Docker Compose allows you to configure volumes and bind mounts using a short syntax. A few examples:

./public:/usr/share/nginx/html

/var/lib/postgresql/data

/some/content:/usr/share/nginx/html

~/configs:/etc/configs

postgresql:/var/lib/postgresql/data

Which of these are volumes and which are a bind mounts?

Whenever I had to read a docker-compose.yml file, I had to look up the official documentation or run a quick local experiment to figure out how Docker Compose would mount the directories into the container.

I wrote this article so that next time you read a Docker Compose file, you won’t have to guess anymore. You’ll simply know by looking at the syntax whether a volume or a bind mount is used behind the scenes.

The different variations are essentially three unique forms. I list and explain them in this article below.

Two volumes keys in docker-compose.yml

Before we talk about the different ways to specify volumes, let’s first clarify which volumes key we’re referring to. In docker-compose.yml, the volumes key can appear in two different places.

version: "3.7"

services:
  database:
    # ...
    volumes: # Nested key. Configures volumes for a particular service.

volumes: # Top-level key. Declares volumes which can be referenced from multiple services.
  # ...

In this article, we’ll talk about the nested volumes key. That’s where you configure volumes for a specific service/container such as a database or web server. This configuration has a short (and a long) syntax format.

Short syntax format and its variations

The volume configuration has a short syntax format that is defined as:

[SOURCE:]TARGET[:MODE]

SOURCE can be a named volume or a (relative or absolute) path on the host system. TARGET is an absolute path in the container. MODE is a mount option which can be read-only or read-write. Brackets mean the argument is optional.

This optionality leads to three unique variations you can use to configure a container’s volumes. Docker Compose is smart about recognising which variety is used and whether to use a volume or bind mount.

#docker #syntax #volume #bind mount