When you install software on Ubuntu, the package manager automatically downloads the required software packages from a software repository, a location on the Internet which stores collections of packages ready to be downloaded.
There are thousands of programs available to install on Ubuntu. These programs are stored in software repositories and are made freely available for all Ubuntu users. This makes it very easy to install new programs, and it is also very secure because each program you install is built specially for Ubuntu and checked before it is allowed into the repositories.
To organise the software, Ubuntu repositories are categorised into four groups: Main, Restricted, Universe, and Multiverse. The rationale used to determine which software goes into which category is based on the level of support that software development teams provide for a program and the level of compliance the program has to the Free Software Philosophy.
The standard Ubuntu Install CD contains some software from the Main and Restricted categories.
If your system is connected to the Internet, many more software programs are made available for installation. For example, the “Universe” and “Multiverse” repositories are only available over the Internet.
![[Warning]](../../libs/admon/warning.png)
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The Multiverse repository contains software which has been classified as non-free. This software may not be permitted in some jurisdictions. When installing each package from this repository, you should verify that the laws of your country permit you to use it. Also, this software may not include security updates. |
You can find more information about the Repositories available on the website.
It is possible to add extra repositories, such as those provided by third parties or those for older version of Ubuntu.
![[Caution]](../../libs/admon/caution.png)
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Be careful when installing software from third-party repositories. It may not have been tested with Ubuntu and could cause your system to break. |
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Open → → and press Third Party Software.
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Press to add a new repository.
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Enter the APT line for the extra repository. This should be available from the website of the repository or similar, and should look similar to the following:
deb http://ftp.debian.org etch main -
Press and then click to save your changes.
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You will be notified that the information about available software is out-of-date. Press .
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Most software repositories use a GPG key to digitally sign the files they provide, which makes it easy to check that the files have not been tampered with since their creation. In order for apt to be able to check this, you need the public key that corresponds to the signatures. The key should be available for download on the repository's website.
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Once you have downloaded the GPG key, import the key by selecting the Authentication tab, clicking on , and then selecting the GPG key to be imported.
