JIYIK CN >

A Dockerfile is a text document that contains all the commands used to build a Docker image. Recently, we have seen Docker being widely used as the default tool for managing configurations and automating deployments.

Advanced features such as automated builds using Dockerfiles are reasons developers are adopting Docker. Others include isolating applications from the environment they run in and quickly ensuring security.

When creating a Docker container, you often need to transfer files from the host machine to the Docker image. These files can include libraries or property files that your application needs to run.

Difference between COPY and ADD commands in Dockerfile

In a Dockerfile, we can copy these files using the COPY or ADD commands. These commands are functionally identical; however, there are some differences.

The COPY and ADD commands follow the following syntax.

COPY <src> <dest>
ADD <src> <dest>

Both instructions copy <src>files or directories located in the local host and add them to a location in the container's file system <dest>. For example, in the following Dockerfile, we copy files from the current directory to the /var/www directory in the Docker image.

#  base image
FROM python

# Set your working directory
WORKDIR /var/www/
# Copy the necessary files
COPY ./app.py /var/www/app.py
ADD./requirements.txt /var/www/requirements.txt

# Install the necessary packages
RUN pip install -r /var/www/requirements.txt
ADD mkdir -p /var/www/new_directory
# Run the app
CMD ["echo", "Hello, Developer"]

We want to copy the app.py and requirements.txt files. Now, if we build this Docker image and use it to create a Docker container, we will definitely be able to find these two files in the file system of the Docker container.

The following example will build a Docker image based on the above Dockerfile.

~/my-app$ Docker build -t new-image .

Now that we have the image, we will docker runcreate a Docker container using the command. In addition to this, we will also start bash inside the container.

Code:

~/my-app$ Docker run -it new-image bash

If we list the files in the /var/www directory, we should see both app.py and requirements.txt files.

Code:

root@841d1e8d8c25:/var/www# ls
app.py  new_directory  requirements.txt

In the Docker container above, both the ADDand COPYcommands allow us to copy files from a host directory to a Docker directory.

However, when copying files into a Docker container, we recommend using COPYthe command.

COPYAccording to docker-file best practices, commands are more suitable when we don't need more functionality than copying local files .

On the other hand, the ADD command has more features. For example, you can use this command to extract a local tar file into a Docker image.

In addition to this, the ADD command supports remote URLs, two operations that are not possible with the COPY command. The previous command may not be ideal if you are trying to reduce the size of the Docker image you are building.

This is because the ADD command can significantly increase the size of a Docker image, especially when it fetches packages from a remote URL.

Using the ADD command to simply copy files from the host machine can result in files being accidentally copied into the Docker image's file system.

In conclusion, while the two commands have similarities and can be used interchangeably, you should stick with COPYthe command. ADDThe command, on the other hand, should be used only when required and with extreme caution.