GNU/Linux is an  operating system that uses the Linux Kernel. It is similar to the Unix operating system. It is an open source operating system which basically means you can view and change the code.  

The Linux Kernel written by Linus Torvalds in 1991. Although written only for x86 architecture , it was ported to many other architectures.The whole operating system contains the kernel and several other system and application software contributed by many different projects. A major contributor has been the GNU project. GNU project was started by Richard Stallman in 1983. Richard Stallman wrote the GNU General Public License which gave the first impetus to the free software movement leading up do development of the family of Linux operating systems that we see today . 

Linux is a modular operating system, deriving much of its basic design from principles established in Unix earlier. The kernel  manages the systems resources like process control, networking, peripherals and file system access. Application Software written on top of it gives higher level functionality. 

- Linux can be deployed easily on clusters for parallel and distributed computing 

There are many distributions of Linux meant for clusters. One of the popular ones is Rocks Cluster Distribution.   

The security model used in Linux is based on the UNIX idea of security, which is known to be robust and of proven quality. 

- Linux contains a tools for scientific computing

Linux contains many tools like latex for reading and writing scientific text. It also contains many softwares like scilab , python and fortran used for scientific computing needs. 

Linux supports multiple users on a machine. Each user must log in with his or her username and password.

In order to work on a Linux system directly, one needs to provide a username and password. You always need to authenticate to the system. After booting , you will see a login screen/prompt asking for username and password , enter the username and password , if it is correct you will be logged in . One can logout by typing logout on the prompt or navigating to logout button if using Graphical User Interface . 

When you see the login screen again, asking to enter username and password, logout was successful.

What files do I have on my computer?

-------------------------------------

All content in Linux  is kept on data structure called files.We can list those files to know what all is there.

	jeeves.rst psmith.html blandings.html Music

How do I move around the file system?

-------------------------------------

“ cd -” will return you to the previous directory.

You can also use cd [absolute path] or cd [relative path] (see below):

Absolute Path is the path of the directory from root i.e / . / is the top most level in file system.

For example to get to /var/www you would type::

	$cd /var/www

   Releative Path is path in relation to your current location . 

    For example if you are in Music directory and want to get to Rock directory inside Music, you type::

	Music$ cd Rock

Linux is multiuser system so *who* all are using my system now?

--------------------------------------------------------

The standard Unix command *who* displays a list of users who are currently logged into a computer.::

	user       tty7         2009-09-08 10:50 (:0)

	harry      pts/0        2009-09-08 11:25 (:0.0)

The columns represent user, current terminal , date and time of login and the host from which he is logged in respectively. 

How do I organize my files?

---------------------------

Where am I now in the filesystem?

--------------

Usage of pwd command::

      $ cd Examples

      $ pwd

      /home/user/Examples

 I wish some commads were a bit smarter ? 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

       * *ls with flag -l* displays the result in long format, displaying Unix file types, permissions, number of hard links, owner, group, size, date, and filename ::

       $ls ­a 

       * *ls with flag -a*  lists all files including hidden files

Similarly, mkdir with -p option automatically creates parent directory even if it does not exist.::

	   $mkdir -p this/path/never/existed/earlier/

How do I find what a command does?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

More extensive Documentation

----------------------------

Looking at man pages is a very good way to actually check flags and other help related to a command. 

Most GNU commands support the --help, which gives a short explanation about how to use the command and a list of available options. Below is the output of this option with the *mkdir* command::

	$ mkdir --help

	Usage: mkdir [OPTION]... DIRECTORY...

        Create the DIRECTORY(ies), if they do not already exist.

        Mandatory arguments to long options are mandatory for short options too.

          -m, --mode=MODE   set file mode (as in chmod), not a=rwx - umask

          -p, --parents     no error if existing, make parent directories as needed

          -v, --verbose     print a message for each created directory

          -Z, --context=CTX  set the SELinux security context of each created

                            directory to CTX

          --help     display this help and exit

          --version  output version information and exit

        Report mkdir bugs to bug-coreutils@gnu.org

        GNU coreutils home page: <http://www.gnu.org/software/coreutils/>

        General help using GNU software: <http://www.gnu.org/gethelp/>

        Report mkdir translation bugs to <http://translationproject.org/team/>

Copying Files

-------------

*cp* is the command to copy a file from one place to another including different file system(#change? ellaborate). The original file remains unchanged, and the new file may have the same or a different name.

To copy a file to another file ::

In case target Directory has a file of the same name::

	TargetDirectory$ls

	jeeves.rst psmith.html

	SourceDirectory$ls

	jeeves.rst index.html

	$cp -i 	jeeves.rst TargetDirectory/

	cp: overwrite 'TargetDirectory/jeeves.rst'? 

-i option is for interactive usage.

*-r* (recursive) – copy directories (recursively copying all the contents)

Moving Files

------------

*mv* (short for move) is a Unix command that moves one or more files or directories from one place to another. The original file is deleted, and the new file may have the same or a different name.An interesting usage of mv is actualy to rename it by moving it in same directory under a different name. 

Usage

To rename a file ::

   $ mv myfile mynewfilename  

To move to a different directory ::

   $ mv myfile otherdir/     

To move a directory ::

   $mv mydir otherdir

Using -i to avoid overwrite(just like cp)::

   $mv -i mydir otherdir

   mv: overwrite `otherdir/mydir'?

Removing files

--------------

*rm*  is used to delete files from a filesystem. 

*rm* is often aliased to "rm -i" so as to avoid accidental deletion of files. 

Working with text

=================

How do I look into a file?

~~~~~~~~~~~~~~~~~~~~~~~~~~

more

-----

In computing, *more* is a command to view  contents of a text file one screen at a time 

Usage

~~~~~

The command-syntax is::

	$ more [options] [file_name]

Traversing the pages ::

     SPACE       Display next k lines of text.  Defaults to current screen

                 size.

     RETURN      Display next k lines of text.  Defaults to 1.  Argument

                 becomes new default.

     /pattern    Search for kth occurrence of regular expression.  Defaults to

                 1 .

less

-----

*less*  is similar to *more* in the sense that it is used to view files , but has the extended capability of allowing both forward and backward navigation through the file. Unlike most Unix text editors/viewers, *less* does not need to read the entire file before starting, resulting in faster load times with large files.

Usage

~~~~~~

*less* can be invoked with options to change its behaviour, for example, the number of lines to display on the screen. A few options vary depending on the operating system. While *less* is displaying the file, various commands can be used to navigate through the file. These commands are based on those used by both *more* and *vi*. It is also possible to search for character patterns in the file.

By default, *less* displays the contents of the file to the standard output (one screen at a time). If the file name argument is omitted, it displays the contents from standard input (usually the output of another command through a pipe). If the output is redirected to anything other than a terminal, for example a pipe to another command, less behaves like cat.

The command-syntax is ::

	$ less [options] file_name

Frequently Used Commands

~~~~~~~~~~~~~~~~~~~~~~~~

    * [Arrows]/[Page Up]/[Page Down]/[Home]/[End]: Navigation.

    * [Space bar]: Next page.

    * b: Previous page.

    * ng: Jump to line number n. Default is the start of the file.

    * nG: Jump to line number n. Default is the end of the file.

    * /pattern: Search for pattern. Regular expressions can be used.

    * '^ or g: Go to start of file.

    * '$ or G: Go to end of file.

    * =: File information.

    * h: Help.

    * q: Quit.

cat

---

The *cat* command is a standard Unix program used to concatenate and display files. The name is from "catenate", a synonym of *concatenate*.

The Single Unix Specification specifies the behavior that the contents of each of the files given in sequence as arguments will be written to the standard output in the same sequence, and mandates one option, -u, where each byte is printed as it is read.

If the filename is specified as -, then *cat* will read from standard input at that point in the sequence. If no files are specified, *cat* will read from standard input entered.

Usage ::

        $ cat foo boo

	This is file foo

	This is file boo.

Text Processing 

---------------

How do look at part of a file?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Monitoring a continously changing file(example: A log file) 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

	$ tail -f /var/log/dmesg

More serious Text Processing:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Problem get the names of people in the following file

-----------------------------------------------------

::

	Foot in Mouth:Bully:Fat:Peter

	Rich:Simple:Peabrain:Lois

	Self-concious:Wannabe:Adolescent:Meg

	Dim-witted:Fat:evil-monkey:Chris

	Matricidal:Over-Ambitious:Infant:Stewy

	Anthropomorphic:Democrat:Sensible:Brian

 	$cut -d : -f 4- file