Top 30 Jenkins Interview Questions and Answers in 2023

Everything You Need to Know About Jenkins in Software SDLC Process

Jenkins plays a vital role as an automation tool in the Software Development Life Cycle (SDLC) process. In this article, we will discuss the fundamental aspects of using Jenkins in the SDLC process, with a particular emphasis on Jenkins interview questions.

Continuous Integration and Continuous Deployment have dramatically changed software development life cycle processes. By using a set of open-source tools, software teams can deliver developed software much faster to the end-users than with traditional approaches. Among these tools, Jenkins is an excellent open-source software that enables continuous and fast-paced software development in this Dev-Ops era.

Jenkins has several benefits over other available automation software. First, it is easy to install and configure. Second, it is pluggable with custom plugins, making it highly extensible. Lastly, Jenkins is inherently distributed in nature, which enables it to support any scale workload through node and agent distribution.

Jenkins can be used for several software-based automations that run on-premise or on the cloud. With its plugin-based architecture, Jenkins has a broad range of capabilities, which can be leveraged to automate various aspects of the SDLC process.

Introduction to Jenkins:

Jenkins is an open-source automation server developed in Java. Its primary function is to automate various aspects of the SDLC process, allowing developers to focus on developing code and building applications.

Jenkins has a plugin-based architecture, which makes it highly customizable. It integrates with popular software development tools such as GitHub, Jira, and Docker. It can automate tasks like building, testing, and deployment of software.

Basic Jenkins Interview Questions:

1. What is Jenkins?

Understanding Continuous Integration, Continuous Delivery, and Continuous Deployment

Continuous Integration (CI), Continuous Delivery (CD), and Continuous Deployment (CD) are all related to software development and deployment processes.

Continuous Integration is the practice of frequently merging code changes from multiple developers into a central repository. This helps to identify bugs and conflicts early in the development process.

Continuous Delivery involves continuously testing and deploying software changes to ensure that the code is always ready to be deployed into production. This can be done manually or through automated testing.

Continuous Deployment is the process of automatically deploying changes to production as soon as they pass automated testing. This ensures that new features or bug fixes are quickly available to users.

These three processes are interrelated and together they help to streamline the software development and deployment process, minimizing errors and reducing the time it takes to get new features to users.

Common Use Cases for Jenkins

Jenkins is widely used in the software development industry to automate the building, testing, and deployment of code. Below are some of the common use cases of Jenkins:

- Continuous Integration (CI) - Continuous Delivery (CD) - Automated testing - Scheduled jobs and backups - Building and packaging code - Deploying code to production or staging environments

By employing Jenkins, teams can automate repetitive tasks, reduce manual error, and speed up the software development and deployment process.

Ways to Install Jenkins

There are several ways to install Jenkins:

1. Using the Jenkins installer

You can download the Jenkins installer from the official Jenkins website and run it on your system. The installer will guide you through the installation process.

2. Using Docker

You can use Docker to install Jenkins. Docker is a container platform that allows you to run applications in isolated containers. You can find the Jenkins Docker image on the Docker Hub website.

3. Using package managers

Jenkins can be installed using package managers such as apt-get (for Ubuntu) or yum (for CentOS/RHEL). This method is suitable for systems that use these package managers.

Choose the installation method that best suits your needs and requirements.

Understanding the Concept of Jenkins Job

In software development, a Jenkins job refers to a specific task that is executed as part of a larger build process in Jenkins automation server. It could be anything from compiling and testing code to deploying software to a production environment. Essentially, a Jenkins job is a set of instructions and configurations that define how a particular task should be performed within the Jenkins environment. Understanding how Jenkins jobs work is important for anyone working in software development or DevOps.

What is a Jenkins Pipeline?

A Jenkins Pipeline is a powerful tool, which provides a way to define and automate all aspects of continuous delivery pipelines. It is a set of plugins which is integrated into Jenkins and allows us to create, manage and visualize the entire build process. It enables the creation of code pipelines as code, which means that the complete workflow or process is defined in a Jenkinsfile which is checked in alongside the code. This makes it easy to create, manage and evolve the entire process as a code review process.

Types of Jenkins Pipelines

There are two types of Jenkins pipelines:

1. Scripted Pipeline: This is a traditional way of writing Jenkins pipelines using Groovy code. It provides maximum flexibility to developers, but may be complicated for beginners to use.
2. Declarative Pipeline: This is a newer approach that provides a more structured way of writing pipelines. It uses a predefined syntax and is easier to read and understand than the Scripted Pipeline.

Both types of pipelines offer several stages for Continuous Integration and Continuous Delivery of software applications.

Explanation of Jenkins Multibranch Pipeline

Jenkins Multibranch Pipeline is a feature that allows automated continuous integration and delivery (CI/CD) for multiple branches of a project repository. It is usually used with Git, but also supports other version control systems. The Multibranch Pipeline scans a repository for branches and automatically creates a pipeline for each branch. This makes it easier to manage and monitor different branches of a project separately with their own build and testing pipelines. When a new branch is added to the repository, a new pipeline is automatically created, and when a branch is deleted, its pipeline is also automatically deleted. The Multibranch Pipeline also supports pull requests, automatically creating a pipeline for the new pull request branch and monitoring its build status. This saves time and effort in setting up and maintaining separate pipelines for different branches manually.

Securely Storing Credentials in Jenkins

To securely store credentials in Jenkins, you can follow the steps below:

1. Click on "Credentials" in the sidebar of the Jenkins dashboard. 2. Under "Stores scoped to Jenkins", click on "Global credentials (unrestricted)". 3. Click on "Add Credentials" and select the type of credential you want to store (e.g. username and password, SSH private key, etc.). 4. Fill in the required information for the credential and click "OK". 5. To use the stored credential in a job, select "Credentials" from the dropdown menu in the appropriate build step and select the stored credential from the list.

By following these steps, you can ensure that your credentials are securely stored in Jenkins and can be accessed only by authorized users.

How to temporarily stop a scheduled job from running?

If you want to temporarily halt a scheduled job from executing, you can follow these steps:

1. Access the system where the scheduled job is located.

2. Open the task scheduler or the program that manages the scheduled task.

3. Locate the scheduled job that you want to stop temporarily.

4. Disable the scheduled job by either right-clicking on it and selecting "disable" or by selecting it and clicking the "disable" button.

5. If you want to enable the scheduled job to run again, repeat steps 2-4, but select "enable" instead of "disable".

By temporarily disabling the scheduled job, it will not run at the specified time and date. Once you are ready for the job to run again, you can re-enable it and it will execute at the next scheduled time.

Ways to Trigger a Jenkins Job/Pipeline

In Jenkins, there are several ways to trigger a job/pipeline:

1. Manual build trigger: This is the most common way to trigger a job manually by clicking the build button on the Jenkins web interface.
2. Scheduled Build Trigger: We can schedule a build at a specific time, date, or interval on a cron pattern.
3. Trigger builds remotely: Jenkins exposes an API that can trigger a build remotely via HTTP POST requests.
4. Trigger builds when changes are pushed to the code repository: Jenkins can automatically detect changes that are pushed to the code repository and trigger a build automatically. This requires setting up the appropriate webhooks on the code repository.
5. Trigger builds when other builds complete: Jenkins has a feature called "Build other projects" which allows us to trigger another job/pipeline when the current job/pipeline completes.

Note: The choice of which trigger option to use depends on the specific use case and requirements for the job/pipeline.

Jenkins Build Cause

In Jenkins, build cause refers to the reason or event that triggered a specific build to occur. These build causes could be manual triggers by a user, scheduled builds, upstream project builds, or even changes pushed to a repository. By understanding the build cause, Jenkins users can better track and manage their builds and ensure they are running as intended.

How Jenkins Determines When to Execute a Scheduled Job/Pipeline and How it is Triggered?

Jenkins has a scheduler that enables it to execute jobs or pipelines at particular times and intervals. The default time zone is UTC, but you can set it to your preferred time zone. You can configure a scheduled job to execute at specific times or intervals using the build triggers feature within the job/pipeline settings.

In addition to scheduled builds, Jenkins can be triggered through other events like changes in source code management systems, such as Git or SVN, or by external tools that send webhook notifications. You can also configure Jenkins to be triggered manually using the "build now" option.

Jenkins can be integrated with other plugins and tools to add more triggers and flexibility for job execution. These plugins can help in triggering jobs/pipelines based on other factors like the success of other builds, specific calendar events or even the weather.

Supported Credential Types in Jenkins

In Jenkins, there are several types of credentials that can be used:

- Username and password - Secret text - Secret file - SSH username with private key - Certificate - Docker registry key - Kubernetes configuration (kubeconfig) file - AWS credentials - Google Service Account Key

These credentials can be used in Jenkins jobs and pipelines for authentication and access to external resources.

Scopes of Jenkins Credentials

Jenkins Credentials can have different scopes, depending on where and how they are used in the Jenkins environment.

There are three main scopes for Jenkins Credentials:

1. System Scope - These are global credentials that can be used by all jobs and plugins in the Jenkins environment. They are stored in the Jenkins Master and can be accessed by any node in the Jenkins cluster.

2. Global Scope - These credentials can be used by all jobs and plugins within a particular Jenkins instance. They are stored in the Jenkins Master but are only accessible by nodes within the same Jenkins instance.

3. Item Scope - These credentials are specific to a single job or plugin within the Jenkins environment. They are stored within the job or plugin configuration and are only accessible by that particular job or plugin.

Using multiple scopes for Jenkins Credentials can help to ensure that sensitive information is only accessible to the jobs and plugins that need it, and can help to improve overall security within the Jenkins environment.

Understanding Jenkins Shared Library and Its Benefits

A Jenkins Shared Library, also known as a "Pipeline Shared Library", is a powerful communication tool that enables developers to use global shared code in their pipelines.

Instead of duplicating code logic across pipelines, the shared library stores it in a single place, which simplifies maintenance and makes it easier to update the code base. This feature is particularly beneficial for large-scale, complex projects where there is a lot of reuse of pipeline code.

Furthermore, Jenkins Shared Libraries increase reusability, enhance consistency, and reduce time-consuming code duplication. By centralizing code logic, developers can collaborate efficiently, save time on development tasks, and create better quality software products.

How to Trigger, Stop, and Control Jenkins Jobs Programmatically?

Code:

python
import jenkins 

# Define Jenkins URL and credentials 
jenkins_url = 'http://localhost:8080' 
username = 'your_username' 
password = 'your_password' 

# Connect to Jenkins server
server = jenkins.Jenkins(jenkins_url, username, password) 

# Trigger job
server.build_job('job_name')

# Stop job
server.stop_build('job_name', build_number)

# Check if job is running
is_running = server.get_build_info('job_name', build_number)['building']

# Get console output
console_output = server.get_build_console_output('job_name', build_number)

This Python code uses the Jenkins Python API to connect to a Jenkins server, trigger a job, stop a job, check if the job is running, and get the console output of the job.

Note: Replace 'jenkins_url', 'your_username', 'your_password', 'job_name', and 'build_number' with your own values.

Programmatically Retrieving the Jenkins Version in Jobs/Pipelines or Nodes other than Master

Code:

 
def jenkins = Jenkins.getInstance()
def version = jenkins.getVersion().toString()
echo "Jenkins version is ${version}"

To retrieve the Jenkins version programmatically in jobs/pipelines or nodes other than the master, you can use the above code snippet, which uses the Jenkins API to get the instance of the Jenkins object and retrieves its version.
First, we get the instance of the Jenkins object using the Jenkins.getInstance() method. Then, we retrieve the version of the Jenkins object using the getVersion() method, which returns a Version object. Finally, we convert the Version object to a string using the toString() method and print it out using the echo command.
By using this code in your job/pipeline or node, you can easily retrieve the version of Jenkins and use it in your scripts or workflows.
What Occurs When a Jenkins Agent is Offline and What is the Best Practice in such a Situation?
When a Jenkins agent goes offline, it becomes unavailable to perform any task assigned to it. In such a situation, Jenkins automatically reroutes the pending jobs to the other available agents.  
The best practice is to set up multiple agents, which can distribute the workload among themselves and prevent any single point of failure. Also, using a master-slave setup can ensure that the master node takes over in case of an agent failure, and the work does not suffer a complete shutdown. It is crucial to maintain a healthy and responsive infrastructure so that delays can be avoided and timelines can be met.  Proper monitoring and alerts can also notify the team when an agent goes offline, and corrective actions can be taken immediately.
What is the Blue Ocean?
The Blue Ocean is a business strategy that involves creating uncontested market space and making competition irrelevant. It focuses on creating new market demand and generating innovative ideas rather than competing in existing market demand with traditional players.
 Understanding Jenkins User Content Service 
The Jenkins User Content Service is a feature that allows users to store files and other resources used in Jenkins job builds. These resources can include things like scripts, configuration files, and plugins. The Jenkins User Content Service makes it easy for users to manage these resources and ensures that they are available to job builds when required.
When a user uploads a file to the Jenkins User Content Service, it is stored in a specific directory on the Jenkins server. This directory is then made available to job builds through a special URL. Job builds can then access these resources by using the URL to download them as needed.
The Jenkins User Content Service is an important feature for users who frequently use external resources in their job builds. By centralizing the storage and management of these resources, the Jenkins User Content Service makes it easier for users to manage their builds and improve overall job performance.
How to Achieve Continuous Integration Using Jenkins?
Continuous Integration (CI) is a software engineering practice that involves merging code changes frequently into a shared repository to enable early detection and resolution of any integration conflicts. Jenkins is an open-source automation tool that can facilitate CI by automating the build, testing, and deployment of code changes.
To achieve Continuous Integration using Jenkins, you can follow these steps:

Install and configure Jenkins on your system.
Create a project in Jenkins to monitor changes in the source code repository.
Set up a build automation process that can compile the code, run unit tests, and generate any necessary reports.
Execute the build process whenever there is a code change in the repository.
Store and analyze the build results to identify any issues or bugs early on.
Deploy the code changes to a staging environment for testing and feedback.
Automatically trigger the final deployment to the production environment once the code changes pass the necessary tests and checks.

By automating these processes, Jenkins can help you achieve Continuous Integration and enable faster and more reliable software delivery.
Understanding Artifact Archival and its Implementation in Pipelines
Artifact archival is the process of preserving and storing the output generated during a software development process. In the context of pipelines, artifacts may include compiled code binaries, documentation files, test results, and other files that are generated during the software development lifecycle.
Implementing artifact archival in pipelines involves specifying the output files that need to be archived and selecting a storage location where the artifacts will be stored. This can be done using various tools and services that are commonly used in software development, such as Artifactory, GitHub, and Jenkins.
To implement artifact archival in pipelines, a series of steps can be followed:

Identify the files that need to be archived.
Configure the pipeline to include artifact archival as a step.
Select a storage location for the artifacts.
Specify access controls and permissions for the stored artifacts.
Test the pipeline to ensure that artifacts are being archived correctly.

Implementing artifact archival in pipelines can provide numerous benefits, such as enabling quick and easy access to previous versions of the software, facilitating collaboration among team members, and ensuring that important files are not lost or deleted.
How to Configure Artifact Archival Inclusions and Exclusions?
To configure inclusions and exclusions in artifact archival, follow these steps:

Open the settings page for the artifact archival tool you are using.
Navigate to the "Inclusions and Exclusions" section.
Specify the files, directories, or patterns that you want to include or exclude from the archival process.
Save the settings and run the archival process.

By specifying inclusions and exclusions, you can control which files and directories are included in the artifact archival process. This allows you to save storage space and only retain the files that are truly necessary for your projects.
Remember to regularly review and update your inclusion and exclusion settings to ensure that you are only archiving the files that are really needed.
  

Sharing Information between Different Build Steps or Stages in a Jenkins Job
In Jenkins, there are various ways to share information between different build steps or stages of a job:
1. Environment Variables - Information can be passed as environment variables from one build step to another. The 'env' object can be used to access these variables.
2. Temporary files - Information can be stored in a file and passed between build steps using the 'archive' and 'copyArtifact' steps.
3. Build Parameters - Parameters created within the job can be used to pass information between build steps or stages. This is particularly useful for passing user input or choices between stages.
4. Plugins - Various Jenkins plugins can be used to share information between build steps. For example, the 'Build With Parameters' plugin can be used to pass parameters between build steps.
Overall, Jenkins provides many powerful ways to manage information flow between build steps or stages, and choosing the right one depends on the specific use case.
Measuring and Tracking Code Coverage using Jenkins in a CI/CD Environment
Code coverage is an essential metric to measure the effectiveness of test suites and to identify which areas of the code need additional testing. Jenkins, a popular CI/CD tool, provides plugins that can help automate the process of measuring and tracking code coverage.
There are various plugins that allow for code coverage measurement in Jenkins, such as the Jacoco Plugin, which is widely used. This plugin integrates the JaCoCo library to collect code coverage metrics from unit tests and integration tests and generate a coverage report.
To use the Jacoco Plugin in Jenkins, first, we need to add it to the plugins list on the Jenkins server and configure it in the build settings of the project. Once configured, Jenkins can automatically run the tests and generate code coverage reports on each build. These reports can then be viewed in Jenkins or exported in different formats like HTML, XML, or CSV.
To ensure that the code coverage reports are accurate, it's important to write comprehensive test suites that cover all aspects of the code. Further, we can set a minimum threshold for code coverage to fail the build if the code coverage falls below the set limit. This ensures that the code is well tested and maintains a high level of quality.
In conclusion, utilizing Jenkins plugins such as Jacoco, helps automate the process of measuring and tracking code coverage in a CI/CD environment and helps ensure our code is robust and of high quality.
Default Environment Variables in Jenkins and How to Add Custom Environment Variables
Jenkins comes with several default environment variables that can be accessed by jobs running within it. These include variables like BUILD_NUMBER, JOB_NAME, and WORKSPACE. 
To add custom environment variables, you can do so in the "Manage Jenkins" section by navigating to "Configure System" and scrolling down to the "Global properties" section. Here, you can click on the "Environment variables" checkbox and add your custom variables and their values.
You can also add custom environment variables within individual jobs by going to the job's configuration page and selecting the "This project is parameterized" checkbox. From here, you can add a new "Environment variable" parameter and specify its name and value.
By adding custom environment variables, you can customize the behavior of your Jenkins jobs and access important information within your build scripts.
Resetting Job Configuration to an Earlier Version/State
To reset a job configuration to an earlier version or state, follow these steps:
1. Navigate to the job configuration page on the website or platform where it is located.
2. Look for a “Version History” or “Revision History” section, usually located towards the bottom of the page.
3. Select the version or state that you want to revert to.
4. Click on the “Restore” or “Revert” button, which should be located next to the version or state you selected.
5. Confirm the action, if prompted.
6. Wait for the job configuration to be restored to the earlier version or state.
It is important to note that restoring to an earlier version or state may result in the loss of any changes or updates made since that version or state. Therefore, it is recommended to carefully review the version history and only revert if necessary.
Global Configuration of Tools in Jenkins
To configure global tools in Jenkins, follow these steps:
1. Open your Jenkins dashboard and click on "Manage Jenkins."
2. Click on "Global Tool Configuration" to open the configuration page.
3. Here, you can add, remove, or modify tool installations for different categories like JDK, Git, Maven, etc.
4. To add a new tool installation, scroll to the category for which you want to add the tool and click on "Add Installer."
5. Select the installer type from the dropdown list and provide the necessary details like the name and path of the tool installation.
6. Once you have configured the tools, click on "Save" to save the changes.
This will configure all the tools globally so that they can be used across all the Jenkins jobs in your system.
Creating and Using a Shared Library in Jenkins
Jenkins is a popular automation tool used in software development. Shared libraries in Jenkins allow you to reuse common code across different Jenkins pipelines. Here are the steps to create and use a shared library in Jenkins:

Create a new repository in your Git hosting tool to store the shared library code.
Create a new file named "vars/customStep.groovy" in the repository. This file will contain the code for the custom step that you want to use in your Jenkins pipelines.
Commit the changes and push them to the Git hosting tool.
In the Jenkins dashboard, go to "Manage Jenkins" > "Configure System" > "Global Pipeline Libraries" and add the URL of the Git repository in the "Library" section. Give it a name and version number.
In your Jenkins pipeline, you can now use the custom step defined in the shared library by calling it with the syntax `customStep()`. Jenkins will automatically load the shared library and execute the code in the "vars/customStep.groovy" file.

By using shared libraries in Jenkins, you can simplify your pipeline code and improve code reuse across your projects.
Installing a Custom Jenkins Plugin or a Version of a Plugin Not Available in Jenkins Update Center
If a desired Jenkins plugin is not available in the Jenkins Update Center or if you want to install a custom version of a plugin, you can follow these steps:

Download the desired plugin or desired version of the plugin in the form of a .hpi file.
Copy the .hpi file to the $JENKINS_HOME/plugins directory on your Jenkins server.
Once the plugin has been copied, restart Jenkins by running the command: 
sudo service jenkins restart

The new plugin or updated version of the plugin will now be available in your Jenkins configuration.

Note: It is important to ensure that the custom plugin or version of the plugin is compatible with your version of Jenkins.
How to Programmatically Download Console Log for a Specific Jenkins Build?
To download the console log for a specific Jenkins build programmatically, you can use the Jenkins API. Here is an example script in Python:

import requests<br>
build_number = "123"<br>
jenkins_url = "http://yourjenkinsurl.com"<br>
console_log_url = jenkins_url + "/job/YourJobName/" + build_number + "/consoleText"<br>
response = requests.get(console_log_url)<br>
console_log_text = response.text<br>
print(console_log_text)<br>

Replace "YourJobName" with the name of your Jenkins job and "build_number" with the build number of the Jenkins build you want to download the console log for. This script will print out the console log text to the console.
Understanding Jenkins Remote Access API
Jenkins Remote Access API is a feature that allows users to remotely access and control Jenkins functions through a RESTful API. With this API, users can perform various operations on Jenkins, such as triggering builds, checking build status, and even creating and editing jobs. It provides a simple and convenient way for developers to automate their Jenkins tasks and integrate it with their toolchain. By using the Jenkins Remote Access API, developers can save time and streamline their development workflows.
In-Process Script Approval: Understanding How it Works

In-process script approval is a process in which scripts or code undergo a review and approval process during development. It is a quality control measure designed to catch errors or issues in the code before it is launched. Developers submit their code for approval to a designated individual or team, and the code is evaluated against established criteria. If the code meets the requirements, it is approved; otherwise, it is sent back for revisions. In-process script approval helps ensure that code is stable, functional, and secure before it goes live.

Can Jenkins be Monitored with Common Observability Tools?
Yes, it is possible to monitor Jenkins with commonly used observability tools. By utilizing tools such as Prometheus and Grafana, you can monitor the performance and health of Jenkins, as well as gather metrics and track trends over time. Additionally, plugins are available within Jenkins itself for monitoring purposes. With proper monitoring, you can quickly identify potential issues and address them before they turn into bigger problems.
Understanding Ping Thread in Jenkins and Its Functionality
In Jenkins, a Ping Thread is a background job that periodically checks if a node (machine) connected to the Jenkins server is still reachable. It sends a "ping" command to the node, and if the node responds back, it means that the node is still connected and Jenkins can continue to send build jobs to that node. 
The Ping Thread is crucial in ensuring that nodes are available and properly functioning. If a node goes offline or becomes unresponsive, the Ping Thread will detect it and mark the node as offline. The Jenkins server will then stop sending build jobs to that node until it comes back online.
Overall, the Ping Thread helps to maintain the stability and reliability of Jenkins automation, and ensures that build jobs are properly distributed to connected nodes.