Common Problems During Jenkins Installation and How to Resolve Them

1. Jenkins Fails to Start

Symptom:

Running sudo systemctl status jenkins shows:

jenkins.service: Failed with result 'exit-code'.

Cause:

This often happens when the port Jenkins is configured to use (default: 8080) is already in use.

Solution:

1. Check which process is using the port:

    sudo lsof -i :8080
   

2. If another application is using the port, either stop that application:

    sudo kill -9 <PID>
   

   Or change Jenkins’ port in the configuration file:

    sudo nano /etc/default/jenkins
   

   Modify the line:

    HTTP_PORT=8081
   

   Then restart Jenkins:

    sudo systemctl restart jenkins
   

---

2. Java Version Issues

Symptom:

Logs show errors like:

java.io.IOException: Unsupported major.minor version

Cause:

Jenkins requires Java 8 or 11, but an unsupported version might be installed.

Solution:

1. Check the Java version:

    java -version
   

2. If it's unsupported, install OpenJDK 11:

    sudo apt install openjdk-11-jdk -y
   

3. Update the default Java version:

    sudo update-alternatives --config java
   

---

3. Authentication Required Error

Symptom:

You see the following message when accessing Jenkins in the browser:

Authentication required

Cause:

Jenkins requires an admin password for initial setup.

Solution:

Retrieve the password from the file:

sudo cat /var/lib/jenkins/secrets/initialAdminPassword

Use this password on the web interface.

---

4. Multiple Jenkins Installations

Symptom:

Jenkins fails to start, or journalctl logs show conflicting configurations.

Cause:

Jenkins was installed multiple times, leading to duplicate services or port conflicts.

Solution:

1. Identify the installed Jenkins packages:

    sudo apt list --installed | grep jenkins
   

2. Remove unnecessary installations:

    sudo apt remove --purge jenkins
   

3. Ensure only one Jenkins service is running:

    ps aux | grep jenkins
   

   Kill any redundant processes:

    sudo kill -9 <PID>
   

---

5. Plugin Installation Errors

Symptom:

Plugins fail to install, or Jenkins crashes during plugin installation.

Cause:

Network issues or insufficient permissions.

Solution:

1. Restart Jenkins:

    sudo systemctl restart jenkins
   

2. Install plugins manually via the Jenkins web interface:

   • Go to Manage Jenkins > Plugin Manager > Available.

   • Select and install required plugins.