Docker Debug

Docker Debug

You are now operating in Docker container debugging mode.

Container Logs

# View logs for a running container
docker logs myapp

# Follow logs in real time
docker logs -f myapp

# Last N lines
docker logs --tail=100 myapp

# With timestamps
docker logs --timestamps myapp

# Logs since a time (minutes ago)
docker logs --since=10m myapp

# Logs within a time window
docker logs --since="2026-03-10T14:00:00" --until="2026-03-10T15:00:00" myapp

# Filter stderr only
docker logs myapp 2>&1 1>/dev/null | grep ERROR

Execute Commands in Running Containers

# Interactive shell in a running container
docker exec -it myapp sh
docker exec -it myapp bash

# Run a specific command
docker exec myapp ls -la /app
docker exec myapp cat /etc/resolv.conf

# Run as root (even if container uses non-root user)
docker exec -it --user root myapp sh

# Check environment variables
docker exec myapp env

# Check running processes
docker exec myapp ps aux

Inspect Container Details

# Full container metadata (JSON)
docker inspect myapp

# Extract specific fields with format
docker inspect --format='{{.State.Status}}' myapp
docker inspect --format='{{.NetworkSettings.IPAddress}}' myapp
docker inspect --format='{{json .Config.Env}}' myapp | jq .
docker inspect --format='{{json .Mounts}}' myapp | jq .

# Show exposed ports and bindings
docker inspect --format='{{json .NetworkSettings.Ports}}' myapp | jq .

# Show restart policy
docker inspect --format='{{.HostConfig.RestartPolicy.Name}}' myapp

# Show resource limits
docker inspect --format='{{.HostConfig.Memory}}' myapp
docker inspect --format='{{.HostConfig.NanoCPUs}}' myapp

Resource Monitoring (Stats)

# Live resource usage (CPU, memory, network, disk)
docker stats myapp

# One-shot (no live update)
docker stats --no-stream myapp

# All containers, formatted
docker stats --no-stream --format "table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}\t{{.NetIO}}"

# Check disk usage by container
docker system df -v

Network Troubleshooting

# Show container network info
docker network inspect bridge

# List networks and which containers are on them
docker network ls
docker network inspect <network-name>

# Test DNS resolution from inside a container
docker exec myapp nslookup postgres
docker exec myapp ping -c 3 postgres

# Test connectivity to another container
docker exec myapp curl -s http://redis:6379 || echo "no HTTP on redis port"
docker exec myapp nc -zv postgres 5432 && echo "port open" || echo "port closed"

# Check if a container can reach the internet
docker exec myapp curl -s https://httpbin.org/ip

Image Debugging

# Examine image layers
docker history myapp:latest
docker history --no-trunc myapp:latest

# Run a shell in a stopped or crashing image
docker run --rm -it --entrypoint sh myapp:latest

# Override entrypoint for debugging
docker run --rm -it --entrypoint bash myapp:latest

# Inspect what changed in a container vs its base image
docker diff myapp

Common Crash Patterns

# Container keeps restarting? Get exit code
docker inspect myapp --format='{{.State.ExitCode}}'
# Exit 1 = application error
# Exit 137 = OOM kill (out of memory)
# Exit 139 = segfault

# Check restart count
docker inspect myapp --format='{{.RestartCount}}'

# View last log lines after a crash
docker logs --tail=50 myapp

# Run with more verbose logging
docker run --rm -e LOG_LEVEL=debug myapp:latest

Filesystem Debugging

# Copy a file from a container for inspection
docker cp myapp:/app/config.yaml ./config-from-container.yaml

# Copy a file into a container
docker cp ./fix.yaml myapp:/app/config.yaml

# Check disk usage inside a container
docker exec myapp df -h
docker exec myapp du -sh /app/*

Container Events

# Monitor Docker events (start, stop, die, OOM)
docker events --filter container=myapp

# Events in the last 10 minutes
docker events --since=10m --filter container=myapp

Cleanup After Debugging

# Remove all stopped containers
docker container prune

# Remove dangling images (untagged)
docker image prune

# Full system cleanup (removes stopped containers, unused images, unused networks)
docker system prune

# Aggressive cleanup including volumes (DESTROYS DATA)
docker system prune -a --volumes

Interpreting Common Errors