环境为mac， 总共报了三种错误。
对应三种不同的解法。
1. 第一种如下， 提示 1.8 的版本找不到
$ brew update
/usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory
/usr/local/bin/brew: line : /usr/local/Library/brew.rb: Undefined error:
这种比较简单。 原因是mac 升级到 某一个版本之后， ruby版本升级到2.0 了， 但是 brew.rb 中的版本号是 写死了1.8 ， 需要修改下。
先检查一下自己 ruby版本 $ cd /System/Library/Frameworks/Ruby.framework/Versions/
一般来说， 都是 2.0
之后 $ vi /usr/local/Library/brew.rb 将第一行的 1.8 ， 修改成 2.0 即可。
详细见： http://stackoverflow.com/questions/24225959/how-to-get-ruby-homebrew-rvm-to-work-on-yosemite
2. 第二种情况， 错误提示如下。
$ brew update
/Library/Ruby/Site/2.0./rubygems/ext/builder.rb::in `run': ERROR: Failed to build gem native extension. (Gem::Ext::BuildError) /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
checking for rb_thread_blocking_region()... yes
checking for sys/select.h... yes
checking for poll.h... yes
checking for sys/epoll.h... no
checking for sys/event.h... yes
checking for sys/queue.h... yes
checking for port.h... no
checking for sys/resource.h... yes
creating Makefile make "DESTDIR=" clean make "DESTDIR="
compiling monitor.c
In file included from monitor.c::
In file included from ./nio4r.h::
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/include/ruby-2.0./ruby/backward/rubyio.h::: warning: use "ruby/io.h" instead of "rubyio.h" [-W#warnings]
#warning use "ruby/io.h" instead of "rubyio.h"
^
warning generated.
In file included from monitor.c::
In file included from ./nio4r.h::
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/include/ruby-2.0./ruby/backward/rubyio.h::: warning: use "ruby/io.h" instead of "rubyio.h" [-W#warnings]
#warning use "ruby/io.h" instead of "rubyio.h"
^
warning generated.
compiling nio4r_ext.c
In file included from nio4r_ext.c::
In file included from ./nio4r.h::
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/include/ruby-2.0./ruby/backward/rubyio.h::: warning: use "ruby/io.h" instead of "rubyio.h" [-W#warnings]
#warning use "ruby/io.h" instead of "rubyio.h"
^
In file included from nio4r_ext.c::
./../libev/ev.c::: warning: '/*' within block comment [-Wcomment]
/*#define MIN_INTERVAL 0.00000095367431640625 /* 1/2**20, good till 2200 */
^
./../libev/ev.c::: error: '_Noreturn' keyword must precede function declarator
ecb_inline void ecb_unreachable (void) ecb_noreturn;
^~~~~~~~~~~~
_Noreturn
./../libev/ev.c::: note: expanded from macro 'ecb_noreturn'
#define ecb_noreturn _Noreturn
^
./../libev/ev.c::: warning: 'extern' variable has an initializer [-Wextern-initializer]
EV_API_DECL struct ev_loop *ev_default_loop_ptr = ; /* needs to be initialised to make it a definition despite extern */
make failed, exit code 2
Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/nio4r-1.0.0 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-16/2.0.0/nio4r-1.0.0/gem_make.out
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:51:in `block in make'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:43:in `each'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:43:in `make'
from /Library/Ruby/Site/2.0.0/rubygems/ext/ext_conf_builder.rb:43:in `block in build'
from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/tempfile.rb:324:in `open'
from /Library/Ruby/Site/2.0.0/rubygems/ext/ext_conf_builder.rb:17:in `build'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:161:in `block (2 levels) in build_extension'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:160:in `chdir'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:160:in `block in build_extension'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:159:in `synchronize'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:159:in `build_extension'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:198:in `block in build_extensions'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:195:in `each'
from /Library/Ruby/Site/2.0.0/rubygems/ext/builder.rb:195:in `build_extensions'
from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:1436:in `block in build_extensions'
from /Library/Ruby/Site/2.0.0/rubygems/user_interaction.rb:45:in `use_ui'
from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:1434:in `build_extensions'
from /Library/Ruby/Site/2.0.0/rubygems/stub_specification.rb:60:in `build_extensions'
from /Library/Ruby/Site/2.0.0/rubygems/basic_specification.rb:56:in `contains_requirable_file?'
from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:925:in `block in find_inactive_by_path'
from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:924:in `each'
from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:924:in `find'
from /Library/Ruby/Site/2.0.0/rubygems/specification.rb:924:in `find_inactive_by_path'
from /Library/Ruby/Site/2.0.0/rubygems.rb:185:in `try_activate'
from /Library/Ruby/Site/2.0.0/rubygems/core_ext/kernel_require.rb:132:in `rescue in require'
from /Library/Ruby/Site/2.0.0/rubygems/core_ext/kernel_require.rb:144:in `require'
from /usr/local/Library/Homebrew/extend/pathname.rb:2:in `<top (required)>'
from /Library/Ruby/Site/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Library/Ruby/Site/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /usr/local/Library/Homebrew/global.rb:3:in `<top (required)>'
from /Library/Ruby/Site/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /Library/Ruby/Site/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /usr/local/Library/brew.rb:17:in `<main>'
这种情况， 需要升级 gem 的版本。
一般来说是用 $ sudo gem update --system 来进行升级， 但是会报错。
$ sudo gem update --system
Password:
Sorry, try again.
Password:
ERROR: While executing gem ... (Gem::RemoteFetcher::FetchError)
bad response Not Found (http://ruby.taobao.org/specs.4.8.gz)
这种错误， 因为 gem源换了， 貌似原来的淘宝源不维护了。 具体见： https://ruby.taobao.org/ 。 需要跟换一下gem源
$ sudo gem update --system
Password:
Updating rubygems-update
Fetching: rubygems-update-2.6..gem (%)
ERROR: While executing gem ... (Errno::EPERM)
Operation not permitted - /usr/bin/update_rubygems
这种错误也没明白， 提示是 没有操作权限。 在网上查了一个方法， 修改命令成如下方式就可以了。
$ sudo gem update -n /usr/local/bin --system
执行完成之后， gem -v 可以看到 版本更新了。
这样， 第二个 gem update的错误，就快要解决了。
第三个问题， 继续 brew update, 出现如下错误。
$ brew update
/Library/Ruby/Site/2.0./rubygems/core_ext/kernel_require.rb::in `require': cannot load such file -- mach (LoadError)
from /Library/Ruby/Site/2.0./rubygems/core_ext/kernel_require.rb::in `require'
from /usr/local/Library/Homebrew/extend/pathname.rb::in `<top (required)>'
from /Library/Ruby/Site/2.0./rubygems/core_ext/kernel_require.rb::in `require'
from /Library/Ruby/Site/2.0./rubygems/core_ext/kernel_require.rb::in `require'
from /usr/local/Library/Homebrew/global.rb::in `<top (required)>'
from /Library/Ruby/Site/2.0./rubygems/core_ext/kernel_require.rb::in `require'
from /Library/Ruby/Site/2.0./rubygems/core_ext/kernel_require.rb::in `require'
from /usr/local/Library/brew.rb::in `<main>'
根据 http://stackoverflow.com/questions/24652996/homebrew-not-working-on-osx 的最高票答案就可以解决。
$ sudo chown -R $(whoami):admin /usr/local
如果这个指令没有解决， 则采用下面的步骤。
1. open terminal
2. $ cd /usr/local/Homebrew ， 这里的路径我是： /usr/local/Library/
3. $ git reset --hard
4. $ git clean -df
5. $ brew update
至此， 这个问题解决了。 
