What is ADD in Dockerfile – Practical Examples

The ADD instruction in Dockerfile allows you to copy files or directories from a source on your local system to a destination in the Docker image. But there’s more. Unlike the COPY command, ADD can do a few extra things. It can automatically extract compressed files and even download files from URLs.

In this guide, we will explain what is ADD in Dockerfile with practical examples.

When to Use ADD vs. COPY

Both ADD and COPY serve the basic purpose of copying files from the host machine into the Docker image, but ADD offers more flexibility with extra features like handling URLs and compressed files. However, using ADD unnecessarily can lead to performance issues or unintentional behavior, so it’s crucial to know when to use each.

Use COPY when:

• You just need to copy files or directories from the local filesystem into the container. It’s faster, simpler, and clearer in intent.

Use ADD when:

• You need to download files from a URL.
• You automatically want to extract compressed files during the build process.

Basic Syntax

The syntax of ADD is straightforward.

ADD source destination

Explanation.

• Source: Specifies the source file, directory, or URL. This can either be from your local machine or a remote URL.
• Destination: Defines the destination path inside the Docker container where the content will be placed.

Now that we understand the syntax, let’s look at how to use ADD in practical scenarios.

Example 1: Copying Local Files to a Docker Container

You can use ADD to copy files or directories from your local machine to the Docker container:

ADD ./app /usr/src/app

In this example, the content of the local app directory is copied into the /usr/src/app directory inside the container. This simple use case is similar to how the COPY instruction works.

Example 2: Extracting a Compressed Archive into a Container

ADD also supports automatic extraction of compressed files like .tar.gz into the specified directory:

COPY archive.tar.gz /usr/src/app/

In this example, the archive.tar.gz file is extracted into the /user/src/app/ directory without needing to be manually extracted using a separate command. This is a powerful feature when dealing with large compressed datasets.

Example 3: Downloading a File from a URL

One of the unique features of ADD is the ability to download a file from a URL directly into the Docker image:

ADD https://example.com/file.tar.gz /tmp/

Here, the file from the URL https://example.com/file.tar.gz is downloaded and placed in the /tmp/ directory inside the container. This feature makes it convenient to fetch resources from remote servers during image build time.

Example 4: Copying Multiple Files and Directories

You can use ADD to copy multiple files and directories in one go:

ADD ./config.json ./app ./logs /var/www/

This command copies config.json, app, and logs into the /var/www/ directory inside the container. Using ADD in this way can simplify the Dockerfile by reducing the number of separate instructions.

Example 5: Copying Files with Wildcards

ADD supports wildcards, which is useful for copying multiple files with a specific pattern:

ADD ./static/* /usr/share/nginx/html/

This example copies all the files from the static folder into the /usr/share/nginx/html/ directory in the container. This feature is especially useful when you want to copy a specific subset of files into the container.

Conclusion

The ADD instruction in Dockerfile is a versatile tool. It allows you to copy, extract, and even download files directly into your Docker image. However, with great power comes great responsibility. Always consider whether you need ADD’s extra features or if COPY would be a better choice.

Experiment with ADD in your Dockerfiles and see how it can streamline your build process. Happy Dockerizing!

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