The Ultimate Docker Commands List for DevOps Pros

Here is a guide on how to troubleshoot Docker issues-

1. Container Won’t Start

• Error: The container fails to start or immediately exits.
• Solution:
  • Check logs:
  • docker logs [container_id]
  • Inspect configuration: Check the container configuration for errors, such as incorrect environment variables or missing dependencies.
  • Try interactive mode:
  • docker run -it [image_name] /bin/bash
  • This allows you to interact with the container and diagnose issues directly.

2. “Image Not Found” Error

• Error: Docker cannot find the image locally or in the Docker registry.
• Solution:
  • Check image name and tag: Ensure that you are using the correct image name and version tag.
  • Pull the image again:
  • docker pull [image_name]
  • Check Docker Hub availability: Sometimes, Docker Hub might be down, or the image is no longer available.

3. Container Exits Immediately

• Error: The container starts and immediately exits.
• Solution:
  • Check the command: Ensure the entrypoint command in the Dockerfile is correct. The container may exit if the command finishes too quickly.
  • Check logs:
  • docker logs [container_id]
  • Use interactive mode:
  • docker run -it [image_name] /bin/bash

4. Network Connectivity Issues

• Error: Containers can’t communicate with each other or external services.
• Solution:
  • Check container IPs:
  • docker inspect [container_id] | grep IPAddress
  • Inspect Docker networks:
  • docker network inspect [network_name]
  • Create a custom network if communication between containers is required:
  • docker network create [network_name]
  • Connect containers to the custom network:
  • docker network connect [network_name] [container_name]

5. Disk Space Issues

• Error: Docker images, containers, or volumes are taking up too much disk space.
• Solution:
  • Prune unused images and containers:
  • docker system prune
  • Remove unused volumes:
  • docker volume prune
  • Remove stopped containers:
  • docker container prune

6. Permission Denied Errors

• Error: You encounter “permission denied” when running Docker commands.
• Solution:
  • Run with sudo:
  • sudo docker [command]
  • Add your user to the Docker group:
  • sudo usermod -aG docker [your_user]
  • Then, log out and back in for the changes to take effect.

7. High Resource Usage

• Error: Containers are consuming too much CPU, memory, or disk space.
• Solution:
  • Monitor resource usage:
  • docker stats
  • This command shows real-time resource consumption.
  • Limit resources:
  • Use flags like –memory and –cpus to limit resource usage:
  • docker run –memory=”512m” –cpus=”1.0″ [image_name]

8. Port Conflicts

• Error: The container cannot start due to a port conflict.
• Solution:
  • List running containers to identify the one using the port:
  • docker ps
  • Change the port mapping: Use the -p flag to map the container port to an available host port:
  • docker run -p [host_port]:[container_port] [image_name]

9. Image Build Failures

• Error: Errors occur while building a Docker image.
• Solution:
  • Check the Dockerfile: Ensure there are no syntax errors or incorrect instructions.
  • Examine the build context: Large contexts can slow down the build. Limit the context by using a .dockerignore file.
  • Use build logs:
  • docker build -t [image_name] .
  • Review the output for specific error messages.

10. DNS Resolution Failures

• Error: Container can’t resolve domain names.
• Solution:
  • Check Docker’s DNS configuration:
  • docker network inspect bridge
  • If the DNS server is missing or incorrect, you can specify a custom DNS server using:
  • docker run –dns=[dns_server_ip] [image_name]