what linux is?

Information Networking Security and Assurance LabNational Chung Cheng University 1

What Linux is?

Free Unix Like Open Source Network operating system


Who developed the Linux?

In 1991, Linus Torvalds studied Unix at the University, where he used special educational experimental purpose operating system called Minix (small version of Unix and used in Academic environment).

But Minix had it's own limitations. Linus felt he could do better than the Minix. So he developed his own version of Minix, which is now know as Linux.

Linux is Open Source From the start of the day. For more information on Linus Torvalds, please visit http://www.cs.helsinki.fi/u/torvalds/.


http://www.freeos.com/guides/lsst/

How to get Linux?


What's Kernel

The hart of Linux OS. Manage resource of Linux OS.

Kernel decides who will use this resource, for how long and when.

Perform the following tasks I/O management Process management Device management File management Memory management

http://files.nixp.ru/books/programming/Linux%20Shell%20Scripting%20Tutorial.pdf

Kernel

ShellApplications


Shell

A command language interpreter that executes commands read from the standard input device (keyboard) or from a file.


Most Popular Shells

$ echo $SHELL


Input – Output Redirection (I)

> Redirector Symbol Syntax: Linux-command > filename To output Linux-commands result to file. If file already exist, it will be overwritten else new fi

le is created.

EX:$ ls > myfiles


Input – Output Redirection (II)

>> Redirector Symbol Syntax: Linux-command >> filename To output Linux-commands result to END of file. If file exist , it will be opened and new information

will be written to END of file (appendix), without losing previous information, And if file is not exist, then new file is created.

EX:$ date >> myfiles


Input – Output Redirection (III)

< Redirector Symbol Syntax: Linux-command < filename To take input to Linux-command from file instead of

key-board.

EX:$ cat < myfiles


Pipe

A pipe is a way to connect the output of one program to the input of another program without any temporary file.

Pipes are used to run more than two commands ( Multiple commands) from same command line. Syntax: command1 | command2


Pipe Examples


Variables in Linux

Variable (a.k.a memory variable): Programmer can give a unique name to this memory location/address.

In Linux, there are two types of variable System variables - Created and maintained by Linux

itself. This type of variable defined in CAPITAL LETTERS.

User defined variables (UDV) - Created and maintained by user.

This type of variable defined in lower LETTERS.


Some System variables

You can change system variables by giving command like $ set,

You can print any of the variables via “echo” Ex: $ echo $USERNAME; $ echo $HOME


How to define and print User defined variables (UDV)

Syntax: variablename=value Ex:

$ no=10 # this is ok $ 10=no # Error, NOT Ok, Value must be on right side of = sign.

Q.1.How to Define variable x with value 10 and print it on screen $ x=10 $ echo $x

Q.2.How to Define variable xn with value Rani and print it on screen $ xn=Rani $ echo $xn


Rules for Naming variable name (Both UDV and System Variable)

Variable name must begin with Alphanumeric character or underscore character (_), followed by

one or more Alphanumeric character. EX:

HOME SYSTEM_VERSION no

Don't put spaces on either side of the equal sign when assigning value to variable. $ no=10 () $ no =10 (X) $ no= 10 (X) $ no = 10 (X) Variables are case-sensitive, just like filename in Linux.

$ no=10 $ NO=11


Some Good Questions (I)

Q.1: How to print sum of two numbers, let's say 6 and 3 $ echo 6 + 3 This will print 6 + 3, not the sum 9 Try $ expr 6 + 3 How about $ expr 6+3 (please note that no space bet

ween 6 and 3)?


Some Good Questions (II)

Q.2:How to define two variable x=20, y=5 and then to print division of x and y (i.e. x/y) $x=20 $ y=5 $ expr x / y

Or $ x=20 $ y=5 $ z=`expr x / y` $ echo $z


What is Shell Scripts?

Shells are interactive. It means shell accept command from you (via

keyboard) and execute them.

If you want to execute the sequence of commands , the you can store this sequence of command to text file and tell the shell to execute this text file instead of entering the commands. This is know as shell script.


Why to Write Shell Script ?

Shell script can take input from user, file and output them on screen.

Useful to create our own commands.Save lots of time.To automate some task of day today life.System Administration part can be also

automated.


How to write shell script

Write a script that will print "Knowledge is Power" on screen.

$ cat > first## My first shell script#clearecho "Knowledge is Power"

$ cat > first## My first shell script#clearecho "Knowledge is Power"


How to Run Shell Scripts

Shell Script does not get execution permission by default. Two Ways to run it

1. Use chmod command to give execution permission Syntax: chmod +x shell-script-name OR Syntax: chmod 777 s

hell-script-name

2. Run our script as Syntax: ./your-shell-program-name Ex: $ ./firstOR /bin/sh &nbsh;&nbsh; your-shell-program-name Ex: $ bash first $ /bin/sh first


More on Running Shell Scripts

Need to run the scripts in same directory where you created your script, if you are in different directory your script will not run.

To Overcome this problem, there are two ways specify complete path of your script when ever you

want to run it from other directories like giving following command.

Ex: $ /bin/sh /home/vivek/first This take time and you have to remember complete

path.


Another Way To Run Shell Scripts

Can be directly executed from prompt from any directory All our executables files are installed in directory called

/bin and /bin directory is set in your PATH setting. Principles: It will look for current directory, if found

shell will execute command from current directory, if not found, then Shell will Look PATH setting, and try to find our requested commands executable file in all of the directories mentioned in PATH settings,

Procedures: Create bin directory in your home directory and then

copy your tested version of shell script to this bin directory. After this you can run you script as executable file without using $ ./shell

$ cd$ mkdir bin$ cp first ~/bin$ first


Use -v and -x option with sh or bash command to debug the shell script.

Syntax:sh option { shell-script-name }ORbash option { shell-script-name }

-v: Print shell input lines as they are read.

-x: After expanding each simple-command, bash displays the expanded value of PS4 system variable, followed by the command and its expanded arguments.

How to de-bug the shell script?

$ cat > dsh1.sh## Script to show debug of shell#tot=èxpr $1 + $2ècho $tot

$ chmod 755 dsh1.sh$ ./dsh1.sh 4 59$ sh -x dsh1.sh 4 5## Script to show debug of shell#tot=èxpr $1 + $2èxpr $1 + $2++ expr 4 + 5+ tot=9echo $tot+ echo 99


One Small Test

Q.6.Point out error if any in following script $ vi variscript

### Script to test MY knowledge about variables!#myname=Vivekmyos = TroubleOSmyno=5echo "My name is $myname"echo "My os is $myos"echo "My number is myno, can you see this number"


The read Statement

Use to get input (data from user) from keyboard and store (data) to variable.

Syntax: read variable1, variable2,...variableNFollowing script first ask user, name and then

waits to enter name from the user via keyboard.

##Script to read your name from key-board#echo "Your first name please:"read fnameecho "Hello $fname, Lets be friend!"

$ chmod 755 sayH$ ./sayHYour first name please: vivekHello vivek, Lets be friend!


Wild cards


Shorthand at the Command Prompt (I)

/ - root directory ./ - current directory ./command_name - run a command in the current directory wh

en the current directory is not on the path ../ - parent directory ~ - home directory $ - typical prompt when logged in as ordinary user # - typical prompt when logged in as root or superuser ! - repeat specified command !! - repeat previous command


Shorthand at the Command Prompt (II)

& - run a program in background mode [Tab][Tab] - prints a list of all available commands. T

his is just an example of autocomplete with no restriction on the first letter.

x[Tab][Tab] - prints a list of all available completions for a command, where the beginning is ``x''

[Ctrl]c - kill the current process [Ctrl]d - logout from the current terminal [Ctrl]z - send current process to the background reset - restore a terminal to its default settings


Important Bash Shell Variables

HOME - home directory, abbreviated as ~ PATH - directory paths to search for

executable files. PS1 - prompt string. Things that can be put in

the prompt string include \h (hostname), \u (username), \w (absolute pathname of working directory), \W (name of working directory w/o path), \d (date), \t (time).


Linux Command Related with Process

Linux is multi-user, multitasking OS. It means you can run more than two process simultaneously if you wish.

what linux is?

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