Ubuntu includes the convenient Software Center, which provides centralized software management and lets you search for new software and install it with a mouse click. With an Ubuntu One account, you can purchase software simply with a credit card.

The Debian packages (ending in .deb ) that Software Center offers are actually archive files containing libraries, configuration files, and executable programs. If you install the Firefox package, for example, the package manager distributes the necessary files to the correct places in the filesystem.

Often dependencies exist, which means that installing program A requires that you have program B installed also. The package manager identifies the dependent packages automatically during installation and loads them onto the computer. Usually, you will notice the dependencies when you try to install a single Debian package over the Internet.

The package you are looking for, such as a video codec, may be missing in the Software Center. In this case, you can find the package in external repositories created by the software's users or developers. The repositories contain not only the software, but also the packages on which they depend. (How to tap into these repositories is covered in the "What Are Repositories?" section.)

Package Manager in Action

Getting to repositories and installing software is often faster from the command line. The "Quick Sources" section goes into greater detail. To begin, I'll look at managing packages using Ubuntu's Software Center.

To access the Software Center, press the Super key [1] and enter softw . The start page (Figure 1) features a few What's New and Top Rated applications. Clicking More on the far right gives you additional choices.

Figure 1: Software Center encourages you to browse.

If you click Turn On Recommendations at the bottom, Ubuntu suggests opening a user account at Ubuntu One. Then, your computer regularly sends lists of software you've installed to Canonical and the company returns a list of recommended software that might interest you. These recommendations appear on the start page and can be deactivated with View | Turn Off Recommendations in the global menu. In the meantime, you can purchase programs using your Ubuntu One account.

Use the arrow icons in the upper left to return to the start window. Starting with the release of Ubuntu 12.10, Ubuntu developers have placed ads for free and commercial software from the Software Center across the broad surface of the screen under the heading Our Favorites .

The left side of the Software Center screen organizes the applications by category. Clicking a category opens subcategories (Figure 2). All other controls are at the top of the package management window. The All Software menu lists all the software in all the available repositories. If you click the small white triangle next to the menu item (Figure 3), you will find the entries Provided by Ubuntu (mostly free software), Canonical Partners (free and commercial software without source code, e.g., Skype and Flash), and For Purchase (commercial software, including many games).

Figure 2: Clicking a category opens up subcategories.

Figure 3: Clicking the little white arrow next to "All Software" shows the integrated repositories.

Clicking Installed on the menu bar at the top displays only the installed applications, sorted by category. Click the little triangle to the right to view the packages from a single source. At the far right is the History menu item that tells you which software you downloaded. Check here if an upgrade suddenly isn't working. Downgrading, unfortunately, doesn't work in Software Center. To do that, you must install the Synaptic Package Manager and specify an earlier Version in Properties .

If you already know the name of the software, you can search for it in the search field at the upper right. Results come from the repository that you use for All Applications . Next to the software names are the ratings in the form of stars. Once you select a package, the More Info and Install buttons appear.

The first button leads to a detailed description of the program, often showing a screenshot (Figure 4). Look under the Add-ons section to see software that could enhance the program. Here People also installed displays other programs to consider. You can also filter reviews by language is. Finally, click the Install button to install the program. Once installed, you can click the Write your own review link to evaluate the software. This action requires a Launchpad account [1].

Figure 4: Once you choose an installable package, you'll receive further information: ratings, screenshots, descriptive text, and more.

The Software Center global menu includes other interesting options. In the File menu, you can choose Reinstall Previous Purchases or Sync Between Computers to move your package selections to other computers over the Ubuntu One cloud service.

Also, you can use Edit | Software Sources to add other repositories. If you don't want to display new applications automatically in the Launcher, unmark the New Applications in Launcher check box in the View menu.

What Are Repositories?

Software Center gives you access to thousands of packages, but the one you want might be missing or available only in an older version. In this case, external repositories and the PPAs can help.

The Opera repository [3] is one of the external repositories, and it provides the Opera web browser that is missing from Ubuntu's standard repositories.

The VirtualBox project [4] provides the proprietary version of its own software, which supports the virtual USB 2.0 ports and Remote Desktop Protocol (RDP), from its own repository.

Personal Package Archives (PPAs) are special versions of external repositories that you can find only on Ubuntu's Launchpad. When experienced Ubuntu users or developers create Debian packages out of their favorite software, they frequently offer them in their personal package archive on Launchpad. Using a search function [5], you can peruse the platform for new versions of current software with great success.

Complementing Package Sources

In Software Center, you can download from external repositories using Edit | Software Sources . To use the Opera project's external sources, for example, click Other Software and Add , then enter deb http://deb.opera.com/opera/ stable non-free . This address wasn't invented: Usually, the address is the website for the external repository. Click the Add Source button to accept the repository (Figure 5).

Figure 5: Obtain additional packages from external repositories.

The search won't find all programs. Software Center hides libraries, developer files, and even programs without graphical interfaces, such as the mplayer command interface. To install these programs, choose Show technical items at the bottom of the window or enter the full name of the software in the search field (see Figure 6).

Figure 6: Additional libraries, developer files, and programs are listed as technical items.

$ sudo add-apt-repositoryppa:inkscape.dev/stable
$ sudo apt-get update
$ sudo apt-get install inkscape

This sequence happens to mount the PPA for the Inkscape vector drawing solution. For other software, simply replace the parts after ppa: and sudo apt-get install . Take the text for the part after ppa: from the website (Figure 7). It may take a while for the repository to appear in Software Center.

Figure 7: In this PPA, you'll find the current packages for Raring Ringtail to install the Inkscape design application.

Removing Software

To remove software, open Software Center again, search for the name of the software, and click Remove . The configuration data is thereby unaffected. The advantage is that you don't need to reconfigure the software if you decide you need it again later. If these remnants (such as you might find in the /etc or /var directories) were to disappear, use the command line to remove the package.

Quick Sources

The Software Center is a graphical interface for apt, the Advanced Packaging Tool, which manages the list of software in the repositories in the background. As already noted, you can also run apt over the command line; many experienced Ubuntu users prefer this method, because the commands work with any Ubuntu derivative.

Here are four important commands:

$ sudo apt-get update
$ sudo apt-cache search (--names-only) inkscape
$ sudo apt-get install inkscape
$ sudo apt-get remove (--purge) inkscape

The first command brings the package list up to date, an important thing considering how quickly program versions change. The next command searches for a package whose name or description includes the term inkscape . Add the --names-only option (without the parentheses) to search for the package name only if too many hits are returned.

The third command installs the package, and the fourth removes it. Use the optional --purge option to purge the configuration files in the home directory along with the package down to the local configuration, which you have to remove yourself.

To remove all the packages installed through a PPA, you can either use Software Center and choose the PPA next to Installed or stay on the command line, install the ppa-purge package, and use the following command:

$ sudo ppa-purge ppa:inkscape/stable

for the Inkscape example.

Single Consignments

Finally, some websites provide single Debian packages to download. To install the software, double-click the DEB package. Ubuntu then calls Software Center and recommends installing the package. Once you click OK , the package appears together with a screenshot and description in Software Center. Finally, you can download the package with Install .

But, a word of caution: For safety reasons, it's preferable that you use the official repositories.