl x Decrements numbers under the cursor by 1
J Joining the two adjacent lines
(n)gg Move cursor to nth
line

Editing Mode
Command Mode Name Insertion Point
a Append just after the current character
A Append end of the current line
i Insert just before the current character
I Insert beginning of the current line
o Open new line below the current line
O Open new line above the current line

Editing Mode – Editing Text
Command Operation
dd For deleting a line
(n)dd For deleting a n lines
x To delete a single character
D Delete contents of line after cursor
dw Delete word
(n)dw Delete n words
● Deleting Text Sometimes you will want to delete some of
the text you are editing. To do so, first move the cursor
so that it covers the first character of the group you want
to delete, then type the desired command from the table
below.

Visual Mode – Editing Text
● Visual Mode
– Visual mode helps to visually select some text, may be
seen as a sub mode of the command mode to switch
from the command mode to the visual mode type one
of
● v - visual mode
● ctrl+v - Go's to visual block mode.
● d or y Delete or Yank selected text
● I or A Insert or Append text in all lines (visual block
only)

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Linux Internals - Part I

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Linux Internals - Part I