LINUX
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution.The development of Linux is one of the most prominent examples of free and open source software collaboration: the underlying source code may be used, modified and distributed commercially or non commercially by anyone under licenses such as the GNU General Public License. Typically Linux is packaged in a format known as a Linux distribution for desktop and server use.
Users operate a Linux-based system through a command line interface (CLI), a graphical user interface (GUI), or through controls attached to the associated hardware, which is common for embedded systems. For desktop systems, the default mode is usually a graphical user interface, by which the CLI is available through terminal emulator windows or on a separate virtual console. Most low-level Linux components, including the GNU userland, use the CLI exclusively. The CLI is particularly suited for automation of repetitive or delayed tasks, and provides very simple inter-process communication. A graphical terminal emulator program is often used to access the CLI from a Linux desktop. A Linux system typically implements a CLI by a shell, which is also the traditional way of interacting with a Unix system. A Linux distribution specialized for servers may use the CLI as its only interface.
The primary difference between Linux and many other popular contemporary operating systems is that the Linux kernel and other components are free and open source software. Linux is not the only such operating system, although it is by far the most widely used.[58] Some free and open source software licenses are based on the principle of copyleft, a kind of reciprocity: any work derived from a copyleft piece of software must also be copyleft itself. The most common free software license, the GNU GPL, is a form of copyleft, and is used for the Linux kernel and many of the components from the GNU project.