How To Attach A Volume To A Running Docker Container

Table of contents

• Introduction
• Attaching a volume to a new container
• Attaching volumes to a running container

Introduction

A Docker volume [↗] allows you to access folders from your local system from within a Docker container [↗].

This allows you to persist data and files to your host machine instead of the "virtual" one (container).

For example, a volume that maps to your local /var/lib/mysql allows the MYSQL database files to be persisted to your local machine, which means if you squash and start a new container with the same volume mapping your data should be available in the new container.

Attaching a volume to a new container

If you are about to spin a new container from an image with a volume attached this is straightforward:

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$ docker run  -v <local_path>:<container_path>

Attaching volumes to a running container

You can't attach a volume to a running container, you can however start a container with one.

Let's say you already have a running container and starting a new one will mean losing some changes you made to the running container.

We can follow a two-step process to get around this.

Step 1 (Container to image)

Since we can only attach a volume to a new container, we need to convert our running container into an image then we can start a new container based on the new image.

We use the Docker commit [↗] command for this.

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01: docker commit <container_id> <new_image_name>
02: 
03: # eg
04: docker commit 277356 my_new_image

• : This is the ID of the running container. You can view details of all running containers using docker ps.
• : This will be the name of the new image that will be created.

Step 2 (From an image to a container with a volume attached)

To spin a new container with a volume we will be using the -v flag.

Here is the command structure docker run -v :< container_path>

Example

Let's assume the following:

• Saved image name: my_new_image
• Local path: /home/user/mySite
• Container path: /var/www/

The resulting command will be:

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$ docker run  -v /home/user/mySite:/var/www my_new_image

You can and should of course add all the other command flags you need.

Note that if you are running a different OS (host OS) from the one the container is based on, you might not be able to mount a volume, especially if the path you are mapping is a systems directory. A mapping for a path like /var/www might work fine, but maybe not /var/lib.

Here is another article you might like 😊 How To Copy Files/folders Between A Docker Container And Your Local Filesystem