Is there a default value for localstack's ${LOCALSTACK_VOLUME_DIR} variable if one is not defined?

In a previous SO question I asked what some environment variables are inside a LocalStack's docker-compose file; for example, a variable reference ${LOCALSTACK_VOLUME_DIR:-./volume}.

For the variable ${LOCALSTACK_VOLUME_DIR} : if this is not defined in an environment variable, what will happen?

The docs says this (but I don't understand / find it ambiguous):

> When using the CLI to start LocalStack, the volume directory can be configured via the LOCALSTACK_VOLUME_DIR. It should point to a directory on the host which is then automatically mounted into /var/lib/localstack. The defaults are:
>
> Mac: ~/Library/Caches/localstack/volume
> Linux: ~/.cache/localstack/volume
> Windows: %LOCALAPPDATA%/localstack/cache/volume

• Firstly, I'm not sure if using docker compose is considered "CLI to start LocalStack"?

• Secondly, is this suggesting that if this value is not provided anywhere (ie. no env var) then it will replace the empty/missing value, with one of those from the list above ☝🏻 based on the host OS?

The ${LOCALSTACK_VOLUME_DIR:-./volume} syntax provides a default: if the LOCALSTACK_VOLUME_DIR environment variable is either not set or set to an empty string in the host environment when docker-compose runs, then the text ./volume is used instead.

Compose supports a limited number of environment variable expansion syntaxes: ${VARIABLE:-default} provides a default value, and ${VARIABLE:?error} prints an error if a value is empty or isn't set. Both syntaxes can be used without the colon :, in which case a set-but-empty value is accepted. This is a subset of the standard Bourne shell parameter expansion syntaxes; the remainder are not supported.

Your original question had a PNG file that looked like it contained this more complete Compose fragment:

volumes:
  - ${LOCALSTACK_VOLUME_DIR:-./volume}:/var/lib/localstack

So if $LOCALSTACK_VOLUME_DIR is set on the host, that directory is mounted into /var/lib/localstack; if nothing is set, then ./volume is (relative to the current directory, created if necessary).

This specific directory name doesn't match the documentation you quote, but the behavior (that the directory is mounted on to /var/lib/localstack) is correct. There isn't Compose syntax to specify an OS-specific default like what that documentation would imply.