When you look for a particular program to install via Synaptic or any other package manager, and it doesn’t seem to exist, it’s no use typing sudo apt-get install nameofpackage in a terminal, since all of them rely on the APT Package Management system. So when Synaptic can’t find it, it means the package is not in the standard repositories, though chances are it exists in a “universe” or “multiverse” repo. What this means is rather than search for the developer’s site, and download and install their program, you might find it in Synaptic if you give it more places to look.
The first thing you should always do is enable universe (“Community-maintained Open Source software“) repos via System > Administration > Software Sources. It resides in the first tab, Ubuntu Software, as does multiverse (“Software restricted by copyright or legal issues“). Make sure both of those are enabled, and your package manager will find a lot more in searches.
The next tab, Third-Party Software, is where you can add more repos yourself. You will see an “Add CD-ROM” button at the bottom, which is useful for adding discs like the Ubuntu Alternate CD as software sources. And “+ Add” is what you use to add other sources manually. These repos can range from those that just take care of obtaining hard-to-find packages, through to ones for keeping specific programs updated, and look like this:
deb http://download.virtualbox.org/virtualbox/debian jaunty non-free
In the case of the example, the repo is to keep Virtualbox up to date, and is added automatically, but when you download programs from sites, and they advise you to add their repo to your software sources, you now know how.
Just remember that when adding repos others have recommended you may need to change the version. In the example used, the “jaunty” refers to Ubuntu 9.04 “Jaunty Jackalope”, so replace it with your current one. For example, if you’re running 9.10 “Karmic Koala”, you would replace it with “karmic“. One thing to remember is that when the sources are being checked, if the repo you added doesn’t exist yet, it will come up with the error that it couldn’t be located; you can either wait for it to eventually be active (ie: just ignore the error), delete it from your software sources (if the error really annoys you), or edit it back to what it was. In other words, if you added deb http://repoubuntusoftware.info karmic all, but it is invalid, change it back to deb http://repoubuntusoftware.info jaunty all.
Probably the best way to get a whole lot of useful repos added at once is to add the Medibuntu repository. This will give you easy access to lots of hard-to-find software within Synaptic, not to mention “non-free” packages (copyrighted or other legal issues) which can often be seemingly impossible to get any other way. It will also make it possible to install popular apps like Skype and Google Earth without downloading from the site (and usually having to register). Medibuntu also includes even more multimedia codec packs than ubuntu-restricted-extras gives you, like proprietary non-native Win32/64 codecs. It also gives you access to libdvdcss2 for playing encrypted DVDs. For more info on this, read my post, or just go straight to the documentation page.
♣ For additional information on Ubuntu repositories, check out the official help page.
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