How search trough software
My question is how and where to search trough software repositories by function when it's not in the name? E.g. contrast, brightness. Or calculator, text editor.
You can browse by "sections" (function) and search the packages using the graphical Synaptic package manager.
Or you can search on the command line with
apt-cache search QUERY
Your suggestions seem better. One step instead of two.
On the other hand there there are packages on the FSF directory which aren't found in the Trisquel repositories so also your advice is better that way.
You can search for software by category (function) at directory.fsf.org
Specifically: http://directory.fsf.org/wiki/Category/Use
Once you have program names, could you use the names to search repositories?
When searching for packages in Synaptic there's the option to search in package descriptions, which usually does what you want. There's the same functionality in <http://packages.trisquel.info/> (It's linked from the navigation bar at the right). When you do this. you're using the information contained in Trisquel only, but you can also use information from other sources to find a free software program that does what you need, and then search the Trisquel package archive for that program. For instance, there's the free software directory <http://directory.fsf.org/> and Libregamewiki <https://libregamewiki.org/>. You can also search for programs that do what you need using any Internet search engine. All of the software listed in the above links is free software; but by searching with an Internet search engine you may find proprietary software; so you must check the software license before installing in order to keep your system fully free.