Docker agent in Jenkinsfile - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Jenkinsfile with Docker agent

What will be the output of the following Jenkinsfile snippet when run on a Jenkins server with Docker installed?

Jenkins

pipeline {
  agent {
    docker {
      image 'alpine:3.14'
      args '-v /tmp:/tmp'
    }
  }
  stages {
    stage('Test') {
      steps {
        sh 'echo Hello from Docker container'
      }
    }
  }
}

A Jenkins pipeline skipped

B Error: Docker image not found

C Hello from Docker container

D Permission denied error

💡 Hint

Think about what the sh step inside a Docker agent does.

The Jenkinsfile uses a Docker agent with the Alpine image. The sh step runs inside the container, so the echo command outputs the string inside the container environment.

❓ Configuration

intermediate

Correct Docker agent syntax in Jenkinsfile

Which of the following Jenkinsfile snippets correctly defines a Docker agent with the image node:16 and mounts the workspace directory?

A

agent {
  docker {
    image 'node:16'
    args "-v ${WORKSPACE}:/workspace"
  }
}

B

agent {
  dockerfile {
    filename 'node:16'
  }
}

C

agent {
  docker {
    image 'node:16'
    args '-v /var/jenkins_home:/workspace'
  }
}

D

agent {
  docker {
    image 'node:16'
    args '-v $WORKSPACE:/workspace'
  }
}

💡 Hint

Remember to use the correct environment variable syntax for mounting volumes.

Option A correctly uses ${WORKSPACE} to mount the Jenkins workspace directory inside the container. Option A uses $WORKSPACE which may not expand correctly in Jenkinsfile Groovy syntax. Option A uses dockerfile incorrectly. Option A mounts a fixed path which is not the workspace.

❓ Troubleshoot

advanced

Troubleshooting Docker agent failure in Jenkinsfile

A Jenkins pipeline using a Docker agent fails with the error: Cannot connect to the Docker daemon. What is the most likely cause?

A The Docker image name is misspelled in the Jenkinsfile

B Docker daemon is not running on the Jenkins agent machine

C The Jenkinsfile syntax is invalid

D The Jenkins user does not have permission to read the Jenkinsfile

💡 Hint

Think about what is needed to run Docker commands on the Jenkins agent.

The error indicates Jenkins cannot communicate with Docker. This usually means the Docker daemon is not running or Jenkins cannot access it. Misspelled image names or syntax errors cause different errors. Permission to read Jenkinsfile would cause pipeline load failure.

🔀 Workflow

advanced

Jenkins pipeline with Docker agent and environment variables

Consider this Jenkinsfile snippet:

pipeline {
  agent {
    docker {
      image 'python:3.9'
      args '-e ENV=production'
    }
  }
  stages {
    stage('Print Env') {
      steps {
        sh 'echo $ENV'
      }
    }
  }
}

What will be printed when this pipeline runs?

A Error: ENV variable not found

B $ENV

C An empty line

D production

💡 Hint

Check how environment variables passed via args are available inside the container.

The -e ENV=production argument sets the environment variable inside the Docker container. The sh 'echo $ENV' command prints the value of ENV, which is 'production'.

✅ Best Practice

expert

Best practice for using Docker agent in Jenkinsfile for caching dependencies

You want to speed up your Jenkins pipeline builds by caching dependencies inside a Docker container used as an agent. Which approach is best?

A Use a Docker agent with a volume mount to cache dependencies between builds

B Pull the Docker image fresh every build without caching

C Use a Docker agent without volume mounts and install dependencies every build

D Run the pipeline on the Jenkins master node without Docker

💡 Hint

Think about how Docker volumes can help keep data between container runs.

Mounting a volume to cache dependencies allows the container to reuse downloaded packages, speeding up builds. Pulling fresh images or reinstalling dependencies every build slows down the process. Running on master without Docker loses container benefits.