How to Share Data Across the Docker Container and the Host

 

Prerequisites

To follow this article, you will need an Ubuntu 16.04 server with the following:

• A non-root user with sudo privileges
• Docker installed with the instructions from the article of How to Install and Use Docker on Ubuntu 16.04

 

Bindmounting a Volume

The following command will create a directory call nginxlogs in the home directory of the localhost user and bindmount it to /var/log/nginx in the container:

docker run --name=nginx -d -v ~/nginxlogs:/var/log/nginx -p 5000:80 nginx 

Let's take a moment to examine this command in detail:

• --name=nginx names the container so we can refer to it more easily.
• -d detaches the process and runs in the background. Otherwise, we would just be watching an empty Nginx prompt and wouldn't be able to use this terminal until we killed Nginx.
• -v ~/nginxlogs:/var/log/nginx Sets up a bindmount volume that links the /var/log/nginx directory from inside the Nginx container to the ~/nginxlogs directory on the host machine. Docker uses a : to split the host's path from the container path, and the host path always comes first.
• -p 5000:80 sets up a port forward. The Nginx container is listening on port 80 by default. This flag maps the container's port 80 to port 5000 on the host system.
• nginx specifies that the container should be built from the Nginx image, which issues the command nginx -g "daemon off to start Nginx.

Note: The -v flag is very flexible. It can bindmount or name a volume with just a slight adjustment in syntax. If the first argument begins with a / or ~/ you're creating a bindmount. Remove that, and you're naming the volume.

• -v /path:/path/in/container mounts the host directory, /path at the /path/in/container
• -v path:/path/in/container creates a volume named path with no relationship to the host.

For more on named volumes, see How to Share Data Between Docker Containers

 

Accessing Data on the Host

We now have a copy of Nginx running inside a Docker container on our machine, and our host machine's port 5000 maps directly to that copy of Nginx's port 80.

Load the address in a web browser, using the IP address or hostname of your server and the port number, http://203.0.113.0:5000. You should see:







More interestingly, if we look in the ~/nginxlogs directory on the host, we'll see the access.log created by the container's nginx which will show our request:

cat ~/nginxlogs/access.log 

This should display something like:

Output

203.0.113.0 - - [11/Nov/2016:00:59:11 +0000] "GET / HTTP/1.1" 200 612 "-"
"Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 
(KHTML, like Gecko) Chrome/54.0.2840.99 Safari/537.36""-"

If you make any changes to the ~/nginxlogs folder, you'll be able to see them from inside the Docker container in real-time as well.

 

Conclusion