Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How do containers interact with my local file system?

Objectives

• Copy files to and from the docker container

• Mount directories to be accessed and manipulated by the docker container

Copying

Copying files between the local host and Docker containers is possible. On your local host, either find a file that you want to transfer to the container or create a new one. Below is the procedure for creating a new file called io_example.txt and then copying it to the container:

touch io_example.txt
# If on Mac need to do: chmod a+w io_example.txt
echo "This was written on local host" > io_example.txt
docker cp io_example.txt <NAME>:/home/docker/data/

and then from the container check and modify it in some way

pwd
ls
cat io_example.txt
echo "This was written inside Docker" >> io_example.txt

Permission issues

If you run into a Permission denied error, there is a simple and quick fix:

exit  # exit container
chmod a+w io_example.txt  # add write permissions for all users

And continue from the docker cp ... command above.

/home/docker/data
io_example.txt
This was written on local host

and then on the local host copy the file out of the container

docker cp <NAME>:/home/docker/data/io_example.txt .

and verify if you want that the file has been modified as you wanted

cat io_example.txt

This was written on local host
This was written inside Docker

Volume mounting

What is more common and arguably more useful is to mount volumes to containers with the -v flag. This allows for direct access to the host file system inside of the container and for container processes to write directly to the host file system.

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

For example, to mount your current working directory ($PWD) on your local machine to the data directory in the example container

docker run --rm -it -v $PWD:/home/docker/data matthewfeickert/intro-to-docker

From inside the container you can ls to see the contents of your directory on your local machine

ls

and yet you are still inside the container

pwd

/home/docker/data

You can also see that any files created in this path in the container persist upon exit

touch created_inside.txt
exit
ls *.txt

Permission issues

If you are using Linux with SELinux enabled, you might run into a Permission denied error. Note that SELinux is enabled if the output of the command getenforce status is Enforcing. To fix the permission issue, append :z (lowercase!) at the end of the mount option, like this:

docker run --rm -it -v $PWD:/home/docker/data:z ...

If this still does not fix the issue you can disable SELinux by running sudo setenforce 0, or you can try using sudo to execute docker commands, but both of these methods are not recommended.

created_inside.txt

This I/O allows for Docker images to be used for specific tasks that may be difficult to do with the tools or software installed on only the local host machine. For example, debugging problems with software that arise on cross-platform software, or even just having a specific version of software perform a task (e.g., using Python 2 when you don’t want it on your machine, or using a specific release of TeX Live when you aren’t ready to update your system release).

Key Points

• Copy files with docker cp

• Mount volumes with docker run -v <path on host>:<path in container> <image>