Docker ADD vs. COPY: What are the Differences?

Introduction

When creating a Dockerfile, there are two commands that you can use to copy files/directories into it – ADD and COPY. Although there are slight differences in the scope of their function, they essentially perform the same task.

So, why do we have two commands, and how do we know when to use one or the other?

In this article, we explain each command, analyze Docker ADD vs COPY, and tell you which one to use.

Docker ADD Command

Let’s start by noting that the ADD command is older than COPY. Since the launch of the Docker platform, the ADD instruction has been part of its list of commands.

The command copies files/directories to a file system of the specified container.

The basic syntax for the ADD command is:

ADD <src> … <dest>

It includes the source you want to copy (<src>) followed by the destination where you want to store it (<dest>). If the source is a directory, ADD copies everything inside of it (including file system metadata).

For instance, if the file is locally available and you want to add it to the directory of an image, you type:

ADD /source/file/path  /destination/path

ADD can also copy files from a URL. It can download an external file and copy it to the wanted destination. For example:

ADD http://source.file/url  /destination/path

An additional feature is that it copies compressed files, automatically extracting the content in the given destination. This feature only applies to locally stored compressed files/directories.

Type in the source and where you want the command to extract the content as follows:

ADD source.file.tar.gz /temp

Bear in mind that you cannot download and extract a compressed file/directory from a URL. The command does not unpack external packages when copying them to the local filesystem.

Docker Copy Command

Due to some functionality issues, Docker had to introduce an additional command for duplicating content – COPY.

Unlike its closely related ADD command, COPY only has only one assigned function. Its role is to duplicate files/directories in a specified location in their existing format. This means that it doesn’t deal with extracting a compressed file, but rather copies it as-is.

The instruction can be used only for locally stored files. Therefore, you cannot use it with URLs to copy external files to your container.

To use the COPY instruction, follow the basic command format:

COPY <src> … <dest> 

For example:

COPY /source/file/path  /destination/path 

Docker Copy vs ADD

Why was there a need to add a new, similar command?

The fact that ADD had so many functionalities proved to be problematic in practice, as it behaved extremely unpredictable. The result of such unreliable performance often came down to copying when you wanted to extract and extracting when you wanted to copy.

Docker couldn’t completely replace the command due to its many existing usages. To avoid backward compatibility, the safest option was to add the COPY command – a less diverse yet more reliable command.

Which to Use (Best Practices)

Considering the circumstances in which the COPY command was introduced, it is evident that keeping ADD was a matter of necessity. Docker released an official document outlining best practices for writing Dockerfiles, which explicitly advises against using the ADD command.

Docker’s official documentation notes that COPY should always be the go-to instruction as it is more transparent than ADD.

If you need to copy from the local build context into a container, stick to using COPY.

The Docker team also strongly discourages using ADD to download and copy a package from a URL. Instead, it’s safer and more efficient to use wget or curl within a RUN command. By doing so, you avoid creating an additional image layer and save space.

Let’s say you want to download a compressed package from a URL, extract the content, and clean up the archive.

Instead of using ADD and running the following command:

ADD http://source.file/package.file.tar.gz /temp
RUN tar -xjf /temp/package.file.tar.gz /
  && make -C /tmp/package.file /
  && rm /tmp/ package.file.tar.gz

You should use:

RUN curl http://source.file/package.file.tar.gz /
  | tar -xjC /tmp/ package.file.tar.gz /
  && make -C /tmp/ package.file.tar.gz

Conclusion

To sum up – use COPY. As Docker itself suggests, avoid the ADD command unless you need to extract a local tar file.

To learn more about creating Dockerfiles check out this article on How to Create Docker Images With Dockerfile.