Linux Sysadmin Notes

Docker Operations

2 min read

Docker Operations

Concept

Docker is a platform for building, shipping, and running containers.
Even in Kubernetes environments, understanding Docker (or the underlying runtime) is still valuable for image work, local debugging, and legacy systems.

Why it matters

Common operational tasks:

  • Inspecting running containers
  • Viewing logs
  • Debugging image and layer issues
  • Cleaning up disk space used by images/containers/volumes
  • Understanding resource usage

Mental Model

Image (read-only layers)
    ↓ docker run / create
Container (writable layer + namespaces + cgroups)
    ↓
Process(es) running inside the container

Key Commands

# Running containers
docker ps
docker ps -a                    # include stopped
 
# Logs
docker logs <container>
docker logs -f --tail 100 <container>
 
# Inspect
docker inspect <container>
docker inspect <image>
 
# Execute into a container
docker exec -it <container> /bin/sh
 
# Resource usage
docker stats
 
# Images
docker images
docker pull <image>
docker rmi <image>
 
# Cleanup (careful in production)
docker system df
docker system prune
docker system prune -a --volumes

Common Failure Modes & Symptoms

SymptomFirst checks
Container exits immediatelydocker logs, docker inspect (ExitCode)
Cannot pull imageNetwork, registry auth, image name
Disk full on Docker hostdocker system df, prune, volumes
Port already allocateddocker ps, host port conflicts
Permission / volume issuesMounts, user namespaces, SELinux/AppArmor

Investigation Tips

  • Always look at logs before removing a failed container — evidence disappears when the container is removed.
  • docker system df is the starting point for disk usage problems related to Docker.
  • Prefer docker inspect over guessing configuration.
  • On modern Kubernetes nodes the runtime is usually containerd; Docker commands may not be available or may talk to a different daemon.

Personal Lessons Learned

Graph View

Backlinks