Basic tasks for new Linux users

Help! I got my Linux installed. Now what?

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If you're a Windows user who wants to try Linux, and you are new to Linux, this tutorial is for you. We'll cover a few basic tasks that may be different from what you are used to. We'll cover these tasks:

These instructions and examples focus on three popular distributions and their default graphical desktops:

Fedora 14 using a GNOME desktop

OpenSUSE 11.3 using a KDE 4 desktop

Ubuntu 10.10 using a GNOME desktop

Other distributions, desktops, and window managers are available, and we encourage you to explore further on your own. Even if some of the information here is specific to a particular distribution and even a particular version of a distribution, what you learn here should help you navigate a strange distribution. Because this tutorial simply aims to help you get started, you won't find information on more advanced tasks like recompiling your kernel or installing software. We recommend our no-charge Linux (LPI) certification self-study guides for deeper information.

At the time of writing, GNOME 3 is likely to be available soon. Expect changes in that user interface from what is described here.

Before using these instructions, install a Linux system and create at least one non-root user as part of your installation process.

Login

Once you have installed your Linux distribution and booted your system, either you will be automatically logged in or you will see a login screen. The next section shows how to switch between these two startup methods and discusses the security implications of automatic login. For this section, we'll assume you are logging in. The three systems we consider in this tutorial implement login a little differently from each other, but all will prompt with the name of one or more users. Once a user is selected, you need to enter a password.

The login screen

The appearance of the screen varies according to your Linux distribution, and it will probably come as no surprise that you can customize it further, although we won't cover that in this short tutorial. A typical login screen for an Ubuntu 10.10 system is shown in Figure 1, and a typical one for Fedora 14 is shown in Figure 2. These both illustrate the default GNOME desktop used on these systems.

Figure 1. Ubuntu 10.10 login screen

Figure 2. Fedora 14 login screen

A typical login screen for an OpenSUSE 11.3 system with the default KDE 4 desktop is shown in Figure 3 or

Figure 3. OpenSUSE Linux 11.3 login screen

When you enter or select an id and press Enter, you will be prompted for your password. If a password field is showing on the login screen, as in the OpenSUSE example here, you can tab to the password field and enter your password. If there is no entry field for an id showing and you need to enter an id that is not in the list of available ids, you will usually find an entry for something like "other..." as shown in Figure 2. Select that and you should see an entry field for the id. We'll talk more about this in the section Becoming superuser (or root).

Login screens may have other items on them, including a clock, perhaps the name of the system, and icons or named menus that allow you to shut down or restart the system.

Accessibility

On the GNOME desktops is a small icon that looks something like a figure of a person inside a circle. Click this and you will see a dialog for accessibility options such as an on-screen keyboard or larger font. An example from our Fedora system is shown in Figure 4. We used the option to make text larger and easier to read for the login screen shown in Figure 2.

Figure 4. Fedora login accessibility options

The desktop screen

After you type in your password and press Enter again, you should be logged in and see your desktop. Figure 5 shows what you might see as user ian on an Ubuntu 10.10 system, with a panel along the top and another along the bottom. To explore the desktop, move your mouse over the icons or click on them.

Note: The next three images are intended to give you an impression of the way your desktop will look. Don't worry if you can't read the tiny print on them.

Figure 5. Sample initial window for Ubuntu 10 and GNOME desktop

Fedora 14 also uses a GNOME desktop. In Figure 6, we clicked the System icon on the left part of the top panel and then selected Preferences. As we hover over the Desktop Effects choice in the subsidiary menu, the hover help shows Select desktop effects. This is where you select desktop effects such as having your window edges wobble as you move them around the desktop. Note that this requires 3-D graphics capabilities and possibly a graphics driver that is not open source.

Figure 6. Sample initial window for Fedora 14 and GNOME desktop

Figure 7 shows what you might see with OpenSUSE and a KDE 4 desktop.

Figure 7. Sample initial window for SUSE Linux and KDE desktop

The relatively new KDE 4 desktop uses a different navigation metaphor, which we'll discuss more in the section Navigation and settings. Note that the window menu for Desktop Folder slides out to the left or right of the window, rather than being fixed at the top.

Navigation and settings

Let's spend a moment exploring the Linux panels on the desktop and then look at navigation and an example of how you can customize your desktop by switching to left-handed mouse usage.

Desktop panels

Panels give you a starting place for interacting with your desktop and provide information about your system. You will usually find one or two panels on your desktop. Typically these will default to being on the top, the bottom, or both the top and bottom edges of the screen. You can move them to the left or right edges if you wish, typically by right clicking and changing the properties.

Different distributions and different desktops often differ in panel layout, so expect differences here. Our Ubuntu GNOME panels are shown in Figure 8. We've shown the ends of the panels for clarity.

Figure 8. GNOME panel features on Ubuntu

The left part of the top panel provides a launching place for accessing programs, folders (directories), or system settings and information.

The right part of the top panel provides information such as time and date, along with several quick-access buttons for functions like setting volume control, opening chat windows, and logging out.

The left part of the bottom panel has a button to hide all windows and show the desktop, along with buttons for active windows.

The right part of the bottom panel contains a trash bin and four rectangles that allow you to navigate between your virtual desktops. Most Linux systems set up multiple distinct desktops, so you can keep your email and web browsing on one desktop, while doing program development on another and testing on yet another, for example. You switch between them by clicking the appropriate smaller square or by using a key combination. For GNOME, Ctrl-Alt-left arrow or Ctrl-Alt-right arrow usually cycles through them in the same way that Alt-tab will cycle through application windows on a given desktop. For OpenSUSE, ctrl-F1 through ctrl-F4 will directly select desktops 1 through 4. When exploring, make sure you log in as a non-root user to avoid accidents; such mistakes may be more serious when you have unlimited authority.

The panels for our Fedora system are shown in Figure 9. The System item in the upper left is selected as we used the image from Figure 6 to create this image. Notice that we do not have quick access to the logout function in the upper right, the hide all windows function in the lower left, or the trash icon that we saw in the lower right of the Ubuntu panels. Otherwise, the panel layout is reasonably similar.

Figure 9. GNOME panel features on Fedora

The OpenSUSE system has a panel across the bottom only, as shown in Figure 10. Access to programs as well as folders and system functions starts with the large button at the far left, which we will refer to as the Start button. Also on the left are quick-access buttons to a browser and desktop navigator, followed by buttons for each of the five virtual desktops. On the right end of the panel, you find a clock and several convenience buttons similar to those at the right end of the upper Ubuntu GNOME panel.

Figure 10. KDE 4 panel features on OpenSUSE

Navigating

The GNOME 2.3x desktop uses the cascading menus that have now become familiar. Figure 11 illustrates how to access the mouse settings from the Fedora System icon. Different distributions may arrange these menus differently. For example, you will find the mouse settings on Ubuntu in the same location, but if you are looking for the Desktop Effects preferences, which you find under System->Preferences->Desktop Effects on our Fedora system, you find it as the Visual Effects tab under System->Preferences->Appearance. Exploring graphical applications is often like turning over different rocks to see what is hiding underneath them.

Figure 11. Accessing mouse settings in Fedora

In contrast, the KDE 4 desktop uses a different metaphor for the Start menu. Menu panels replace each other, and you navigate by clicking on items in the menu or by mousing over the icons at the bottom of the menu. Figure 12 illustrates the Favorites and Applications menus.

Figure 12. Changing KDE 4 menus by mousing over icons

When submenus are selected, such as from Start->Applications->Utilities, a back button opens along the left of the menu so you can return to the previous menu level. Figure 12 illustrates this.

Figure 13. KDE 4 menu back button

Some Start menu items on OpenSUSE open a dialog box, possibly containing further selections. An example is the Start->Applications->Configure Desktop menu, which opens a window like that in Figure 14, where we show the hover help for the Keyboard & Mouse settings menu item.

Figure 14. KDE 4 Configure Desktop menu

Switching to left-handed mouse usage

A right-handed user is generally assumed, but you can change your mouse configuration for left-handed use, along with many other desktop settings. Refer back to Figure 11 or Figure 14 to navigate to the mouse settings dialog.

On an Ubuntu system, you should see a window similar to Figure 15 where you can change your mouse settings. In addition to basic left-handed or right-handed use, there are several other settings you can change and a tab of additional settings for accessibility. Settings take effect immediately, so once you click the left-handed choice, your mouse is set for left-handed use and you'll have to use the right mouse button as button 1 to close the dialog. The dialog on a Fedora system is similar.

Figure 15. GNOME dialog to change mouse settings

On an OpenSUSE system with KDE, you should see a window similar to Figure 16 where you can change your mouse settings. Note the mouse image in the dialog. If you switch to left-handed use, the right button in the mouse illustration will be highlighted to indicate that it is your primary selection button. Switch back to right-handed use, and the left button will be highlighted. As with the Ubuntu dialog, there are several other options that you can set. Unlike the Ubuntu dialog, the changes only take effect when you click the Apply button.

Figure 16. KDE dialog to change mouse settings

Command line access

Sometimes on a Linux system you need to enter Linux commands rather than using a GUI. A Linux Terminal window is similar to a DOS command prompt under Windows. On our KDE desktop, you can reach a list of available terminal programs using Start->Applications->System->Terminal as shown in Figure 17.

Figure 17. Opening a terminal on OpenSUSE with KDE 4

On the Ubuntu system, you can find a terminal window either under Applications->Accessories->Terminal as shown in Figure 18 or under Applications->System->Terminal. On Fedora systems, look under Applications->System Tools->Terminal.

Note: If you right-click (or use the appropriate button if you reconfigured your mouse) the terminal menu choice, you will usually see an option to add this icon to your panel or desktop. Adding it to your panel gives you quick access to a terminal without going through the menus.

Figure 18. KDE shell icon

On the OpenSUSE system, select the Terminal (konsole) choice and you will get a terminal window something like Figure 19. In both Figure 19 and Figure 20, we've included some commands and their output that we'll discuss below.

Figure 19. KDE shell icon

The Ubuntu terminal window will look something like Figure 20.

Figure 20. KDE shell icon

The default appearance of your shell window depends on your distribution and your choice of desktop. To resize it, you can use the left mouse button to drag the corners or sides of the window. To scroll back through the most recent history, you can use the scroll bar. The command prompt typically ends with a $ character for users other than root . Usually the command prompt will end with a # character indicating that the user of this shell is user root or has root authority. You can use the up arrow to recall previous commands and modify them if necessary. You will usually find a Settings or Terminal menu where you change things like window colors and fonts.

Figure 19 and Figure 20 show a few commands and their output:

whoami Shows who is using this terminal window, ian in this case. pwd Prints the full name of the current working directory, which is /home/ian in this case. Note that the tilde (~) before the $ in the command prompt shows that the user is currently in his or her home directory. cd Changes the current or working directory. We illustrate changing to the / (or root) directory and then to the /tmp directory, which is usually used for storing temporary files. Note that / is the root of the whole file system, and /root is the home directory of the root user. Using cd without any directory name returns the user to the home directory, and using cd - returns you to the last directory you were in before the current one. Users other than root will normally have a home directory under /home. For example, /home/ian is my home directory on a system where my id is ian . Remember that tilde (~) is shorthand for the home directory of the current user. Add the name of a specific user to reference that user's home directory. For example, the home directory for user ian can also be referenced as ~ian . uname Without parameters, shows the name of the operating system: Linux. With the -a parameter, displays additional information about your system. which Scans the directories in your PATH environment variable, and shows the full path to an executable program that would be executed if you typed the command at the shell prompt. In this case, we see that the xclock program would be run from /usr/bin/xclock. Note: This application is not always installed in a default Linux installation. It is usually part of a package with a name like xorg-x11-apps, so you may have to find and install the appropriate package to use it. xclock Launches a new window on your desktop with a clock. Note the trailing & on the command, which indicates that the command processor should return control to the terminal window rather than wait for the command to finish. Note also that this is the first such process spawned by this terminal window, and it has a process id ( PID ) of 1774. ps With the -T option, displays all processes started by this terminal. On some systems, the default display from the ps command includes a process status. See the man pages for details on all possible options and output columns. If the status were displayed in this example, you would see the bash shell program waiting for input (status S for sleeping) as would be the xclock process. The ps command is running and would have status R for runnable.

The output from these commands is shown in the two terminal windows above. A text form from the Ubuntu system is shown in Listing 1.

Listing 1. Ubuntu output from some basic commands

ian@pinguino:~$ whoami ian ian@pinguino:~$ pwd /home/ian ian@pinguino:~$ cd / ian@pinguino:/$ cd /tmp ian@pinguino:/tmp$ uname Linux ian@pinguino:/tmp$ uname -a Linux pinguino 2.6.35-27-generic #48-Ubuntu SMP Tue Feb 22 20:25:29 UTC 2011 i686 GNU/Linux ian@pinguino:/tmp$ which xclock /usr/bin/xclock ian@pinguino:/tmp$ xclock& [1] 2072 ian@pinguino:/tmp$ ps -T PID SPID TTY TIME CMD 2049 2049 pts/1 00:00:00 bash 2072 2072 pts/1 00:00:00 xclock 2073 2073 pts/1 00:00:00 ps ian@pinguino:/tmp$

Some other commands that you might find useful include:

info cmd_name Displays information about the command named cmd_name. Try info info to find out about the info documentation system. man cmd_name Is an interface to the online manual (man) pages about the command named cmd_name. Some information is in info format, while some is available only in man page format. Try man man to find out more about manual pages.

Becoming superuser (or root)

For many tasks on Linux, you need root or superuser authority. The root user, sometimes called the superuser, is the user that is normally used for administrative tasks like configuring the system or installing software. Use root only when you need to do administrative tasks; avoid using root for your normal work. The root user can do anything, including accidentally destroying your system, which is usually not a good thing. Normal users have fewer privileges, and the system is much more protected from being inadvertently damaged by normal users.

Most administrative applications that have a graphical interface now ask for the root password before allowing non-root users access to the function. When you need to run commands from a terminal window as root, this doesn't help.

Your first thought may be to switch to another userid by logging out of the current userid and logging in as the new userid. But what if you only need to run a couple of quick commands as another user? Linux has a solution for you: The su (substitute user) and sudo commands allow you to temporarily run one or more commands as another user. This is often used for tasks that require root access. Indeed, if you connect in remotely to a system using a terminal program such as ssh (or the very insecure telnet ), then many Linux distributions will prevent you from signing in as root. This is a good security practice, and we encourage you to not try to circumvent it. Rather, you should sign in as a non-privileged user and then use the su or sudo command to do the work you need to do with root authority.

To summarize, there are two main ways to run an arbitrary command with root authority.

Use the su command, usually with the - option to become root. Use the sudo command to execute a single command with root authority.

On systems such as Fedora or OpenSUSE, both methods are available, although using su is perhaps more common. On Debian-based systems such as Ubuntu, the security model prevents root login, so you can neither log in as root nor use su to become root, so you must use sudo .

Using su

Suppose you are logged in and looking at a terminal window, and you are not the root user but need to run a command, such as fdisk , which requires root authority. You switch to root using the su command alone, or, more commonly, add the - option.

The su command without the - option simply switches you to become root, but does not change your environment variables, including your path. The - option, which may also be typed as -l or -login if you really like typing extra letters, allows the login startup files for the substitute user to be read, thus setting things such as the path, environment, and prompt to those of the target user. Listing 2 shows examples of these two forms on our Fedora system. We've used the pwd (print working directory) command to show the current working directory in each case. Note how the prompts differ. If you'd like to understand more about how to customize your own prompts or what makes these prompts appear as they do, check out the "Prompt magic" tip on developerWorks.

Listing 2. Switching to the root user

[ian@echidna ~]$ su Password: [root@echidna ian]# pwd /home/ian [root@echidna ian]# su - [root@echidna ~]# pwd /root

You will notice, not surprisingly, that you had to provide a password to switch to root. Once you have root authority, you can use su or su - to switch to another user or to switch to root with the login option. If you want to switch to a non-root user, just add the id. As before, you can use the - option or not, according to your needs. For example:

su - db2inst1

To return to the previous id, press Ctrl-d or type exit and press Enter if you are using the bash shell, which is the default on most Linux systems.

Now that we've learned how to use su , let's put it into practice with the fdisk command.

Listing 3. Running the fdisk command with su

[ian@echidna ~]$ fdisk /dev/sda Unable to open /dev/sda [ian@echidna ~]$ su - Password: [root@echidna ~]# fdisk /dev/sda Command (m for help): m Command action a toggle a bootable flag b edit bsd disklabel c toggle the dos compatibility flag d delete a partition l list known partition types m print this menu n add a new partition o create a new empty DOS partition table p print the partition table q quit without saving changes s create a new empty Sun disklabel t change a partition's system id u change display/entry units v verify the partition table w write table to disk and exit x extra functionality (experts only) Command (m for help): q [root@echidna ~]# exit logout [ian@echidna ~]$

Using sudo

Like the su command, the sudo command allows you to run commands with the authority of another user. The commands that a given user or class of users may execute are listed in the /etc/sudoers file. In contrast to the su command, you do not need to know the password of the root, or other user, although you will need to provide your own password. The /etc/sudoers file is maintained by root and can be edited using the visudo command.

Usually, if you are executing multiple sudo commands in rapid succession, you will not need to reenter your password for each one. An alternative is to run sudo with the -s option, which runs a shell for you, from which you can run many commands as the target user until you close the shell. Listing 4 illustrates both of these methods.

Listing 4. Using the sudo command on Ubuntu

ian@pinguino:~$ fdisk /dev/sda Unable to open /dev/sda ian@pinguino:~$ sudo fdisk /dev/sda [sudo] password for ian: WARNING: DOS-compatible mode is deprecated. It's strongly recommended to switch off the mode (command 'c') and change display units to sectors (command 'u'). Command (m for help): p Disk /dev/sda: 120.0 GB, 120034123776 bytes 255 heads, 63 sectors/track, 14593 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x54085408 Device Boot Start End Blocks Id System /dev/sda1 * 1 2611 20972826 7 HPFS/NTFS /dev/sda2 2612 2624 104422+ 83 Linux /dev/sda3 2625 14593 96140962 5 Extended /dev/sda5 2625 2689 522081 82 Linux swap / Solaris /dev/sda6 2690 5180 20008926 83 Linux /dev/sda7 5181 9341 33423201 83 Linux /dev/sda8 9342 14593 42186658+ 83 Linux Command (m for help): q ian@pinguino:~$ sudo -s root@pinguino:~# fdisk /dev/sda WARNING: DOS-compatible mode is deprecated. It's strongly recommended to switch off the mode (command 'c') and change display units to sectors (command 'u'). Command (m for help): q root@pinguino:~# exit ian@pinguino:~$

If you are not authorized in the sudoers file, you will receive an error message similar to that in Listing 5.

Listing 5. Attempting to use sudo without authority

[ian@echidna ~]$ sudo fdisk /dev/sda [sudo] password for ian: ian is not in the sudoers file. This incident will be reported.

Using a GUI application as another user

You may have noticed in the discussion of the su command in the previous section that we ran only commands that displayed output in the terminal window. Usually you will be able to run GUI commands too. For example, some installation programs require you to have root authority to install a program and have a GUI installer. If you find you cannot start GUI applications as another user, then read on, as you may have to take additional steps on some distributions in order to run GUI applications as another user.

Note: Recent distributions often let you have multiple desktops open at once and switch between them using a key sequence such as Ctrl-Alt-F7 or Ctrl-Alt-F8. Depending on what you need to do, this may be another alternative. See the section on Logout for details on this menu option.

GUI applications on Linux use the X Window System, a client-server system designed to allow multiple users to access a computer across a network using windowed applications. An X display is known by a name of the form hostname:displaynumber.screennumber. For Linux running on a workstation such as a PC, there is typically only one display with a single screen. In this case, the displayname may be, and usually is, omitted so the display is known as :0.0, or sometimes just :0.

The X Window System server needs to know the display and also whether you are authorized to connect to the server. Authorization is most commonly done using the MIT-MAGIC-COOKIE-1, which is a long random string that is regenerated whenever the server is reset. So that applications can pass this information to the X server, you will have DISPLAY and XAUTHORITY variables set in your environment. The XAUTHORITY variable will point to a file that is usually only able to be read or written by the owning user as a security precaution. We assume you are using a graphical login if you are reading this, so your startup should have already set these for you. The example in Listing 6 is from our Ubuntu system and shows the values of the DISPLAY and XAUTHORITY variables as well as the ownership for the file pointed to by the XAUTHORITY variable.

Listing 6. DISPLAY and XAUTHORITY

ian@pinguino:~$ echo $DISPLAY :0.0 ian@pinguino:~$ echo $XAUTHORITY /var/run/gdm/auth-for-ian-WoeKHn/database ian@pinguino:~$ ls -l $XAUTHORITY -rw------- 1 ian ian 53 2011-04-01 16:24 /var/run/gdm/auth-for-ian-WoeKHn/database

Now we attempt to use sudo to run the xclock command as user editor , again on our Ubuntu system. As Listing 7 shows, the values for DISPLAY and XAUTHORITY variables are the same as for user ian , but the xclock command fails.

Listing 7. DISPLAY and XAUTHORITY with sudo

ian@pinguino:~$ sudo -u editor echo $DISPLAY [sudo] password for ian: :0.0 ian@pinguino:~$ sudo -u editor echo $XAUTHORITY /var/run/gdm/auth-for-ian-WoeKHn/database ian@pinguino:~$ sudo -u editor xclock No protocol specified Error: Can't open display: :0.0

In this case, user editor has the XAUTHORITY variable set to /var/run/gdm/auth-for-ian-WoeKHn/database, but we already saw that the permissions on that file only allow user ian to read or write it. The variable may as well not be set if user editor cannot read the file it points to. Before we look at how to address this issue, let's look at what happens if we unset either the DISPLAY or XAUTHORITY variables for user ian. . We'll do this by running the xclock command with an environment variable modified using the env command's -u option to unset an environment variable before running xclock . Our results are in Listing 8.

Listing 8. Unsetting DISPLAY and XAUTHORITY

ian@pinguino:~$ env -u DISPLAY xclock Error: Can't open display: ian@pinguino:~$ env -u XAUTHORITY xclock

You may be surprised to see that the xclock command runs, even though we unset the XAUTHORITY environment variable.

So far, we have mentioned the MIT-MAGIC-COOKIE-1 security method. If the token is not provided, the X server will also check a list of authorized hosts. Use the xhost command to display or update the list. Use the + option to add entries and the - option to remove entries. Use the special family entry local: (note the ':') to allow access to the display by any local user on the system. Since you are a single user system, this means that you can su to an arbitrary non-root user and can now launch xclock or other X applications. We illustrate the use of the xhost command in Listing 9.

Listing 9. Using xhost

ian@pinguino:~$ xhost access control enabled, only authorized clients can connect SI:localuser:ian ian@pinguino:~$ xhost +local: non-network local connections being added to access control list ian@pinguino:~$ xhost access control enabled, only authorized clients can connect LOCAL: SI:localuser:ian ian@pinguino:~$ sudo -u editor xclock ian@pinguino:~$ # Close the xclock window to return here ian@pinguino:~$ xhost -local: non-network local connections being removed from access control list ian@pinguino:~$ xhost access control enabled, only authorized clients can connect SI:localuser:ian

On a single user system, enabling the display for use by all local users is usually a reasonable and simple solution. If you need to be more restrictive, use the xauth to extract the cookie from your XAUTHORITY file and give it to the user who needs access to the display. In Listing 10 we perform the following tasks:

Use xauth as user ian to display the cookie in a format that can be emailed or otherwise sent to the desired other user. Use sudo -s and switch to user editor to run several commands. Use xauth to create a new authorization file. Note that we used echo to pipe the data to stdin while splitting the command over several lines by using a trailing backslash (\). We export a new value for the XAUTHORITY variable so it now points to our newly created authorization file. Finally, we run the xclock command using a trailing ampersand (&) to run it in the background and retain control of our terminal window.

Listing 10. Using xauth

ian@pinguino:~$ xauth -f $XAUTHORITY nextract - :0 0100 0008 70696e6775696e6f 0001 30 0012 4d49542d4d414749432d434f4f4b49452d31 0010 3c4bc87 c2ce4ce5e97f8199c213b4ec9 ian@pinguino:~$ sudo -s -u editor editor@pinguino:~$ echo "0100 0008 70696e6775696e6f 0001 30 0012"\ > " 4d49542d4d414749432d434f4f4b49452d31"\ > " 0010 3c4bc87c2ce4ce5e97f8199c213b4ec9" | > xauth -f ~editor/temp-xauth nmerge - xauth: creating new authority file /home/editor/temp-xauth editor@pinguino:~$ export XAUTHORITY=~editor/temp-xauth editor@pinguino:~$ xclock& [1] 4827

This brief introduction will probably get you started with running X applications as another user. Although we have used Ubuntu as an example, the basic principles we have demonstrated here apply to all distributions. For more details on using the xauth and xhost commands, you can use any of these commands as appropriate to view the online manual pages:

info xauth

man xauth

info xhost

man xhost

Removable devices

On Linux and UNIX® systems, all files are accessed as part of a single large tree that is rooted at /. To access the files on a CD-ROM, you need to mount the CD-ROM device at some mount point in the file tree. On current distributions, this is usually automated for you; you need only insert the disc and it will be recognized and mounted. Once mounted, it is important to properly unmount the device to avoid data loss.

Mounting removable devices

When you insert a CD or DVD disc into a SUSE 11.3 system, or attach a USB drive, you will see a pop-up window similar to Figure 21. If you miss the pop-up before it closes, you can use the panel icon that we've shown in the top left of the figure to open it again. Hovering over the icon shows the tooltip indicating that the device is not yet mounted.

Figure 21. Pop-up when CD or DVD loaded on SUSE system

Hovering over the plug icon to the right of the image reveals a tooltip saying "Click to access this device from other applications". Clicking over the text "4 actions for this device" expands the image to look like Figure 22. The available actions may differ on your system if you have installed different software packages.

Figure 22. Available actions for CD or DVD

If you click the "Open with File Manager" selection, you will see a window something like Figure 23. From this window you can navigate around the DVD, open files, or run applications. Hover over an item to see a description in the right part of the window.

Figure 23. SUSE File Manager

On Ubuntu and Fedora systems, the default action on inserting a disc is slightly different. Usually, an icon similar to that shown in Figure 24 will appear on your desktop. The file manager (Nautilus) may also open automatically. If it does not, you can double-click the icon to open the file manager. Depending on your system, you may also be prompted as to whether to run the autorun file in the root of the disc or not.

Figure 24. Inserted CD on Ubuntu

If you clicked to access the device from other applications (OpenSUSE), or opened it with File Manager or another application on any distribution, then the device will have been mounted for you. It will usually be mounted in the /media directory and will probably use the disc label as a mount point. You can verify by opening a terminal window and running the mount command as shown for our OpenSUSE system in Listing 11. Other systems are very similar, although the mount options may differ slightly.

Listing 11. Using the mount command

ian@lyrebird:~> mount /dev/sdb12 on / type ext4 (rw,acl,user_xattr) proc on /proc type proc (rw) sysfs on /sys type sysfs (rw) ... /dev/sr0 on /media/openSUSE-DVD-x86_64.0702..001 type iso9660 (ro,nosuid,nodev,uid=1000, utf8)

In this case our disc is mounted at /media/openSUSE-DVD-x86_64.0702..001 and you can explore it or change to a directory on the disc from the terminal window. Note that the CD/DVD device is /dev/sr0. Devices on a Linux system also appear in the filesystem tree. To put this another way, the filesystem that is on the media in the device /dev/sr0 is accessible to applications starting at the mount point /media/openSUSE-DVD-x86_64.0702..001. If you'd like to learn more about how devices are mounted, see our article Learn Linux, 101: Control mounting and unmounting of filesystems.

Safely removing devices

Once a DVD or CD is mounted, you can use the files on the disk as they are now part of your file system. While a CD-ROM is mounted, Linux will lock the CD so that it cannot be ejected with the Eject button. Of course, this doesn't stop you from unplugging an external CD or USB drive, pulling a USB stick out of the USB socket, or ejecting a floppy disk. To avoid potential data loss, you should always remove the device safely by first unmounting it.

You can unmount devices from the graphical desktop, or from the command line. The umount command (note umount without an 'n') unmounts a device, and the eject command first unmounts a device and then tries to eject it, for example by opening a CD drawer. Traditionally, mounting and unmounting devices required root authority. Nowadays, it is common to allow user mounting, so that a non-root user is able to mount and unmount devices. You may have noticed back in Listing 11 the option uid=1000 . The uid of 1000 corresponds to user ian on this system as shown in Listing 12.

Listing 12. Using the id command for the current user

iian@lyrebird:~> id uid=1000(ian) gid=100(users) groups=33(video),100(users)

If you use the graphical desktop tools to unmount the device, you may only need to worry about ids if you have logged out and then back in as a different user. So let's look at the graphical tools, then the command line ones.

On Ubuntu or Fedora, if you right click on the icon for the removable media you will see a context menu similar to that in Figure 25. You will have an option to either unmount or eject the device. Select the eject option for a CD or DVD and the device will be unmounted, the icon will disappear from your desktop, and your drawer will open. For a USB drive, the option may be unmount rather than eject, and you can safely unplug the device after it is unmounted. If you use an external hard drive with multiple partitions, you need to make sure that all partitions are unmounted before removing or powering off the drive.

Figure 25. SUSE File Manager

In Figure 26 we show our OpenSUSE system with an attached USB drive as well as the DVD we had used earlier. The DVD and one partition of the hard drive are mounted. OpenSUSE distinguishes the mounted from the unmounted by changing the plug icon (when a device is not mounted) to an eject button (when it is mounted). You will also note that the small square at the bottom left of the device icon also changes from having a diagonal line to having an asterisk. Note that for writeable media, such as our hard drive, the available space is also displayed.

Figure 26. OpenSUSE File Manager

To unmount or eject a mounted drive, simply click the eject button (shown above for the 2006R1 partition). If you eject a device such as a CD or DVD where a drawer opens or some other similar physical disconnection takes place, then the device will be removed from your list of plugged devices.

Safe removal from the command line

We mentioned the umount and eject commands earlier. Let's now look at how to use them and the lsof that you may also want to know about. You will find some differences between systems in this area, so be prepared for things not to be exactly as we illustrate here.

To begin, we'll look at using the unmount command to unmount the CD on our Ubuntu system as shown in Listing 13. We first use grep to filter out the entries from the output of mount to show only those entries that contain 'media', the usual mount point for removable media. We then use umount to unmount the device using its mount point (/media/Ubuntu 10.10 i386). Finally, we repeat the mount + grep filter to confirm that the device is no longer mounted. Note that you can use either the mount point or the device name (/dev/sr0 in this case) as the argument to umount .

Listing 13. Unmounting a CD on Ubuntu from the command line

ian@pinguino:~$ mount | grep media /dev/sr0 on /media/Ubuntu 10.10 i386 type iso9660 (ro,nosuid,nodev,uhelper=udisks, uid=1000,gid=1000,iocharset=utf8,mode=0400,dmode=0500) ian@pinguino:~$ umount /media/Ubuntu\ 10.10\ i386 ian@pinguino:~$ mount | grep media

As you can see, Ubuntu allows you to use the command line to unmount a drive that was automatically mounted. Repeating the above scenario on OpenSUSE 11.3 is likely to result in something like Listing 14.

Listing 14. Unmounting a CD on OpenSUSE from the command line (1)

ian@lyrebird:~> mount | grep media /dev/sr0 on /media/openSUSE-DVD-x86_64.0702..001 type iso9660 (ro,nosuid,nodev,uid=1000, utf8) /dev/sdc6 on /media/2006R1 type ext3 (rw,nosuid,nodev) ian@lyrebird:~> umount /media/openSUSE-DVD-x86_64.0702..001/ umount: /media/openSUSE-DVD-x86_64.0702..001 is not in the fstab (and you are not root) ian@lyrebird:~> umount /dev/sr0 umount: /dev/sr0 is not in the fstab (and you are not root)

Sometimes you will find a disconnect between what you can do graphically and what you might expect to do at a command line. As Linux matures, such disconnects become less frequent, but you will sometimes find them as we have done here. The obvious solution, given what you have already learned in this tutorial, is to use su or sudo to run the required command with root privileges. So let's try it using su as shown in Listing 15.

Listing 15. Unmounting a CD on OpenSUSE from the command line (2)

ian@lyrebird:~> su - Password: lyrebird:~ # mount | grep media /dev/sr0 on /media/openSUSE-DVD-x86_64.0702..001 type iso9660 (ro,nosuid,nodev,uid=1000, utf8) /dev/sdc6 on /media/2006R1 type ext3 (rw,nosuid,nodev) lyrebird:~ # umount /dev/sdc6 lyrebird:~ # umount /media/openSUSE-DVD-x86_64.0702..001/ umount: /media/openSUSE-DVD-x86_64.0702..001: device is busy. (In some cases useful info about processes that use the device is found by lsof(8) or fuser(1))

We were able to unmount /dev/sdc6 successfully, but were unable to unmount /media/openSUSE-DVD-x86_64.0702..001 because Linux claimed it was busy. We would get a similar error message if we tried to use the eject command. Remember that we said that Linux will lock a CD or DVD closed while it is in use. More generally, you cannot unmount a filesystem if it is in use by some other user. As suggested in the error message above, you can use the lsof or fuser command to find which user is causing your unmount problem. Typical usage is illustrated in Listing 16.

Listing 16. Using lsof and fuser (as root)

lyrebird:~ # lsof /media/openSUSE-DVD-x86_64.0702..001/ COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME bash 3824 ian cwd DIR 11,0 2048 2048 /media/openSUSE-DVD-x86_64.0702..001/ boot lyrebird:~ # fuser -um /media/openSUSE-DVD-x86_64.0702..001/ /media/openSUSE-DVD-x86_64.0702..001: 3824c(ian)

The lsof command shows the actual file or directory (/media/openSUSE-DVD-x86_64.0702..001/boot in this case) that is open as well as the process id (3824) and user id (ian) using the file or directory. The fuser command shows the user and process id using the /media/openSUSE-DVD-x86_64.0702..001 filesystem, but not the specific open file or directory.

Normally you would use this information to cleanly close the open process or window using the filesystem. On some Linux systems there is also an -L option to umount that allows a lazy unmount where the filesystem is detached from the filesystem hierarchy immediately, and all references to the filesystem are cleaned up when the filesystem is no longer busy. On OpenSUSE 11.3 this option is not available, so you need to clean up the filesystem references, either by terminating the offending processes or by ensuring they are no longer using affected resources, before unmounting.

We mentioned back in Listing 14 that the OpenSUSE system does not support command line use of umount for a device that was mounted from the desktop, so we attacked the problem by using root authority. There is another approach using the halmount command to unmount the device. HAL is a Hardware Abstraction Layer, which provides a consistent application interface to various hardware devices. If you use halmount , you will still be unable to unmount a busy device. Listing 17 shows how to use halmount and then switch to root to forcibly terminate the process that is keeping the disc busy. Note that killing the process in this way may cause data loss, so we do not recommend it unless you are sure you will not lose data. We then switch back to normal user mode and use halmount to unmount the filesystem that is no longer busy. Finally we use eject to open the drawer and eject the DVD.

Listing 17. Using halmount, kill, and eject

ian@lyrebird:~> halmount -u /dev/sr0 /dev/sr0: org.freedesktop.Hal.Device.Volume.Busy: umount: /media/openSUSE-DVD-x86_64.0702 ..001: device is busy. ian@lyrebird:~> lsof /dev/sr0 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME bash 3824 ian cwd DIR 11,0 2048 2048 /media/openSUSE-DVD-x86_64.0702..001/ boot ian@lyrebird:~> su - Password: lyrebird:~ # kill -9 3824 lyrebird:~ # ps -ef | grep 3824 root 5542 5507 0 22:47 pts/2 00:00:00 grep 3824 lyrebird:~ # logout ian@lyrebird:~> halmount -u /dev/sr0 ian@lyrebird:~> eject /dev/sr0

We hope this brief introduction to removable media use on Linux will help you enjoy your removable media files and avoid data loss.

Logout

The tasks of logging out, locking your screen, shutting down, and restarting the system are usually related in the sense that they are accessed from the same or similar menus. Frequently your distribution will add a quick launch button to access these tasks. If not, you can add one yourself.

OpenSUSE logout

To log out, click Start and then Leave. You will see a menu like Figure 27 with options for logging out, locking the screen, and shutting down or restarting the system. Click the appropriate selection.

Figure 27. Adding a logout button with Red Hat enterprise Linux 3

You will also see icons for leave and lock at the bottom right of the OpenSUSE panel. Click the Leave icon on the right, and you will see a dialog box like Figure 28. Again, make your selection.

Figure 28. Using the OpenSUSE panel logout icon

Ubuntu

You access Ubuntu logout and restart functions from a panel icon at the right end of the top Ubuntu panel. This icon is similar to the Leave icon on OpenSUSE. Click it to reveal a dialog similar to Figure 29 where you can make your choice.

Figure 29. Using the Ubuntu panel logout icon

Fedora logout

Access the Fedora logout and restart functions from the System menu on the top panel as shown in Figure 30. Your choices are similar to the OpenSUSE and Ubuntu choices that you have seen above.

Figure 30. Logging out of the Fedora GNOME desktop

Adding a logout button to your panel

Fedora does not install a panel icon for logout or screen lock similar to those on OpenSUSE or Ubuntu. However, you can add your own quite easily.

Start by right-clicking a blank part of the panel. From the context menu, select Add to Panel..., and you will see a selection of items that you may add. Scroll down to the Log Out selection and click on it to add it to your panel. You can add the Lock Screen icon in the same manner.

Figure 31. Adding a logout button to the Fedora GNOME desktop

A logout icon will be added to your panel.

If an application is not in the list, and you can start it from the Applications menu or you can open the menu to the point where you would start the application, then right-click to open a context menu that includes choices to add a launcher to the panel or to your desktop.

Adding users and groups

Sometimes you will need to define new users of your system and new groups for those users. For example, you may need to define a user called mqm and a group also called mqm (as well as another group called mqbrkrs when installing the embedded messaging component in WebSphere Application Server), or you may need to create users to administer databases.

If you do not specify a user number, graphical tools will usually assign the next available user number. For Fedora and Red Hat systems, user numbers start at 500, so the user you created when you installed your system is probably user 500. For OpenSUSE and Ubuntu, the numbers start at 1000. If you use the same id on several systems as I do, you will probably find it convenient to use the same id and group numbers on each system too.

For the purpose of this section, we will add a user called testuser with id 2000 and group 2000. Usually you will define the group first and then define the users who will use the group, so that's what we will do here. You can either use the graphical tools for user administration or enter commands in a terminal window. We'll give an overview of the graphical process here using OpenSUSE's system administration tools. Then we'll tell you where to find the corresponding tools on a Fedora or Ubuntu system. Finally we'll give you the commands if you really want to do it from the command line.

Adding users and groups to your OpenSUSE system

On an OpenSUSE system with KDE, you access the YaST (Yet Another System Tool) control center using Start->Applications, then select System and scroll down to Administrator Settings as shown in Figure 32.

Figure 32. YaST2 Control Center

Open this application and click Security and users in the left panel to view the tasks shown in the main panel of Figure 33.

Figure 33. YaST2 Control Center

Select User and Group Management. If you have not recently received root authority, you will be prompted for the root password. On the next screen, you will see any existing users. Select the Groups tab, and you will see something like Figure 34.

Figure 34. Group list in YaST2 Control Center

Click the Add button to add a new group. You will see something like Figure 35. Note that there are a number of groups that were created when you installed your system. Enter 'testuser' for the Group Name and '2000' for the Group ID. Click OK to return to the group list display and see the new group listed. At this point your group has not yet been saved to the system, so it will be lost if you cancel.

Figure 35. Adding a group in YaST2 Control Center

Click the Users tab to return to the user display, then click Add to add a new user. Enter 'Test User' for the User's Full Name, 'testuser' for the User Name, then type and retype an initial password for the user. See Figure 36.

Figure 36. Adding a user in YaST2 Control Center

Click on the Details tab and enter '2000' for the User ID (uid), and select 'testuser' from the Default Group drop-down menu. This panel is where you can change the default home directory and default login shell, among other items. You can also select additional groups that this user will be a member of. When you have finished, click OK to return to the list of users where you will see your new user. Click OK, and YaST will process all your changes and save them to the system.

Figure 37. User details in YaST2 Control Center

Adding users and groups to your Fedora or Ubuntu system

On GNOME systems, such as our Fedora and Ubuntu systems, you start user management from the System->Administration->Users and Groups menu as shown in Figure 38.

Figure 38. Starting user and group management on GNOME

However, Fedora and Ubuntu have different dialogs once you open the User and Group management. Fedora starts the system-config-users application, while Ubuntu starts the users-admin application. We'll illustrate the Fedora usage here and then summarize the differences for Ubuntu.

If you are not logged in or recently authenticated as root, you will need to provide the root password when prompted. You will then see the User Manager screen, opened at the Users tab as in Figure 39. By default, only normal users and groups are shown. To view system users and groups, uncheck the Hide system users and groups checkbox under Edit->Preferences.

Figure 39. Fedora User Manager

We could do as we did above for the OpenSUSE system and define our groups first. However, the Red Hat User Manager has a convenient feature that lets you create a private group for a user with the group name being the same as the user name. So click the Add User button and fill in the details for the testuser user as for OpenSUSE above. However, this time, check the Specify user ID manually and Specify group ID manually check boxes and fill in 2000 for each of these values. Our screen now looks like Figure 40.

Figure 40. Adding a user in Fedora

After you click OK, you will return to the User Manager. Your new user will already be added to the system, unlike the case for OpenSUSE. Click Add Group to add any additional groups you may need. To make users members of additional groups, you can either select a group and use its properties to add users or select a user and use the properties to add groups. When you are done with properties, click OK to return to User Manager and then File->Quit to close the User Manager.

Ubuntu user manager

Now that you are familiar with adding users on both OpenSUSE and Fedora, you will be able to manage the Ubuntu process. In general, Ubuntu will lead you through the process of adding a user and set up the user with a default id and group. Once you have added testuser in this way, you should see a screen like Figure 41.

Figure 41. User management in Ubuntu

At this point you will need to use the Manage Groups button to add the new group (or you could add it before you add the user). Once you have added the testuser group, you will need to come back to the screen of Figure 41, select the testuser id, and click the Advanced button to change both the id number and primary group for the user.

Adding users and groups using the command line

You can add or change users and groups from the command line. These tasks require root authority.

Information on groups is stored as a flat file in /etc/group. You may use the groupadd command to add a new group. This is fairly simple. Adding a new user is a little more complex as there are more parameters, and you will need the numerical number of the user's group. Let's use the groupadd command to add our testuser group, with group id 2000, and then use the grep command to search /etc/group and verify the settings. Note: If you do not provide a group id, the system will assign the next one that is higher than any existing group id.

root@pinguino:~# groupadd -g 2000 testuser root@pinguino:~# grep testuser /etc/group testuser:x:2000:

As you see, the testuser group is 2000. Now let's use the useradd command to add the testuser user. The -c option allows us to specify a comment that is usually a user's real name. The -u option allows us to specify the numerical id (2000) for the user. The -d option allows us to specify the home directory for the user. The -g option specifies the user's primary group. Here we use 2000, which is the testuser group we just created. The last option we use is the -G option to specify additional groups for this user. Here we can use the group name. In this case, we'll add testuser to the group ian .

Once you have added the user, you can use the grep command again, and you will see that user testuser has been added to the testuser and ian groups. At this point you have created a new user, but the user does not have a password and cannot log on to the system. Some users do not need to log on, so that would be alright for those users. The root user has the authority to set (or reset) passwords for other users. To do this, you use the passwd command and give the username as a parameter. You will be prompted for the new password, and then you will be prompted to retype it for verification.

root@pinguino:~# useradd -c"Test User" -u 2000 -d/home/testuser -g 2000 -G ian \ > testuser root@pinguino:~# grep testuser /etc/group ian:x:1000:testuser testuser:x:2000: root@pinguino:~# passwd testuser Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully

Finally, you may need to add users to an existing group. You can use the usermod command to do this, but you need the list of existing groups for the user as you will replace the list of additional groups. It is often easier to edit /etc/group directly. Make a backup copy first, just in case you make a mistake. To add the editor user to both the ian and testuser groups, edit /etc/group and update the lines for ian and testuser so they look as follows:

ian:x:1000:testuser,editor testuser:x:2000:editor

You will find much of the user information is stored in /etc/passwd, but you should not edit this file yourself. Use the useradd , usermod , and userdel commands instead. If you are not a full time system administrator, you will probably find it easier to do occasional manipulation of users and groups through the graphical interfaces.

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