系统升级到最新版本（10.13.x）之后发现cocoapods更新失败了，重新安装之后也提示如下错误：
$ pod setup
-bash: /usr/local/bin/pod: /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby: bad interpreter: No such file or directory
说明pod的路径没有指向usr/local/bin目录，所以只要更改一下install命令参数就好：
$ sudo gem install -n /usr/local/bin cocoapods
Successfully installed cocoapods-1.3.1
Parsing documentation for cocoapods-1.3.1
Done installing documentation for cocoapods after 2 seconds
1 gem installed
