If other operating systems, such as Windows, are already installed on your computer, you can press the spacebar at startup to display a boot loader menu showing those systems as boot options. If a Windows system is listed, you can choose to start that instead of Ubuntu.
The GNOME Display Manager: GDM
In Ubuntu, the GNOME Display Manager (GDM) manages the login interface along with authenticating the username and password, and then Ubuntu starts up a selected desktop.
On startup, the GDM displays a login window with a Username text box. Various GDM themes are available, which you can select using the GDM configuration tool. The default theme currently used is the Ubuntu Human theme. An Options pop-up menu at the lower-left corner of the screen shows entries for Restart, Shut Down, Suspend, and Hibernate your system. In addition, Select Language and Select Session entries display dialogs for selecting the language or user interface you want to use, such as GNOME or KDE.
Enter your username and press enter. Then enter your password and press enter. By default, the GNOME desktop starts up.
When you log out from the desktop, you return to the GDM login screen. To shut down your Linux system, choose Options | Shut Down. To restart, choose Options | Restart. You can also shut down the system from GNOME: Choose System | Quit, or click the Quit button on the top panel at the right. GNOME will display a dialog box with two rows of buttons: Suspend, Hibernate, Restart, or Shut Down options appear in the bottom row. Click Restart to shut down and restart your system.
Choose Options | Select Sessions to select the desktop or window manager you want to start up. In the dialog box that shows all installed user interfaces, you can choose KDE to start up the K Desktop, for example, instead of GNOME. The KDE option will not be shown unless you have already installed KDE.
Failsafe entries for both GNOME and the terminal provide a stripped down interface you can use for troubleshooting. For example, the Run Xclient script lets you run just your X Window System configuration script. This script will normally start up GNOME or KDE, but it could be specially configured for other desktops or window managers. Normally, the last session entry would be selected, which starts up the interface you used previously. Once you have selected your interface, click the Change Session button (or you can opt
out of any change by clicking the Cancel button) to return to the login screen.
Selecting Language from the Options menu opens a dialog with a drop-down menu that lists a variety of languages supported by Linux. Choose one and click the Change Language button to change the language used by your interface.
If you encounter problems in using the GUI, you can force an exit by pressing CTRL-ALTBACKSPACE to return to the login screen. In addition, from the GDM, you can switch to the command line interface by pressing CTRL-ALT-F1, and then switch back to the GUI by pressing CTRL-ALT-F7. You can also force your system to reboot at the login prompt by holding down the CTRL and ALT keys and then pressing the DEL key (CTRL-ALT-DEL). Your system will go through the standard shutdown procedure and then your computer will reboot.
You can configure your GDM login window with different features such as background images and user icons. The GDM even has its own selection of themes from which to choose. Choose System | Administration | Login Window to configure your login window.
The User Switcher
The user switcher at the upper-right of the top panel of the GNOME desktop lets you switch to another user without having to log out or end your current user session. The user switcher is installed automatically as part of your basic GNOME desktop configuration. Click the username to see a list of all other users, as shown next. Check boxes appear next to the names to indicate which users are logged in and running. To switch to a different user, simply select the user from this list. If the user is not already logged in, the login screen (GDM) will appear, where you can enter the user’s password.
If the user is already logged in, the login screen for the lock screen will appear (you can disable the lock screen). Enter the user’s password here. The user’s original session will continue, and any open windows and applications running when the user switched off will still be open and running. You can easily switch back and forth between logged in users, with each user retaining his session where he left off. When you switch to a new user, the former user’s running programs will continue in the background. Right-clicking the switcher will list several user management options, such as configuring the login screen, managing users, or changing the user’s password and personal information. The Preferences item lets you configure how the user switcher is displayed on your screen. For example, instead of the username, you could use the term Users or a user icon. You can also choose whether to use a lock screen when the user is switched. Disabling the lock screen option will let you switch seamlessly between logged in users.
Source of Information : McGraw Hill Ubuntu The Complete Reference
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