Linux systems - Getting started with setting up and embedded platform

Team Emertxe
Linux Systems
Getting started with setting up an Embedded platform

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
●
Today right from hand-held devices (ex: Android)
to high end systems (ex: Stock exchange servers)
use Linux

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
The word 'Free' in Open Source should be interpreted as in
'Freedom' not as 'Free Beer'. This also explains the spirit of
creating Open Source software.

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
What has made Linux so popular to scale from mobile devices to
powering 90% of world’s super computer? Here are the key
properties of Linux
●
Multitasking
– Ability to handle multiple tasks across single / multiple processors
●
Multi-user
– Have got users with different level of privileges for secured access
●
Protected Memory
– Clear distinction called ‘user-space’ and ‘kernel’ space thereby having protected memory
access. This makes Linux Super secure comparing with other operating systems
●
Hierarchical File System
– Well organized file system that handles various types of files. This also makes handling
various inputs very simple

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
●
In graphical mode the user will be given a GUI using which he / she will be
able to use the system using mouse
●
Similar to windows based system that exist in other operating systems like
MS Windows & Apple MAC OS

User Interfaces
CLI
●
Textual mode used to execute requested commands
Our focus is to be in the CLI mode by executing various commands by invoking shells.
We will also create programs using this environment called ‘Shell scripts’

User Interfaces
The Shell - Introduction
● Shell is an application, works as a command interpreter
● Gets a command from user, gets it executed from OS
● Gives a programming environment to write scripts using
interpreted language
●
It has been inherited from UNIX operating system, which was
predecessor to Linux

User Interfaces
The Shel - Types
●
Login
– Starts after a successful login
– It is executed under user ID during login process
– It picks up user specific configuration and loads them
●
Non Login
– A Non login shell is started by a program without a login
– In this case, the program just passes the name of the shell executable
– For example, for a Bash shell it will be simply bash
– Following are examples of Non-login shells:
● sh
●
bash
● ksh
●
csh

User Interfaces
The Shel - Types
●
Try the following on your command prompt
$ 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
Error
report

User Interfaces
The Shell - bash
●
Bash – The command interpreter
– .bash_profile (Login Shell, During login)
– .bashrc (Non Login Shell, New instance)
– .bash_logout (Login Shell, Logout)
– .bash_history (Non Login Shell, Command history)

User Interfaces
The Shell - bash
●
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
Variable Description
SHELL Describes the shell that will be interpreting user commands
TERM This specifies the type of terminal to emulate when running
the shell
USER The current logged in user
PWD The current working directory
OLDPWD The previous working directory
MAIL The path to the current user's mailbox
PATH A list of directories that the system will check when looking for
commands
HOME The current user's home directory
HOSTNAME The hostname of the computer
PS1 The primary command prompt definition

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>

Basic Shell Commands
Command Meaning
ls List's all the files
pwd Gives present working directory
cd Change directory
man Gives information about command
df Disk free
du Disk usage
which Shows full path of command

Path
●
Path is the location where a particular file is located in the
directory (tree) structure
●
It starts with the root (‘/’) directory and goes into
appropriate directory
●
The path depends on the reference point from where you
take it up:
– Absolute Path : Specifies the location with reference from
root directory
– Relative Path : Specifies the location with reference to
present working directory (pwd)
●
As the name says relative path will vary depending on your
pwd

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)

Shell Scripting - Basics
Programming Languages
●
There are various types of programming languages, compared on various
parameters
●
From Embedded system engineer’s view it should be seen how close or how
much away from the hardware the language is
●
Based on that view programming languages can be categorized into three
areas:
– Assembly language (ex: 8051)
– Middle level language (ex: C)
– High level / Scripting language (ex: Shell)
●
Each programming language offers some benefits with some shortcomings
●
Depending on the need of the situation appropriate language needs to be
chosen
●
This make language selection is a key criteria when it comes to building real
time products!

Programming Languages – A Comparison
Language
parameter
Assembly C Shell
Speed High Medium Medium
Portability Low Medium High
Maintainability Low Medium High
Size Low Medium Low
Easy to learn Low Medium High
Shell or any scripting language is also called as ‘interpreted’ language as it doesn’t
go through compilation phase. This is to keep the language simple as the purpose is
different than other languages.

Shell Script
●
Any collection of shell commands can be stored
in a file, which is then called as shell script
●
Programming the scripts is called shell scripting
●
Scripts have variables and flow control
statements like other programming languages
●
Shell script are interpreted, not compiled
●
The shell reads commands from the script line
by line and searches for those commands on the
system

Shell Script – Where to use?
● System Administration
– Automate tasks
– Repeated tasks
● Development
– Allows testing a limited sub-set
– Testing tools
● Daily usage
– Simple scripts
– Reminders, e-mails etc…

Shell Script – Invocation
●
Example:
$ vi hello.sh
and type the following inside it:
#!/bin/bash
echo “ Hello World”
Then, make the script executable:
$ chmod 700 hello.sh
$ ./hello.sh
●
First line tells Linux to use BASH interpreter
●
Second line prints the “Hello world” into standard I/O

Variables
● Variables are a way of storing information temporarily.
NAME=”Emertxe”
X=10
● A couple of conversions we need to follow
– Variables usually appear in uppercase
– There is no white space between the variable name and the equal sign
● Variable substitution
●
Variable assignment
●
Bash variables & special variables

Whitespace & Line-breaks
● Bash shell scripts are very sensitive to whitespace & line-
breaks
● Because the “keywords” of this programming language are
actually commands evaluated by the shell
●
Need to separate arguments with whitespaces
● Likewise a line-break in the middle of a command will mislead
the shell into thinking the command is incomplete.
● Example: x=10; x = 10; x = “ok”; x=”ok”

Special Characters
Character Meaning
~ The current user's home directory
$ used to access a variable (e.g. : $HOME)
& used to put a command in the background
* wildcard, matching zero or more characters (e.g. : ls doc_*)
? wildcard, matching exactly one character (e.g.: ls doc_?)
${#} No of arguments passed to shell script
${@} Value of all arguments passed
$0 contains the name of the script user executed

Quotes
● Using single quotes causes the variable name to be used
literally, and no substitution will take place.
$var=’test string’
$newvar=’Value of var is $var’
echo $newvar
● Using double quotes to show a string of characters will allow a
ny variables in the quotes to be resolved
$var=“test string”
$newvar=“Value of var is $var”
echo $newvar

Expressions
● expr: Evaluates simple math on the command line calculator.
● bc: An arbitrary precision calculator language
●
Available operators: +, , /, *, %
1 #!/bin/bash
2
3 NUM1=5
4 NUM2=3
5
6 ADD=$((${NUM1} + ${NUM2}))
7 SUB=$((${NUM1} - ${NUM2}))
8 MUL=$((${NUM1} * ${NUM2}))
9 DIV=$((${NUM1} / ${NUM2}))
10 MOD=$((${NUM1} % ${NUM2}))
11
12 echo "Addition of two numbers is: ${ADD}"
13 echo "Substraction of two numbers is: ${SUB}"
14 echo "Multiplication of two numbers is: ${MUL}"
15 echo "Division of two numbers is: ${DIV}"
16 echo "Modulum of two numbers is: ${MOD}"
Example

Conditions – if else
● The if statement chooses between alternatives each of which
may have a complex test
● The simplest form is the if-then statement
1 #!/bin/bash
2
3 NUM1=5
4 NUM2=3
5
6 if [ ${NUM1} -gt ${NUM2} ]
7 then
8 echo "NUM1 is greater than NUM2"
9 else
11 fi
if [ condition ]
then
expression
else
expression
fi
Syntax Example

Conditions – if else if
● Multiple if blocks can be strung together to make an elaborate
set of conditional responses
1 #!/bin/bash
2
3 NUM1=5
4 NUM2=3
5
6 if [ ${NUM1} -eq ${NUM2} ]
7 then
8 echo "NUM1 is equal to NUM2"
9 elif [ ${NUM1} -gt ${NUM2} ]
10 then
12 else
13 echo "NUM1 is less than NUM2"
11 fi
if [ condition_a ]
then
condition_a is true
elif [ condition_b ]
then
condition_b is true
else
both false
fi
Syntax Example

Shell Scripting - Intermediate

String Tests
● String comparison, Numeric comparison, File operators and logical operators
● Comparison operations are provided below
1 #!/bin/bash
2
3 echo “Enter the first string”
4 read STR1
5 echo “Enter the second string”
6 read STR2
7
8 if [ -z ${STR1} ]; then
9 echo "First string is empty"
10 else
11 echo "First string is not empty"
12 fi
13 if [ -n ${STR2} ]; then
14 echo "First string is not empty"
15 else
16 echo "First string is empty"
17 fi
18 if [ ${STR1} = ${STR2} ]; then
19 echo "Both strings are equal"
20 else
21 echo "Both strings are not equal"
22 fi
Example
Operator Meaning
= Compare if two strings
are equal
!= Compare if two strings
are not equal
-n Evaluate if string
length is greater than
zero
-z Evaluate if string
length is equal to zero

Numeric Tests
1 #!/bin/bash
2
3 NUM1=5
4 NUM2=3
5
6 if [ ${NUM1} -gt ${NUM2} ]
7 then
9 else
11 fi
Example
Operator Meaning
-eq
Compare if two
numbers are equal
-ge
Compare if one number
is greater than are
equal to num
-le
Compare if one number
is less than or equal to
a num
-ne
Compare if two
numbers are not equal
-gt
Compare if one
number is greater than
another number
-lt
Compare if one
number is less than ano
ther number

Logical Operators
Operator Meaning
!
Negate (NOT) a logical
expression
-a
Logically AND two
logical expression
-o
Logically OR two logical
expressions
1 #!/bin/bash
2
3 echo “Enter the first number A”
4 read A
5 echo “Enter the second number B ”
6 read B
7 echo “Enter the third number C ”
8 read C
9
10 if [ ${A} -gt ${B} -a ${A} -gt ${C} ]; then
11 echo "A is the greatest of all"
12 elif [ ${B} -gt ${A} -a ${B} -gt ${C} ]; then
13 echo "B is the greatest of all"
14 elif [ ${C} -gt ${A} -a ${C} -gt ${B} ]; then
15 echo "C is the greatest of all"
16 else
17 echo "Invalid Input"
18 fi
Example

Loop – for
●
Sequential loop with expressions
● First arithmetic expr EXPR1 is evaluated;EXPR2 evaluated
repeatedly until it evaluates to 0; Each time EXPR2 is evaluated to
a non-zero value, statements are executed & EXPR3 is evaluated
1 #!/bin/bash
2
3 for ((i=1; i<=5; i++))
4 do
5 echo "Loop counter is ${i}"
6 done
for ((expr1; expr2; expr3))
do
Code Block
done
Syntax Example

Loop – while
● The structure is a looping structure. Used to execute a set of commands
while a specified condition is true
● The loop terminates as soon as the condition becomes false. If condition
never becomes false, loop will never exit
● Any valid conditional expression will work in the while loop.
1 #!/bin/bash
2
3 LOOP=1
4
5 while [ ${LOOP} -le 5 ]
6 do
7 echo "Looping : ${LOOP}"
8 LOOP=$((${LOOP} + 1))
9 done
while [ condition ]
do
Code Block
done
Syntax Example

Case Statements
●
The case statement compares the value of the variable
($var in this case) to one or more values
● Once a match is found, the associated commands are
executed and the case statement is terminated
● Used to execute statements based on specific values
●
Often used in place of an if statement if there are a large
number of conditions.
●
Each set of statements must be ended by a pair of
semicolon
● *) is used for not matched with list of values

Case Statements
1 #!/bin/bash
2
3 echo “Enter a number:”
4 read NUM
5
6 case ${NUM} in
7 1)
8 echo "You entered One"
9 ;;
10 2) echo "You entered Two" ;;
11 *) echo "Obey my orders please"
12 ;;
13 esac
case ${VAR} in
value_1)
commands;
;;
value_2)
commands;
;;
*)
commands;
;;
esac
Syntax Example

Functions
● Writing functions can greatly simplify a program
● Improves modularity, readability and maintainability
● However speed will get slowed down
● Arguments are accessed as $1, $2, $3…
function name()
{
<command>
<statments>
<expression>
}
Syntax
1 #!/bin/bash
2
3 function sum()
4 {
5 x=`expr $1 + $2`
6 echo $x
7 }
8
9 y=`sum 5 3`
10 echo "The sum is 5 and 3 is $y"
11 echo "The sum is 6 and 2 is `sum 6 2`"
Example

Arrays
● An array is a variable containing multiple values may be of
same type or of different type
● There is no maximum limit to the size of an array
● Array index starts with zero

Arrays
declare -a array_name=(element1 element2 element3)
Syntax
1 #!/bin/bash
2
3 declare -a LINUX_DISTROS=('Debian' 'Redhat' 'Ubuntu' 'Suse' 'Fedora');
4
5 echo "Number of elements in the array: ${#LINUX_DISTROS[@]}"
6 echo "Printing elements of array in one shot: ${LINUX_DISTROS[@]}"
7 echo "Printing elements of array in one shot: ${LINUX_DISTROS[*]}"
8 echo "Printing elements of array in using a loop:"
10 for ((i = 0; i < ${#LINUX_DISTROS[@]}; i++))
11 do
12 echo ${LINUX_DISTROS[$i]}
13 done
Example
Here declare –a is optional, arrays can be declared without that also

Command Line Arguments
● Shell script can accept command-line arguments & options
just like other Linux commands
● Within your shell script, you can refer to these arguments as
$1,$2,$3,.. & so on.
● Then the command line arguments are executed like
●
Read all command line arguments and print them

Command Line Arguments
1 #!/bin/bash
2
3 if [ $# != 2 ]
4 then
5 echo "Usage: Pass 3 arguments"
6 exit 0
7 fi
8
9 echo "The arguments of the script you passed are:"
10 echo "Total number of arguments you passed are : $#"
11 echo "The name of the script is : $0"
12 echo "The first argument is : $1"
13 echo "The second argument is : $2"
Example

Linux Systems – The Bigger Picture

Linux System – A Bigger Picture
How things fit together?
● Shell commands: ls, date, whoami etc.
●
Shell scripting: Operators, Loop, Arrays etc..
● Editor (vi): Insertion, commands, visual mode
Shell
commands
Shell
scripting
Editor
(vi)
Shell
programs
in Linux
Now let us learn some more Linux shell commands (advanced) for making
our shell scripts more powerful

Linux System – A Bigger Picture
Advanced Shell Commands
Shell
commands
User
Interface
File
Filters
Pattern
matching

Shell Commands - Advanced
User Specific Commands
● All Accesses into a Linux System are through a User
● Super user (root) will have higher privileges
● User related Shell commands
Command Meaning
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 To find out who is logged in
whoami Who are you

Remote Login and Remote Copy
● ssh is a program for logging into a remote machine
and for executing commands on a remote machine.
$ ssh username@ipaddress
● scp copies files between hosts on a network.
$ scp filename username@ipaddress:/path/

File Related - Redirection
● Out put redirection ( > )
$ ls > /tmp/outputfile
● Redirecting to append ( >> )
$ ls -l >> /tmp/outputfile
●
Redirecting the error (2>)
$ ls 2> /tmp/outputfile

File Related - 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
$ ls -al /bin | less

File Related
● Every thing is viewed as a file in Linux. Even a Directory is a
file.
● Basic Shell Command Set
Command Meaning
cp <source> <dest> Copy file from one to another
mv <source> <dest> Rename a file
rm <file> Remove a file
stat File related statistics (i-node)
ln Linking between files (-s option for soft link)

File Related
Command Meaning
mkdir <dir_name> Make directory
rmdir <dir_name> Remove a particular directory
touch Change file timestamps
wc Counts the number of lines in a file
cat Display contents of the file in standard output
more Display contents, navigate forward
head Display first 10 lines of the file (-n to change)
tail Display last 10 lines of the file (-n to change)
sort Sort lines of text files

File 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

File Listing - 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

File Listing - Permission
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

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

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

find
Find options Meaning
find . –print Find all files from current directory & sub-
directory & print their path
find . –name *.sh Find all files that are having *.sh extension
starting from current directory
find / -type d –name tech Search for directories with name ‘tech’ from
the ‘root’ directory
find . –type f –empty Find all files that are empty in current
directory
find <where-to-look> <criteria> <what-to-do>
Syntax

cut
cut <option> <file>
Option ‘-c’ character
Option ‘-d’ delimiter and much more
Syntax
cut options Meaning
cut –c3 <file> Outputs third character of every line of <file>
cut –c1-3 <file> Outputs the first three characters of every line
of <file>
cut -d':' -f1 /etc/passwd Outputs the first field of the file /etc/passwd,
where fields are delimited by a colon (':'). The
first field of /etc/passwd is the username, so
this will output every username in
the passwd file
cut -d':' -f1,6 /etc/passwd Output first and sixth fields of /etc/passwd

split
split <option> <file> <newfile>
Option ‘-b’ bytes
Option ‘-l’ lines and much more
Syntax
split options Meaning
split –b <file> <newfile> Split ‘b’ bytes from ‘file’ and put them into
newfileaa, newfileab, newfileac etc..
split –l <file> <newfile> Split <file> into ‘l’ number of lines and put
them into newfileaa, newfileab, newfileac etc.
There are three more file related commands as follows:
• cmp – Compares two files and stops where difference is found
• diff – Reports differences between two files
• uniq – Reports/Filters repeated pattern in a file
Let us read through the man pages and understand them (Self-study)

tr
tr <options> <input>
Syntax
tr options Meaning
tr –d <char> Delete occurrences of given character
tr –s <char> Squeeze repetition characters
:upper: Upper case characters
:lower: Lower case characters
:space: Space character
:digit: Numerical numbers

tr
# Delete all occurrences of character ‘h’
$ echo hello how are you | tr -d h
ello ow are you
# Replace all spaces with tabs
$ echo "whats up with you guys" | tr [:space:] 't'
whats up with you guys
# Remove all numbers from the input
$ echo "my age is 99" | tr -d [:digit:]
my age is
# Replace all lower case with upper case
$ tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
hello
HELLO

File Compression
●
Compression is needed to conserve the disk space
●
When there is a need to send large files as an attachment via
the email, it is good practice to compress first
●
Compression & Decompression utilities - gzip & gunzip(.gz)
●
The degree of compression depends on
– The type of the file
– Its size
– Compression program used
●
Example
– Html files compress more
– GIF & JPEG image files compress very less, as they are already in
compressed form

File Compression - Flow
file
gzip
file.gz
file.gz
gunzip
file
Compression flow De-compression flow
• Recursive compression and de-compression (-r option), will
come handy
• gzip –r <directory> : To compress whole directory
• gunzip –r <directory> : To de-compress whole directory

File Compression - Example
Compression
Input Program Output
file gzip file file.gz
sample.html gzip sample.html sample.html.gz
De-compression
Input Program Output
file.gz gunzip file.gz file
sample.html.gz gunzip sample.html.gz sample.html

File Archival
● Used for creating disk archive that contains a group of files or
an entire directory structure
● An archive file is a collection of files and directories that are
stored in one file
●
Archive file is not compressed, it uses the same amount of disk
space as all the individual files and directories
● In case of compression, compressed file occupies lesser space
● Combination of archival & compression also can be done
• File archival is achieved using ‘tar’ with the following commands:
• tar -cvf <archive name> <file-names>
• tar -xvf <archive name>

File Archival - Flow
file1, file2,
file3
tar -cvf
file.tar
gzip file.tar
file.tar.gz
file.tar.gz
gunzip
file.tar
tar -xvf
file1, file2,
file3
Archival
De-compression
Compression Untar

Regular Expressions
●
Regular expressions = search (and
replace/modify/remove) pattern
●
In theoretical computer science regular expressions are
called as regex or regexp
●
It is a sequence of characters that forms a search
pattern using some special characters
●
Popular applications in Linux (Vi editor, Grep, Sed, Lex
& Yacc etc..) extensively use regular expressions
●
Extensively used in compiler design and implementation
●
Our idea is to understand them from Linux commands

Regular Expressions
● Each character in a regular expression is either understood to
be a meta-character with its special meaning
● Or a regular character with its literal meaning
● Together they form a pattern. Some popular & most frequently
used examples are provided below
Meta-character Meaning
? Zero or one occurrence
* Zero or more occurrence
+ One or more occurrence

Pattern Matching - grep
● Get Regular Expression And Print (GREP)
● Grep is pattern matching tool used to search the name input
file
Option Meaning
grep a* <file> Search for lines starting with name ‘a’ in <file>
grep –x <pattern> <file> Exactly match for <pattern> in <file>
grep –v <pattern> <file> Print non-matching of <pattern> in <file>
grep <reg-exp> <file>
Syntax

Pattern Matching - sed
● Stream Editor (SED)
●
Sed perform basic text transformations on an input stream
● It can be a file, or input from a pipe
sed <reg-exp> <file>
Syntax
Option Meaning
sed –n ‘1,5p’ <file> Print line numbers ranging from 1-5 in the
given input file
sed ‘1,5d’ <file> > out Delete line number ranging from 1-5 in the
given input file and re-direct into another
file called out
sed ‘s/<old_string>/<new_string>/’
<file>
Replace <old_string> with <new_string> in
input <file>

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