1. first of all, you need to download the right version of rar for mac os x.
the download url:www.rarlab.com
2.it is really a pity that the rar is just for command.
using the tar command to decompress the file
3.using command "sudo install -c -o $USER rar /bin" to install
4.using unrar command to decompress the rar files
for instance:
unrar x compressed-files.rar
