我搜索了很多内容，但仍然找不到是什么原因，我确定 rvm 文件夹权限应该是正确的。
$ rvm get stable
A RVM version 1.20.13 (master) is installed yet 1.20.13 () is loaded.
Please do one of the following:
* 'rvm reload'
* open a new shell
* 'echo rvm_auto_reload_flag=1 >> ~/.rvmrc' # for auto reload with msg.
* 'echo rvm_auto_reload_flag=2 >> ~/.rvmrc' # for silent auto reload.
并检查 rvm 版本：
$ rvm --version
rvm 1.20.13 (master) by Wayne E. Seguin <wayneeseguin@gmail.com>, Michal Papis <mpapis@gmail.com> [https://rvm.io/]
然后安装 ruby 2.0.0：
$ rvm install 2.0.0
mkdir: /Users/xudannie/.rvm/log/ruby-2.0.0-p195: Permission denied
mkdir: /Users/xudannie/.rvm/log/ruby-2.0.0-p195: Permission denied
Searching for binary rubies, this might take some time.
No binary rubies available for: osx/10.7/x86_64/ruby-2.0.0-p195.
Continuing with compilation. Please read 'rvm mount' to get more information on binary rubies.
Installing requirements for osx, might require sudo password.
Certificates in '/usr/local/etc/openssl/cert.pem' already are up to date.
Installing Ruby from source to: /Users/xudannie/.rvm/rubies/ruby-2.0.0-p195, this may take a while depending on your cpu(s)...
ruby-2.0.0-p195 - #downloading ruby-2.0.0-p195, this may take a while depending on your connection...
mkdir: /Users/xudannie/.rvm/tmp/rvm_src_32045: Permission denied
mkdir: /Users/xudannie/.rvm/log/ruby-2.0.0-p195: Permission denied
tee: /Users/xudannie/.rvm/log/ruby-2.0.0-p195/1370079408_extract.log: No such file or directory
tee: /Users/xudannie/.rvm/log/ruby-2.0.0-p195/1370079408_extract.log: No such file or directory
ruby-2.0.0-p195 - #extracting ruby-2.0.0-p195 to /Users/xudannie/.rvm/src/ruby-2.0.0-p195....
Error running '__rvm_package_extract /Users/xudannie/.rvm/archives/ruby-2.0.0-p195.tar.bz2 /Users/xudannie/.rvm/tmp/rvm_src_32045',
please read /Users/xudannie/.rvm/log/ruby-2.0.0-p195/1370079408_extract.log
There has been an error while trying to extract the source. Halting the installation.
There has been an error fetching the ruby interpreter. Halting the installation.
我刚刚开始学习ROR，所以我需要面对的问题太多了。
希望您的帮助！