Transferring Files to and from Docker Containers

Docker containers offer an isolated environment for applications. However, there are often scenarios where you need to transfer files between the host machine and a running container. This tutorial explores various methods for achieving this, covering both copying files into a container and retrieving files from a container.

Understanding the Approaches

There are several ways to transfer files, each with its own advantages and disadvantages. Here’s an overview:

docker cp command: This is the most straightforward and commonly used method for simple file transfers. It directly copies files between the host and the container.
Volume Mounting: A more persistent and efficient approach, especially for development, involves mounting a directory on your host machine into the container. Changes made in either location are immediately reflected in the other.
docker exec with redirection: This involves executing a command inside the container to redirect the input of a file, effectively copying it.
Using tar with docker exec: A slightly more complex but versatile method that can copy entire directories.

1. Using the `docker cp` Command

The docker cp command is designed for simple file transfers. It works both ways:

Copying from the host to the container:

docker cp <source_path_on_host> <container_id>:<destination_path_in_container>

Example:

docker cp my_local_file.txt my_container:/app/

This command copies my_local_file.txt from the current directory on the host to the /app/ directory inside the container named my_container.

Copying from the container to the host:

docker cp <container_id>:<source_path_in_container> <destination_path_on_host>

Example:

docker cp my_container:/app/data.txt ./backup/

This copies the file /app/data.txt from the container my_container to the ./backup/ directory on the host.

Copying Directories: You can copy entire directories using the docker cp command, but you need to include a trailing slash (/) after the source directory on the host.

docker cp my_local_directory/ my_container:/app/

2. Volume Mounting

Volume mounting is a powerful technique for sharing files between the host and the container. It’s particularly useful for development, as changes made in either location are instantly reflected in the other.

When you run a container, you can specify a volume mount using the -v or --mount option:

docker run -v /path/on/host:/path/in/container <image_name>

Example:

docker run -v /home/user/my_project:/app my_image

This mounts the /home/user/my_project directory on the host to the /app directory inside the container. Now, any files you create or modify in /home/user/my_project on the host will be immediately visible in /app inside the container, and vice versa.

Volume mounting creates a persistent link between the host directory and the container directory.

3. Using `docker exec` with Redirection (Less Common)

While less common, you can use docker exec to redirect the contents of a file into a container:

docker exec -i <container_id> cat > /path/in/container < /path/on/host

Example:

docker exec -i my_container cat > /app/new_file.txt < my_local_file.txt

This effectively copies the contents of my_local_file.txt from the host to the /app/new_file.txt file inside the container my_container. The -i flag keeps STDIN open even if not attached.

Choosing the Right Method

For simple, one-time file transfers, docker cp is the easiest option.
For ongoing file sharing and development, volume mounting is the most efficient and convenient.
docker exec with redirection is a less common, but sometimes useful, alternative for specific scenarios.