Docker agents for isolation in Jenkins - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the Docker agent in a Jenkins pipeline.

Jenkins

pipeline {
  agent {
    docker {
      image '[1]'
    }
  }
  stages {
    stage('Build') {
      steps {
        echo 'Building inside Docker container'
      }
    }
  }
}

Drag options to blanks, or click blank then click option'

Aubuntu:latest

Bnodejs:14

Calpine:3.12

Dpython:3.8

Attempts:

3 left

💡 Hint

Common Mistakes

Using an invalid or misspelled image name.

Omitting the image tag (like :latest).

The image field specifies the Docker image to use for the agent. Here, ubuntu:latest is a common base image.

2fill in blank

medium

Complete the code to run a shell command inside the Docker agent.

Jenkins

pipeline {
  agent {
    docker {
      image 'python:3.8'
    }
  }
  stages {
    stage('Test') {
      steps {
        sh '[1]'
      }
    }
  }
}

Drag options to blanks, or click blank then click option'

Apython --version

Bpip install requests

Cecho Hello World

Dls -l

Attempts:

3 left

💡 Hint

Common Mistakes

Using commands that require prior installation.

Using commands unrelated to the container's environment.

The sh step runs a shell command inside the Docker container. Here, python --version checks the Python version inside the container.

3fill in blank

hard

Fix the error in the Docker agent declaration to correctly specify the Docker image.

Jenkins

pipeline {
  agent {
    docker {
      image [1]
    }
  }
  stages {
    stage('Deploy') {
      steps {
        echo 'Deploying application'
      }
    }
  }
}

Drag options to blanks, or click blank then click option'

A"nginx:latest"

Bnginx:latest

C'nginx:latest'

Dnginx_latest

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the image name.

Using invalid image names.

The image value must be a string enclosed in quotes. Using single quotes like 'nginx:latest' is correct.

4fill in blank

hard

Fill both blanks to define a Docker agent with a custom label and reuse the same image in a stage.

Jenkins

pipeline {
  agent {
    docker {
      image [1]
      label [2]
    }
  }
  stages {
    stage('Build') {
      agent {
        docker {
          image 'myapp:latest'
        }
      }
      steps {
        echo 'Building inside Docker'
      }
    }
  }
}

Drag options to blanks, or click blank then click option'

A'myapp:latest'

B'docker-agent'

C'ubuntu:20.04'

D'build-node'

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted strings for image or label.

Mismatching image names between agent and stage.

The image is set to 'myapp:latest' to specify the Docker image. The label is set to 'docker-agent' to identify the agent node.

5fill in blank

hard

Fill all three blanks to create a Docker agent with arguments, reuse the image, and run a command inside the container.

Jenkins

pipeline {
  agent {
    docker {
      image [1]
      args [2]
    }
  }
  stages {
    stage('Test') {
      steps {
        sh '[3]'
      }
    }
  }
}

Drag options to blanks, or click blank then click option'

A'node:16'

B'-u root:root'

Cnpm test

D'--privileged'

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting the image or args values.

Using incorrect shell commands.

The image is 'node:16'. The args specify running as root user with '-u root:root'. The shell command npm test runs tests inside the container.

Practice

1. What is the main benefit of using Docker agents in Jenkins for running jobs?

easy

A. They provide isolated environments to avoid conflicts between builds.

B. They speed up the Jenkins UI loading time.

C. They allow Jenkins to run without any plugins installed.

D. They automatically backup Jenkins configuration files.

Solution

1. Step 1: Understand Docker agent purpose

   Docker agents run Jenkins jobs inside containers, isolating each build environment.

2. Step 2: Identify the benefit of isolation

   This isolation prevents conflicts and keeps builds clean and reproducible.

3. Final Answer:

   They provide isolated environments to avoid conflicts between builds. -> Option A

4. Quick Check:

   Docker agents = isolated environments [OK]

Hint: Docker agents isolate builds to avoid conflicts [OK]

Common Mistakes:

• Thinking Docker agents speed up Jenkins UI
• Believing Docker agents remove need for plugins
• Assuming Docker agents backup Jenkins configs

2. Which of the following is the correct syntax to specify a Docker agent with image python:3.12 in a Jenkins pipeline?

easy

A. agent { docker { image 'python:3.12' } }

B. agent { docker.image('python:3.12') }

C. agent { docker('python:3.12') }

D. agent { docker { image: 'python:3.12' } }

Solution

1. Step 1: Recall Jenkins pipeline Docker agent syntax

   The correct syntax uses agent { docker { image 'image_name' } } format.

2. Step 2: Match the syntax with options

   agent { docker { image 'python:3.12' } } matches the correct syntax exactly with single quotes and no colon.

3. Final Answer:

   agent { docker { image 'python:3.12' } } -> Option A

4. Quick Check:

   Correct Docker agent syntax = agent { docker { image 'python:3.12' } } [OK]

Hint: Use agent { docker { image 'name' } } syntax [OK]

Common Mistakes:

• Using dot notation instead of block syntax
• Adding colon inside image declaration
• Using parentheses incorrectly

3. Given this Jenkins pipeline snippet:

pipeline {
  agent {
    docker {
      image 'node:18'
      args '-p 3000:3000'
    }
  }
  stages {
    stage('Run') {
      steps {
        sh 'node -v'
      }
    }
  }
}

What will be the output of the sh 'node -v' command?

medium

A. No output, command skipped

B. Error: node command not found

C. v12.x.x (Node.js version 12 output)

D. v18.x.x (Node.js version 18 output)

Solution

1. Step 1: Identify Docker image used

   The pipeline uses the Docker image node:18, which includes Node.js version 18.

2. Step 2: Understand command execution inside container

   The sh 'node -v' runs inside the container, so it will output Node.js version 18.

3. Final Answer:

   v18.x.x (Node.js version 18 output) -> Option D

4. Quick Check:

   Docker image node:18 runs Node.js v18 [OK]

Hint: Command runs inside specified Docker image environment [OK]

Common Mistakes:

• Assuming host Node.js version runs instead
• Expecting error due to missing node
• Confusing image tag with different Node version

4. You wrote this Jenkins pipeline snippet to use a Docker agent:

pipeline {
  agent {
    docker {
      image 'ubuntu:latest'
      args '-v /var/run/docker.sock:/var/run/docker.sock'
    }
  }
  stages {
    stage('Test') {
      steps {
        sh 'docker ps'
      }
    }
  }
}

But the build fails with docker: command not found. What is the likely cause?

medium

A. The Jenkins agent cannot mount volumes.

B. The ubuntu:latest image does not have Docker installed.

C. The args option is incorrect syntax.

D. The docker ps command is invalid inside containers.

Solution

1. Step 1: Check the Docker image capabilities

   The ubuntu:latest image is a minimal OS image and does not include Docker by default.

2. Step 2: Understand why command fails

   Since Docker is not installed inside the container, docker ps command is not found, causing the error.

3. Final Answer:

   The ubuntu:latest image does not have Docker installed. -> Option B

4. Quick Check:

   Missing Docker in image = command not found error [OK]

Hint: Check if Docker is installed inside the container image [OK]

Common Mistakes:

• Blaming volume mount syntax
• Assuming docker commands always work in containers
• Thinking docker ps is invalid inside containers

5. You want to run a Jenkins job inside a Docker container with a custom environment variable MY_VAR=hello and mount the workspace directory to /app inside the container. Which Jenkins pipeline snippet correctly sets this up?

hard

A. agent { docker { image 'python:3.12' args '-e MY_VAR=hello -v /app:$WORKSPACE' } }

B. agent { docker { image 'python:3.12' args '-v $WORKSPACE:/app -e MY_VAR=hello' } }

C. agent { docker { image 'python:3.12' args '-e MY_VAR=hello -v $WORKSPACE:/app' } }

D. agent { docker { image 'python:3.12' args '-v /app -e MY_VAR=hello' } }

Solution

1. Step 1: Understand Docker args for environment and volume

   To set environment variables, use -e VAR=value. To mount volumes, use -v host_path:container_path.

2. Step 2: Match correct volume and env var syntax

   agent { docker { image 'python:3.12' args '-e MY_VAR=hello -v $WORKSPACE:/app' } } correctly uses -e MY_VAR=hello and mounts $WORKSPACE to /app inside container.

3. Final Answer:

   agent { docker { image 'python:3.12' args '-e MY_VAR=hello -v $WORKSPACE:/app' } } -> Option C

4. Quick Check:

   Env var and volume mount syntax = agent { docker { image 'python:3.12' args '-e MY_VAR=hello -v $WORKSPACE:/app' } } [OK]

Hint: Use '-e VAR=value' and '-v host:container' in args [OK]

Common Mistakes:

• Reversing volume mount paths
• Omitting environment variable flag
• Incorrect order or missing colon in volume