**Q&A for Jenkins Interview Preparation**

**1. What is Jenkins?**
- Jenkins is a self-contained, open-source automation server.
- It automates tasks related to building, testing, and delivering/deploying software.
- It can be installed via native system packages, Docker, or standalone with a Java Runtime Environment.

**2. What are Continuous Integration (CI), Continuous Delivery (CD), and Continuous Deployment (CD)?**
- CI: Integrating code changes into the main codebase continuously to ensure readiness for building, testing, and deployment.
- CD: Delivering integrated changes into a specific environment, usually through a manual release process.
- Continuous Deployment: Automatically deploying integrated changes into the target environment after successful quality checks.

**3. What are common use cases for Jenkins?**
- Software build jobs
- Testing jobs (sanity/smoke/CI/regression)
- Web/data scraping tasks
- Code coverage measurement
- General-purpose automation
- Reverse engineering
- Key decoding

**4. What are the ways to install Jenkins?**
- Native system package manager (e.g., apt, brew)
- Docker
- Kubernetes (via Helm chart)
- Standalone (requires Java Runtime Environment)

**5. What is a Jenkins job?**
- A fundamental unit of work in Jenkins, representing tasks like software builds, automation, or test execution.
- Can be of types: freestyle project, multi-configuration project, pipeline project, multi-branch project, etc.

**6. What is a Jenkins Pipeline?**
- A sequence of steps controlled by defined logic in Jenkins.
- Orchestrates long-running activities across multiple build agents.
- Suitable for building pipelines or organizing complex activities.

**7. What are the types of Jenkins pipelines?**
- Declarative Pipeline
- Scripted Pipeline

**8. Explain Jenkins Multibranch Pipeline.**
- Creates a set of pipeline projects based on detected branches in a single SCM repository.
- Useful for configuring pipelines for different branches (e.g., production code branches).

**9. How do you store credentials securely in Jenkins?**
- Use the Credentials plugin to store credentials securely.
- Supports various types of credentials like username/password, SSH keys, AWS credentials, etc.

**10. How can we temporarily stop a scheduled job from being executed?**
- Disable the job from the job details page.
- Enable it back to resume scheduled executions.
- Alternatively, delete the job if not required permanently.

**11. What are the ways to trigger a Jenkins Job/Pipeline?**
- API request
- Manual trigger from Jenkins web application
- Jenkins CLI
- Time-based scheduled triggers (cron job)
- Event-based triggers (SCM actions, webhooks)
- Upstream/downstream triggers by other Jenkins jobs

**12. What is Jenkins Build Cause?**
- A text attribute representing what triggered a job's build.
- Can be Jenkins user, timer, upstream job, etc.

**13. How does Jenkins know when to execute a scheduled job/pipeline and how is it triggered?**
- Jenkins master has cron entries set up for the jobs.
- As scheduled time approaches, Jenkins commands agents to execute the job.

**14. What credential types are supported by Jenkins?**
- Secret text
- Username and password
- Secret file
- SSH username with a private key
- Certificate (PKCS#12)
- Docker host certificate authentication credentials

**15. What are the Scopes of Jenkins Credentials?**
- Global: Usable across all Jenkins jobs
- System: Used by Jenkins core functionalities and some plugins

**16. What is a Jenkins Shared Library and how is it useful?**
- Set of code common for multiple pipeline jobs, maintained separately.
- Improves maintenance, modularity, and readability of pipeline code.

**17. How can Jenkins jobs be Triggered/Stopped/Controlled programmatically?**
- Using Jenkins Remote Access API
- CURL commands for triggering builds, stopping/aborting builds, enabling/disabling jobs, etc.

**18. How to get the Jenkins version programmatically in Jobs/Pipelines or nodes other than master?**
- Check the 'X-Jenkins' response header from the top-level page or API paths.

**19. What happens when a Jenkins agent is offline and what is the best practice in that situation?**
- Jobs tied to offline agents wait indefinitely.
- Best practice: Tie jobs to a group of nodes/agents using a label to reduce wait time, add more nodes/agents if needed.

**20. What is the Blue Ocean?**
- Redefined user experience for Jenkins, focusing on Pipeline projects.
- Features sophisticated visualizations, pipeline editor, personalization, pinpoint precision, and native integration for branch and pull requests.

**21. What is the Jenkins User Content service?**
- Mechanism to serve static resources like images and stylesheets from the $JENKINS_HOME/userContent folder.

**22. How is continuous integration achieved using Jenkins?**
- Developer makes a change in a feature/dev branch.
- SCM plugins detect events and trigger build/test jobs in Jenkins.
- CD/CD tools take integrated changes forward based on test results.

**23. What is Artifact Archival & how to do it in Pipelines?**
- Archiving exportable/storable results of a job build.
- Configured using the Copy Artifact Plugin in pipelines.

**24. How to configure inclusions & exclusions in Artifacts Archival?**
- Use patterns for including and excluding files/directories.

**25. How can we share information between different build steps or stages in a Jenkins Job?**
- Use files, database entries, environment variables, or post-build actions to share information.

**26. How is code coverage measured/tracked using Jenkins in a CI environment?**
- Using language-specific code coverage plugins or tools like JaCoCo, CodeCov, Sonarqube, etc.

**27. What are the default Environment Variables by Jenkins & How to introduce custom environment variables?**
- Default variables include BRANCH_NAME, BUILD_NUMBER, WORKSPACE, etc.
- Custom variables can be introduced via Jenkinsfile or pipeline script.

**28. How can a job configuration be reset to an earlier version/state?**
- Use Job Config History to review and revert job configurations from the history of changes.

**29. How to do Global Tools Configuration in Jenkins?**
- Install tool plugins, configure global tools in Jenkins, and use them in jobs/pipelines.

**30. How to create & use a Shared Library in Jenkins?**
- Maintain shared library code in SCM repository.
- Configure global shared library in Jenkins.
- Include and use shared library methods in pipeline code.

**31. How to install a Custom Jenkins Plugin or a Version of Plugin Not available in Jenkins Update Center?**
- Use Jenkins CLI, web UI, or manually place .hpi file in the $JENKINS_HOME/plugins directory.

**32. How to download the Console log for a particular Jenkins build programmatically?**
- Use Jenkins CLI console command to retrieve console output for a specific build.

**33. What is Jenkins Remote Access API?**
- Provides remote access to Jenkins functionalities in XML, JSON, or Python formats.

**34. What is In-process Script Approval and how it works?**
- Allows administrators to approve unsafe methods used in user-provided scripts to improve security.
- Appears in "Manage Jenkins" for administrators to make decisions.

**35. Can we monitor Jenkins using common Observability tools?**
- Yes,