Variables in Shell

To process our data/information, data must be kept in computers RAM memory. RAM memory is divided into small locations, and each location had unique number called memory location/address, which is used to hold our data. Programmer can give a unique name to this memory location/address called memory variable or variable (Its a named storage location that may take different values, but only one at a time).

In Linux (Shell), there are two types of variable:
(1) System variables - Created and maintained by Linux itself. This type of variable defined in CAPITAL LETTERS.
(2) User defined variables (UDV) - Created and maintained by user. This type of variable defined in lower letters.

You can see system variables by giving command like $ set, some of the important System variables are:

System Variable	Meaning
BASH=/bin/bash	Our shell name
BASH_VERSION=1.14.7(1)	Our shell version name
COLUMNS=80	No. of columns for our screen
HOME=/home/vivek	Our home directory
LINES=25	No. of columns for our screen
LOGNAME=students	students Our logging name
OSTYPE=Linux	Our Os type
PATH=/usr/bin:/sbin:/bin:/usr/sbin	Our path settings
PS1=[\u@\h \W]\$	Our prompt settings
PWD=/home/students/Common	Our current working directory
SHELL=/bin/bash	Our shell name
USERNAME=vivek	User name who is currently login to this PC

NOTE that Some of the above settings can be different in your PC/Linux environment. You can print any of

the above variables contains as follows:

How to write shell script

Following steps are required to write shell script:

(1) Use any editor like vi or mcedit to write shell script.

(2) After writing shell script set execute permission for your script as follows
syntax: 
chmod permission your-script-name

Examples:
$ chmod +x your-script-name
$ chmod 755 your-script-name

Note: This will set read write execute(7) permission for owner, for group and other permission is read and execute only(5).

(3) Execute your script as
syntax: 
bash your-script-name
sh your-script-name
./your-script-name

Examples:
$ bash bar
$ sh bar
$ ./bar

NOTE In the last syntax ./ means current directory, But only . (dot) means execute given command file in current shell without starting the new copy of shell, The syntax for . (dot) command is as follows
Syntax:
. command-name

Linux Timeline

Linux.org presented one of the most detailed important events in the history of the Linux operating system development
1983
September    Richard M. Stallman announces the GNU Project, an attempt at creating a completely free operating system.
1984
January    Work begins on the GNU operating system
1985
October    Free Software Foundation established as a non-profit organization to promote the development of Free Software. Sponsors the GNU Project.
1987
January    Computer science professor Andrew Tannenbaum publishes the textbook Operating Systems: Design and Implementation which includes a copy of a teaching version of Unix called Minix.

Install Centos

[1]	Insert Install media and restart Computer. After booting, following screen is shown, then Push Enter key.

[2]	This is the section to check a media, Skip it if you don't need it.

Download Centos 6

[1]

Here is the configuration Examples for CentOS 6 that is compatible with Red Hat Enterprise Linux 6. Download ISO file ( CentOS-6.3-x86_64-bin-DVD1.iso ) for installation from the following site. ⇒ http://ftp.jaist.ac.jp/pub/Linux/CentOS/6.3/isos/x86_64/ It's OK to download 64 bit version commonly. But if your computer is old and not supported 64 bit computing, download 32 bit version from below. ⇒ http://ftp.jaist.ac.jp/pub/Linux/CentOS/6.3/isos/i386/ * The examples on this site uses 64 bit version.

[2]	Burn ISO file to DVD with an application on your computer and Make an Installation Disk. After finishing it, proceed to next step for installing CentOS 6.

Introduction to MPLS .
pls watching to video.

this is one more video for MPLS tutorials.