Linux is a modular Unix-like operating system. It derives much of its basic design from principles established in Unix during the 1970s and 1980s. Linux uses a monolithic kernel, the Linux kernel, which handles process control, networking, and peripheral and file system access. Device drivers are integrated directly with the kernel.

Much of Linux's higher-level functionality is provided by separate projects which interface with the kernel. The GNU userland is an important part of most Linux systems, providing the shell and Unix tools which carry out many basic operating system tasks. Atop these tools graphical user interfaces can be placed, usually running via the X Window System. This short section requires expansion.

Linux is coupled to a text-based command line interface (CLI), though this is usually hidden on desktop computers by a graphical user interface (GUI). On small devices, input may be handled through controls on the device itself, and direct input to Linux might be hidden entirely.

The X Window System (X) is the predominant graphical subsystem used in Linux. X provides network transparency, enabling graphical output to be displayed on machines other than that which a program runs on. For desktop machines X runs locally.

Early GUIs for Linux were based on a stand-alone X window manager such as FVWM, Enlightenment, or Window Maker, and a suite of diverse applications running under it. The window manager provides a means to control the placement and appearance of individual application windows, and interacts with the X window system. Because the X window managers only manage the placement of windows, their decoration, and some inter-process communication, the look and feel of individual applications may vary widely, especially if they use different graphical user interface toolkits.

This model contrasts with that of platforms such as Mac OS, where a single toolkit provides support for GUI widgets and window decorations, manages window placement, and otherwise provides a consistent look and feel to the user. For this reason, the use of window managers by themselves declined with the rise of Linux desktop environments. They combine a window manager with a suite of standard applications that adhere to human interface guidelines. While a window manager is analogous to the Aqua user interface for Mac OS X, a desktop environment is analogous to Aqua with all of the default Mac OS X graphical applications and configuration utilities. KDE, which was announced in 1996, along with GNOME and Xfce which were both announced in 1997, are the most popular desktop environments.

Linux systems usually provide a CLI of some sort through a shell, the traditional way of interacting with Unix systems. Even on modern desktop machines, some form of CLI is almost always accessible. Linux distributions specialized for servers may use the CLI as their only interface, and Linux machines can run without a monitor attached. Such “headless systems” may be controlled by command line via a protocol such as SSH or telnet.

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. Graphical terminal emulator programs can be used to access the CLI from a Linux desktop.