📋 Logging & Debugging

Container Log Basics

# View container logs
docker logs myapp

# Follow logs in real time (like tail -f)
docker logs -f myapp

# Show only the last 100 lines
docker logs --tail 100 myapp

# Show only logs from the last hour
docker logs --since 1h myapp

# With timestamps
docker logs -t myapp

# Combined: follow + timestamps + last 50 lines
docker logs -f -t --tail 50 myapp

# Where are logs stored?
docker inspect --format='{{.LogPath}}' myapp
/var/lib/docker/containers/abc123.../abc123...-json.log

Log Driver Configuration

Docker uses the json-file driver by default, storing logs in JSON format on the host disk. Without log rotation configured, logs will grow indefinitely and eventually fill up the disk!

# Global log rotation configuration (/etc/docker/daemon.json)
cat > /etc/docker/daemon.json << 'EOF'
{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}
EOF
systemctl restart docker

# Per-container configuration
docker run -d --log-opt max-size=10m --log-opt max-file=3 myapp

# Common log drivers
# json-file  — default, JSON format, supports docker logs
# syslog     — sends to syslog, suitable for existing centralized logging
# journald   — sends to systemd journal
# fluentd    — sends to Fluentd
# none       — disables logging (not recommended)

# Clean up logs from stopped containers (free disk space)
docker system prune -f

# Check Docker disk usage
docker system df
TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          12        5         2.8GB     1.5GB (53%)
Containers      8         5         120MB     35MB (29%)
Local Volumes   15        8         1.2GB     400MB (33%)
Build Cache     0         0         0B        0B

Debugging Tips

# Container exits on startup? Check the exit code
docker inspect --format='{{.State.ExitCode}}' myapp
1
# Exit code meanings: 0=normal, 1=app error, 137=OOM/killed, 139=segfault, 143=SIGTERM

# Enter a crashing container for debugging
docker run -it --entrypoint /bin/sh myapp

# View a container's environment variables
docker inspect --format='{{range .Config.Env}}{{println .}}{{end}}' myapp

# View a container's mount points
docker inspect --format='{{json .Mounts}}' myapp | python3 -m json.tool

# View container networking
docker inspect --format='{{json .NetworkSettings.Networks}}' myapp | python3 -m json.tool

# Copy files out of a container
docker cp myapp:/app/config.yml ./config.yml

# Real-time resource monitoring for all containers
docker stats --no-stream
CONTAINER ID   NAME     CPU %   MEM USAGE / LIMIT    MEM %   NET I/O         BLOCK I/O
a1b2c3d4e5f6   nginx    0.50%   12.3MiB / 512MiB     2.40%   1.2MB / 800kB   0B / 0B
f6e5d4c3b2a1   mysql    45.20%  1.2GiB / 2GiB        58.59%  5.6MB / 3.2MB   12MB / 8MB