what is Linux ?

* Linux is Unix based operating system.
  
* Unix  is the 1st operating system came in word in 1917's.

* Linux is one of the flavor among  Solaris ,HP-unix and AIX .

* Linux is open source software. it means it can be freely available .

* Linux is the Linux kernel, an operating System kernel   first released on October 5, 1991 by Linus Torvalds.

*Most of the companies are using Linux instead Windows .

*Linux Kernel is designed in such a manner that Virus is wont effect to it  .

* From Security perspective, In Linux we have IP Tables, which is inbuilt firewall.

* In Linux systems we have file level security which provides SELinux.

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for more deeper about Linux please refer https://en.wikipedia.org/wiki/Linux