1、最近在使用cocoapods时出现下面的报错：

ERROR:  SSL verification error at depth 0: certificate has expired (10)ERROR:  Certificate /C=CN/ST=ZheJiang/L=HangZhou/O=Alibaba (China) Technology Co., Ltd./CN=*.taobao.org expired at 2018-12-05T03:16:02ZERROR:  SSL verification error at depth 0: certificate has expired (10)ERROR:  Certificate /C=CN/ST=ZheJiang/L=HangZhou/O=Alibaba (China) Technology Co., Ltd./CN=*.taobao.org expired at 2018-12-05T03:16:02ZERROR:  SSL verification error at depth 0: certificate has expired (10)ERROR:  Certificate /C=CN/ST=ZheJiang/L=HangZhou/O=Alibaba (China) Technology Co., Ltd./CN=*.taobao.org expired at 2018-12-05T03:16:02ZERROR:  Could not find a valid gem 'cocoapods' (= 1.5.0), here is why:          Unable to download data from https://ruby.taobao.org/ - SSL_connect returned=1 errno=0 state=error: certificate verify failed (certificate has expired) (https://ruby.taobao.org/specs.4.8.gz)ERROR:  SSL verification error at depth 0: certificate has expired (10)ERROR:  Certificate /C=CN/ST=ZheJiang/L=HangZhou/O=Alibaba (China) Technology Co., Ltd./CN=*.taobao.org expired at 2018-12-05T03:16:02ZERROR:  SSL verification error at depth 0: certificate has expired (10)ERROR:  Certificate /C=CN/ST=ZheJiang/L=HangZhou/O=Alibaba (China) Technology Co., Ltd./CN=*.taobao.org expired at 2018-12-05T03:16:02Z

出现这个的原因是因为原来淘宝的ruby源无法使用了

我们先删除原来的ruby源：gem sources --remove https://ruby.taobao.org/

然后添加新的ruby源： gem sources -a https://gems.ruby-china.com/ 

gem sources 查看当前ruby源状态是否替换成功

最后 sudo gem install -n /usr/local/bin cocoapods -v 1.5.3（任意版本）升级或降级到自己需要的版本

2、什么是gem

Gem是一个管理Ruby库和程序的标准包，它通过Ruby Gem（如 http://rubygems.org/）源来查找、安装、升级和卸载软件包，非常的便捷。

常用的命令有：

$ gem --version (查看gem版本)

$ gem update --system(更新gem)

$ gem sources (查看数据源)

$ gem sources --remove  https://rubygems.org/(删除数据源)

$ gem sources -a https://ruby.taobao.org/(添加数据源)

$ gem search 软件包关键字(搜索软件包)

$ gem install 软件包名称(安装软件包)

$ gem install cocoapods --pre(安装上一个版本软件包)

$ gem uninstall 软件包名称(卸载安装包)