ubuntu進行apt-get時候出現Package ssh is not available, but is referred to by another package 錯誤

今天在ubuntu進行ssh安裝的時候，出現如下錯誤。

 

Reading package lists... Done

Building dependency tree... Done

Package ssh is not available, but is referred to by another package.

This may mean that the package is missing, has been obsoleted, or

is only available from another source

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這個問題的原因是ubuntu的/etc/apt/source.list中的源比較舊了，需要更新一下，更新方法：

$ sudo apt-get -y update

更新完畢之後，在使用apt-get就沒有問題了。