Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How do containers interact with my local file system?

Objectives

• First learning objective.

Copying

Copying files between the local host and Docker containers is possible. On your local host find a file that you want to transfer to the container and then

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

/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 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

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

• First key point. Brief Answer to questions.