sed: How to replace variable line if found or append to end of file if not found?

sed: How to replace variable line if found or append to end of file if not found? Helpful? Please support me on Patreon: With thanks praise to God and with thanks to the many people who have made this project possible! Content except music images licensed under CC BY-SA Music: Images: others With thanks to user Wade S superuser.comusers456870 user Kamil Maciorowski superuser.comusers432690 and the Stack Exchange Network superuser.comquestions1361102. Trademarks are property of their respective owners. Disclaimer: All information is provided AS IS without warranty of any kind. You are responsible for your own actions. Please contact me if anything is amiss at Roel D.OT VandePaar A.T gmail.com,

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