Docker Debugging

# Docker Debugging Commands Reference
# Version: 1.0.0
# Last Updated: 2025-12-29

container_inspection:
  # View container details
  inspect: "docker inspect <container_id>"
  # View running processes
  top: "docker top <container_id>"
  # View resource usage
  stats: "docker stats <container_id> --no-stream"
  # View port mappings
  port: "docker port <container_id>"

log_analysis:
  # View all logs
  all_logs: "docker logs <container_id>"
  # Follow logs in real-time
  follow: "docker logs -f <container_id>"
  # Last N lines
  tail: "docker logs --tail 100 <container_id>"
  # With timestamps
  timestamps: "docker logs -t <container_id>"
  # Since specific time
  since: "docker logs --since 30m <container_id>"

interactive_debugging:
  # Start bash in running container
  exec_bash: "docker exec -it <container_id> /bin/bash"
  # Start sh (if bash not available)
  exec_sh: "docker exec -it <container_id> /bin/sh"
  # Run specific command
  exec_cmd: "docker exec <container_id> <command>"
  # As root user
  exec_root: "docker exec -u root -it <container_id> /bin/bash"

network_debugging:
  # View container networks
  network_inspect: "docker network inspect <network_name>"
  # Test DNS resolution
  dns_test: "docker exec <container_id> nslookup <hostname>"
  # Test connectivity
  ping_test: "docker exec <container_id> ping -c 3 <target>"
  # Check open ports
  netstat: "docker exec <container_id> netstat -tlnp"

resource_analysis:
  # Disk usage
  system_df: "docker system df -v"
  # Container size
  container_size: "docker ps -s"
  # Image layers
  image_history: "docker history <image>"

health_checks:
  # View health status
  health_status: "docker inspect --format='{{.State.Health.Status}}' <container_id>"
  # View health logs
  health_log: "docker inspect --format='{{range .State.Health.Log}}{{.Output}}{{end}}' <container_id>"

common_issues:
  container_not_starting:
    - "Check logs: docker logs <container_id>"
    - "Check exit code: docker inspect -f '{{.State.ExitCode}}' <container_id>"
    - "Review entrypoint/cmd in Dockerfile"

  high_memory_usage:
    - "Check limits: docker stats"
    - "Add memory limit: --memory=512m"
    - "Review application memory leaks"

  network_issues:
    - "Check network mode: docker inspect -f '{{.NetworkSettings.Networks}}' <container_id>"
    - "Verify DNS: docker exec <container_id> cat /etc/resolv.conf"
    - "Test inter-container: docker exec <container_id> ping <other_container>"


# Docker Debugging Guide

## Quick Reference

### Container Won't Start

1. **Check exit code:**
   ```bash
   docker inspect -f '{{.State.ExitCode}}' <container>
   ```
   - Exit 0: Normal termination
   - Exit 1: Application error
   - Exit 137: OOM killed (out of memory)
   - Exit 139: Segmentation fault
   - Exit 143: SIGTERM received

2. **View logs:**
   ```bash
   docker logs <container>
   ```

3. **Debug entrypoint:**
   ```bash
   docker run -it --entrypoint /bin/sh <image>
   ```

### High Memory Usage

1. **Check current usage:**
   ```bash
   docker stats <container>
   ```

2. **Set memory limits:**
   ```bash
   docker run -m 512m --memory-swap 1g <image>
   ```

3. **Find memory hogs:**
   ```bash
   docker exec <container> ps aux --sort=-%mem | head
   ```

### Network Issues

1. **Container can't reach internet:**
   ```bash
   # Check DNS
   docker exec <container> cat /etc/resolv.conf

   # Test connectivity
   docker exec <container> ping -c 3 8.8.8.8
   ```

2. **Containers can't communicate:**
   ```bash
   # Ensure same network
   docker network inspect <network>

   # Use container name as hostname
   docker exec <container1> ping <container2_name>
   ```

### Disk Space Issues

1. **Check Docker disk usage:**
   ```bash
   docker system df -v
   ```

2. **Cleanup unused resources:**
   ```bash
   # Remove unused containers, networks, images
   docker system prune -a

   # Remove unused volumes
   docker volume prune
   ```

## Best Practices

1. **Always use health checks**