docker logs
# Print all logs
docker logs my-container
# Follow (tail -f equivalent)
docker logs -f my-container
# Last N lines
docker logs --tail 50 my-container
# With timestamps
docker logs -t my-container
# Since a point in time
docker logs --since 5m my-container
docker logs --since 2024-01-15T10:00:00 my-container
# Combine: follow last 20 lines with timestamps
docker logs -f -t --tail 20 my-container
Logs come from container stdout/stderr. If your app writes to a file instead, you will not see anything here. Always write to stdout in containers.
docker exec
# Open a shell in a running container
docker exec -it my-container sh # Alpine / minimal images
docker exec -it my-container bash # Debian/Ubuntu-based
# Run a one-off command
docker exec my-container ls /app
docker exec my-container cat /etc/os-release
# Common debugging patterns
docker exec -it my-container sh -c "env | grep DATABASE"
docker exec -it my-container sh -c "ps aux"
docker exec -it my-container sh -c "ls -la /app"
docker exec -it my-container sh -c "curl -s localhost:3000/health"
# Check what process is PID 1 (the one that gets SIGTERM)
docker exec my-container cat /proc/1/cmdline
Debugging a crashed container
# Container exited - check the exit code
docker ps -a
# EXITED (1) means the process failed
# EXITED (137) means OOM killed (or SIGKILL)
# Check logs of a stopped container
docker logs my-container
# Inspect the container metadata
docker inspect my-container | jq '.[0].State'
# Run the image with a shell override (skip CMD/ENTRYPOINT)
docker run -it --entrypoint sh my-app
Scratch containers have no shell - docker exec will not work. Build a separate :debug image tag that uses FROM busybox as the final stage instead of FROM scratch.
Try it - debug a running container
# Start a container
docker run -d --name debug-me nginx:alpine
# Explore from inside
docker exec -it debug-me sh
# Inside the container:
# ps aux - what's running
# env - environment variables
# ls /etc/nginx - nginx config location
# cat /etc/nginx/nginx.conf
# exit
# Check resource usage
docker stats debug-me # live CPU/memory
# Clean up
docker rm -f debug-me
Image naming
registry/repository:tag
docker.io/library/nginx:alpine
│ │ │ │
│ │ │ └── tag (version/variant)
│ │ └──────── image name
│ └──────────────── user/org (library = Docker official)
└─────────────────────────── registry host (docker.io = Docker Hub)
# Short form (defaults apply)
nginx → docker.io/library/nginx:latest
nginx:alpine → docker.io/library/nginx:alpine
myuser/myapp → docker.io/myuser/myapp:latest
Tag, push, pull
# Build and tag
docker build -t myapp:1.0.0 .
# Tag with a registry prefix
docker tag myapp:1.0.0 myregistry.example.com/myteam/myapp:1.0.0
# Push to registry (must be logged in)
docker push myregistry.example.com/myteam/myapp:1.0.0
# Pull on another machine
docker pull myregistry.example.com/myteam/myapp:1.0.0
# Login to a registry
docker login myregistry.example.com
# or with credentials
echo $TOKEN | docker login myregistry.example.com -u $USER --password-stdin
Try it - tag and inspect
# Pull an image and inspect its full name
docker pull nginx:alpine
docker inspect nginx:alpine | jq '.[0].RepoDigests'
# Build a tiny image and tag it
cat > /tmp/Dockerfile.hello << 'EOF'
FROM alpine:latest
CMD ["echo", "hello from my-registry"]
EOF
docker build -f /tmp/Dockerfile.hello \
-t my-registry.example.com/demo/hello:1.0.0 .
# Verify the tag appears
docker images | grep my-registry
# Add a second tag (same image, different name)
docker tag my-registry.example.com/demo/hello:1.0.0 \
my-registry.example.com/demo/hello:latest
docker images | grep hello
# Inspect before pushing (dry-run)
docker image inspect my-registry.example.com/demo/hello:1.0.0 \
| jq '.[0] | {Id, RepoTags, Size}'
# Clean up
docker rmi my-registry.example.com/demo/hello:1.0.0 \
my-registry.example.com/demo/hello:latest