Inspecting container details in Docker - Commands & Configuration

Introduction

Sometimes you need to see what is happening inside a running container or check its settings. Inspecting container details helps you understand its status, configuration, and resource usage.

When you want to check the IP address or network settings of a running container

When you need to see environment variables or volumes attached to a container

When you want to verify the container's current state and resource limits

When debugging why a container is not behaving as expected

When you want to see logs or the command used to start the container

Commands

This command lists all running containers so you can find the container ID or name to inspect.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED         STATUS         PORTS                    NAMES

b1f3c2a4d5e6   nginx:1.23.3   "/docker-entrypoint.…"   2 minutes ago   Up 2 minutes   0.0.0.0:8080->80/tcp     example-nginx

→

-a - Show all containers, including stopped ones

This command shows detailed JSON information about the container named 'example-nginx', including network, volumes, and environment variables.

Terminal

docker inspect example-nginx

Expected Output

[

  {

    "Id": "b1f3c2a4d5e6f7g8h9i0jklmnopqrstuv",

    "Created": "2024-06-01T12:00:00.000000000Z",

    "Path": "/docker-entrypoint.sh",

    "Args": ["nginx", "-g", "daemon off;"],

    "State": {

      "Status": "running",

      "Running": true,

      "Paused": false,

      "Restarting": false,

      "OOMKilled": false,

      "Dead": false,

      "Pid": 12345,

      "ExitCode": 0,

      "StartedAt": "2024-06-01T12:00:05.000000000Z",

      "FinishedAt": "0001-01-01T00:00:00Z"

    },

    "Image": "sha256:abcdef1234567890",

    "NetworkSettings": {

      "IPAddress": "172.17.0.2",

      "Ports": {

        "80/tcp": [

          {

            "HostIp": "0.0.0.0",

            "HostPort": "8080"

          }

        ]

      }

    },

    "Mounts": []

  }

]

This command shows the logs output by the container, useful for debugging or checking what the container is doing.

Terminal

docker logs example-nginx

Expected Output

172.17.0.1 - - [01/Jun/2024:12:01:00 +0000] "GET / HTTP/1.1" 200 612 "-" "curl/7.68.0"

→

-f - Follow the log output live

Key Concept

If you remember nothing else from this pattern, remember: docker inspect shows all the detailed settings and state of a container in JSON format.

Common Mistakes

Trying to inspect a container using its image name instead of container name or ID

docker inspect requires a container name or ID, not an image name, so it will fail or show wrong info

Use 'docker ps' to find the container name or ID, then run 'docker inspect' with that name or ID

Running 'docker logs' on a container that is not running

If the container is stopped or does not exist, logs command will fail or show no output

Check container status with 'docker ps -a' before running logs

Summary

Use 'docker ps' to list running containers and find the container name or ID.

Use 'docker inspect <container>' to see detailed JSON info about the container's settings and state.

Use 'docker logs <container>' to view the container's output logs for debugging.

Practice

1. What does the command docker inspect CONTAINER do?

easy

A. Shows detailed JSON information about the specified container

B. Starts the specified container

C. Deletes the specified container

D. Lists all running containers

Solution

1. Step 1: Understand the command purpose

   The docker inspect command is used to get detailed information about Docker objects like containers.

2. Step 2: Identify the output type

   It returns detailed JSON data about the container's configuration and state.

3. Final Answer:

   Shows detailed JSON information about the specified container -> Option A

4. Quick Check:

   docker inspect = detailed JSON info [OK]

Hint: Remember: inspect means see detailed info [OK]

Common Mistakes:

• Confusing inspect with start or delete commands
• Expecting a simple list instead of JSON output
• Using inspect without specifying a container

2. Which of the following is the correct syntax to get only the container's IP address using docker inspect?

easy

A. docker inspect --format='{{.NetworkSettings.IPAddress}}' CONTAINER

B. docker inspect --ip CONTAINER

C. docker inspect -ip CONTAINER

D. docker inspect --format='{{IPAddress}}' CONTAINER

Solution

1. Step 1: Recall the --format option usage

   The --format flag uses Go templates to extract specific fields from the JSON output.

2. Step 2: Identify the correct template for IP address

   The IP address is under .NetworkSettings.IPAddress, so the correct syntax is {{.NetworkSettings.IPAddress}}.

3. Final Answer:

   docker inspect --format='{{.NetworkSettings.IPAddress}}' CONTAINER -> Option A

4. Quick Check:

   Use --format with correct JSON path [OK]

Hint: Use --format with full JSON path in double braces [OK]

Common Mistakes:

• Using incorrect flags like --ip or -ip
• Missing the dot before NetworkSettings in template
• Not quoting the template string properly

3. Given a container named webapp, what will the command docker inspect --format='{{.State.Status}}' webapp output if the container is running?

medium

A. created

B. paused

C. exited

D. running

Solution

1. Step 1: Understand the .State.Status field

   The .State.Status field shows the current status of the container, such as running, exited, paused, or created.

2. Step 2: Match the container state

   If the container is running, the output will be the string running.

3. Final Answer:

   running -> Option D

4. Quick Check:

   .State.Status = running when container runs [OK]

Hint: Check .State.Status for container current state [OK]

Common Mistakes:

• Confusing status with container name or ID
• Expecting JSON output instead of plain string
• Using wrong field like .Status instead of .State.Status

4. You run docker inspect --format='{{.Config.Env}}' mycontainer but get an error. What is the most likely cause?

medium

A. The container name is misspelled

B. The format string syntax is incorrect

C. The .Config.Env field does not exist in the container JSON

D. The container is not running

Solution

1. Step 1: Check the format string syntax

   The .Config.Env field is an array, so printing it directly requires proper formatting.

2. Step 2: Identify the syntax error

   Using {{.Config.Env}} tries to print an array directly, which causes a template error. You need to format it, for example with {{range .Config.Env}}{{println .}}{{end}}.

3. Final Answer:

   The format string syntax is incorrect -> Option B

4. Quick Check:

   Arrays need special formatting in --format [OK]

Hint: Arrays need loops in --format templates [OK]

Common Mistakes:

• Assuming all fields print directly without formatting
• Ignoring container name correctness
• Thinking container must be running to inspect

5. You want to list all container names and their IP addresses in a simple format. Which command correctly achieves this?

hard

A. docker ps -q | xargs -n1 docker inspect --format='{{.Name}} {{.NetworkSettings.IPAddress}}'

B. docker ps --format='{{.Name}} {{.NetworkSettings.IPAddress}}'

C. docker inspect --format='{{.Name}} {{.NetworkSettings.IPAddress}}' $(docker ps -q)

D. docker inspect $(docker ps) --format='{{.Name}} {{.NetworkSettings.IPAddress}}'

Solution

1. Step 1: Understand how to get container IDs

   docker ps -q lists container IDs only, which can be passed to docker inspect.

2. Step 2: Use inspect with multiple container IDs and format

   Passing IDs as arguments to docker inspect with --format outputs info for each container. docker inspect --format='{{.Name}} {{.NetworkSettings.IPAddress}}' $(docker ps -q) uses this correctly.

3. Step 3: Check other options for errors

   docker ps -q | xargs -n1 docker inspect --format='{{.Name}} {{.NetworkSettings.IPAddress}}' uses xargs but may break if container names have spaces. docker ps --format='{{.Name}} {{.NetworkSettings.IPAddress}}' is invalid because docker ps does not support .NetworkSettings.IPAddress. docker inspect $(docker ps) --format='{{.Name}} {{.NetworkSettings.IPAddress}}' uses wrong syntax for docker ps.

4. Final Answer:

   docker inspect --format='{{.Name}} {{.NetworkSettings.IPAddress}}' $(docker ps -q) -> Option C

5. Quick Check:

   Use docker inspect with IDs from docker ps -q [OK]

Hint: Use docker inspect with IDs from docker ps -q [OK]

Common Mistakes:

• Trying to get IP from docker ps directly
• Using wrong command substitution syntax
• Not passing container IDs correctly to inspect