I'm currently researching a similar trip. From Tunisia is almost impossible, the visa's for Libya and Algeria are almost impossible to get for a solo cyclist and I get the impression that you dont want to be in south Algeria by yourself (unless you want to get famous on youtube with a black hood on for your beheading
You have to pretty much choose if you want to go down the east or the west of the continent. i.e via Morocco or Egypt. But 6 months is very ambitious, I would recommmend taking more time and enjoying it, or as peter said join in with the Tour de afrique
and be done in 100 days. I plan to take 12-18 months for the same trip.
In the mean time here are some links which may be of help:http://takeonafrica.com/
Helen Lloyd's solo trip through africa.http://www.thebigafricacycle.com/
Peter Gostelow's solo trip through africa.http://www.horizonsunlimited.com/hubb/sahara-travel-forum/
Motorcycle forum for africa which has alot of usefull tips which are also relevant to cyclists.http://www.shanecycles.com/
and of course my own website, I'll be cycling in Uganda early next year, then the big trip will start sometime in 2012. I hope before that time to have alot of useful info/links on my site about africa mainly for my own use but also for others....
Good luck and have fun