I opted for this instead

#!/bin/sh

[[ -n "${BUILD_VERSION-}" ]] || export BUILD_VERSION=local

I guess this is to have a default in bootstrap, which you can override from the env. I have this in one of my bootstrap files, which I think reads cleaner 😉

export BRANCH=${BRANCH:-$(git rev-parse --abbrev-ref HEAD)}
export BRANCH_NAME=${BRANCH_NAME:-${BRANCH//\//_}}
export BUILD_NUMBER=${BUILD_NUMBER:-0}
export GIT_PREVIOUS_SUCCESSFUL_COMMIT=${GIT_PREVIOUS_SUCCESSFUL_COMMIT:-$(git rev-parse HEAD)}
export TEST_DB_CONNECTION=${TEST_DB_CONNECTION:-sqlite://}