How to use the command 'docker tag' (with examples)

The docker tag command in Docker allows users to assign a human-readable identifier (known as a “tag”) to a Docker image. This is particularly useful for versioning your images, organizing them, or simply renaming them for better identification. Tags are critical in workflows that involve multiple image versions or when preparing images for distinct environments. They help in maintaining clarity and accessibility in the image management process.

Use case 1: Assign a name and tag to a specific image ID

Code:

docker tag bc3f7c23a6b3 myapp:1.0

Motivation:

Assigning a name and tag to an image ID is fundamental when managing Docker images because image IDs are long hashes which are cumbersome to use and remember. By assigning a descriptive name and a version tag, it is easier to identify the image, version it, and ensure that the correct image is being used in your Docker operations or when sharing with others.

Explanation:

• bc3f7c23a6b3: This is the image ID, a unique identifier for the image stored in the Docker engine. Since IDs are not human-friendly, tagging helps in making them identifiable.
• myapp: This is the chosen name for the image, representing the application packaged in it. Using a familiar name can help quickly understand what the image contains.
• 1.0: This denotes the version of the image. By tagging this way, you can keep track of different versions or iterations of the same application more effectively.

Example output:

There will be no direct output in the shell. However, you can verify the success of the operation by running docker images, which will list an entry for myapp:1.0 associated with the image ID bc3f7c23a6b3.

Use case 2: Assign a tag to a specific image

Code:

docker tag myapp:latest myapp:stable

Motivation:

Sometimes there is a need to maintain multiple tags for the same image to denote different stages of the deployment and operational cycle, like latest, stable, development, etc. This use case is crucial for identifying images according to their intended use or the phase in the production pipeline. For example, an image tagged as latest might be the newest build but untested, whereas stable might be the last version that was confirmed to be bug-free.

Explanation:

• myapp:latest: This is the source image name along with its existing tag. The tag latest typically refers to the most recent build of this image that’s available.
• myapp:stable: This is the new tag being applied to the same source image. By naming it stable, you effectively communicate that this image has passed certain quality checks or testing stages and is suitable for production use.

Example output:

No direct shell output will be shown. You can run docker images and observe myapp:stable listed alongside myapp:latest, both pointing to the same IMAGE ID, signifying they are the same image.

Use case 3: Display help

Code:

docker tag

Motivation:

Understanding how a command works and learning about its options is critical for effective usage. Displaying help information provides a quick overview of the command usage, including potential flags and syntax that might enhance image tagging operations. It is especially helpful for newcomers or when trying to remember less frequently used options.

Explanation:

• This input by itself shows the usage of the docker tag command and provides explanations for its syntax. It does not involve additional arguments or options but offers guidance on structuring command inputs properly.

Example output:

The command output will display the general syntax:

Usage: docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]

This usage information helps clarify how the docker tag command is structured, reiterating that the command involves specifying images with optional tags.

Conclusion:

The docker tag command is a simple yet powerful tool within the Docker suite, facilitating better image management. With this command, users can navigate Docker’s image ecosystem efficiently by naming, tagging, and organizing images based on their unique workflows and project requirements. Whether you’re marking an image’s version, indicating its stability, or simply organizing your Docker repository, the docker tag command proves indispensable in day-to-day operations.