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While having a fancy graphical desktop interface is nice, there are drawbacks. The extra processing power required to interact with the graphics card takes away crucial CPU time that can be used for other programs. Nowhere is this more important than in a server environment.

Because of that, many Linux servers don't load a graphical desktop and instead rely on a text‐based interface, called the command‐line interface (CLI). The CLI provides a way for users to start programs, manage files on the filesystem, and manage processes running on the Linux system using simple text commands. The CLI is produced by a program called a shell. The shell allows you to enter text commands, and then it interprets the commands and then executes them in the kernel.

The shell contains a set of internal commands that you use to control things such as copying files, moving files, renaming files, displaying the programs currently running on the system, and stopping programs running on the system. Besides the internal commands, the shell also allows you to enter the name of a program at the command prompt. The shell passes the program name off to the kernel to start it.

You can also group shell commands into files to execute as a program. Those files are called shell scripts. Any command that you can execute from the command line can be placed in a shell script and run as a group of commands. This provides great flexibility in creating utilities for commonly run commands, or processes that require several commands grouped together.

There are quite a few Linux shells available to use on a Linux system. Different shells have different characteristics, some being more useful for creating scripts and some being more useful for managing processes. The default shell used in all Linux distributions is the Bash Shell. The Bash Shell was developed by the GNU project as a replacement for the standard Unix shell, called the Bourne shell (after its creator). The Bash Shell name is a play on this wording, referred to as the “Bourne again shell.”

In addition to the Bash Shell, there are several other popular shells you could run into in a Linux environment. Table 1.4 lists the more popular ones.

TABLE 1.4: Linux Shells

SHELL	DESCRIPTION
ash	A simple, lightweight shell that runs in low‐memory environments but has full compatibility with the Bash Shell
korn	A programming shell compatible with the Bourne shell but supporting advanced programming features like associative arrays and floating‐point arithmetic
tcsh	A shell that incorporates elements from the C programming language into shell scripts
zsh	An advanced shell that incorporates features from bash, tcsh, and korn, providing advanced programming features, shared history files, and themed prompts

Most Linux distributions include more than one shell, although usually they pick one of them to be the default. If your Linux distribution includes multiple shells, feel free to experiment with different shells and see which one fits your needs.