gem-问题

• Gem::FilePermissionError
  问题：

$ gem install metamodel --verbose
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
或者
$ sudo gem install metamodel --verbose
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /usr/bin directory.

原因：没有权限。
解决：

1. 方案1:
   export GEM_HOME="$HOME/.gem"

2. 方案2：使用-n选项
   sudo gem install -n /usr/local/bin metamodel -verbose
   参考：https://stackoverflow.com/questions/18599889/error-while-executing-gem-gemfilepermissionerror

3. 方案3：推荐
   gem install cocoapods --user-install