准备将 ubuntu 16.04 升级 18.04，在执行 apt-get update && apt-get upgrade 命令之后执行 do-release-upgrade 报错
apt-get update && apt-get upgrade
Please install all available updates for your release before upgrading.
再次执行 apt-get upgrade 出现提示
The following packages have been kept back:
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
专门升级 fish 后问题解决
apt install fish