想用brew安装wget，但是提示失败，然后想先 brew update，然后再重新试一下，结果在brew update后出现了这个情况：
$ 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>'不管执行什么brew命令提示依旧，这种情况多发生在 El Capitan系统上，原因大概是更新失败，下面是stackoverflow找到的答案，亲测有效：
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 如果运行完上面的命令还没有起作用的话，那么再运行下面的命令应该就可以了。这可能是因为homebrew还没有升级的原因。
1. 首先打开终端
2. $ cd /usr/local
3. $ git reset --hard
4. $ git clean -df
5. $ brew update