What Is Linux?

Technically speaking, Linux refers only to an operating system kernel originally writtenby Linus Torvalds. The Linux kernel provides a variety of core system facilities required for any system based upon Linux to operate correctly. Application software relies upon specific features of the Linux kernel, such as its handling of hardware devices and its provision of a variety of fundamental abstractions, such as virtual memory, tasks
(known to users as processes), sockets, files, and the like. The Linux kernel is typically started by a bootloader or system firmware, but once it is running, it is never shut down (although the device itself might temporarily enter a low-powered suspended state).

These days, the term “Linux” has become somewhat overloaded in everyday communication. In large part, this is due to its growing popularity—people might not know what an operating system kernel is or does, but they will have perhaps heard of the term Linux. In fact, Linux is often used interchangeably in reference to the Linux kernel itself, a Linux system, or an entire prebuilt (or source) software distribution built upon the Linux kernel and related software. Such widely varying usage can lead to difficulties when providing technical explanations. For example, if you were to say, “Linux provides TCP/IP networking,” do you mean the TCP/IP stack implementation in the Linux kernel itself, or the TCP/IP utilities provided by a Linux distribution using the Linux kernel, or all of the above?

The broadness of the usage of the term has led to calls for a greater distinction between uses of the term “Linux.” For example, Richard Stallman and the Free Software Foundation often prefix “GNU/” (as in “GNU/Linux”) in order to refer to a complete system running a Linux kernel and a wide variety of GNU software. But even terms such as these can be misleading—it’s theoretically possible to build a complete Linux-based system without GNU software (albeit with great difficulty), and most practical Linux systems make use of a variety of both GNU and non-GNU software. Despite the confusion, as more people continue to hear of Linux, the trend is toward a generalization of the term as a reference to a complete system or distribution, running both GNU and non-GNU software on a Linux kernel. If a friend mentions that her development team is using Linux, she probably means a complete system, not a kernel.

A Linux system may be custom built, as you’ll see later, or it can be based on an already available distribution. Despite a growth in both the availability of Linux distributions targeted at embedded use, and their use in embedded Linux devices, your friend’s development team may well have custom built their own system from scratch . Conversely, when an end user says she runs Linux on the desktop, she most likely means that she installed one of the various distributions, such as Red Hat Enterprise Linux (RHEL), SuSE Linux Enterprise Server (SLES),

Ubuntu Linux, or Debian GNU/Linux. The end user’s running Linux system is as much a Linux system as that of your friend’s, but apart from the kernel, their systems most likely have very different purposes, are built from very different software packages, and run very different applications.

When people use the term Linux in everyday conversation, they usually are referring to a Linux distribution, such as those just mentioned. Linux distributions vary in purpose, size, and price, but they share a common goal: to provide the user with a prepackaged, shrinkwrapped set of files and an installation procedure to get the kernel and various overlaid software installed on a certain type of hardware for a certain purpose.
In the embedded space, a variety of embedded Linux distributions are available, such as those from MontaVista, Wind River, Timesys, Denx, and other specialist vendors. These specialist embedded Linux distributions are generally not targeted at generic desktop, workstation, or server use like their “mainstream” counterparts. This means that they typically won’t include software that is not suited for embedded use.
Beginning with the next chapter and throughout the remainder of this book, we will frequently avoid referring to the word “Linux” on its own. Instead, we will generally refer directly to the object of discussion, so rather than talking about the “Linux kernel,” the “Linux system,” and the “Linux distribution,” we will generally refer only to the “kernel,” the “system,” and the “distribution,” respectively. In each of these circumstances,
“Linux” is obviously implied. We will use the term “Linux,” where appropriate, to designate the broad range of software and resources surrounding the kernel.