60 Jenkins Interview Questions for DevOps Engineer

In this article, We are going to cover Jenkins Interview Questions for DevOps Engineer and Real Time Jenkins Interview Questions and Answers, Jenkins Troubleshooting Interview Questions and Answers.

Introduction

If you’re a DevOps engineer, then you will probably face the Jenkins interview questions at least once during your professional life. But have no fear, because we are here to help guide you through the top questions you need to know, that are usually asked by professionals and prepared by some of the best mentors in the field. This guide will help you nail the interview in 2020, and even further!

Jenkins Interview Questions for DevOps Engineer

  1. What is Jenkins for DevOps?

We are starting easy with this one. Jenkins is a free, open-source, continuous integration tool, written in Java and helps the monitoring process for integration and delivery. It also enables developers to check any errors in code and analyze the processes of coding, testing, and deployment.

  1. Are there any per-requisites for using Jenkins?

Yes, there are:

  • Access to source code management repository like SVN or GIT.
  • A build script that should work like mavens builds script.
  1. Which are the main advantages of Jenkins? Why do we need it?

There’s quite a lot of advantages to this tool, but the main ones you can list on your interview are:

  • There is no need to check code or start to build manually, Jenkins does that for you.
  • An automatic build gets started whenever there are changes in source code.
  • Jenkins sends an automatic email if there are any build failures you need to know about.
  • Bugs or defects are easily resolved in the early stages of development.
  • It supports Continuous Integration in agile and test-driven development.
  • It has Built Pipeline Support.
  • Free open-source, which might be its biggest advantage.
  1. What’s the actual use of pipelines in Jenkins? What is their relation to Jenkins File?

This is a plugin that essentially serves to give a view of the stages or tasks that need to be performed one after the other, like a pipeline. Teams can edit, review and iterate upon the tasks. Further, the pipeline details are stored in a file that is connected to the source code repository in general, which is the Jenkins file. They can be different types, like CI CD, scripted and declarative.

  1. What is continuous integration in Jenkins?

When a big team works on a single project, they usually need to run integrations tests to blend all modules in one. This is a feature where isolated changes are tested frequently and reported at the same time. Also, the integration is validated through automated builds and tests.

  1. Which command is used to start Jenkins? How is a manual start performed?

In command Prompt, you can run D:\>Java –jar Jenkins.war, and that is used to start Jenkins with a command. On the other hand, for a manual start, you need to follow these steps:

  • (Jenkins_url)/restart: Force to restart without waiting for build completion.
  • (Jenkin_url)/safeRestart: Waits until the build gets completed before restarting.
  1. Mention some of the most useful plugins known on Jenkins.

There are many, of which we can mention:

  • Maven (build tool)
  • GIT (SCM)
  • Selenium (continuous testing)
  • Amazon EC2
  • Puppet (configuration management)
  • Nagios (Continuous monitoring)
  • Copy artifact
  • HTML publisher
  1. There are some steps in ensuring that your project build doesn’t break. Mention them:
  • First, successfully install Jenkins on your machine and all related units.
  • All code changes should be reflected successfully.
  • Check the repository synchronization as well, just to make sure that all the differences and changes are saved.
  1. How do you set up a Jenkins job?
  • First, select ‘New Job’ on the Jenkins page;
  • Enter the name you want to call it and choose ‘build free-style project’, and then click Ok;
  • Put details of SVN or GIT repository, where the source code has been placed.
  • Give information regarding the plugins you need to use in a project, like for unit testing, code coverage, code quality.
  • Mention the build tool script like Maven script, that we mentioned before.
  • Collect the information regarding results.
  • Configure the steps to send the notifications via emails, etc.
  1. Is there any relation between Hudson and Jenkins? What about Maven and Jenkins?

Hudson is the traditional name for Jenkins, which means they are the same thing.

As for Maven, it is a built tool that consists of a pom.xml file, specified in Jenkins to run the code. It defines how the software is built and its dependencies.

Real Time Jenkins Interview Questions ans Answers

1. You have a Jenkins pipeline that is not triggering automatically when changes are pushed to a Git repository?
Answer: Check if the correct webhook is configured in the Git repository. Verify if the webhook URL is correctly pointing to your Jenkins instance. Ensure that the Jenkins job is configured to “Poll SCM” or use the “GitHub hook trigger for GITScm polling” option in the build triggers section.

2. A Jenkins build fails because of missing dependencies, but the build runs perfectly fine on your local machine?
Answer: Ensure that all required dependencies are installed on the Jenkins server (e.g., libraries, SDKs, or tools). Verify that the build environment on Jenkins is consistent with your local environment (e.g., Java version, etc.). Check the build agent configuration to ensure it has access to the necessary tools or libraries. If you’re using Docker, make sure the correct image with the necessary dependencies is used in the pipeline.

3. A Jenkins pipeline takes too long to complete due to resource-heavy tasks. How can you optimize the pipeline to reduce the execution time?
Answer: Identify tasks in your pipeline that can run in parallel. Modify your Jenkinsfile to execute independent stages concurrently. Use caching mechanisms to avoid repeated downloading or compiling of dependencies. For example, cache dependencies (Maven/npm packages) across builds. Implement incremental builds where only the changed parts of the codebase are built, rather than the entire application. Docker containers can help isolate builds and ensure a consistent environment, which can speed up build times.

4. Your Jenkins server runs out of disk space, causing builds to fail. How do you resolve this issue?
Answer: Use the Jenkins “Workspace Cleanup Plugin” to automatically clean up workspaces after the build is complete. Configure jobs to retain fewer build artifacts. Configure log rotation policies to discard old build logs and free up space. Manually or automatically clean up old builds and artifacts on build nodes.

5. Your Jenkins master is unable to connect to a build agent, and builds are stuck in the queue ?
Answer: Ensure that the agent configuration in Jenkins is correct (e.g., IP address, port, credentials).Verify that the Jenkins master can connect to the agent over the network e.g., via SSH. Ensure that the necessary ports are open in firewalls between the master and the agent.

6. A Jenkins pipeline is stuck in a “waiting” state on one of the stages and not proceeding ?
Answer: Check if the required resources are available. The pipeline might be waiting for an available agent or executor. Check if there’s a input step in the pipeline that requires manual approval before proceeding to the next stage. Ensure that no other pipelines or jobs are holding resources that your job requires. Use resource locks to handle dependencies properly.

7. A Jenkins pipeline fails consistently at a specific stage, but the preceding and following stages work fine?
Answer: Review the logs of the failing stage to understand the exact cause of the failure. Ensure that the necessary environment variables are properly set and passed to that specific stage. Confirm that the stage has access to the required resources. Check if there are errors in the script or commands executed within that stage. Try running that stage in isolation to replicate the error and diagnose the issue.

8. Your Jenkins job is failing when accessing a third-party service (e.g., AWS, DockerHub) due to authentication failure?
Answer: Ensure the correct credentials are stored in Jenkins. If you’re using the Jenkins Credentials Plugin, verify that the correct credential ID is being used. If the third-party credentials have expired or changed, update them in the Jenkins credentials store. Ensure that the service account or user associated with the credentials has the necessary permissions to access the third-party service.

9. One stage in your Jenkins pipeline is taking significantly longer than expected. What steps would you take reduce its execution time?
Answer: Review CPU, memory, and I/O utilization on the build agent during the execution of this stage. If the machine is under heavy load, consider increasing resources or running the job on a different agent. Analyze the code or script being executed in the slow stage. There could be inefficient operations or unnecessary loops causing delays. If the stage is executing multiple tasks, consider splitting it into multiple smaller stages. This allows better parallelism and easier troubleshooting of specific tasks. Add performance logging to the stage to measure where time is being spent.

10. A Jenkins job fails sporadically even though no changes are made to the job configuration or code?
Answer: If the job relies on external services (e.g., databases, APIs), those services might be experiencing intermittent issues. Monitor network connectivity and service availability. Compare the logs of failed and successful builds. Look for patterns or specific errors that appear only in failed runs. If running on a shared build agent, other jobs might be competing for resources (CPU, memory).

11. After restarting Jenkins, it remains stuck at the “Preparing Jenkins…” or “Initializing” stage?
Answer: Review the jenkins.log file located in the Jenkins home directory (/var/log/jenkins/jenkins.log by default). Look for errors indicating configuration issues or plugin problems. If you suspect a plugin issue, restart Jenkins in safe mode. This loads Jenkins without any installed plugins. Verify if any recently updated or installed plugins are causing the issue. Try disabling or downgrading problematic plugins. Ensure the Jenkins server has sufficient disk space. Low disk space can prevent Jenkins from starting correctly.

12. A Jenkins job is queued but doesn’t start because it is “Waiting for next available executor.” What could be the problem, and how would you address it?
Answer: Go to Jenkins “Manage Nodes” and check the number of available executors. If no executors are free, either increase the number of executors on your nodes or wait for the current jobs to finish. Ensure that the build nodes are online and connected to the Jenkins master. If nodes are offline, investigate the cause

13. Your Jenkins pipeline is configured to send notifications (via email, Slack, etc.) on build failure, but you are not receiving them ?
Answer: Verify that the notification plugin (e.g., Email Extension Plugin, Slack Notification Plugin) is installed and configured correctly. Ensure that the correct email address, Slack channel, or other recipient details are set in the job or Jenkinsfile. If using email notifications, ensure that the SMTP server settings are correctly configured in Jenkins global settings (Manage Jenkins > Configure System > E-mail Notification). Review the post-build actions or post section in the Jenkinsfile to ensure the notification steps are included.

14. A Jenkins job was manually cancelled, but it still appears in the “Cancelled” state and is occupying an executor?
Answer: Go to the job’s build page and use the “Terminate” button if available. This will attempt to forcibly kill the job. If the job doesn’t terminate, Jenkins might be under heavy load. Restart the Jenkins service after ensuring no other critical jobs are running. If the job is running on an agent, log into the agent machine and manually kill any lingering processes associated with the build.

15. The “Build Now” button is disabled for a specific Jenkins job, and you cannot manually trigger the build ?
Answer: Check if your Jenkins user has the necessary permissions to trigger the build. If not, an administrator can grant the appropriate permissions via Manage Jenkins > Configure Global Security > Authorization. Ensure that the job is not disabled. If it is, re-enable it by going to the job’s configuration page and unchecking “Disable this project.” If the job configuration restricts concurrent builds and another build is running, the button may be disabled until the current build finishes.

16. Environment variables defined in the pipeline are not being recognized in certain stages?
Answer: Ensure that the environment variable is defined in the correct scope. Use the environment block in the Jenkinsfile to set variables either globally or for specific stages. Ensure that environment variables are referenced using the correct case, as variable names are case-sensitive. Add echo or sh “env” steps to print the environment variables and verify if they are being set correctly.

Conclusion:

Here We have covered common Jenkins Interview Questions for DevOps Engineer.

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