bash - What is ${DEBUG- } means in yaml config?

Question

This question already has answers here: question from:https://stackoverflow.com/questions/65623205/what-is-debug-means-in-yaml-config

Answer 1

Not sure about YAML, but in bash, the expression ${VARNAME-DEFAULT} means: Replace by the contents of the variable VARNAME if it exists, otherwise use DEFAULT

Here's an example:

$ echo "Hello ${PL-Earth}"
Hello Earth
$ PL=Neptune
$ echo "Hello ${PL-Earth}"
Hello Neptune
$ unset PL
$ echo "Hello ${PL-Earth}"
Hello Earth

When I look at your yaml file, it looks very similar: If DEBUG is set, use whatever is set, otherwise use a single space, etc. This is actually a common usage, adding the line

DEBUG=${DEBUG- }

ensures that a variable with the name DEBUG exists, even if it contains only a space.

Well, as for how to use it, set the corresponding environment variables before you execute whichever program uses this yaml file.

$ export DEBUG='-g -O0 -fbacktrace -Wall'
$ <Call to program>

I don't know much about docker, but there is almost certainly a way to set environment variables in a docker configuration.