What Are the Key Differences Between the 'COPY' and 'ADD' Commands in a Dockerfile?

Understanding the Key Differences Between the COPY and ADD Commands in a Dockerfile

We need to understand the key differences between the ‘COPY’ and ‘ADD’ commands in a Dockerfile. This is important for creating Docker images well. The ‘COPY’ command is simple. It moves files and folders from the host to the Docker image. On the other hand, ‘ADD’ has more features. It can work with remote URLs and also unpack compressed files. These differences can change how we build Docker images. So, it is important for developers to pick the right command for their needs.

In this article, we will look closely at the differences between ‘COPY’ and ‘ADD’ commands in a Dockerfile. We will explain what each command does. We will see how ‘COPY’ handles file context. We will also look at when to use ‘ADD’ and how both commands affect performance. Plus, we will answer common questions to help us understand how to make our Dockerfile better.

  • What Are the Key Differences Between the COPY and ADD Commands in a Dockerfile
  • Understanding the COPY Command in a Dockerfile
  • Exploring the ADD Command in a Dockerfile
  • How Does COPY Handle File Context in a Dockerfile
  • What Are the Use Cases for ADD in a Dockerfile
  • Performance Considerations for COPY and ADD in a Dockerfile
  • Frequently Asked Questions

Understanding the COPY Command in a Dockerfile

We use the COPY command in a Dockerfile to move files and folders from our host system into the Docker image. This command is simple. It lets us set the source and destination paths.

Syntax

COPY <source> <destination>

Example

If we want to copy a local folder called app into the /usr/src/app folder in the image, we write:

COPY app /usr/src/app

Key Points

  • The <source> can be a file or folder path. This path is relative to the build context.
  • The <destination> is where the files go inside the image.
  • We can use wildcards in the source path.
  • The COPY command keeps the ownership and permissions of the files.

Context

The files or folders we mention in the COPY command need to be in the build context. The build context is usually the folder where the Dockerfile is located.

Limitations

  • The COPY command cannot extract tar files. If we need to do that, we should use the ADD command.
  • It also does not support downloading files from remote URLs. The files must be in the build context.

Using the COPY command helps us manage files in Docker images in a clean way. It keeps the original file permissions and attributes. For more details on Dockerfile basics, check out what is a Dockerfile and how do you create one.

Exploring the ADD Command in a Dockerfile

The ADD command in a Dockerfile helps us copy files and folders from a source on our host to the Docker image’s filesystem. It has some special features compared to the COPY command.

Syntax

ADD [source] [destination]

Features of ADD

  • Local and Remote Sources: We can use ADD for both local files and URLs. This means we can download files from the internet while building the image.

    Example:

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

  • Automatic Extraction: If the source is a compressed file like .tar, .tar.gz, or .zip, ADD will automatically unpack the contents to the destination we choose.

    Example:

    ADD archive.tar.gz /app/

Limitations of ADD

  • Less Predictable: The automatic extraction feature might give us unexpected results. This can happen if the file types are not clear.

  • No Cache: Unlike COPY, when we use ADD with a URL, it does not use the Docker cache well. It does not check the content of the URL.

Use Cases for ADD

  • When we need to get files from a URL that we need during the image build.
  • When we want to copy and unpack compressed files directly into the image.

Example

Here’s an example Dockerfile that uses the ADD command:

FROM ubuntu:latest

# Add a local file
ADD ./local-file.txt /app/local-file.txt

# Add a remote file and extract it
ADD https://example.com/archive.tar.gz /app/

In summary, the ADD command gives us extra features compared to COPY. But we should use it carefully because it can be unpredictable and not work well with caching. For most cases, we should use COPY unless we really need what ADD offers. For more details about Dockerfile commands, we can check the differences between COPY and ADD.

How Does COPY Handle File Context in a Dockerfile

In a Dockerfile, the COPY command copies files and folders from the source path on the host to the container’s filesystem at the destination path we choose. It is important to understand how COPY works with file context. This knowledge helps us build Docker images better.

File Context

  • Build Context: The build context is the folder we send to the Docker daemon during the build process. Only files in this context can be used by commands like COPY and ADD.
  • Limitation: The COPY command cannot reach files outside of the build context. For instance, if our build context is /app, and we try to copy a file from /home/user/config.json, it will not work.

Syntax

The basic way to write the COPY command is:

COPY <source_path> <destination_path>

Example

Let’s say our build context is /app. We can use this command to copy a file named example.txt from the context into the container:

COPY example.txt /usr/src/app/

If we want to copy a folder and what is inside, we write:

COPY my_folder/ /usr/src/app/my_folder/

Handling Wildcards

The COPY command can also use wildcards. For example:

COPY *.txt /usr/src/app/

This command copies all .txt files from the build context to the specified folder in the container.

Permissions

The files we copy keep their original permissions. If we need to change permissions, we can do that in a later RUN command.

Best Practices

  • Always write the destination path clearly.
  • Keep the number of files in the build context low. This helps speed up the build.
  • Use a .dockerignore file to leave out files and folders that we do not need in the image.

For more details on Docker commands and how to use them, check out this complete guide on Docker commands.

What Are the Use Cases for ADD in a Dockerfile

The ADD command in a Dockerfile is useful and can be used in many situations. It does more than just copy files. Here are the main uses for ADD:

  • Copying Files and Directories: Just like COPY, we can use ADD to copy files and folders from our computer into the Docker image.

    ADD myfile.txt /app/

  • Extracting Tar Archives: When we add a .tar, .tar.gz, or similar file, ADD will automatically unpack it into the folder we choose.

    ADD myarchive.tar.gz /app/

  • Fetching Remote Files: ADD can get files from a website and put them in the image. This is very helpful for downloading things we need during the build.

    ADD https://example.com/myfile.txt /app/

  • Handling Local Context: If we need to add files that are not directly in the build context but can be accessed through links or archives, we should use ADD.

    ADD https://example.com/data.tar.gz /data/

  • Simplifying Multi-Stage Builds: In complex Dockerfiles, especially with multi-stage builds, ADD can help us get and use files from different places easily.

    FROM base AS builder
ADD source.tar.gz /src/

FROM runtime
COPY --from=builder /src/bin /app/bin

Even if ADD has these extra features, we often suggest to use COPY for simple file copying tasks. This helps us avoid surprises, like automatic unpacking or getting files from the internet. For more about Docker commands, you can check this article.

Performance Considerations for COPY and ADD in a Dockerfile

When we think about the performance of the COPY and ADD commands in a Dockerfile, we see some important factors. Both commands copy files from our host system to the Docker image. But they have different effects on performance.

  1. File Size and Layering:
    • Both COPY and ADD make a new layer in the Docker image. We usually like to use COPY because it is simple and works well, especially for copying files without extra steps.
    • ADD is stronger since it can unpack compressed files. But it can slow down the process if the files are big.
  2. Context Handling:
    • COPY follows the build context strictly. This means it only looks at files in the context directory we choose. This way, we can make the build faster by not moving too much data.
    • ADD can get files from remote URLs. This may slow down our builds because of network delays and possible problems when downloading.
  3. Cache Efficiency:
    • The Docker build cache prefers COPY commands. It can easily cache these based on when the files were changed.
    • With ADD, if the source file changes, it can break the cache. This can happen even if the change does not affect the final image. So, it can take longer to build.
  4. Example Usage:

    • Here is how we use COPY for local files:

      COPY ./localfile.txt /app/localfile.txt

    • Here is how we use ADD for unpacking a tarball:

      ADD ./archive.tar.gz /app/
  5. Best Practices:
    • We should use COPY when we just need to copy files without other features.
    • We should use ADD only when we need its special features like unpacking files or getting files from the internet.

By knowing these performance points, we can choose better between COPY and ADD in our Dockerfile. This helps us make the build process smoother and makes our Docker images work better.

For more tips on Docker commands, we can check this article.

Frequently Asked Questions

1. What is the main difference between the COPY and ADD commands in a Dockerfile?

The main difference between the COPY and ADD commands in a Dockerfile is in what they can do. Both commands move files from our computer to the Docker image. But ADD has extra features. It can extract tar files and get files from the internet. For most cases, we should use COPY because it is simpler and clearer.

2. When should I use ADD instead of COPY in my Dockerfile?

We should use the ADD command in a Dockerfile when we need its special features. This includes when we want to automatically extract compressed files or get files from a URL. But for just copying files, the COPY command is better. It is simple and keeps us from pulling in files from the internet by mistake, which is safer.

3. How does the file context affect the behavior of COPY and ADD commands?

The file context in a Dockerfile decides which files we can use with the COPY and ADD commands. Both commands can only see files in the build context. This context is set when we run the docker build command. If we try to access files outside of this context, we will get an error. This keeps our build environment safe and contained.

4. Are there performance implications when using COPY vs. ADD in Dockerfiles?

Yes, there are performance issues when we use COPY and ADD. The COPY command is usually faster because it just copies files. In contrast, ADD can slow things down because it might extract files or download them from URLs. For the best performance, we should use COPY for simple file transfers unless we need extra features.

5. Can I use the ADD command to copy files from a remote server in a Dockerfile?

Yes, the ADD command lets us copy files from a remote server into our Docker image. But this can cause problems if the remote files change. For stable builds, it is better to download files first and then use the COPY command to add them to our Docker image. This way, we keep the build context controlled.

These questions and answers help us understand the key differences between the COPY and ADD commands in Dockerfiles. This knowledge helps us make better choices when we create Docker images. For more information about Docker basics, we can read articles like What is Docker and Why Should You Use It or What Are Docker Images and How Do They Work.