You can use a terminal window to issue shell commands. However, you can issue shell commands even when X is not running or available. To do so, you use the Linux virtual console feature.
Linux provides six virtual consoles for interactive use; a seventh virtual console is associated with the graphical user interface. You can use special keystrokes to switch between virtual consoles. The keystroke Alt-Fn, where n is the number of a virtual console (1-6), causes Linux to display virtual console n. For example, you can display virtual console 2 by typing Alt-F2. You can view only a single console at a time, but you can switch rapidly between consoles by using the appropriate keystroke. The keystroke Alt-F7 causes Linux to enter graphical mode using virtual console 7.
Virtual consoles also have a screensaver feature like that found on Windows. If a virtual console is inactive for an extended period of time, Linux blanks the monitor. To restore the screen without disturbing its contents, simply press the Shift key.
To log in using a virtual console, type your user ID and press Enter. The system prompts you for the password associated with your account. Type the proper password and press Enter. To prevent anyone nearby from learning your password, Linux does not display your password as your enter it. If you suspect you've typed it incorrectly, you can either hit the Backspace key a number of times sufficient to delete the characters you've entered and type the password again or simply press Enter and start over. If you type the user ID or password incorrectly, Linux displays the message "login incorrect" and prompts you to try again.
When you've successfully logged in, you'll see a command prompt that looks something like this:
If you logged in as the root user, you'll see a prompt that contains a hash mark (#); if you've logged in as an ordinary user, you'll see a dollar sign ($). The prompt tells you that the Linux bash shell is ready to accept your commands.
When you're done using a virtual console, you should log out by typing the command exit and pressing Enter. When you log out, the system frees memory and other resources that were allocated when you logged in, making those resources available to other users.
When the system logs you out, it immediately displays a login prompt. If you change your mind and want to access the system, you can log in simply by supplying your username and password.