set-release-name command: Use branch name or fall back to "production" if it's a tag

[Rade333]

Let's break down this shell variable assignment:

BRANCHNAME="${CIRCLE_BRANCH:-${CIRCLE_TAG:+production}}"

This is using shell parameter expansion with fallback and conditional logic. Let's break it down from inside out:

${CIRCLE_TAG:+production} 
    This is using the :+ operator
    If CIRCLE_TAG is set and not empty, it evaluates to "production"
    If CIRCLE_TAG is unset or empty, it evaluates to empty string
     

${CIRCLE_BRANCH:-...} 
    This is using the :- operator
    If CIRCLE_BRANCH is set and not empty, use its value
    If CIRCLE_BRANCH is unset or empty, use the value of what follows after :-

So putting it all together:

If CIRCLE_BRANCH is set:  
    BRANCHNAME = value of CIRCLE_BRANCH
     

If CIRCLE_BRANCH is empty/unset AND CIRCLE_TAG is set: 
    BRANCHNAME = "production"
     

If both CIRCLE_BRANCH and CIRCLE_TAG are empty/unset: 
    BRANCHNAME = empty string