Jenkins not starting in Ubuntu - jenkins

Question

In my AWS Ubuntu machine, I started a Jenkins build and somehow due to this machine hang so I forcefully stop the instance and then start again after that Jenkins is not starting up. Getting below.
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jenkins.service - LSB: Start Jenkins at boot time
Loaded: loaded (/etc/init.d/jenkins; generated)
Active: failed (Result: exit-code) since Tue 2020-12-01 15:56:48 UTC; 4min 6s ago
Docs: man:systemd-sysv-generator(8)
Process: 27605 ExecStart=/etc/init.d/jenkins start (code=exited, status=7)
Dec 01 15:56:47 ip-172-31-7-133 systemd[1]: Starting LSB: Start Jenkins at boot time...
Dec 01 15:56:47 ip-172-31-7-133 jenkins[27605]: Correct java version found
Dec 01 15:56:47 ip-172-31-7-133 jenkins[27605]: * Starting Jenkins Automation Server jenkins
Dec 01 15:56:48 ip-172-31-7-133 jenkins[27605]: ...fail!
Dec 01 15:56:48 ip-172-31-7-133 systemd[1]: jenkins.service: Control process exited, code=exited status=7
Dec 01 15:56:48 ip-172-31-7-133 systemd[1]: jenkins.service: Failed with result 'exit-code'.
Dec 01 15:56:48 ip-172-31-7-133 systemd[1]: Failed to start LSB: Start Jenkins at boot time.
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Answer 1

I imagine it can't start because there is already a Jenkins process running which is consuming that same port. You should be able to verify that by looking at the Jenkins logfile.
You can do:
ps auxw | grep jenkins
If it returns a process, then you can kill -9 PID.
For example:
[user#server~]$ ps auxw | grep jenkins
jenkins 4656 0.3 33.2 5070780 2716228 ? Ssl Nov05 144:15 /etc/alternatives/java -Djava.awt.headless=true -DJENKINS_HOME=/var/lib/jenkins -jar /usr/lib/jenkins/jenkins.war --logfile=/var/log/jenkins/jenkins.log --webroot=/var/cache/jenkins/war --httpPort=8080 --debug=5 --handlerCountMax=100 --handlerCountMaxIdle=20
user 14665 0.0 0.0 119416 908 pts/0 S+ 23:08 0:00 grep --color=auto jenkins
[user#server~]$ kill -9 4656
Then try and start your Jenkins instance. Depending on how your Jenkins server is setup you would most likely need to do the above via sudo.

Answer 2

I don't know if this applies to you, but it was my problem. It is similar to Terry Sposato's answer.
We had a scenario where our Ubuntu node was running a standalone instance of Jenkins as wells an SSH started child instance managed from another node.
We saw these sort of service start errors if the SSH started child instance was running.
I resolved this by
Accessing the parent Jenkins instance and selecting disconnect for the Ubuntu node
Immediately invoking sudo /etc/init.d/jenkins start on the Ubuntu node to start it's standalone instance.
After that the jenkins parent in step 1 would eventually reconnect to the the Ubuntu node.
We have not seen this sort of behavior with our CentOS node that also has a standalone and child instance of Jenkins running. I suspect it's a defect with Ubuntu's lsb startup scripts, my debugging showed that the problem happened when /etc/init.d/jenkins sourced /lib/lsb/init-functions.