ubuntu进行apt-get时候出现Package ssh is not available, but is referred to by another package 错误

今天在ubuntu进行ssh安装的时候，出现如下错误。

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

这个问题的原因是ubuntu的/etc/apt/source.list中的源比较旧了，需要更新一下，更新方法：

$ sudo apt-get -y update

更新完毕之后，在使用apt-get就没有问题了。