Linux Command Line and Shell Scripting Bible. Christine Bresnahan
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and viewing text output. These days, with graphical desktop environments, it's getting harder to find a shell prompt on the system in order to enter shell commands. This chapter discusses what is required to reach a command line environment. It walks you through the terminal emulation packages that you may run into in the various Linux distributions.
Reaching the Command Line
Before the days of graphical desktops, the only way to interact with a Unix system was through a text command line interface (CLI) provided by the shell. The CLI allowed text input only and could display only text and rudimentary graphics output.
Because of these restrictions, output devices were not very fancy. Often, you needed only a simple dumb terminal to interact with the Unix system. A dumb terminal was usually nothing more than a monitor and keyboard connected to the Unix system via a communication cable (usually a multi-wire serial cable). This simple combination provided an easy way to enter text data into the Unix system and view text results.
As you well know, things are significantly different in today's Linux environment. Just about every Linux distribution uses some type of graphical desktop environment. However, to enter shell commands, you still need a text display to access the shell's CLI. The problem now is getting to one. Sometimes finding a way to get a CLI in a Linux distribution is not an easy task.
One way to get to a CLI is to take the Linux system out of graphical desktop mode and place it in text mode. This provides nothing more than a simple shell CLI on the monitor, just like the days before graphical desktops. This mode is called the Linux console because it emulates the old days of a hard-wired console terminal and is a direct interface to the Linux system.
When the Linux system starts, it automatically creates several virtual consoles. A virtual console is a terminal session that runs in Linux system memory. Instead of having several dumb terminals connected to the computer, most Linux distributions start five or six (or sometimes even more) virtual consoles that you can access from a single computer keyboard and monitor.
The alternative to using a virtual console terminal is to use a terminal emulation package from within the Linux graphical desktop environment. A terminal emulation package simulates working on a console terminal, but within a desktop graphical window. Figure 2.1 shows an example of a terminal emulator running in a Linux graphical desktop environment.
Figure 2.1 A simple terminal emulator running on a Linux desktop
Graphical terminal emulation is responsible only for a portion of the Linux graphical experience. As a whole, the experience is accomplished via several components, including graphical terminal emulation software (called a client). Table 2.1 shows the different components in the Linux graphical desktop environment.
Table 2.1 Graphical Interface Elements
For dealing with the command line from the desktop, the focus is on the graphical terminal emulator. You can think of graphical terminal emulators as CLI terminals “in the GUI” and virtual console terminals as CLI terminals “outside the GUI.” Understanding the various terminals and their features can enhance your command line experience.
Accessing CLI via a Linux Console Terminal
In the early days of Linux, when you booted up your system you would see a login prompt on your monitor, and that's all. As mentioned earlier, this is called the Linux console. It was the only place you could enter commands for the system.
Even though several virtual consoles are created at boot time, many Linux distributions switch to a graphical environment after the boot sequence completes. This provides the user with a graphical login and desktop experience. Therefore, in this case, accessing a virtual console is done manually.
In most Linux distributions, you can access one of the Linux virtual consoles using a simple keystroke combination. Usually, you must hold down the Ctrl+Alt key combination and then press a function key (F1 through F7) for the virtual console you want to use. Function key F2 produces virtual console 2, key F3 produces virtual console 3, key F4 produces virtual console 4, and so on.
Note
Linux distributions typically use the Ctrl+Alt key combination with either F1 or F7 to reach the graphical interface. Ubuntu uses F7, while RHEL uses F1. It is best to test and see where your distribution puts the graphical interface.
Text mode virtual consoles use the whole screen and start with the text login screen displayed. An example of a text login screen from a virtual console is shown in Figure 2.2.
Figure 2.2 Linux virtual console login screen
Notice in Figure 2.2 the words tty2 at the end of the first text line. The 2 in tty2 indicates that it is virtual console 2 and was reached by pressing the Ctrl+Alt+F2 key sequence. tty stands for teletypewriter. Teletypewriter is an old term, indicating a machine used for sending messages.
Note
Not all Linux distributions show the virtual console's tty number at the login screen.
You log into a console terminal by entering your user ID after the login: prompt and typing your password after the Password: prompt. If you have never logged in this way before, be aware that typing your password is a different experience than in a graphical environment. In a graphical environment, you may see dots or asterisks indicating the password characters as you type. However, at the virtual console, nothing is displayed when you type your password.
After logging into a virtual console, you are taken to the Linux CLI. Keep in mind that, within the Linux virtual console, you do not have the ability to run any graphical programs.
After you have logged in to a virtual console, you can keep it active and switch to another virtual console without losing your active session. You can switch between all the virtual consoles, with multiple active sessions running. This feature provides a great deal of flexibility while you work at the CLI.
Additional flexibility deals with the virtual console's appearance. Even though it is a text mode console terminal, you can modify the text and background colors.
For example, it may be easier on your eyes to set the background of the terminal to white and the text to black. After you have logged in, you can accomplish this modification in a couple of ways. One way is to type in the command setterm – inversescreen on and press the Enter key, as shown in Figure 2.3. Notice in the figure that the inversescreen feature is being turned on using the option on. You can also turn it off using the off option.
Figure 2.3 Linux virtual console with inversescreen being turned on
Another way is to type two commands, one after the other. Type setterm – background white and press Enter, and then type setterm – foreground black and press Enter. Be careful because, when you change your terminal background first, it may be hard to see the commands you are typing.
With the commands in the preceding paragraph, you are not turning features on and off, as with inversescreen. Instead, you have a choice of eight colors. The choices are black, red, green, yellow, blue, magenta, cyan, and white (which looks gray on some distributions). You can get rather creative with your plain text mode console terminals. Table 2.2 shows some options you can use with the setterm command to help improve your console terminal's readability or appearance.