Variable substitution in docker-compose .yml file

Question

I was trying to use variable substitution for a volume declaration. This led to the fallowing error:

ERROR: for php  Cannot create container for service php: create .: volume name is too short, names should be at least two alphanumeric characters

Docker's Compose file:

services:        
  php:
      image: php5-apache-composer
      volumes:
        - ${DIR}:/var/www/html

Is variable substitution supported for the volume declaration? I can use environment variables for any other declaration except this one.

Answer 1

You need to use .env file in folder where docker-compose.yaml is in order to declaring default environment variables for docker-compose.yaml file.

Just create .env file with the following content:

DIR=/var/www/html

Of course you can do that dinamically on each build like:

echo "DIR=/var/www/html" > .env && docker-compose up

Answer 2

I've run export DIR="/var/www/html" and sudo docker-compose run php -e DIR="/var/www/html"

The sudo creates a different environment for the command it's running, the DIR variable doesn't exist there. You can check that with a simple env command:

$ export USER_VAR=test
$ sudo env | grep USER_VAR
$

To get this to work, you'll need to either:

1. Create a .env and let docker-compose source the variable from that.
2. Run a shell with sudo -s and run both commands as root from there.
3. Run a shell as the sudo command, eg: sudo /bin/sh -c "DIR=/var/www/html docker-compose run php -e DIR=/var/www/html"