http://www.beginninglinux.com/home/data-compression/unrar-all-files-from-multiple-subdirectories-at-once 

Unrar all files from multiple subdirectories at once
  cd /home/username/Videos/TV-show
  unrar e -r *.rar

To understand this command, unrar is command line application to decompress .rar files, e is command to extract files to current directory, -r is switch to recurse subdirectories.
TIP
You can run this command on any number of subdirectories, even whole Downloads folder, if you want to unrar everything and some archives are already unrar-ed, consider option switch -o- which is do not overwrite existing files, makes it faster and you do not have to confirm anything manually. For more information $ man unrar.

  cd /home/username/Videos
  unrar e -r -o- *.rar

To keep the same naming standards as with rar4, use the following command:
  rar a -ma4 -v1024m -vn ./the.rar.file.rar ./Some_file_somewhere.mkv