Inspecting a Docker Image

Understanding how Docker images are built from multiple layers can help you optimize, debug, and secure your container workflows. In this guide, we’ll explore essential Docker CLI commands to:

• List local images
• Examine image history
• Inspect detailed metadata
• Filter output with JSONPath

Table of Contents

1. Prerequisites
2. Listing Local Images
3. Viewing Image History
4. Inspecting Image Metadata
5. Filtering with JSONPath
6. References

Prerequisites

Listing Local Images

To see all images stored on your host machine, use:

docker image list

Example output:

REPOSITORY    TAG       IMAGE ID       CREATED       SIZE
httpd         latest    c2aa7e16edd8   2 weeks ago   165MB
ubuntu        latest    549b9b86cb8d   4 weeks ago   64.2MB

Viewing Image History

Examining an image’s history reveals each layer and the command that created it. This is especially valuable if the Dockerfile isn’t available.

docker image history ubuntu

Sample output:

IMAGE          CREATED        CREATED BY                                      SIZE
549b9b86cb8d   4 weeks ago    /bin/sh -c #(nop) CMD ["/bin/bash"]             0B
<missing>      4 weeks ago    /bin/sh -c mkdir -p /run/systemd && echo 'do…   7B
<missing>      4 weeks ago    /bin/sh -c set -xe && echo '#!/bin/sh' > ./…    745B
<missing>      4 weeks ago    /bin/sh -c [ -z "$(apt-get indextargets)" ]      987kB
<missing>      4 weeks ago    /bin/sh -c #(nop) ADD file:53f100793e6c0adfc…   63.2MB

Inspecting Image Metadata

The inspect command returns comprehensive image metadata in JSON format, including environment variables, exposed ports, volumes, and more.

docker image inspect ubuntu

Abbreviated example:

[
  {
    "Id": "549b9b86cb8d...",
    "RepoTags": ["ubuntu:latest"],
    "Created": "2020-09-15T23:05:57.348340124Z",
    "ContainerConfig": {
      "ExposedPorts": {
        "80/tcp": {}
      }
    },
    "DockerVersion": "18.09.7",
    "Architecture": "amd64",
    "Os": "linux",
    "Size": 137532780
  }
]

docker image inspect ubuntu > ubuntu-inspect.json

Filtering with JSONPath

Docker’s inspect supports the -f flag with JSONPath templates to extract specific fields.

Common Filters

docker image inspect httpd -f '{{.Os}}'
docker image inspect httpd -f '{{.Architecture}}'
docker image inspect httpd -f '{{.Architecture}} {{.Os}}'
# Output: amd64 linux

Retrieving Exposed Ports

To list all ports exposed by an image:

docker image inspect httpd \
  -f '{{range $p := .ContainerConfig.ExposedPorts}}{{printf "%s " $p}}{{end}}'

Example output:

80/tcp 

References

• Docker CLI Reference
• Docker Inspect
• JSONPath Guide