I just created a new Jenkins instance and I could not start the services.  Based on the logs it shows that 8080 was already used.  See below

Dec 05 22:10:19 localhost systemd[1]: Started Jenkins Continuous Integration Server.
...skipping...
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.Server.lambda$doStart$0(Server.java:402)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:992)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.Server.doStart(Server.java:398)
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:93)
Dec 05 21:54:11 localhost jenkins[82427]:         at winstone.Launcher.<init>(Launcher.java:205)
Dec 05 21:54:11 localhost jenkins[82427]:         ... 6 more
Dec 05 21:54:11 localhost jenkins[82427]: Caused by: java.net.BindException: Address already in use
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.Net.bind0(Native Method)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.Net.bind(Net.java:555)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.ServerSocketChannelImpl.netBind(ServerSocketChannelImpl.java:337)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:294)
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.ServerConnector.openAcceptChannel(ServerConnector.java:339)
Dec 05 21:54:11 localhost jenkins[82427]:         ... 21 more
Dec 05 21:54:11 localhost jenkins[82427]: 2023-12-05 21:54:11.995+0000 [id=1]        SEVERE        winstone.Logger#logInternal: Container startup failed
Dec 05 21:54:11 localhost jenkins[82427]: java.net.BindException: Address already in use
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.Net.bind0(Native Method)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.Net.bind(Net.java:555)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.ServerSocketChannelImpl.netBind(ServerSocketChannelImpl.java:337)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:294)
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.ServerConnector.openAcceptChannel(ServerConnector.java:339)
Dec 05 21:54:11 localhost jenkins[82427]: Caused: java.io.IOException: Failed to bind to 0.0.0.0/0.0.0.0:8080
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.ServerConnector.openAcceptChannel(ServerConnector.java:344)
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.ServerConnector.open(ServerConnector.java:304)
Dec 05 21:54:11 localhost jenkins[82427]:         at org.eclipse.jetty.server.Server.lambda$doStart$0(Server.java:402)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:992)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
Dec 05 21:54:11 localhost jenkins[82427]:         at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)

Based on this logs, I follow the command

systemctl edit jenkins and try to update the port using port 8090 using below values in the file.

[Service]
Environment="JENKINS_PORT=8090"

Saved the file, restarted the services, stopped and started the services but error kept coming back to the same 8080 port already in use.  After restart i go back and put the update in and still will not take it.

most of the Google answers were to use the above command until I found this.  This is the solution that worked.

If jenkins installed as linux service, open the file /usr/lib/systemd/system/jenkins.service,search for the content "Environment="JENKINS_PORT=8080". Change the port number to your desired. Execute the commands

 systemctl daemon-reload
 systemctl restart jenkins.service `

Access the Jenkins in browser with the new port number

Source: https://askubuntu.com/questions/1398910/changed-jenkins-port-doesnt-applied

I know it should not work based on your document but it worked.

The fix i did and that worked, is this good for future use or did not go around the core issue?