For Rar files:
My friend sent me a link to a rar file just today. After downloading, I found that rar file wasn’t supported natively on Ubuntu. I tried different softwares but none was good. So finally I installed rar and unrar, both of them aren’t free. They provide integration with the native gnome archive manager.
To install them, type the following line in terminal and press enter:
sudo apt-get install rar unrar
When you double click a rar file it will automatically open with gnome archive manager (file roller). Also, you’ll be able to see the rar format option available in the create archive dialog box.
For 7z files:
I also wanted to have 7z support while I was at this. So I just installed p7zip-full for this. It also provided integration with default archiver of gnome. To install it, type the following in terminal:
sudo apt-get install p7zip-full
This will give 7z support. Try double clicking a 7z file and it will open directly in archive manager. You will also find the option to select 7z in create archive dialog box.