escaping - How can I escape a $ dollar sign in a docker compose file?

Question

I have a YAML scalar that is throwing the following error when I try to evaluate my docker-compose.yml file:

ERROR: Invalid interpolation format for "environment" option in service "time_service": "${Time.now}"

YAML:

---
version: '2'
services:
  time_service:
    build: "."
    environment:
      TIME: "${Time.now}"

How can I maintain the same string output as written, but avoid having the docker-compose interpret it as faulty string interpolation?

Answer 1

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign.

You are hitting the docker-compose variable substition, which is well documented here:

Both $VARIABLE and ${VARIABLE} syntax are supported. Extended shell-style features, such as ${VARIABLE-default} and ${VARIABLE/foo/bar}, are not supported.

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign. This also prevents Compose from interpolating a value, so a $$ allows you to refer to environment variables that you don’t want processed by Compose.

docker-compose is written in Python, as you see on github, the doubling mechanism to get the original meaning of special characters can be found in many programs, I needed to use this myself, while programming, as far back in 1984.