Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How do containers interact with my local file system?

Objectives

• Better understand I/O with containers

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 may need to do: chmod a+x io_example.txt
echo "This was written on local host" > io_example.txt
docker cp io_example.txt <CONTAINER ID>:/home/docker/data/

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

pwd
ls io_example.txt
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 <CONTAINER ID>:/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 -ti -v $PWD:/home/docker/data matthewfeickert/intro-to-docker:latest

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).

Flag choices

What will be the result of running the following command?

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

Solution

Outwardly it would appear that there is no affect! You are returned to your starting terminal. However, something did happen. Look again at the flags: --rm -v …but no -ti for interactive. So the container got spun up by docker run, wasn’t given any command and and so executed a Bash shell with /bin/bash, wasn’t put into an interactive state and finished, and then cleaned itself up with --rm.

Running Jupyter from a Docker Container

You can run a Jupyter server from inside of your Docker container. First run a container (with Jupyter installed) while exposing the container’s internal port 8888 with the -p flag

docker run --rm -ti -p 8888:8888 matthewfeickert/intro-to-docker:latest /bin/bash

Then start a Jupyter server with the server listening on all IPs

jupyter lab --allow-root --no-browser --ip 0.0.0.0

though for your convince the example container has been configured with these default settings so you can just run

jupyter lab

Finally, copy and paste the following with the generated token from the server as <token> into your web browser on your local host machine

http://localhost:8888/?token=<token>

You now have access to Jupyter running on your Docker container.

Key Points

• Learn how docker cp works

• Learn about volume mounts

• Show port forwarding of applications