Inspecting Containers

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LEVEL 0

The Problem

You need to know details about a running container:

What’s its IP address?
What ports are mapped?
What environment variables are set?
When was it created?
What image is it using?

You can’t just look at the container — you need to query this metadata.

LEVEL 1

The Concept — The Container's ID Card

The Concept

Every container has an “ID card” with complete information about itself. This isn’t information the container shows you — it’s information Docker keeps about the container.

docker inspect lets you read this ID card. It’s like asking “tell me everything you know about this container.”

LEVEL 2

The Mechanics — Inspecting Containers

The Mechanics

Full Inspection

docker inspect mycontainer

Returns a JSON document with everything: state, configuration, network settings, mounts, and more.

Getting Specific Values

The --format flag (Go templates) extracts specific values:

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

# Get mapped ports
docker inspect --format '{{.NetworkSettings.Ports}}' mycontainer

# Get environment variables
docker inspect --format '{{.Config.Env}}' mycontainer

# Get container state
docker inspect --format '{{.State.Status}}' mycontainer

# Get start time
docker inspect --format '{{.State.StartedAt}}' mycontainer

LEVEL 3

Key Information in Inspect Output

State Information

"State": {
    "Status": "running",
    "Running": true,
    "Paused": false,
    "StartedAt": "2024-01-15T10:00:00.000000000Z",
    "FinishedAt": "0001-01-01T00:00:00Z",
    "ExitCode": 0
}

Network Information

"NetworkSettings": {
    "IPAddress": "172.17.0.2",
    "Ports": {
        "80/tcp": [{ "HostIp": "0.0.0.0", "HostPort": "8080" }]
    },
    "Networks": { ... }
}

Mount Information

"Mounts": [
    {
        "Type": "volume",
        "Source": "/var/lib/docker/volumes/mydata/_data",
        "Destination": "/data"
    }
]

Configuration

"Config": {
    "Image": "nginx:latest",
    "Env": ["PATH=...", "NGINX_VERSION=1.24.0"],
    "Cmd": ["nginx", "-g", "daemon off;"],
    "WorkingDir": "/",
    "ExposedPorts": { "80/tcp": {} }
}

LEVEL 4

Practical Use Cases

Debugging Network Issues

# What's the container's IP?
docker inspect --format '{{.NetworkSettings.IPAddress}}' mycontainer

# What network is it on?
docker inspect --format '{{.NetworkSettings.Networks}}' mycontainer

Checking Port Mappings

docker inspect --format '{{range $p, $conf := .NetworkSettings.Ports}}{{$p}} -> {{(index $conf 0).HostPort}}{{"\n"}}{{end}}' mycontainer

Finding the Image Used

docker inspect --format '{{.Config.Image}}' mycontainer