container-debug

# Container Debug

Debug running Docker containers and Compose services. Covers logs, exec, networking, resource inspection, multi-stage builds, health checks, and common failure patterns.

## When to Use

- Container exits immediately or crashes on start
- Application inside container behaves differently than on host
- Containers can't communicate with each other
- Container is using too much memory or CPU
- Multi-stage Docker build produces unexpected results
- Health checks are failing
- Compose services start in wrong order or can't connect

## Container Logs

### View and filter logs

```bash
# Last 100 lines
docker logs --tail 100 my-container

# Follow (stream) logs
docker logs -f my-container

# Follow with timestamps
docker logs -f -t my-container

# Logs since a time
docker logs --since 30m my-container
docker logs --since "2026-02-03T10:00:00" my-container

# Logs between times
docker logs --since 1h --until 30m my-container

# Compose: logs for all services
docker compose logs -f

# Compose: logs for specific service
docker compose logs -f api db

# Redirect logs to file for analysis
docker logs my-container > container.log 2>&1

# Separate stdout and stderr
docker logs my-container > stdout.log 2> stderr.log
```

### Inspect log driver

```bash
# Check what log driver a container uses
docker inspect --format='{{.HostConfig.LogConfig.Type}}' my-container

# If json-file driver, find the actual log file
docker inspect --format='{{.LogPath}}' my-container

# Check log file size
ls -lh $(docker inspect --format='{{.LogPath}}' my-container)
```

## Exec Into Containers

### Interactive shell

```bash
# Bash (most common)
docker exec -it my-container bash

# If bash isn't available (Alpine, distroless)
docker exec -it my-container sh

# As root (even if container runs as non-root user)
docker exec -u root -it my-container bash

# With specific environment variables
docker exec -e DEBUG=1 -it my-container bash

# Run a single command (no interactive shell)
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