brew update失敗提示：/System/Library/Frameworks/Ruby.framework/。。。解決方法

狀況大概是這樣的：執行sudo brew update以後，提示：

 /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- utils/popen (LoadError)
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /usr/local/Library/Homebrew/utils.rb:6:in `<top (required)>'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /usr/local/Library/Homebrew/global.rb:9:in `<top (required)>'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /usr/local/Library/brew.rb:16:in `<main>'

無論執行什麼brew命令提示依舊，這種狀況多發生在 El Capitan系統上，緣由大概是更新失敗，下面是stackoverflow找到的答案，親測有效：

The problem mainly occurs after updating OS X to El Capitan (OS X 10.11). This is mainly because of file permission issues with file El Capitan’s new SIP process. Try changing permissions of/usr/local directory.

$ sudo chown -R $(whoami):admin /usr/local 

If it still doesn't work, use following steps and everything will be fine. This may be because homebrew is not updated.

1. open terminal  
2. $ cd /usr/local  
3. $ git reset --hard  
4. $ git clean -df
5. $ brew update