情報
-HirsuteHippo/ReleaseNotes/Ja - Ubuntu Wiki
--https://wiki.ubuntu.com/HirsuteHippo/ReleaseNotes/Ja
-Hirsute Hippo Release Notes - Release - Ubuntu Community Hub
--https://discourse.ubuntu.com/t/hirsute-hippo-release-notes/19221
-Ubuntu 21.04を公開 | Ubuntu
--https://jp.ubuntu.com/blog/ubuntu-21-04%E3%82%92%E5%85%AC%E9%96%8B
-HirsuteUpgrades - Community Help Wiki
--https://help.ubuntu.com/community/HirsuteUpgrades
-nilog: From Ubuntu 20.10 Groovy Gorilla to Ubuntu 21.04 Hirsute Hippo (2021-07-21)
--http://www.nilab.info/nilog/?type=twitter&id=1417836218654461958
-->$ sudo apt update
-->$ sudo apt upgrade
-->$ sudo apt dist-upgrade
-->$ sudo apt autoremove
-->$ sudo iptables -I INPUT -p tcp --dport 1022 -j ACCEPT
-->$ sudo do-release-upgrade -c
-->$ sudo do-release-upgrade
-Ubuntu
--http://www.nilab.info/wiki/Ubuntu.html