[SOLVED] Copying files from Docker container to host - docker

[SOLVED] Easy Ways to Copy Files from a Docker Container to Host

In this article, we will look at simple ways to copy files from a Docker container to your host system. This task is normal for developers and system admins. They often need to get data that is created or changed in a container. Knowing how to move files between Docker containers and the host can help us a lot with our work and data handling. We will cover many solutions that fit different situations. This way, we have the right tools for managing Docker files.

Solutions We Will Talk About:

• Solution 1: Use the docker cp command
• Solution 2: Use volume mounts when we run a container
• Solution 3: Use tar command inside the container
• Solution 4: Copy files with a Dockerfile using the COPY command
• Solution 5: Use rsync for fast file transfer
• Solution 6: Get files from a stopped container

If we work with containerized apps or manage Docker images, knowing how to copy files from a Docker container to our host is very important. For more info about how Docker works and what it can do, we can check our articles on Docker architecture and what is Docker. Now, let’s start with the ways to copy files from Docker containers to the host!

Solution 1 - Using docker cp command

We can use the docker cp command to copy files from a running or stopped Docker container to our host machine. This command is easy to use. It works like the Unix cp command. We can specify where to copy from and where to copy to.

Syntax

The basic way to write the docker cp command is:

docker cp <container_id>:<path_in_container> <path_on_host>

Steps to Copy Files

1. Identify the Container: First, we need to find the ID or name of the container. This is the container we want to copy files from. We can list all running containers with:

   docker ps

   To see all containers, even the stopped ones, we can use:

   docker ps -a

2. Copy the File: Now we can copy files. We use the docker cp command for this. For example, if we want to copy a file called example.txt from the /app folder in the container my_container to our current host folder, we can run:

   docker cp my_container:/app/example.txt .

3. Copy Directories: We can also copy whole directories. If we want to copy the /app/data directory from the container to our host, we write:

   docker cp my_container:/app/data ./data

Important Notes

• We should check if we have permission to access the destination path on the host.
• The destination path can be absolute or relative. It depends on where we are currently working.
• This method works well for both running and stopped containers.

Using the docker cp command is one of the easiest and best ways to transfer files from our Docker containers to our host system. For more details about Docker commands, we can check the Docker commands documentation.

Solution 2 - Using volume mounts during container run

We can copy files from a Docker container to the host easily by using volume mounts when we start the container. This way, we share a folder between the host and the container. It makes transferring files simple in both ways.

Step-by-Step Guide to Using Volume Mounts

1. Create a folder on the host: First, we need to create a folder on our host system to store the files from the container. For example:

   mkdir -p /path/to/host/directory

2. Run the Docker container with a volume mount: We use the -v or --mount option to set the volume mount. This links the host folder to a folder in the container. Here’s how we do it:

   docker run -v /path/to/host/directory:/path/in/container -it your-docker-image

   In this command:

   • /path/to/host/directory is the place on our host where we want to copy files.
   • /path/in/container is the place inside the container where we will put files.

3. Copy files from the container to the mounted folder: Once we are inside the container, we can copy files to the mounted folder. For example:

   cp /path/in/container/yourfile.txt /path/in/container/

   This command will copy yourfile.txt from the container to the shared folder.

4. Access the files on the host: After we copy the files, we can easily access them from the host in the folder we made:

   ls /path/to/host/directory

Additional Considerations

• Read-Only Mounts: If we want the files to be read-only in the container, we add :ro to the volume argument:

  docker run -v /path/to/host/directory:/path/in/container:ro -it your-docker-image

• Multiple Volume Mounts: We can mount many folders by adding more -v options. For example:

  docker run -v /path/to/host1:/path/in/container1 -v /path/to/host2:/path/in/container2 -it your-docker-image

Using volume mounts is a simple and good way to handle file transfers between our Docker container and host. It helps us access and change files without trouble. For more detailed info about Docker volumes, we can check the Docker documentation on Docker Volumes.

Solution 3 - Using tar command within the container

One good way to copy files from a Docker container to the host is to use the tar command. This method helps us transfer many files or folders at once. It also compresses data to save space and bandwidth.

Steps to Use tar Command

1. Access the Docker Container: First, we need to enter the Docker container where our files are. We can do this with the docker exec command. Change container_name_or_id to the real name or ID of your container.

   docker exec -it container_name_or_id /bin/sh

   Or if our container uses bash:

   docker exec -it container_name_or_id /bin/bash

2. Create a Tarball: Once we are inside the container, we can use the tar command to make a tarball (archive) of the files or folders we want to copy. Change /path/to/files to the real path of the files or folders we want to archive.

   tar czf /tmp/myfiles.tar.gz /path/to/files

   Here, c means create, z means gzip compression, and f is for the filename of the archive.

3. Exit the Container: After we make the tarball, we can exit the container:

   exit

4. Copy the Tarball to Host: Now we have the tarball in the container. We can copy it to our host using the docker cp command. Change container_name_or_id and the path on your host as needed.

   docker cp container_name_or_id:/tmp/myfiles.tar.gz /path/on/host/

5. Extract the Tarball on Host: Finally, on our host machine, we go to the folder where we copied the tarball and extract it using the tar command.

   tar xzf myfiles.tar.gz

Example

Here is a complete example of these steps:

1. Enter the container:

   docker exec -it my_container /bin/bash

2. Create a tarball of the /data folder:

   tar czf /tmp/data_backup.tar.gz /data

3. Exit the container:

   exit

4. Copy the tarball to the host:

   docker cp my_container:/tmp/data_backup.tar.gz /home/user/

5. Extract the tarball on the host:

   cd /home/user/
   tar xzf data_backup.tar.gz

Using the tar command is a great way to copy files from a Docker container to our host. This is especially helpful when we deal with many files or folders. For more methods, we can look into using volume mounts or the docker cp command directly.

Solution 4 - Copying files using Dockerfile with COPY command

We can use the COPY command in a Dockerfile to easily copy files from our host machine to the image. This happens while we build the image. It is helpful when we want to add configuration files, application code, or any other important files into our Docker container from the beginning.

Syntax

The basic syntax for the COPY command is:

COPY <src> <dest>

• <src>: This is where the file or folder is on the host.
• <dest>: This is the path in the container where we want to copy the file or folder.

Example

Here is an example of how to use the COPY command in a Dockerfile:

# Use an official base image
FROM ubuntu:latest

# Set the working directory
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . .

# Install dependencies (if needed)
RUN apt-get update && apt-get install -y some-package

# Command to run your application
CMD ["your-command"]

Important Notes

1. Context: The COPY command only works with files and folders in the build context. We must make sure the files we want to copy are in the same folder as our Dockerfile or in a subfolder.

2. Permissions: The files we copy will keep their permissions from the host. But we need to check that the container has the right permissions to access and run these files.

3. Layering: Each COPY command makes a new layer in the image. To reduce image size, we should think about combining many files into one copy action when we can.

4. Ignore Files: If we want to skip certain files from being copied, we can make a .dockerignore file in the same folder as our Dockerfile. This file can list patterns of files and folders to ignore, like .gitignore.

Best Practices

• We should use the COPY command for static files like configuration files, application code, and assets.
• If we need to copy files from a URL or do more complex things, we can think about using the ADD command. But we prefer COPY for simple file transfers.
• For more details about Dockerfile commands, we can check the official Docker documentation.

With the COPY command, we can build Docker images easily with all the needed files included. This makes it easier to deploy and scale our applications.

Solution 5 - Using rsync for easy file transfer

We can use rsync to sync files between a Docker container and the host system. This tool is good because it only sends the changes between the source and the destination. This can make transfer times much shorter, especially when we deal with big datasets.

Prerequisites

• We need to make sure rsync is installed on both the host and the Docker container.
• We must have access to the Docker container’s shell.

Steps to Copy Files Using rsync

1. Install rsync (if it is not installed yet):

   For Debian-based Docker images like Ubuntu, we run:

   apt-get update && apt-get install -y rsync

   For Red Hat-based Docker images like CentOS, we run:

   yum install -y rsync

2. Run the Docker Container:

   If our container is not running yet, we can start it with:

   docker run -d --name my_container your_image

3. Use rsync to Copy Files:

   We can use this command to copy files from the Docker container to the host:

   docker exec -i my_container rsync -avz /path/to/source/ /path/to/destination/on/host

   Here is what the command does:

   • docker exec -i my_container: This runs a command inside the container.
   • rsync -avz: The -a option means archive mode, -v means verbose output, and -z compresses file data during transfer.
   • /path/to/source/: This is the path in the container where we want to copy files from.
   • /path/to/destination/on/host: This is the path on the host where we want to copy files to.

4. Example:

   If we want to copy files from /var/log in the container to /tmp/logs on the host, we run:

   docker exec -i my_container rsync -avz /var/log/ /tmp/logs

5. Using rsync over SSH:

   If we work with a remote server or need to use SSH for the transfer, we must make sure SSH is set up right. Then we can use:

   docker exec -i my_container rsync -avz -e "ssh -p PORT" /path/to/source user@host:/path/to/destination

Benefits of Using rsync

• Efficiency: It only transfers files that are changed.
• Resume Capability: If the transfer stops, we can start it again.
• Bandwidth Limitation: We can limit how much bandwidth we use during transfer.

By following these steps, we can easily copy files from a Docker container to the host using rsync. This way is very helpful for large file transfers or when we need to keep folders in sync. For more details on working with Docker containers, you can check out Docker’s official documentation.

Solution 6 - Extracting files from a stopped container

When we need to get files from a stopped Docker container, we have a few methods. The easiest way is to use the docker cp command. This command lets us copy files directly from a container to our host system. But sometimes, if the container is stopped, we may have trouble using docker cp directly. Here is how we can extract files from a stopped container:

Method 1: Using docker cp

We can still use the docker cp command to copy files from a stopped container. The basic way to do this is:

docker cp <container_id>:/path/to/file/on/container /path/to/destination/on/host

Example:

1. First, we list our stopped containers to find the container ID:

   docker ps -a

2. Then, we find the container ID or name of the stopped container. After that, we run the copy command:

   docker cp <container_id>:/usr/src/app/myfile.txt /home/user/myfile.txt

Here, myfile.txt is copied from the stopped container at /usr/src/app/ to the host’s /home/user/ folder.

Method 2: Using a Temporary Container

If we want to get a whole directory or many files, we can create a temporary container. This temporary container can mount the filesystem of the stopped container. We can do this with the --volumes-from option:

1. We create a temporary container that mounts the volumes of the stopped container:

   docker run --rm --volumes-from <stopped_container_id> -v /host/path:/mnt busybox tar cvf /mnt/backup.tar /path/in/container

2. This command makes a tarball of the files in the chosen path inside the stopped container. It puts this tarball in the chosen path on the host.

Example:

If we want to get all files from a stopped container’s /usr/src/app/ directory, we can run:

docker run --rm --volumes-from <stopped_container_id> -v /home/user:/mnt busybox tar cvf /mnt/app_backup.tar /usr/src/app/

This command will create a file called app_backup.tar in the /home/user folder on the host. This file will contain all files from /usr/src/app/ in the stopped container.

Important Considerations

• We should make sure we have the right permissions on the host path where we want to copy files.
• The busybox image is small and often used for these tasks. If we need a different setup, we can change busybox to another image that works for us.
• If we have problems with accessing files or permissions, we should check the user settings of the container and the permissions of the host directory.

For more details about Docker commands and managing containers, we can check this resource on Docker commands.

Conclusion

In this article, we looked at different ways to copy files from a Docker container to the host. We talked about using the docker cp command and volume mounts. Each method has its own perks. For example, using the tar command or rsync can be useful based on what we need.

Knowing these methods can help us improve our Docker workflow. It also makes file management easier. If we want to learn more about Docker, we should check our guides on Docker networking and Docker volumes.