Objectives

After completing this section, students should be able to log into a Linux system on a local text console and run simple commands using the shell.

The bash shell

A command line is a text-based interface which can be used to input instructions to a computer system. The Linux command line is provided by a program called the shell. Over the long history of UNIX-like systems, many shells have been developed. The default shell for users in Red Hat Enterprise Linux is the GNU Bourne-Again Shell (bash). Bash is an improved version of one of the most successful shells used on UNIX-like systems, the Bourne Shell (sh).

When a shell is used interactively, it displays a string when it is waiting for a command from the user. This is called the shell prompt. When a regular user starts a shell, the default prompt ends with a $ character.

[student@desktopX ~]$ 

The $ is replaced by a # if the shell is running as the superuser, root. This makes it more obvious that it is a superuser shell, which helps to avoid accidents and mistakes in the privileged account.

[root@desktopX ~]# 

Using bash to execute commands can be powerful. The bash shell provides a scripting language that can support automation of tasks. The shell has additional capabilities that can simplify or make possible operations that are hard to accomplish efficiently with graphical tools.

Virtual consoles

Users access the bash shell through a terminal. A terminal provides a keyboard for user input and a display for output. On text-based installations, this can be the Linux machine's physical console, the hardware keyboard and display. Terminal access can also be configured through serial ports.

Another way to access a shell is from a virtual console. A Linux machine's physical console supports multiple virtual consoles which act like separate terminals. Each virtual console supports an independent login session.

If the graphical environment is available, it will run on the first virtual console in Red Hat Enterprise Linux 7. Five additional text login prompts are available on consoles two through six (or one through five if the graphical environment is turned off). With a graphical environment running, access a text login prompt on a virtual console by pressing Ctrl+Alt and pressing a function key (F2 through F6). Press Ctrl+Alt+F1 to return to the first virtual console and the graphical desktop.

Shell basics

Commands entered at the shell prompt have three basic parts:

The command is the name of the program to run. It may be followed by one or more options, which adjust the behavior of the command or what it will do. Options normally start with one or two dashes (-a or --all, for example) to distinguish them from arguments. Commands may also be followed by one or more arguments, which often indicate a target that the command should operate on.

For example, the command line usermod -L morgan has a command (usermod), an option (-L), and an argument (morgan). The effect of this command is to lock the password on user morgan's account.

To use a command effectively, a user needs to know what options and arguments it takes and in what order it expects them (the syntax of the command). Most commands have a --help option. This causes the command to print a description of what it does, a "usage statement" that describes the command's syntax, and a list of the options it accepts and what they do.

Usage statements may seem complicated and difficult to read. They become much simpler to understand once a user becomes familiar with a few basic conventions:

Consider the first usage statement for the date command:

[student@desktopX ~]$ date --help
date [OPTION]... [+FORMAT]

This indicates that date can take an optional list of options ([OPTION]...), followed by an optional format string, prefixed with a plus character, +, that defines how the current date should be displayed ([+FORMAT]). Since both of these are optional, date will work even if it is not given options or arguments (it will print the current date and time using its default format).

When a user is finished using the shell and wants to quit, there are a couple of ways to end the session. The exit command terminates the current shell session. Another way to finish a session is by pressing Ctrl+d.