Introducing Linux

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

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 . 

Design and Implications

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

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. 

Reasons for Using Linux

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

- Linux is free:

As in "free beer". Linux can be downloaded in its entirety from the Internet completely for free. No registration fees, no costs per user, free updates, and freely available source code in case you want to change the behavior of your system.

- 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.   

- Linux was made to keep on running:

As with UNIX, a Linux system expects to run without rebooting all the time. That is why a lot of tasks are being executed at night or scheduled automatically for other times, resulting in higher availability during busier periods and a more balanced use of the hardware. This property allows for Linux to be applicable to environments where people do not have the time or the possibility to control their systems constantly.

- Linux is secure and versatile:

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. 

Getting Started

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

Logging in, activating the user interface and logging out

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

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.

Basic Commands

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

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.

*ls* lists the files in the current working directory. A directory that is not the current working directory can be specified and ls will list the files there.::

	$ ls

	jeeves.rst psmith.html blandings.html Music

How do I move around the file system?

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

This stands for "change directory". When one wants to change the directory .

       $cd Music 

One dot '.' represents the current directory while two dots '..' represent the parent directory.

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

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

Absolute paths:

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

This is an absolute path because you start at the top of the hierarchy and go downwards from there (it doesn't matter where in the filesystem you were when you typed the command).

Relative paths:

   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

Please note that there is no / using the above cd command. Using a / would cause this to be an absolute path, working from the top of the hierarchy downward.

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.::

	$who

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

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

	dumbledore pts/1        2009-09-08 18:11 (potter.xyz.in)

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

The command can be invoked with the arguments *am i* or *am I* (so it is invoked as *who am i* or * who am I*), showing information about the current terminal only (see the *-m* option below, of which this invocation is equivalent).

How do I organize my files?

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

This command is used to make a new directory. Normal usage is as straightforward as follows::

	$mkdir name_of_directory

Where *name_of_directory* is the name of the directory one wants to create. When typed as above (ie. normal usage), the new directory would be created within the current directory. On Unix, multiple directories can be specified, and *mkdir* will try to create all of them.

Where am I now on the filesystem?

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

pwd is a Linux / Unix command which prints the current working directory. If you wish to know the full path of the  directory in which you are in from the Linux console, then the pwd command will come to your rescue. pwd stands for Print Working Directory.

Usage of pwd command::

      $ cd Examples

      $ pwd

      /home/user/Examples

 I wish some commads were a bit smarter ? 

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

The standard commands in Linux have a lot of options also called flags to change or provide some additional functionality to the command For example ::

       $ls -l 

       * *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/

Getting Help

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

How do I find what a command does?

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

A short index of explanations for commands is available using the *whatis* command, like in the examples below::

	$whatis ls

	ls (1) 		 - list directory contents

This displays short information about a command, and the first section in the collection of man pages that contains an appropriate page.

More extensive Documentation

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

Man pages (short for "manual pages") are the extensive documentation that comes preinstalled with almost all substantial Unix and Unix-like operating systems. The Unix command used to display them is *man*. Each page is a self-contained document.

To read a manual page for a Unix command, one can use::

	$ man <command_name>

To see the manual on man itself do::

	$man man

The previous example will take you to the "Manual" page entry about manual pages!

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

--help

-------

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/>

Working with Files

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

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.

Usage

~~~~~

To copy a file to another file ::

	$ cp  SourceFile TargetFile

To copy a file to a directory::

	$ cp  SourceFile  TargetDirectory

To copy a directory to a directory::

	$ cp  -r SourceDirectory  TargetDirectory

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.

Flags

~~~~~

*-i* (interactive) – prompts you with the name of a file to be overwritten. This occurs if the TargetDirectory or TargetFile parameter contains a file with the same name as a file specified in the SourceFile or SourceDirectory parameter. If you enter y or the locale's equivalent of y, the cp command continues. Any other answer prevents the cp command from overwriting the file.

*-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. 

Here's example to remove a file named "foo" from a directory, here shown with the -i option::

  	$ rm -i foo

    	remove foo? y

Options

~~~~~~~

Common options that rm accepts include:

    * *-r*, which removes directories, removing the contents recursively beforehand (so as not to leave files without a directory to reside in) ("recursive")

    * *-i*, which asks for every deletion to be confirmed ("interactive")

    * *-f*, which ignores non-existent files and overrides any confirmation prompts ("force")

    * *-v*, which shows what is being removed as it happens ("verbose")

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

*rm -rf* (variously, rm -rf /, rm -rf `*`, and others) is frequently used in jokes and anecdotes about Unix disasters. The rm -rf variant of the command, if run by a superuser on the root directory, would cause the contents of every writable mounted filesystem on the computer to be deleted.

Permissions

~~~~~~~~~~~

Linux is a proper multi-user environment. In a multi-user environment, security of user and system data is very important. Access should be given only to users who need to access the data. Since Linux is essentially a server OS, good and efficient file security is built right . The permissions are based on whether one is allowed to read, write or execute a file.

Usually, on most filesystems, deleting a file requires write permission on the parent directory (and execute permission, in order to enter the directory in the first place). (Note that, confusingly for beginners, permissions on the file itself are irrelevant. However, GNU rm asks for confirmation if a write-protected file is to be deleted, unless the -f option is used.)

To delete a directory (with rm -r), one must delete all of its contents recursively. This requires that one must have read and write and execute permission to that directory (if it's not empty) and all non-empty subdirectories recursively (if there are any).

Is there a default organization system of files and Directories that Linux follows

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

In the File Hierarchy Standard (FHS) all files and directories appear under the root directory "/", even if they are stored on different physical devices. Note however that some of these directories may or may not be present on a Unix system depending on whether certain subsystems, such as the X Window System, are installed.

The majority of these directories exist in all UNIX operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS, and are not considered authoritative for platforms other thanmajor Linux distros.

+---------------+------------------------------------------------+

|   Directory   |             Description                        |

+===============+================================================+

| /             | Primary hierarchy root and root directory of   |

|               | the entire file system hierarchy.              |

+---------------+------------------------------------------------+

| /bin/         | Essential command binaries that need to be     |

|               | available in single user mode; for all users,  |

|               | e.g., *cat*, *ls*, *cp*.                       |

+---------------+------------------------------------------------+

| /boot/        | Boot loader files, e.g., *kernels*, *initrd*;  |

|               | often a separate partition.                    |

+---------------+------------------------------------------------+

| /dev/         | Essential devices, e.g., /dev/null             |

+---------------+------------------------------------------------+

| /etc/         | Host-specific system-wide configuration files  |

|               | (the name comes from *et cetera*)              |

+---------------+------------------------------------------------+

| /home/        | User's home directories, containing saved      |

|               | files, personal settings, etc.; often a        |

|               | separate partition.                            |

+---------------+------------------------------------------------+

| /lib/         | Libraries essential for the binaries in        |

|               | */bin/* and */sbin/*                           |

+---------------+------------------------------------------------+

| /media/       | Mount points for removable media such as       |

|               | CD-ROMs, external hard disks, USB sticks, etc. |

+---------------+------------------------------------------------+

| /mnt/         | Temporarily mounted file systems               |

+---------------+------------------------------------------------+

| /opt/         | Optional application software packages         |

+---------------+------------------------------------------------+

| /proc/        | Virtual filesystem documenting kernel and      |

|               | process status as text files; e.g., uptime,    |

|               | network. In Linux, corresponds to a *Procfs*   |

|               | mount.                                         |

+---------------+------------------------------------------------+

| /root/        | Home directory for the root user               |

+---------------+------------------------------------------------+

| /sbin/        | Essential system binaries; e.g., *init*,       |

|               | *route*, *mount*.                              |

+---------------+------------------------------------------------+

| /srv/         | Site-specific data which is served by the      |

|               | system.                                        |

+---------------+------------------------------------------------+

| /tmp/         | Temporary files. Often not preserved between   |

|               | system reboots.                                |

+---------------+------------------------------------------------+

| /usr/         | Secondary hierarchy for read-only user data;   |

|               | contains the majority of (multi-)user          |

|               | utilities and applications.                    |

+---------------+------------------------------------------------+

| /var/         | Variable files - files whose content is        |

|               | expected to continually change during normal   |

|               | operation of the system - such as logs, spool  |

|               | files, and temporary e-mail files.             |

|               | Sometimes a separate partition.                |

+---------------+------------------------------------------------+

man hier

---------

This is the manual page on the UNIX filesystem. The syntax for this is::

	$ man hier

Permissions and Ownership

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

drwxr-x--- 2 user group 4096 Dec 28 04:09 tmp

-rw-r--r-- 1 user group 969 Dec 21 02:32 foo

-rwxr-xr-x 1 user group 345 Sep 1 04:12 somefile

d - The file is a directory.

r - Read permission. The file may be read. In the case of a directory, this would mean the ability to list the contents of the directory.

w - Write permission.For a directory, this defines whether you can make any changes to the contents

of the directory. If write permission is not set then you will not be able to delete, rename or create a file.

x - Execute permission. Whether the file may be executed. In the case of a directory, this attribute decides whether you have permission to enter,run a search through that directory or execute some program from that directory.

The Permission are read in the following manner::

    | User | Group | Others |

    |------+-------+--------|

    | drwx | rwx   | rwx    |

Problem: Given a file with only read permission, add execute permission to User, Group and Others.

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

Solution::

	$chmod a+x executablefile

   	$ls -l executablefile

Thats it .

chmod

------

The *chmod* command changes the  file system modes of files and directories. The modes include permissions and special modes.

Usage

~~~~~

The *chmod* command options are specified like this:

::

	$ chmod [options] mode[,mode] file1 [file2 ...]

To view what the permissions currently are, type:

::

	$ ls -l file

Command line options

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

The *chmod* command has a number of command line options that affect its behavior. The most common options are:

    * -R: Changes the modes of directories and files recursively

    * -v: Verbose mode; lists all files as they are being processed

Symbolic modes

+++++++++++++++

To the *chmod* utility, all permissions and special modes are represented by its mode parameter. One way to adjust the mode of files or directories is to specify a symbolic mode. The symbolic mode is composed of three components, which are combined to form a single string of text:

::

	$ chmod [references][operator][modes] file1 ...

The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to “all” but modifies only the permissions allowed by the umask. The references are represented by one or more of the following letters:

+--------------+--------+---------------------------------------------+

| Reference    | Class  |                Description                  |

+==============+========+=============================================+

|      u       | user   | the owner of the file                       |

+--------------+--------+---------------------------------------------+

|      g       | group  | users who are members of the file's group   |

+--------------+--------+---------------------------------------------+

|      o       | others | users who are not hte owner of the file or  |

|              |        | members of the group                        |

+--------------+--------+---------------------------------------------+

|      a       | all    | all three of the above; is the same as *ugo*|

+--------------+--------+---------------------------------------------+