Debugging Running Containers

Essential Debugging Commands

When a container misbehaves, Docker provides several commands to investigate without stopping the container.

Opening a Shell with docker exec

The most common debugging technique is opening a shell inside the running container:

docker exec -it my-app /bin/sh

For Alpine-based images, use /bin/sh since bash is not installed. For Debian/Ubuntu-based images, /bin/bash works.

Run a specific command without an interactive session:

docker exec my-app cat /etc/nginx/nginx.conf
docker exec my-app env
docker exec my-app ls -la /app

Viewing Logs with docker logs

Stream logs in real time with -f (follow):

docker logs -f --tail 200 my-app

Filter by time range for incident investigation:

docker logs --since "2024-01-15T10:00:00" --until "2024-01-15T11:00:00" my-app

Inspecting Container Details

docker inspect returns comprehensive JSON about the container:

# Get IP address
docker inspect --format '{{.NetworkSettings.IPAddress}}' my-app

# Check restart count
docker inspect --format '{{.RestartCount}}' my-app

# View environment variables
docker inspect --format '{{.Config.Env}}' my-app

Monitoring Resource Usage

Watch real-time CPU, memory, and I/O with docker stats:

docker stats --format "table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}\t{{.NetIO}}"