想用brew安裝wget，但是提示失敗，然後想先 brew update，然後再重新試一下，結果在brew update後出現了這個情況：
$ brew update
/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 -- mach (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/extend/pathname.rb:2: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:3: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>'不管執行什麼brew命令提示依舊，這種情況多發生在 El Capitan系統上，原因大概是更新失敗，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 如果運行完上面的命令還沒有起作用的話，那麼再運行下面的命令應該就可以了。這可能是因爲homebrew還沒有升級的原因。
1. 首先打開終端
2. $ cd /usr/local
3. $ git reset --hard
4. $ git clean -df
5. $ brew update